2015-18075. Revision to the Guideline on Air Quality Models: Enhancements to the AERMOD Dispersion Modeling System and Incorporation of Approaches To Address Ozone and Fine Particulate Matter

AGENCY:

Environmental Protection Agency (EPA).

ACTION:

Proposed rule; notice of conference.

SUMMARY:

In this action, the Environmental Protection Agency (EPA) proposes to revise the Guideline on Air Quality Models (“Guideline”). The Guideline has been incorporated into EPA's regulations, satisfying a requirement under the Clean Air Act (CAA) section 165(e)(3) for the EPA to specify, with reasonable particularity models to be used in the Prevention of Significant Deterioration (PSD) program. It provides EPA-preferred models and other recommended techniques, as well as guidance for their use in predicting ambient concentrations of air pollutants. The proposed revisions to the Guideline include enhancements to the formulation and application of the EPA's AERMOD near-field dispersion modeling system and the incorporation of a tiered demonstration approach to address the secondary chemical formation of ozone and fine particulate matter (PM_2.5) associated with precursor emissions from single sources. Additionally, the EPA proposes various editorial changes to update and reorganize information throughout the Guideline to streamline the compliance assessment process.

Within this action, the EPA is also announcing the Eleventh Conference on Air Quality Modeling and invites the public to participate in the conference. The conference will focus on the proposed revisions to the Guideline and part of the conference will also serve as the public hearing for these revisions.

DATES:

Comments must be received on or before October 27, 2015.

Public hearing and conference: The public hearing for this action and the Eleventh Conference on Air Quality Modeling will be held August 12-13, 2015, from 8:30 a.m. to 5:00 p.m.

ADDRESSES:

Submit your comments, identified by Docket ID No. EPA-HQ-OAR-2015-0310, by one of the following methods:

• Federal eRulemaking Portal: http://www.regulations.gov. Follow the online instructions for submitting comments.
• Email: [e-mail]. Include docket ID No. EPA-HQ-OAR-2015-0310 in the subject line of the message.
• Fax: [phone].
• Mail: Environmental Protection Agency, Mail code 28221T, Attention Docket No. EPA-HQ-OAR-2015-0310, 1200 Pennsylvania Ave. NW., Washington, DC 20460. Please include a total of two copies.
• Hand/Courier Delivery: EPA Docket Center, Room 3334, EPA WJC West Building, 1301 Constitution Ave. NW., Washington, DC. Such deliveries are only accepted during the Docket's normal hours of operation, and special arrangements should be made for deliveries of boxed information.

Instructions: Direct your comments to Docket ID No. EPA-HQ-OAR-2015-0310. The EPA's policy is that all comments received will be included in the public docket without change and may be made available online at http://www.regulations.gov, including any personal information provided, unless the comment includes information claimed to be Confidential Business Information (CBI) or other information whose disclosure is restricted by statute. Do not submit information that you consider to be CBI or otherwise protected through http://www.regulations.gov or email. The www.regulations.gov Web site is an “anonymous access” system, which means the EPA will not know your identity or contact information unless you provide it in the body of your comment. If you send an email comment directly to the EPA without going through http://www.regulations.gov, your email address will be automatically captured and included as part of the comment that is placed in the public docket and made available on the Internet. If you submit an electronic comment, the EPA recommends that you include your name and other contact information in the body of your comment and with any disk or CD ROM you submit. If the EPA cannot read your comment due to technical difficulties and cannot contact you for clarification, the EPA may not be able to consider your comment. Electronic files should avoid the use of special characters, any form of encryption, and be free of any defects or viruses. For additional information about the EPA's public docket, visit the EPA Docket Center homepage at http://www.epa.gov/​epahome/​dockets.htm.

Docket: All documents in the docket are listed in the http://www.regulations.gov index. Although listed in the index, some information is not publicly available, e.g., CBI or other information whose disclosure is restricted by statute. Certain other material, such as copyrighted material, will be publicly available only in hard copy. Publicly available docket materials are available either electronically in www.regulations.gov or in hard copy at the Air and Radiation Docket and Information Center, EPA/DC, Room 3334, WJC West Building, 1301 Constitution Ave. NW., Washington, DC. The Public Reading Room is open from 8:30 a.m. to 4:30 p.m., Monday through Friday, excluding legal holidays. The telephone number for the Public Reading Room is [phone] and the telephone number for the Air and Radiation Docket and Information Center is [phone].

Public hearing and conference: The public hearing for this action and the Eleventh Conference on Air Quality Modeling will be held in the EPA Auditorium, Room C111, 109 T.W. Alexander Drive, Research Triangle Park, NC 27711.

FOR FURTHER INFORMATION CONTACT:

Mr. George M. Bridgers, Air Quality Assessment Division, Office of Air Quality Planning and Standards, U.S. Environmental Protection Agency, Mail code C439-01, Research Triangle Park, NC 27711; telephone: [phone]; fax: [phone]; email: [e-mail].

SUPPLEMENTARY INFORMATION:

Table of Contents

The following topics are discussed in this preamble:

I. General Information

A. Does this action apply to me?

B. What should I consider as I prepare my comments for the EPA?

C. Where can I get a copy of this document?

II. Background

A. The Guideline on Air Quality Models and EPA Modeling Conferences

B. The Tenth Conference on Air Quality Modeling

III. Public Participation Regarding Revisions to the Guideline and Notice of Eleventh Conference on Air Quality Modeling

IV. Proposed Changes to the Guideline

A. Proposed Actions

1. Clarifications To Distinguish Requirements From Recommendations

2. Updates to EPA's AERMOD Modeling System

3. Status of AERSCREEN

4. Updates to 3-Tiered Demonstration Approach for NO₂

5. Status of CALINE3 Models

6. Addressing Single-Source Impacts on Ozone and Secondary PM_2.5

7. Status of CALPUFF and Assessing Long-Range Transport for PSD Increment and Regional Haze

8. Role of EPA's Model Clearinghouse

9. Updates to Modeling Procedures for Cumulative Impact Analysis

10. Updates on Use of Meteorological Input Data for Regulatory Dispersion Modeling

11. Transition Period for Applicability of Revisions to the Guideline

B. Proposed Editorial Changes

V. Statutory and Executive Order Reviews

A. Executive Order 12866: Regulatory Planning and Review and Executive Order 13563: Improving Regulation and Regulatory Review

B. Paperwork Reduction Act

C. Regulatory Flexibility Act

D. Unfunded Mandates Reform Act

E. Executive Order 13132: Federalism

F. Executive Order 13175: Consultation and Coordination With Indian Tribal Governments

G. Executive Order 13045: Protection of Children From Environmental Health and Safety Risks

H. Executive Order 13211: Actions Concerning Regulations That Significantly Affect Energy Supply, Distribution, or Use

I. National Technology Transfer and Advancement Act

J. Executive Order 12898: Federal Actions To Address Environmental Justice in Minority Populations and Low-Income Populations

I. General Information

A. Does this action apply to me?

This action applies to federal, state, territorial, and local air quality management programs that conduct air quality modeling as part of State Implementation Plan (SIP) submittals and revisions, New Source Review (NSR), including new or modifying industrial sources under Prevention of Significant Deterioration (PSD), Conformity, and other air quality assessments required under EPA regulation. Categories and entities potentially regulated by this action include:

Category	NAICS a Code
Federal/state/territorial/local/tribal government	924110
a North American Industry Classification System.

B. What should I consider as I prepare my comments for the EPA?

1. Submitting CBI. Do not submit this information to the EPA through http://www.regulations.gov or email. Clearly mark any of the information that you claim to be CBI. For CBI information in a disk or CD ROM that you mail to the EPA, mark the outside of the disk or CD ROM as CBI and then identify electronically within the disk or CD ROM the specific information that is claimed as CBI. In addition to one complete version of the comment that includes information claimed as CBI, a copy of the comment that does not contain the information claimed as CBI must be submitted for inclusion in the public docket. Information so marked will not be disclosed except in accordance with procedures set forth in 40 CFR part 2.

2. Tips for preparing your comments. When submitting comments, remember to:

• Follow directions—The agency may ask you to respond to specific questions or organize comments by referencing a CFR part or section number.
• Explain why you agree or disagree, suggest alternatives, and substitute language for your requested changes.
• Describe any assumptions and provide any technical information and/or data that you used.
• If you estimate potential costs or burdens, explain how you arrived at your estimate in sufficient detail to allow for it to be reproduced.
• Provide specific examples to illustrate your concerns and suggest alternatives.
• Explain your views as clearly as possible, avoiding the use of profanity or personal threats.
• Make sure to submit your comments by the comment period deadline identified.

C. Where can I get a copy of this document?

In addition to being available in the docket, an electronic copy of this proposed rule will also be available on the Worldwide Web (WWW) through the EPA's Technology Transfer Network (TTN). Following signature, a copy of this proposed rule will be posted on the TTN's Support Center for Regulatory Atmospheric Modeling (SCRAM) Web site at the following address: http://www.epa.gov/​ttn/​scram. The TTN provides information and technology exchange in various areas of air pollution control.

II. Background

A. The Guideline on Air Quality Models and EPA Modeling Conferences

The Guideline is used by the EPA, other federal, state, territorial, and local air quality agencies, and industry to prepare and review new source permits, source permit modifications, SIP submittals and revisions, conformity, and other air quality assessments required under EPA regulation. The Guideline serves as a means by which national consistency is maintained in air quality analyses for regulatory activities under 40 CFR 51.112, 51.117, 51.150, 51.160, 51.165, 51.166, 52.21, 93.116, 93.123, and 93.150.

The EPA originally published the Guideline in April 1978 (EPA-450/2-78-027), and it was incorporated by reference in the regulations for the PSD program in June 1978. The EPA revised the Guideline in 1986 (51 FR 32176), and updated it with supplement A in 1987 (53 FR 32081), supplement B in July 1993 (58 FR 38816), and supplement C in August 1995 (60 FR 40465). The EPA published the Guideline as appendix W to 40 CFR part 51 when the EPA issued supplement B. The EPA republished the Guideline in August 1996 (61 FR 41838) to adopt the CFR system for labeling paragraphs. The publication and incorporation of the Guideline by reference into the EPA's PSD regulations satisfies the requirement under the CAA section 165(e)(3) for the EPA to promulgate regulations that specify with reasonable particularity models to be used under specified sets of conditions for purposes of the PSD program.

To support the process of developing and revising the Guideline during the period of 1977-1988, we held the First, Second, and Third Conferences on Air Quality Modeling as required by CAA section 320 to help standardize modeling procedures. These modeling conferences provided a forum for comments on the Guideline and associated revisions, thereby helping us introduce improved modeling techniques into the regulatory process.

In October 1988, we held the Fourth Conference on Air Quality Modeling to advise the public on new modeling techniques and to solicit comments to guide our consideration of any rulemaking needed to further revise the Guideline. We held the Fifth Conference in March 1991, which also served as a public hearing for the proposed revisions to the Guideline. In August 1995, we held the Sixth Conference as a forum to update our available modeling tools with state-of-the-science techniques and for the public to offer new ideas.

The Seventh Conference was held in June 2000, and also served as a public hearing for another round of proposed changes to the recommended air quality models in the Guideline. These changes included the CALPUFF modeling system, AERMOD modeling system, and ISC-PRIME model.

Subsequently, the EPA revised the Guideline on April 15, 2003 (68 FR 18440), to adopt CALPUFF as the preferred model for long-range transport of emissions from 50 to several hundred kilometers and to make various editorial changes to update and reorganize information and remove obsolete models.

We held the Eighth Conference on Air Quality Modeling in September 2005. This conference provided details on changes to the preferred air quality models, including available methods for model performance evaluation and the notice of data availability that the EPA published in September 2003, related to the incorporation of the PRIME downwash algorithm in the AERMOD dispersion model (in response to comments received from the Seventh Conference). Additionally, at the Eighth Conference, a panel of experts discussed the use of state-of-the-science prognostic meteorological data for informing the dispersion models.

The EPA further revised the Guideline on November 9, 2005 (70 FR 68218), to adopt AERMOD as the preferred model for near-field dispersion of emissions for distances up to 50 kilometers.

The Ninth Conference on Air Quality Modeling was held in October 2008, and emphasized the following topics: Reinstituting the Model Clearinghouse, review of non-guideline applications of dispersion models, regulatory status updates of AERMOD and CALPUFF, continued discussions on the use of prognostic meteorological data for informing dispersion models, and presentations reviewing the available model evaluation methods.

B. The Tenth Conference on Air Quality Modeling

The most recent EPA modeling conference was the Tenth Conference on Air Quality Modeling held in March 2012. This conference covered multiple topics which have been vital in the development of the proposed revisions to the Guideline. The conference addressed updates on the regulatory status and future development of AERMOD and CALPUFF, review of the Mesoscale Model Interface (MMIF) prognostic meteorological data processing tool for dispersion models, draft modeling guidance for compliance demonstrations of the PM_2.5 National Ambient Air Quality Standards (NAAQS), modeling for compliance demonstration of the 1-hour nitrogen dioxide (NO₂) and sulfur dioxide (SO₂) NAAQS, and new and emerging models/techniques for future consideration under the Guideline to address single-source modeling for ozone and secondary PM_2.5, as well as long-range transport and chemistry. A transcript of the conference proceedings and a document that summarizes the public comments received are available at EPA's SCRAM Web site at http://www.epa.gov/​ttn/​scram/​10thmodconf.htm.

The EPA promulgated a new 1-hour NAAQS for NO₂ in January 2010, and a new 1-hour NAAQS for SO₂ in June 2010. Although AERMOD evaluations that formed the basis of its promulgation as the EPA's preferred dispersion model demonstrated that AERMOD provides generally unbiased estimates of ambient concentrations, the increased stringency of these new standards resulted in increased scrutiny by the modeling community of AERMOD model performance. In response, the EPA issued several guidance memoranda to clarify the applicability of the Guideline and address initial issues with use of current models and procedures under PSD permitting.^{[1 2 3 4]} However, the situation also necessitated the EPA and the modeling community to more closely evaluate the science and model formulation of AERMOD to better understand the issues being experienced by stakeholders and to address performance issues in its use for PSD permitting under these new standards. As part of this effort, the EPA reconvened the AERMOD Implementation Workgroup (AIWG) with state and local agency modelers to evaluate AERMOD across a variety of hypothetical sources and results from this assessment were also presented at this conference to inform the modeling community of potential implications and areas for improvement in the model and guidance on their use.

Several presentations at the Tenth Modeling Conference addressed issues and challenges associated with demonstrating compliance with these new 1-hour NAAQS for NO₂ and SO₂. This included results from a study sponsored by the American Petroleum Institute (API) that evaluated AERMOD model performance under low wind speed conditions using additional National Oceanic and Atmospheric Administration (NOAA) field studies at Oak Ridge, TN, and Idaho Falls, ID, which were not included in the original 17 databases used to support AERMOD's promulgation in 2005. The API low wind study ^[5] showed significant overprediction of observed concentrations, especially for the Oak Ridge study where observed wind speeds were below 0.5 m/s for 10 of the 11 tracer tests, and included wind speeds as low as 0.15 m/s. The API low wind study also included proposed modifications to the AERMET meteorological processor and AERMOD model to address this bias toward overprediction under stable/light wind conditions.

Prior to the promulgation of the 1-hour NO₂ NAAQS, compliance with the previous annual NO₂ NAAQS was routinely demonstrated based on the Tier 1 assumption of full conversion or a Tier 2 option based on an ambient ratio of 75 percent conversion of nitrogen oxides (NO_X) to NO₂, referred to as the Ambient Ratio Method (ARM). However, compliance with the new 1-hour NAAQS has typically required a more refined treatment of NO_X conversion to NO₂. Therefore, several presentations at the Tenth Modeling Conference focused on issues associated with demonstrating compliance with the new 1-hour NO₂ NAAQS.

These presentations included an overview of an API funded study to develop a Tier 2 ambient ratio method for the 1-hour NO₂ NAAQS, referred to as ARM2. The ARM2 approach was developed based on an extensive analysis of ambient ratios of NO₂/NO_X that were analyzed by land use (urban vs. rural) and geographical areas. Based on these analyses of the ambient NO₂/NO_X ratios, an empirical relationship between ambient concentrations of NO₂ and NO_X was developed. The EPA subsequently reviewed and evaluated this ARM2 approach and then incorporated this screening technique as a non-Default/Beta option in version 13350 of AERMOD in December 2013. Another issue associated with NO₂ NAAQS compliance presented at this conference focused on the use of relative (instantaneous) dispersion coefficients to define the plume volume which determines the amount of ozone available to convert nitrogen (NO) to NO₂ using the Plume Volume Molar Ratio Method (PVMRM) option in AERMOD. The relative dispersion coefficients originally incorporated in AERMOD for PVMRM are best representative of daytime convective conditions and may tend to overestimate plume volumes during stable conditions. Such overestimation of the plume volume will tend to result in PVMRM to overestimate concentrations of NO₂.

In addition, modeling of single-source impacts for ozone and secondarily formed PM_2.5 was a topic of discussion at the Tenth Modeling Conference. On January 4, 2012, the EPA granted a petition submitted on behalf of the Sierra Club on July 28, 2010 ^[6] and committed to engage in rulemaking to evaluate whether updates to the Guideline are warranted and, as appropriate, incorporate new analytical techniques or models for ozone and secondarily formed PM_2.5. As a part of satisfying this commitment, there were presentations of ongoing research at the Tenth Modeling Conference regarding single-source plume chemistry and photochemical grid modeling techniques, as well as several public forums. In addition, the EPA presented an overview along with a panel discussion of its Draft Guideline for PM_2.5 Permit Modeling that addressed the need for consideration of secondary PM_2.5 in demonstrating compliance with the PM_2.5 NAAQS.^[7] Subsequently, written comments pertaining to such modeling were submitted to the EPA.

As introduced at the Tenth Modeling Conference, the Interagency Workgroup on Air Quality Modeling (IWAQM) process was formally reinitiated in June 2013 to inform the EPA's process of updating the Guideline to address chemically reactive pollutants in near-field and long-range transport applications. The IWAQM, which consists of representatives from the EPA, the U.S. Forest Service, the National Park Service, and the U.S. Fish and Wildlife Service, was initially formed to support development of technically sound recommendations regarding assessment of air pollutant source impacts on Federal Class I parks and wilderness areas. Comments received from stakeholders at the Tenth Modeling Conference supported reinitiating this interagency collaborative effort (as “Phase 3”) to provide additional guidance for modeling single-source impacts on secondarily formed pollutants (e.g., ozone and PM_2.5) in the near-field and for long-range transport. Stakeholder comments also support the idea of this collaborative effort working in parallel with stakeholders to further model development and evaluation. This renewed ^[8] effort included the establishment of two separate working groups, one focused on long-range transport of primary and secondary pollutants and the other on near-field single-source impacts of secondary pollutants. The primary objectives of this phase of IWAQM include reviewing existing approaches for estimating single-source secondary pollutant impacts, developing revisions to the Guideline, and the development of guidance for using technical methods to estimate downwind secondary pollutant impacts.

III. Public Participation Regarding Revisions to the Guideline and Notice of Eleventh Conference on Air Quality Modeling

Interested persons may provide the EPA with their views on the proposed revisions to the Guideline in several ways. This includes submitting written comments to the EPA, participating in the Eleventh Conference on Air Quality Modeling, and speaking at the public hearing that will be conducted as part of the conference. Additional information on how to submit written comments on the proposed revisions to the Guideline is provided in the ADDRESSES section above.

The public hearing for this action and the Eleventh Conference on Air Quality Modeling will be held August 12-13, 2015, from 8:30 a.m. to 5:00 p.m., in the EPA Auditorium, Room C111, 109 T.W. Alexander Drive, Research Triangle Park, NC 27711. On August 12, 2015, the first half of the conference will consist of a structured agenda with presentations. The second half of the first and all of the second day (August 13, 2015), is reserved for the public hearing on this proposed rule. Advance requests for reserved time to speak during the public hearing should be submitted by August 7, 2015, to Mr. George M. Bridgers, Air Quality Assessment Division, Office of Air Quality Planning and Standards, U.S. Environmental Protection Agency, Mail code C439-01, Research Triangle Park, NC 27711; telephone: [phone]; fax: [phone]; email: [e-mail]. The EPA will also provide an opportunity for oral presentations by individuals that sign up at the public hearing. Information submitted to the EPA during the conference will be placed in the docket for this rule proposing revisions to the Guideline.

Background information: Preregistration details, additional background information, and a more detailed agenda for the Eleventh Conference on Air Quality Modeling are electronically available at http://www.epa.gov/​ttn/​scram/​11thmodconf.htm. Preregistration for the conference, while not required, is strongly recommended due to heightened security protocols at the EPA-RTP facility.

REAL ID Act: Because this hearing is being held at a U.S. government facility, individuals planning to attend the hearing should be prepared to show valid picture identification to the security staff in order to gain access to the meeting room. Please note that the REAL ID Act, passed by Congress in 2005, established new requirements for entering federal facilities. These requirements took effect July 21, 2014. If your driver's license is issued by Alaska, American Samoa, Arizona, Kentucky, Louisiana, Maine, Massachusetts, Minnesota, Montana, New York, Oklahoma, or the state of Washington, you must present an additional form of identification to enter the federal buildings where the public hearings will be held. Acceptable alternative forms of identification include: Federal employee badges, passports, enhanced driver's licenses and military identification cards. We will list any additional acceptable forms of identification at: http://www.epa.gov/​ttn/​scram/​11thmodconf.htm. In addition, you will need to obtain a property pass for any personal belongings you bring with you. Upon leaving the building, you will be required to return this property pass to the security desk. No large signs will be allowed in the building, cameras may only be used outside of the building and demonstrations will not be allowed on federal property for security reasons.

Conference and Public Hearing: The Eleventh Conference on Air Quality Modeling will be open to the public. No registration fee is charged. The conference will be conducted informally and chaired by an EPA official. As required under CAA section 320, a verbatim transcript of the conference proceedings will be produced and placed in the docket for this proposed rule.

The Eleventh Conference on Air Quality Modeling will begin with introductory remarks by the presiding EPA official. The EPA staff will then provide an overview of the revisions to the Guideline as proposed in this document and present on the research that supports those revisions and supports formulation updates to the preferred models. The following topics will be presented:

I. Overview of the Eleventh Conference on Air Quality Modeling;

II. Review of the proposed revisions to the Guideline; and

III. Review of the proposed revisions to the preferred air quality models.

At the conclusion of the presentations, the EPA will convene the public hearing on the proposed revisions to the Guideline. The public hearing will span the second half of the first day and throughout the second day of the conference.

Those wishing to reserve time to speak at the public hearing, whether to volunteer a presentation on a special topic or to offer general comment on any of the modeling techniques scheduled for presentation, should contact us at the address given in the FOR FURTHER INFORMATION CONTACT section (note the cutoff date). Such persons should identify the organization (if any) on whose behalf they are speaking and the length of the presentation. If a scheduled presentation is projected to be longer than 10 minutes, the presenter should also state why a longer period is needed. Scheduled speakers should bring extra copies of their presentation for inclusion in the docket and for the convenience of the recorder. Scheduled speakers will also be permitted to enter additional written comments into the record.

Any person in attendance wishing to speak at the public hearing who has not reserved time prior to the conference may provide oral comments on the proposed revisions to the Guideline during time allotted on the last day of the conference. These parties will need to sign up to speak on the second day of the hearing and the EPA may need to limit the duration of presentations to allow all participants to be heard.

Additional written statements or comments on the proposed revisions should be sent to the OAR Regulatory Docket (see ADDRESSES section). A transcript of the conference proceedings and a copy of all written comments will be maintained in Docket ID No. EPA-HQ-OAR-2015-0310, which will remain open until October 27, 2015, for the purpose of receiving additional comments after the conference and the public hearing on the proposed revisions to the Guideline.

IV. Proposed Changes to the Guideline

In this action, the EPA is proposing two type of revisions to the Guideline. The first involve substantive changes to address various topics, including those presented and discussed at the Tenth Modeling Conference. These proposed revisions to the Guideline include enhancements to the formulation and application of the EPA's preferred dispersion modeling system, AERMOD, and the incorporation of a tiered demonstration approach to address the secondary chemical formation of ozone and PM_2.5 associated with precursor emissions from single sources. The second type of revision involves editorial changes to update and reorganize information throughout the Guideline. These revisions are not intended to meaningfully change the substance of the Guideline, but rather to make the Guideline easier to use and to streamline the compliance assessment process.

A. Proposed Actions

This section provides a detailed overview of the substantive proposed changes to the Guideline that are intended to improve the science of the models and approaches used in regulatory assessments.

1. Clarifications To Distinguish Requirements From Recommendations

The EPA's PSD permitting regulations specify that “[a]ll applications of air quality modeling involved in this subpart shall be based on the applicable models, data bases, and other requirements specified in appendix W of this part (Guideline on Air Quality Models).” 40 CFR 51.166(l); see also 40 CFR 52.21(l). The applicable models are the preferred models listed in appendix A to appendix W to 40 CFR part 51. However, there has been some ambiguity in the past with respect to the “other requirements” specified in the Guideline that must be used in PSD permitting analysis and other regulatory modeling assessments.

Ambiguity can result because the Guideline generally contains “recommendations” and these recommendations are expressed in non-mandatory language. For instance, the Guideline frequently uses “should” and “may” rather than “shall” and “must.” This approach is generally preferred throughout the Guideline because of the need to exercise expert judgment in air quality analysis and the reasons discussed in the Guideline that “dictate against a strict modeling `cookbook'.” (40 CFR part 51, appendix W, section 1.0(c))

Considering the non-mandatory language used throughout the Guideline, the EPA's Environmental Appeals Board has correctly observed the following:

“Although appendix W has been promulgated as codified regulatory text, appendix W provides permit issuers broad latitude and considerable flexibility in application of air quality modeling. Appendix W is replete with references to “recommendations,” “guidelines,” and reviewing authority discretion.”

In Re Prairie State Generating Company, 13 E.A.D. 1, 99 (EAB 2005) (internal citations omitted).

Although this approach is typical throughout the Guideline, there are instances where the EPA does not believe permit issues should have broad latitude. Some principles of air quality modeling described in the Guideline must always be applied to produce an acceptable analysis. Thus, to promote clarity in the use and interpretation of the revised Guideline, we have, in these cases used mandatory language, and made specific reference to “requirements” throughout the proposed text where appropriate to distinguish requirements from recommendations in the application of models for regulatory purposes. We solicit comment regarding the appropriateness of these revisions in providing the necessary clarity on the requirements under the proposed revisions to the Guideline as distinct from the recommendations in the revised text while noting the continued flexibilities provided for within the Guideline including but not limited to use and approval of alternative models.

2. Updates to EPA's AERMOD Modeling System

Based on studies presented and discussed at the Tenth Modeling Conference, and additional relevant research since 2010, the EPA and other researchers have conducted additional model evaluations and developed changes to the model formulation of the AERMOD modeling system to improve model performance in its regulatory applications. We propose the following updates to the AERMOD modeling system to address a number of technical concerns expressed by stakeholders:

1. A proposed option incorporated in AERMET to adjust the surface friction velocity (u*) to address issues with AERMOD model overprediction under stable, low wind speed conditions. This proposed option is selected by the user with the METHOD STABLEBL ADJ_U* record in the AERMET Stage 3 input file.

2. A proposed low wind option in AERMOD to address issues with model overprediction under low wind speed conditions. The low wind option will increase the minimum value of the lateral turbulence intensity (sigma-v) from 0.2 to 0.3 and adjusts the dispersion coefficient to account for the effects of horizontal plume meander on the plume centerline concentration. It also eliminates upwind dispersion which is incongruous with a straight-line, steady-state plume dispersion model such as AERMOD. The proposed option is selected by specifying “LOWWIND3” on the CO MODELOPT keyword in the AERMOD input file.

3. Modifications to AERMOD formulation to address issues with overprediction for applications involving relatively tall stacks located near relatively small urban areas (no user input is required).

4. Proposed regulatory default options in AERMOD to address plume rise for horizontal and capped stacks based on the July 9, 1993, Model Clearinghouse memorandum,^[9] with adjustments to account for the PRIME algorithm for sources subject to building downwash. These options are selected by the model user specifying “POINTCAP” or “POINTHOR” for source type on the SO LOCATION keyword in the AERMOD input file.

5. A proposed buoyant line source option, based on the BLP model, has been incorporated in AERMOD. This proposed option is selected by the model user with the SOURCE type “BOUYLINE” to specify the individual buoyant line source locations and emissions and the new “BLAVGVAL” keyword to specify average parameters for a composite buoyant line.

6. Proposed updates to the NO₂ Tier 2 and Tier 3 screening techniques coded within AERMOD as described more fully later in this preamble section.

Model performance evaluation and peer scientific review references for the updated AERMOD modeling system are cited, as appropriate. An updated user's guide and model formulation documents for version 15181 have been placed in the docket. We have updated the summary description of the AERMOD modeling system to appendix A of the Guideline to reflect these proposed updates. The essential codes, preprocessors, and test cases have been updated and posted to the EPA's SCRAM Web site, http://www.epa.gov/​ttn/​scram.

We invite comments on whether we have reasonably addressed the technical concerns expressed by the stakeholder community and are on sound footing to recommend these updates to the regulatory default version of the AERMOD modeling system which includes its replacement of BLP as an appendix A model for the intended regulatory applications.

3. Status of AERSCREEN

In the preamble of the 2005 Guideline, we stated that a screening version of AERMOD called AERSCREEN was being developed and, in the meantime, SCREEN3 may be used until AERSCREEN was available. In 2011, the EPA released AERSCREEN, a program that creates inputs and runs AERMOD in screening mode. AERSCREEN also interfaces with AERMOD's terrain processor, AERMAP, the building processor for AERMOD, BPIPPRIME, and can use AERSURFACE surface characteristics in the generation of meteorological data for AERMOD via the MAKEMET utility. In an April 2011 memorandum, the EPA stated that AERSCREEN was the recommended screening model for simple and complex terrain and replaced SCREEN3. Since AERSCREEN invokes AERMOD, AERSCREEN represents the state of the science in screening dispersion models. As part of this proposed update to AERSCREEN, AERSCREEN now includes inversion break-up and coastal fumigation, features that were part of SCREEN3. These fumigation algorithms also take advantage of AERMOD's boundary layer parameterizations for calculating variables needed by the algorithms.

We invite comment on incorporation of AERSCREEN into the Guideline as the screening model for AERMOD that may be applicable in applications in all types of terrain and for applications involving building downwash.

4. Updates to 3-Tiered Demonstration Approach for NO₂

Section 5.2.4 of the 2005 Guideline details a 3-tiered approach for assessing NO_X sources, which was recommended to obtain annual average estimates of NO₂ from point sources for purposes of NSR analysis, including the PSD program and SIP planning purposes. This 3-tiered approach addresses the co-emissions of NO and NO₂ and the subsequent conversion of NO to NO₂ in the atmosphere. The tiered levels include: (1) Assuming that all NO is converted to NO₂ (full conversion), (2) using the Ambient Ratio Method (ARM), which applies an assumed equilibrium ratio of NO₂ to NO_X, based on observed ambient conditions, to the annual results from the Tier 1 full conversion, and (3) detailed screening options focused on determining site-specific ratios of NO₂ to NO_X.

In January 2010, a new 1-hour NO₂ standard was promulgated. Prior to the adoption of the 1-hour NO₂ standard, few PSD permit applications required the use of Tier 3 options and guidance available at the time did not fully address the modeling needs for a 1-hour standard, i.e., tiered approaches for NO₂ in the 2005 Guideline specifically targeted an annual standard. As a result, several guidance memoranda have been issued by the EPA to further inform modeling procedures for sources demonstrating compliance with the new 1-hour standard.^{1 2 3 4}. In response to the 1-hour NO₂ standard, the EPA is proposing several modifications to the Tier 2 and 3 NO₂ screening techniques incorporated into AERMOD.

For the Tier 2 technique, the EPA is proposing to replace the existing ARM with a revised Ambient Ratio Method 2 (ARM2). The existing Tier 2 technique, ARM, was based on a study that focused exclusively on long-term averages.^[10] A recently published study ^[11] presented a new analysis of national levels of ambient ratios of NO₂ to NO_X based on hourly data from the EPA's Air Quality System (AQS). Based on this analysis, a new second tier NO₂ screening technique, ARM2, has been developed and incorporated into AERMOD. Because ARM2 is based on hourly measurements of the NO₂ to NO_X ratios and provides more detailed estimates of this ratio based on the total NO_X present, the EPA is proposing to incorporate a modified version of ARM2 as the new preferred second tier NO_X modeling approach.

For the Tier 3 technique, the EPA proposes that the existing detailed screening options of the Ozone Limiting Method (OLM) ^[12] and PVMRM ^[13] be formally incorporated into the regulatory version of AERMOD. Both OLM and PVMRM have been available as non-regulatory, non-default options in AERMOD for many years, but their usage in a NAAQS compliance demonstration required approval by the reviewing authority. Based on the EPA's evaluation and external studies available on their performance, which show that OLM and PVMRM are capable of modeling 1-hour NO₂ impacts and NO and NO₂ speciation with reasonable accuracy when applied appropriately, both OLM and PVMRM are being proposed as preferred Tier 3 screening methods for NO₂ modeling. In addition, the EPA is proposing to incorporate a revised version of the PVMRM option, referred to as PVMRM2, that utilizes relative dispersion coefficients to estimate plume volume during convective conditions and total dispersion coefficients during stable conditions. These adjustments to the calculation of plume volume are intended to mitigate potential overprediction of NO₂ conversion in multisource applications, especially during stable meteorological conditions. The EPA is proposing to replace the existing PVMRM with the new PVMRM2 with both versions being made available in the proposed version of AERMOD to facilitate testing and evaluation of the EPA's proposed replacement of PVMRM option with new PVMRMR2 option.

We invite comments on whether we have reasonably addressed technical concerns regarding the 3-tiered demonstration approach and specific NO₂ screening techniques within AERMOD and whether we are on sound foundation to recommend the updates described above.

5. Status of CALINE3 Models

The 2005 Guideline identified CALINE3 ^[14] and its variants (CAL3QHC and CAL3QHCR) as the preferred model for mobile source modeling for carbon monoxide (CO), particulate matter (PM), and lead. CALINE3 was developed in the late 1970's using P-G stability classes as the basis for the dispersion algorithms. AERMOD, on the other hand, uses a planetary boundary layer scaling parameter to characterize stability and determine dispersion rates, which has been found to be superior to dispersion parameterizations based on P-G stability classes.^[15] In addition, the LINE and AREA source options in AERMOD implement a full numerical integration of emissions across the LINE or AREA sources, whereas the CALINE3 family of models incorporate a much less refined approach. Thus, AERMOD provides a more scientifically credible and accurate characterization of plume dispersion than CALINE3. Recent model performance studies ^[16] have shown that the CALINE models performed poorly when compared to AERMOD and other modern dispersion models which also employ state-of-the-science dispersion parameters. AERMOD is also able to model multiple years in a single model run, while the CALINE3 variants are limited to either a single meteorological condition (CALINE3 and CAL3QHC) or a single year of meteorological data (CAL3QHCR). Additionally, AERMOD is able to utilize more recent, and more representative, meteorological observations than are readily available for modeling with CAL3QHCR. Based on the more scientifically sound basis for AERMOD, improved model performance over CALINE3, and the availability of more representative meteorological data, the EPA proposes replacing CALINE3 with AERMOD as the preferred appendix A model for determining near-field impacts for primary emissions from mobile sources, including PM_2.5, PM₁₀, and CO hot-spot analyses.^[17]

We solicit comments on our proposal to identify AERMOD as a replacement for CALINE3 as an appendix A model for its intended regulatory applications.

6. Addressing Single-Source Impacts on Ozone and Secondary PM_2.5

On January 4, 2012, the EPA granted a petition submitted on behalf of the Sierra Club on July 28, 2010,^[18] that requested the EPA initiate rulemaking to establish air quality models for ozone and PM_2.5 for use by all major sources applying for a PSD permit. In granting that petition, the EPA explained that the “complex chemistry of ozone and secondary formation of PM_2.5 are well-documented and have historically presented significant challenges to the designation of particular models for assessing the impacts of individual stationary sources on the formation of these air pollutants” and further explained that “[b]ecause of these considerations, the EPA's judgment in the past has been that it was not technically sound to designate with particularity specific models that must be used to assess the impacts of a single source on ozone concentrations,” but rather the EPA had established a process for determining on a case-by-case basis the analytical techniques that should be used for ozone, as well as for secondary formation of PM_2.5.

In the petition grant, the EPA committed to engage in rulemaking to evaluate whether updates to the Guideline are warranted and, as appropriate, incorporate new analytical techniques or models for ozone and secondarily formed PM_2.5. This rulemaking satisfies the EPA's commitment in the petition grant. As a part of this commitment and in compliance with CAA section 320, the EPA conducted the Tenth Modeling Conference in March 2012, where there were presentations of ongoing research of single-source plume chemistry and photochemical grid modeling techniques, as well as several public forums, and the EPA subsequently received written comments pertaining to such modeling.

The EPA initiated Phase 3 of the IWAQM process in June 2013 to inform this process to update the Guideline to address chemically reactive pollutants for near-field and long-range transport applications. Comments received from stakeholders at the Tenth Modeling Conference supported this collaborative effort to provide additional guidance for modeling single-source impacts of secondarily formed pollutants in the near-field and for long-range transport. Stakeholder comments also supported the idea of this collaborative effort occurring in parallel with stakeholders' efforts to further model development and evaluation. The EPA's recommended revisions to the Guideline are largely based on detailed review and assessment of this input.

For this proposed revision to the Guideline, the EPA has determined that advances in photochemical modeling science indicate it is now reasonable to provide more specific, generally-applicable guidance that identifies particular models or analytical techniques that may be used under specific circumstances for assessing the impacts of an individual source on ozone and secondary PM_2.5.

Quantifying secondary pollutant formation requires simulating chemical reactions and thermodynamic partitioning in a realistic chemical and physical environment. Chemical transport models treat atmospheric chemical and physical processes such as deposition and transport. There are two types of chemical transport models, which are differentiated based on a fixed frame of reference (i.e., Eulerian models, specifically photochemical grid models) or a frame of reference that moves with parcels of air between the source and receptor point (i.e., Lagrangian models).^[19]

Comparing these two types of chemical transport models, photochemical grid models are integrated, three-dimensional grid-based models that treat chemical and physical processes in each grid cell and use Eulerian diffusion and transport processes to move chemical species to other grid cells.^[19] While some Lagrangian models also treat in-plume gas and particulate chemistry, to do so these models require time and space varying oxidant concentrations, and in the case of PM_2.5, neutralizing agents such as ammonia, because important secondary impacts happen when plume edges start to interact with the surrounding chemical environment.^[20 21] These oxidant and neutralizing agents are not routinely measured, but can be generated with a three-dimensional photochemical transport model and subsequently input to a Lagrangian modeling system.

In light of these differences between photochemical grid models and Lagrangian models that address chemistry, the EPA believes photochemical grid models are generally most appropriate for addressing ozone and secondary PM_2.5 because they provide a spatially and temporally dynamic realistic chemical and physical environment for plume growth and chemical transformation.^[20 22] Publically available and documented Eulerian photochemical grid models such as the Comprehensive Air Quality Model with Extensions (CAMx) ^[23] and the Community Multiscale Air Quality (CMAQ) ^[24] model treat emissions, chemical transformation, transport, and deposition using time and space variant meteorology. These modeling systems include primarily emitted species and secondarily formed pollutants such as ozone and PM_2.5.^{[25 26 27 28]} These models have been used extensively to support ozone and PM_2.5 SIPs and to explore relationships between inputs and air quality impacts in the United States and elsewhere.^{[26 29 30]}

For assessing secondary pollutant impacts from single sources, the degree of complexity required to assess potential impacts varies depending on the nature of the source, its emissions, and the background environment. In order to provide the user community flexibility in estimating single-source secondary pollutant impacts and given the emphasis on the use of photochemical grid models for these purposes, the EPA is proposing a two-tiered demonstration approach for addressing single-source impacts on ozone and secondary PM_2.5. The first tier involves use of technically credible relationships between precursor emissions and a source's impacts that may be published in the peer-reviewed literature; developed from modeling that was previously conducted for an area by a source, a governmental agency, or some other entity and that is deemed sufficient; or generated by a peer-reviewed reduced form model. The second tier involves application of more sophisticated case-specific chemical transport models (e.g., photochemical grid models) to be determined in consultation with the EPA Regional Office and conducted consistent with new EPA single-source modeling guidance.^[31] The appropriate tier for a given application should be selected in consultation with the appropriate reviewing authority and be consistent with EPA guidance.

To fully implement these proposed changes to the Guideline related to addressing ozone and secondary PM_2.5 impacts, the EPA intends to pursue a separate rulemaking to establish a technical basis and new values for PM_2.5 Significant Impact Levels (SILs) and to introduce a new demonstration tool for ozone and PM_2.5 precursors referred to as Model Emissions Rates for Precursors (MERP). When completed, this rule would differ from the current process recommended in the EPA's Guidance for PM_2.5 Permit Modeling.^[7] A MERP would neither replace the existing Significant Emissions Rates (SERs) for these pollutants nor serve as the basis for the applicability of PSD requirements to sources with emissions above the SER. However, a MERP would represent a level of emissions of precursors that is not expected to contribute significantly to concentrations of ozone or secondarily-formed PM_2.5. Our present understanding of the atmospheric science of ozone and secondary PM_2.5 formation indicates that MERP values will likely be higher than the SERs and more appropriate for evaluating the impacts of these criteria pollutants as precursors to ozone and PM_2.5 formation. As part of the separate rulemaking, the EPA intends to demonstrate that a source with precursor emissions (e.g., NO_X and SO₂ for PM_2.5) below the MERP level will have ambient impacts that will be less than the SIL and, thereby, provide a sufficient demonstration that the source will not cause or contribute to a violation of the PM_2.5 NAAQS or PSD increments. The EPA's Guidance for PM_2.5 Permit Modeling ^[7] provides for a three-tiered approach to address secondary PM_2.5 with (1) a qualitative assessment; (2) a hybrid qualitative/quantitative assessment utilizing existing technical work; and (3) a full quantitative modeling exercise. The EPA expects that MERPs as a demonstration tool will replace the first tier of a qualitative assessment as sources that currently would provide a qualitative assessment are expected to have precursor emissions levels below the MERP. The second and third tier of assessment will then be consistent with the EPA's proposed two-tiered demonstration approach for PM_2.5 reflected in this proposed revisions to the Guideline. To specifically assist the public in commenting on this rule within the overall context of the NSR program, including PSD, the EPA has added two separate memoranda to the docket of this proposed rule. These memoranda provide more details on how this future approach to PSD compliance demonstrations will work for secondary PM_2.5 and also describe our expectations for how such an approach might work for ozone based on a future, separate action to similarly establish a SIL and MERPs (for VOC and NO_X precursors) for ozone using approaches similar to those for PM_2.5.^[32 33]

While the development of MERPs for ozone and secondary PM_2.5 precursors is expected to address a number of PSD permitting situations, the EPA believes that most of the remaining situations in which a source must demonstrate compliance under the proposed Guideline will be addressed sufficiently under the proposed first tier where existing technical information could be used in combination with other supportive information and analysis for the purposes of estimating secondary impacts from a particular source. The existing technical information should provide a credible and representative estimate of the secondary impacts from the project source. In these situations, a more refined approach for estimating secondary pollutant impacts from project sources may not be necessary. The EPA has been compiling and reviewing screening approaches that are based on technically credible tools (e.g., photochemical grid models) that relate source precursor emissions to secondary impacts. In review of existing approaches detailed in peer reviewed journal articles and non-peer reviewed forms (e.g., technical reports, conference presentations), it is not clear that a single approach has been clearly proposed to and evaluated by the modeling community for estimating screening level secondary impacts from single sources. Other screening level alternatives to photochemical grid model application may include the use of existing credible photochemical model impacts for sources deemed to be similar in terms of emission rates, release parameters, and background environment. The EPA will continue to engage with the modeling community to identify credible alternative approaches for estimating single-source secondary pollutant impacts which provide flexibility and are less resource intensive for permit demonstration purposes.

For those situations for which existing modeling or screening estimates are not available or appropriate, the second tier proposed by the EPA would apply and involve use of more sophisticated case-specific chemical transport models (e.g., photochemical grid models) to be determined in consultation with the appropriate EPA Regional Office based upon new EPA single-source modeling guidance.^[31] Based on several scientific studies, the EPA proposes to determine that photochemical grid models are appropriate for assessment of near-field and regional scale reactive pollutant impacts from specific sources ^{[20 22 34 35]} or a group of multiple sources impacting an area.^25 27 28 Even though single-source emissions are injected into a grid volume, photochemical transport models have been shown to adequately capture single-source impacts when compared with downwind in-plume measurements.^20 22 Where set up appropriately for the purposes of assessing the contribution of single sources to primary and secondarily formed pollutants, photochemical grid models can be used with a variety of approaches to estimate these impacts. These approaches generally fall into the category of source sensitivity (how air quality changes due to changes in emissions) and source apportionment (what air quality impacts are related to certain emissions). Source apportionment has been used to differentiate the contribution from single sources on model predicted ozone and PM_2.5 concentrations.^20 34 The direct decoupled method (DDM) has also been used to estimate ozone and PM_2.5 impacts from specific sources ^20 35 as well as the simpler brute-force sensitivity approach.^20 22 35 Limited comparison of single-source impacts between models ^[36] and approaches to differentiate single-source impacts ^20 36 show generally similar downwind spatial gradients and impacts.

Near-source in-plume aircraft based measurement field studies provide an opportunity for evaluating model estimates of (near-source) downwind transport and chemical impacts from single stationary point sources.²¹ Photochemical grid model source apportionment and source sensitivity simulation of a single source downwind impacts compare well against field study primary and secondary ambient measurements made in Tennessee and Texas.^{20 21} This work indicates photochemical grid models and source apportionment and source sensitivity approaches provide meaningful estimates of single-source impacts on ozone and secondarily-formed PM_2.5. Additional evaluations for longer time periods and more diverse environments, both physical and chemical, would be valuable to generate broader confidence in these approaches for this purpose.

We invite comments on whether the proposed two-tiered demonstration approach and related EPA guidance is appropriately based on sound science and practical application of available models and tools to address single-source impacts on ozone and secondary PM_2.5.

7. Status of CALPUFF and Assessing Long-Range Transport for PSD Increment and Regional Haze

The 2003 Guideline recommended CALPUFF as the preferred model for long-range transport (i.e., source-receptor distances of 50 to several hundred kilometers) of emissions from point, volume, area, and line sources for primary criteria pollutants (e.g., PM and SO₂). Since that time, as discussed previously in this preamble, the EPA has received input from stakeholders and has worked through the IWAQM process on analytical techniques to address chemically reactive pollutants for near-field and long-range transport applications. As a result, in order to provide the user community flexibility in estimating single-source secondary pollutant impacts and given the availability of more appropriate modeling techniques, such as photochemical transport models (which address limitations of models like CALPUFF ^[37] ), the EPA is proposing that the Guideline no longer contain language that requires the use of CALPUFF or another Lagrangian puff model for long-range transport assessments. Additionally, the EPA is proposing to remove the CALPUFF modeling system as an EPA-preferred model for long-range transport due to concerns about the management and maintenance of the model code given the frequent change in ownership of the model code since promulgation in the previous version of the Guideline.^[38] The EPA recognizes that long-range transport assessments may be necessary in certain limited situations for PSD increment. For these situations, the EPA is proposing a screening approach where CALPUFF along with other appropriate screening tools and methods may be used to support long-range transport PSD increment assessments.

To determine if a Class I PSD increment analyses may be necessary beyond 50 km (i.e., long-range transport assessment), the EPA is recommending a screening approach to determine if a significant impact will occur with particular focus on Class I areas that may be threatened at such distances. The first step relies upon the near-field application of the appropriate screening and/or preferred model to determine the significance of ambient impact at or about 50 km from the new of modifying source. If this initial analysis indicates there may be significant ambient impacts at that distance, then further analysis is necessary. For assessment of Class I ambient impacts, under the proposed Guideline, there will not a preferred model for distances beyond 50 km. Typically, a Lagrangian model is the type of model appropriate to use for these screening assessments; however, applicants should establish approaches (models and modeling parameters) on a case-by-case basis in consultation with the appropriate reviewing authority, Regional Office, and the affected Federal Land Manager(s) (FLM(s)). If a cumulative increment analysis is necessary, for these limited situations, the selection and use of an alternative model shall occur in agreement with the appropriate reviewing authority (paragraph 3.0(b)) and approval by the EPA Regional Office based on the requirements of section 3.2.2(e).

As previously noted, Phase 3 of the IWAQM process was reinitiated in June 2013 to inform the EPA's commitment to update the Guideline to address chemically reactive pollutants in near-field and long-range transport applications. This Phase 3 effort included the establishment of a working group composed of EPA and FLM technical staff focused on long-range transport of primary and secondary pollutants with an emphasis on use of consistent approaches to those being developed and applied to meet near-field assessment needs for ozone and secondarily-formed PM_2.5. The EPA expects that such approaches will be focused on state of the science chemical transport models (CTMs) as detailed in IWAQM reports ^[39 40] and published literature.

To inform future consideration of visibility modeling in regulatory applications consistent with proposed changes for addressing chemistry for single-source impact on ozone and secondary PM_2.5, the final report ⁴⁰ of the IWAQM long-range transport subgroup identified that modern CTMs have evolved sufficiently and provide a credible platform for estimating potential visibility impacts from a single or small group of emission sources. Chemical transport models are well suited for the purpose of estimating long-range impacts of secondary pollutants, such as PM_2.5, that contribute to regional haze and other secondary pollutants, such as ozone, that contribute to negative impacts on vegetation through deposition processes. These multiple needs require a full chemistry photochemical model capable of representing both gas, particle, and aqueous phase chemistry for PM_2.5, haze, and ozone.

Photochemical transport models are suitable for estimating visibility and deposition since important physical and chemical processes related to the formation and transport of PM are realistically treated. Source sensitivity and apportionment techniques implemented in photochemical grid models have evolved sufficiently and provide the opportunity for estimating potential visibility and deposition impacts from one or a small group of emission sources using a full science photochemical grid model. Photochemical grid models using meteorology output from prognostic meteorological models have demonstrated skill in estimating source-receptor relationships in the near-field ^20 21 and over long distances.^[41]

It is important that modeling tools used for single-source long-range transport impacts assessments demonstrate skill in adequately replicating source-receptor relationships that are not in close proximity. For source-receptor distances greater than 50 km, regional scale photochemical grid models may be applied for the assessment of visibility impacts due to one or a small group of sources. Skill in estimating source-receptor relationships on this scale can be illustrated by evaluating modeling systems against regional scale inert tracer release experiments. Historically, several regional tracer release experiments have been used to demonstrate skill in long-range transport of inert pollutants: 1980 Great Plains Mesoscale Tracer Field Experiment, the 1983 Cross-Appalachian Tracer Experiment (CAPTEX), the 1987 Across North American Tracer Experiment (ANATEX), and 1994 European Tracer Experiment (ETEX).^[41 42] Photochemical grid models have been shown to demonstrate similar skill to Lagrangian models for pollutant transport when compared to measurements made from multiple mesoscale field experiments.⁴¹ Use of CTMs for Air Quality Related Values (AQRV) analysis requirements, while not subject to specific EPA model approval requirements outlined in 40 CFR 51.166(l)(2) and 40 CFR 52.21(l)(2), should be justified for each application following the general recommendations outlined in section 3.2, and concurrence sought with the affected FLM(s).

In 2005, the EPA issued guidelines for implementation of the best available retrofit technology (BART) requirements under the Regional Haze Rule. In these BART Guidelines, the EPA addressed the question of how states could best predict a single source's contribution to visibility impairment.^[43] At the time, the EPA recognized that CALPUFF had not yet been fully evaluated for secondary pollutant formation, but the EPA still considered CALPUFF to be the best application for assessing a single source's impact on visibility in a Class I area for purposes of the regional haze program. The EPA took note of the limitations of CALPUFF for this purpose but concluded that CALPUFF was the best modeling application for use in evaluating BART, especially given how the modeling results would be used. Based on this assessment, the EPA recommended that the states use CALPUFF. The EPA also made clear, however, that states could use other alternative approaches, including photochemical grid models, if done in consultation with the appropriate EPA Regional Office.

The current version of the Guideline does not contain any explicit recommendation regarding the use of CALPUFF in the regional haze program, but in advising states and in making its own BART determinations, the EPA has looked to the Guideline to resolve questions regarding the proper application of the model. In particular, the EPA has guided states to use the applicable regulatory version of CALPUFF for such assessments. Following the EPA's recommendations, states have used the EPA-preferred version of CALPUFF in hundreds of BART determinations since 2005. Although most assessments of BART are now complete, a handful of BART determinations remain outstanding. We expect most of the remaining actions addressing the BART requirements to be completed within the next two years.

The proposed changes to the Guideline do not affect the EPA's recommendation in the 2005 BART Guidelines to use CALPUFF in the BART determination process. Given that the overwhelming majority of BART determinations have been made using CALPUFF, we consider it appropriate for states (or the EPA) to continue to use this application for the remaining assessments under the current Guideline with approved protocols. This approach assures consistency across and within states in the regional haze program. In addition, in many instances, the modeling of visibility impacts has already been completed even though the BART determination process is not yet done. Allowing states to continue to rely on CALPUFF avoids additional time and expense in developing a new assessment of visibility impacts for a SIP initially due in 2007. We intend to continue to advise states with respect to the EPA-preferred version of CALPUFF that should be used in specific BART cases. Consistent with the BART Guidelines, states may also use alternative modeling approaches, in consultation with the appropriate EPA Regional Office.

The EPA is seeking comment on its proposed screening approach to address long-range transport for purposes of assessing PSD increments; its decision to remove CALPUFF as a preferred model in appendix A for such long-range transport assessments; and its decision to consider CALPUFF as a screening technique along with other Lagrangian models to be used in consultation with the appropriate reviewing authority. It is important to note that the EPA's proposed action to remove CALPUFF as an appendix A model in this Guideline does not affect its use under the FLM's guidance regarding AQRV assessments (FLAG 2010) nor previous use of this model as part of regulatory modeling applications required under the CAA. Similarly, this proposed action does not affect EPA's recommendation that States use CALPUFF to determine the applicability and level of BART in regional haze implementation plans.⁴³

8. Role of EPA's Model Clearinghouse

The EPA's Model Clearinghouse has been a fundamental aspect of communication between the EPA Region Offices and with the broader permitting community on technical modeling and compliance demonstration issues for almost three decades. The Model Clearinghouse serves a critical role in helping resolve issues that arise from unique situations that are not specifically addressed in the Guideline or necessitate the consideration of an alternative model or technique for a specific application or range of applications. The Model Clearinghouse ensures that fairness, consistency, and transparency in modeling decisions are fostered among the Regional Offices and the state, local, and tribal agencies.

In this action, we are proposing to codify the long-standing process of the Regional Offices consulting and coordinating with the Model Clearinghouse on all approvals of alternative models or techniques. While the Regional Administrators are the delegated authority to issue such approvals under section 3.2 of the Guideline, all alternative model approvals will only be issued after consultation with the EPA's Model Clearinghouse and formal documentation through a concurrence memorandum which demonstrates that the requirements within section 3.2 for use of an alternative model have been met.

We invite comment on our proposal to codify existing practice of requiring consultation and coordination between the EPA Regional Offices and the EPA's Model Clearinghouse on all approvals under section 3.2 of alternative models or techniques.

9. Updates to Modeling Procedures for Cumulative Impact Analysis

Based on input from the Tenth Modeling Conference and recent permit modeling experiences under new short-term NAAQS for SO₂ and NO₂, the EPA is proposing to make modifications to section 8 of the Guideline regarding model inputs and background concentrations to provide much needed clarity associated with input and database selection for use in PSD and SIP modeling. Many of these revisions are based on the EPA clarification memoranda issued since 2010 that were intended to provide the necessary clarification regarding applicability of the Guideline to PSD modeling for these new standards.^{[1 2 44 45]} The EPA has specifically cautioned against the literal and uncritical application of very prescriptive procedures for conducting NAAQS and PSD modeling compliance demonstrations as described in chapter C of the draft New Source Review Workshop Manual.^[46] Our main concern is that following such procedures in a literal and uncritical manner has led to practices that are overly conservative and unnecessarily complicate the permitting process. The proposed changes to section 8 are intended to modify these practices and provide a more appropriate basis for selection and use of modeling inputs through the Guideline itself and supporting guidance.

We have provided a more definitive definition of the appropriate modeling domain and how to best characterize the various contributions to air quality concentrations within that domain. Specifically, we provide the following recommendations:

• Definition and/or factors to consider in determining appropriate modeling domain for NAAQS and PSD increment assessments and for SIP attainment demonstrations (see section 8.1).
• Revised requirements on how to characterize emissions from nearby sources to be explicitly modeled for purposes of a cumulative impact assessment under PSD and new language regarding how to characterize direct and precursor emissions from modeled sources for SIP attainment demonstrations for ozone, PM_2.5, and regional haze (see section 8.2).
• Revised recommendations on how to determine background concentrations in constructing the design concentration, or total air quality concentration, as part of a cumulative impact analysis for NAAQS and PSD increments. Specific recommendations are proposed for situations involving isolated single-source(s) and multi-source areas (see section 8.3) with an emphasis on how to determine which nearby sources to explicitly model based on the concept of significant concentration gradients and the use of monitored background to adequately represent “other sources” (i.e., that portion of the background attributable to natural sources, other unidentified sources in the vicinity of the project, and regional transport contributions from more distant sources (domestic and international)). It is important to note the interconnectedness of these issues as the question of which nearby sources to include in cumulative modeling is inextricably linked with the question of what ambient monitoring data are available and what these data represent for a specific application.

More specific data requirements and the format required for the individual models are described in detail in the users' guide and/or associated documentation for each model.

Given the added complexity of the technical issues that arise in the context of demonstrating compliance with NAAQS through dispersion modeling, we strongly encourage adherence to the recommendations in section 9.2.1 of the proposed Guideline regarding development of a modeling protocol, i.e., that “[e]very effort should be made by the Regional Office to meet with all parties involved in either a SIP revision or a PSD permit application prior to the start of any work on such a project. During this meeting, a protocol should be established between the preparing and reviewing parties to define the procedures to be followed, the data to be collected, the model to be used, and the analysis of the source and concentration data.” We expect by providing more clarity in the Guideline of the factors to be considered in the cumulative impact assessment, permit applicants and permitting authorities will be able to find the proper balance of the competing factors that contribute to these analyses.

We invite comments on whether the updates proposed in section 8 of the Guideline and associated guidance are appropriate and sufficient to provide the necessary clarification in selecting and establishing the model inputs for conducting the regulatory modeling for PSD and SIP applications.

10. Updates on Use of Meteorological Input Data for Regulatory Dispersion Modeling

For near-field dispersion modeling applications using National Weather Service (NWS) Automated Surface Observing Stations (ASOS), the EPA released a pre-processor to AERMET, called AERMINUTE, in 2011 that calculates hourly averaged winds from 2-minute winds reported every minute at NWS ASOS sites. AERMET substitutes these hourly averaged winds for the standard hourly observations, thus reducing the number of calms and missing winds for input into AERMOD. The presence of calms and missing winds were due to the METAR reporting methodology of surface observations. In March 2013, the EPA released a memorandum regarding the use of ASOS data in AERMOD as well as the use of AERMINUTE. When using meteorological data from ASOS sites for input to AERMOD, hourly averaged winds from AERMINUTE should be used in most cases.

For a near-field dispersion modeling application where there is no representative NWS station, and it is prohibitive or not feasible to collect adequately representative site-specific data, it may be necessary to use prognostic meteorological data for the application. The EPA released the MMIF program that converts the prognostic meteorological data into a format suitable for dispersion modeling applications. The most recent 3 years of prognostic data are preferred. Use of the prognostic data is contingent on the concurrence of the appropriate reviewing authorities and collaborating agencies that the data are of acceptable quality and representative of the modeling application.

We solicit comments on our proposed updates regarding use of meteorological input data for regulatory application of dispersion models.

11. Transition Period for Applicability of Revisions to the Guideline

In previous rulemakings to revise the Guideline, we have traditionally communicated that it would be appropriate to provide 1 year to transition to the use of new models, techniques and procedures in the context of PSD permit applications and other regulatory modeling applications. We invite comments whether it would be appropriate to apply a 1-year transition after promulgation of the revised Guideline (i.e., from its effective date) such that applications conducted under the current Guideline with approved protocols would be acceptable during that period, but new requirements and recommendations should be used for applications submitted after that period or protocols approved after that period.

The EPA believes such a transition period is appropriate to avoid the time and expense of revisiting modeling that is substantially complete, which would cause undue delays to permit applications that are pending when the proposed revisions to the Guideline are finalized. The revisions that the EPA is proposing to the Guideline are intended as incremental improvements to the Guideline, and such improvements do not necessarily invalidate past practices under the previous edition of the Guideline. The requirements and recommendations in the current (2005) version of the Guideline were previously identified as acceptable by the EPA, and they will continue to be acceptable for air quality assessments during the period of transition to the revised version of the Guideline.

Where a proposed revision to the Guideline does raise questions about the acceptability of a requirement or recommendation that it replaces, model users and applicants are encouraged to consult with the appropriate reviewing authority as soon as possible to assure the acceptability of modeling used to support permit applications during this period.

B. Proposed Editorial Changes

The EPA is proposing to make editorial changes to update and reorganize information throughout the Guideline. These revisions are not intended to meaningfully change the substance of the Guideline, but rather to make the Guideline easier to use. One way this is accomplished is by grouping topics together in a more logical manner to make related content easier to find. This in turn should streamline the compliance assessment process.

Editorial changes are described below for each affected section. We invite comment on any of the changes proposed below for the Guideline text.

1. Preface

Only a few minor text revisions are proposed to this section for consistency with the remainder of the Guideline.

2. Section 1

The EPA propose to update the introduction section to reflect the reorganized nature of the revised Guideline. Minor text revisions are proposed throughout this section for additional clarity. Additional information is provided regarding the importance of CAA section 320 to amendments of the Guideline.

3. Section 2

The EPA proposes to revise section 2 to more appropriately discuss the process by which models are evaluated and considered for use in particular applications. We propose to incorporate information from the previous section 9 pertaining to model accuracy and uncertainty within this section to clarify how model performance evaluation is critical in determining the suitability of models for particular application.

We also propose to provide a discussion in section 2.1 (Model Accuracy and Uncertainty) of the three types of models historically used for regulatory demonstrations. For each type of model, some strengths and weaknesses are listed to assist readers in the understanding of the particular regulatory applications to which they are most appropriate.

In addition, the EPA proposes revisions to section 2.2 with respect to the recommended practice of progressing from simplified and conservative air quality analysis toward more complex and refined analysis. In this section, the EPA proposes to clarify distinctions between various types of models that have previously been described as screening models. In addition, this section clarifies distinctions between models used for screening purposes and screening techniques and demonstration tools that may be acceptable in certain applications.

4. Section 3

The EPA proposes minor modifications to section 3 to more accurately reflect current EPA practices and by moving the discussion of the EPA's Model Clearinghouse to a revised section 3.3 for ease of reference and prominence within the Guideline. A change is proposed to require Regional Office consultation with the Model Clearinghouse on all alternative model approvals. Previously, section 3 included various requirements under recommendation subheading that were not clearly identified as requirements. Accordingly, the EPA is proposing to modify section 3 with the incorporation of requirement subsections to eliminate any ambiguity.

5. Section 4

The EPA proposes to significantly revise section 4 to incorporate the modeling approaches recommended for air quality impact analyses for the criteria pollutants of CO, lead, SO₂, NO₂, and primary PM_2.5 and PM₁₀. In many respects, the proposed revisions to section 4 are a combination of the previous sections 4 and 5, reflecting inert criteria pollutants only. The EPA also proposes to modify section 4 to incorporate requirement subsections to provide clarity of the various requirements where previously sections 4 and 5 included various requirements under recommendation subheadings.

As proposed, this section provides an in-depth discussion of screening and refined models, including the introduction of AERSCREEN as the recommended screening model for simple and complex terrain for single sources and options for multi-source screening with AERMOD.^[47] The EPA proposes to include a clear discussion of each appendix A preferred model in section 4.3 (Refined Models). The EPA also proposes to modify the discussion for each preferred model (i.e., AERMOD Modeling System, CTDMPLUS, and OCD) from the previous section 4 with appropriate edits and some streamlining based on information available in the respective model formulation documentation and users guides.

The EPA is proposing to add a subsection specifically addressing the modeling recommendations for SO₂ where, previously, section 4 of the Guideline was generally understood to be applicable for SO_2. Minor updates are proposed with respect to the modeling recommendations for each of the other inert criteria pollutants that were previously found in section 5. For NO₂, the ARM2 is proposed to be added as a Tier 2 option, and the Tier 3 options of OLM and PVMRM are proposed to become part of the regulatory version of AERMOD. For any pollutant that had significant emissions from mobile sources, our previous recommendation to use the CALINE3 models is proposed to be replaced with AERMOD.

6. Section 5

As already stated, much of the previous section 5 with respect to the inert criteria pollutants is proposed to be incorporated into the revised section 4. As proposed, the revised section 5 is now focused only on the modeling approaches recommended for ozone and secondary PM_2.5.

Both ozone and secondary PM_2.5 are formed through chemical reactions in the atmosphere and are not appropriately modeled with traditional steady-state Gaussian plume models, such as AERMOD. Chemical transport models are necessary to appropriately assess the single-source air quality impacts of precursor pollutants on the formation of ozone or secondary PM_2.5.

While the proposed revision to section 5 do not specify a particular EPA-preferred model or technique for use in air quality assessments, a two-tiered screening approach is proposed for ozone and secondary PM_2.5 with appropriate references to the EPA's new single-source modeling guidance. The first tier consists of technically credible and appropriate relationships between emissions and the impacts developed from existing modeling simulations. If existing technical information is not available or appropriate, then a second tier approach would apply, involving use of sophisticated chemical transport models (e.g., photochemical grid models) as determined in consultation with the appropriate EPA Regional Office on a case-by-case basis based upon the EPA's new single-source modeling guidance.

7. Section 6

Revisions to section 6 are proposed to more clearly address the modeling recommendations of other federal agencies, such as the FLM(s), that have been developed in response to EPA rules or standards. While no attempt is made to comprehensively discuss each topic, the EPA proposes to provide appropriate references to the respective federal agency guidance documents.

The proposed revision to section 6 focus primarily on AQRVs, including near-field and long-range transport assessments for visibility impairment and deposition. The interests of the Bureau of Ocean Energy and Management for Outer Continental Shelf permitting situations and of the Federal Aviation Administration for airport and air base permitting situations are represented in proposed section 6.3 (Modeling Guidance for Other Governmental Programs).

The discussion of Good Engineering Practices (GEP) for stack height consideration is proposed to be modified and moved to section 7. The EPA proposed to remove the discussion of long-range transport for PSD Class I increment and references to the previously preferred long-range transport model, CALPUFF, in accordance with the more detailed discussion in the Proposed Actions section of this Preamble.

8. Section 7

We propose to revise section 7 to be more streamlined and appropriate to the variety of general modeling issues and considerations that are not already been covered in sections 4, 5, and 6 of the Guideline. The EPA proposes to move the information concerning design concentrations and receptor sites to section 9. The discussion of stability categories is proposed to be removed from section 7 since it is specifically addressed in the model formulation documentation and guidance for the dispersion models that require stability categories to be defined. As already stated, the GEP discussion from the previous section 6 is proposed to be incorporated into this section.

The EPA proposes to expand the recommendations for determining rural or urban dispersion coefficients to provide more clarity with respect to appropriate characterization within AERMOD, including a discussion on the existence of highly industrialized areas where population density is low that may be best treated with urban rather than rural dispersion coefficients. References to CALPUFF in the Complex Winds subsection are proposed to be removed due to technical issues described in the Proposed Actions section of this preamble. As proposed, if necessary for special complex wind situations, the setup and application of an alternative model should now be determined in consultation with the appropriate reviewing authority. Finally, the EPA proposes to revise section 7 to include a new discussion of modeling considerations specific to mobile sources.

9. Section 8

The EPA propose extensive updates and modifications to section 8 to reflect current EPA practices, requirements, and recommendations for determining the appropriate modeling domain and model input data from new or modifying source(s) or sources under consideration for a revised permit limit, from background concentrations (including air quality monitoring data and nearby and others sources), and from meteorology. As with earlier sections, the EPA proposes to modify section 8 to incorporate requirement subsections where previously section 8 ambiguously included various requirements under recommendation subheadings.

The Background Concentration subsection is proposed to be significantly modified from the existing Guideline to include a more clear and comprehensive discussion of nearby and other sources. This is intended to eliminate confusion of how to identify nearby sources that should be explicitly modeled and all other sources that should be generally represented by air quality monitoring data. In addition to air quality monitoring data, a brief discussion on the use of photochemical grid modeling to appropriately characterize background concentrations has been included in this proposed section. Updates to Tables 8-1 and 8-2 are proposed per changes in the considerations for nearby sources, as discussed in the Proposed Actions section of this Preamble.

The use of prognostic mesoscale meteorological models to provide meteorological input for regulatory dispersion modeling applications is proposed to be incorporated throughout the Meteorological Input Data subsection, including the introduction of the MMIF as a tool to inform regulatory model applications. Other than additional minor modifications to the recommendations through this subsection based on current EPA practices, the most substantive proposed edits relate to the recommendation to use the AERMINUTE meteorological data processor to calculate hourly average wind speed and direction when processing NWS ASOS data for developing AERMET meteorological inputs to the AERMOD dispersion model.

10. Section 9

The EPA proposes to move all of the information previously in section 9 related to model accuracy and evaluation into other sections in the revised Guideline (primarily to the revised section 2 and some to the revised section 4). This provides for greater clarity in those topics as applied to selection of models under the Guideline. However, the EPA proposes to remove subsection on the “Use of Uncertainty in Decision Making.”. After removing this content, the EPA proposes to totally revise section 9 to focus on the regulatory application of models, which would include the majority of the information found previously in section 10.

The EPA proposes to revise the discussion portion of section 9 to more clearly summarize the general concepts presented in earlier sections of the Guideline and to set the stage for the appropriate regulatory application of models and/or, in rare circumstances, air quality monitoring data. The importance of developing and vetting a modeling protocol is more prominently presented in a separate subsection.

The information related to design concentrations is proposed to be updated and unified from previous language found in sections 7 and 10. An expanded discussion of receptor sites is proposed based on language from the previous section 7 and new considerations given past practices of model users tending to define an excessively large and inappropriate number of receptors based on vague guidance.

The recommendations for NAAQS and PSD increment compliance demonstrations are proposed to be overhauled to more clearly and accurately reflect the long-standing EPA recommendation and practice of performing a single-source impact analysis as a first stage of the NAAQS and PSD increment compliance demonstration and, as necessary, conducting a more comprehensive cumulative impact analysis as the second stage. The appropriate considerations and applications of screening and/or refined model are described in each stage.

Finally, the section on Use of Measured Data in Lieu of Model Estimates subsection is proposed to be revised to provide more details on the process for determining the rare circumstances in which air quality monitoring data may be considered for determining the most appropriate emissions limit for a modification to an existing source. As with other portions of the revised section 9, the language throughout this subsection is proposed to be updated to reflect current EPA practices, as appropriate.

11. Section 10

As discussed, the majority of the information found previously in section 10 is proposed to be incorporated into the revised section 9. As proposed, section 10 consists of the references that were in the previous section 12. We also propose to update each reference, as appropriate, based on the text revisions throughout the Guideline.

12. Section 11

In a streamlining effort, the EPA proposes to remove this bibliography section from the Guideline.

13. Section 12

As stated earlier, this references section is now proposed as section 10 with appropriate updates.

14. Appendix A to the Guideline

The EPA proposes to revise appendix A to the Guideline to remove the Buoyant Line and Point Source Dispersion Model (BLP), CALINE3, and CALPUFF as refined air quality models preferred for specific regulator applications. The rational for the removal of these air quality models from the preferred status can be found in the Proposed Actions section of this Preamble.

V. Statutory and Executive Order Reviews

A. Executive Order 12866: Regulatory Planning and Review and Executive Order 13563: Improving Regulation and Regulatory Review

This proposed action is not a “significant regulatory action” under the terms of Executive Order 12866 (58 FR 51735, October 4, 1993) and is, therefore not subject to OMB review under Executive Orders 12866 and 13563 (76 FR 3821, January 21, 2011).

B. Paperwork Reduction Act

This proposed action does not impose an information collection burden subject to OMB review under the provisions of the Paperwork Reduction Act, 44 U.S.C. 3501 et seq.

C. Regulatory Flexibility Act

The Regulatory Flexibility Act (RFA) generally requires an agency to prepare a regulatory flexibility analysis of any rule subject to notice and comment rulemaking requirements under the Administrative Procedure Act or any other statute unless the agency certifies that the rule will not have a significant economic impact on a substantial number of small entities. Small entities include small businesses, small organizations, and small governmental jurisdictions.

For purposes of assessing the impacts of this rule on small entities, small entity is defined as (1) a small business as defined by the Small Business Administration's (SBA) regulations at 13 CFR 121.201; (2) a small governmental jurisdiction that is a government of a city, county, town, school district or special district with a population of less than 50,000; and (3) a small organization that is any not-for-profit enterprise which is independently owned and operated and is not dominant in its field.

The modeling techniques described in this proposed action are primarily used by air agencies and by industries owning major sources subject to NSR permitting requirements. To the extent that any small entities would have to conduct air quality assessments, using the models and/or techniques described in this proposed action are not expect to pose any additional burden (compared to the existing models and/or techniques) on these entities. The proposal features updates to the existing EPA-preferred model, AERMOD, that serves to increase efficiency and accuracy by changing only mathematical formulations and specific data elements. Also, this proposed action will streamline resources necessary to conduct necessary modeling with AERMOD by incorporating model algorithms from the BLP model and replacing CALINE3 for mobile source applications. Although this proposed action calls for new models and/or techniques for use in addressing ozone and secondary PM_2.5, we expect most small entities will generally be able to rely on existing modeling simulations; so, we expect minimal burden associated with these assessments. Therefore, we do not believe that that this proposal poses a significant or unreasonable burden on any small entities.

After considering the economic impacts of this rule on small entities, I certify that this action will not have a significant economic impact on a substantial number of small entities. We continue to be interested in the potential impacts of the proposed rule on small entities and welcome comments on issues related to such impacts.

D. Unfunded Mandates Reform Act

This proposed action contains no federal mandates under the provisions of Title II of the Unfunded Mandates Reform Act of 1995 (UMRA), 2 U.S.C. 1531-1538 for state, local, or tribal governments or the private sector. This action imposes no enforceable duty on any state, local or tribal governments or the private sector. Therefore, this action is not subject to the requirements of sections 202 or 205 of the UMRA. This action is also not subject to the requirements of section 203 of UMRA because it contains no regulatory requirements that might significantly or uniquely affect small governments.

E. Executive Order 13132: Federalism

This proposed action does not have federalism implications. It will not have substantial direct effects on the states, on the relationship between the national government and the states, or on the distribution of power and responsibilities among the various levels of government, as specified in Executive Order 13132. This rule does not create a mandate on state, local or tribal governments nor does it impose any enforceable duties on these entities. This action would add better, more accurate techniques for conducting air quality assessments and does not add any additional requirements for any of the affected parties covered under Executive Order 13132. Thus, the requirements of section 6 of the Executive Order do not apply to this proposal. In the spirit of Executive Order 13132, and consistent with the EPA policy to promote communications between the EPA and state and local governments, the EPA specifically solicits comment on this proposed rule from state and local officials.

F. Executive Order 13175: Consultation and Coordination With Indian Tribal Governments

This proposed action does not have tribal implications, as specified in Executive Order 13175 (65 FR 67249, November 9, 2000). This proposed rule imposes no requirements on tribal governments. Accordingly, Executive Order 13175 does not apply to this action. In the spirit of Executive Order 13175, the EPA specifically solicits additional comment on this proposed action from tribal officials.

G. Executive Order 13045: Protection of Children From Environmental Health and Safety Risks

The EPA interprets Executive Order 13045 as applying only to those regulatory actions that concern environmental health or safety risks that the EPA has reason to believe may disproportionately affect children, per the definition of “covered regulatory action” in section 2-202 of the Executive Order. This action is not subject to Executive Order 13045 because it does not concern an environmental health risk or safety risk.

H. Executive Order 13211: Actions Concerning Regulations That Significantly Affect Energy Supply, Distribution, or Use

This action is not a “significant energy action” as defined in Executive Order 13211 (66 FR 28355 (May 22, 2001)), because it is not likely to have a significant adverse effect on the supply, distribution, or use of energy.

I. National Technology Transfer and Advancement Act

This rulemaking does not involve technical standards.

J. Executive Order 12898: Federal Actions To Address Environmental Justice in Minority Populations and Low-Income Populations

The EPA has determined that this proposed rule will not have disproportionately high and adverse human health or environmental effects on minority or low-income populations because it does not affect the level of protection provided to human health or the environment.

List of Subjects in 40 CFR Part 51

• Environmental protection
• Administrative practice and procedure
• Air pollution control
• Carbon monoxide
• Intergovernmental relations
• Nitrogen oxides
• Ozone
• Particulate Matter
• Reporting and recordkeeping requirements
• Sulfur oxides

Dated: July 14, 2015.

Gina McCarthy,

Administrator.

For the reasons stated in the preamble, title 40, chapter I of the Code of Federal Regulations is proposed to be amended as follows:

PART 51—REQUIREMENTS FOR PREPARATION, ADOPTION, AND SUBMITTAL OF IMPLEMENTATION PLANS

1. The authority citation for part 51 continues to read as follows:

Authority: 23 U.S.C. 101; 42 U.S.C. 7401-7671q.

2. Appendix W to part 51 is revised to read as follows:

APPENDIX W TO PART 51—Guideline on Air Quality Models Preface

a. Industry and control agencies have long expressed a need for consistency in the application of air quality models for regulatory purposes. In the 1977 Clean Air Act (CAA), Congress mandated such consistency and encouraged the standardization of model applications. The Guideline on Air Quality Models (hereafter, Guideline) was first published in April 1978 to satisfy these requirements by specifying models and providing guidance for their use. The Guideline provides a common basis for estimating the air quality concentrations of criteria pollutants used in assessing control strategies and developing emissions limits.

b. The continuing development of new air quality models in response to regulatory requirements and the expanded requirements for models to cover even more complex problems have emphasized the need for periodic review and update of guidance on these techniques. Historically, three primary activities have provided direct input to revisions of the Guideline. The first is a series of periodic EPA workshops and modeling conferences conducted for the purpose of ensuring consistency and providing clarification in the application of models. The second activity was the solicitation and review of new models from the technical and user community. In the March 27, 1980, Federal Register, a procedure was outlined for the submittal to the EPA of privately developed models. After extensive evaluation and scientific review, these models, as well as those made available by the EPA, have been considered for recognition in the Guideline. The third activity is the extensive on-going research efforts by the EPA and others in air quality and meteorological modeling.

c. Based primarily on these three activities, new sections and topics have been included as needed. The EPA does not make changes to the guidance on a predetermined schedule, but rather on an as-needed basis. The EPA believes that revisions of the Guideline should be timely and responsive to user needs and should involve public participation to the greatest possible extent. All future changes to the guidance will be proposed and finalized in the Federal Register. Information on the current status of modeling guidance can always be obtained from EPA's Regional Offices.

Table of Contents

List of Tables

1.0 Introduction

2.0 Overview of Model Use

2.1 Suitability of Models

2.1.1 Model Accuracy and Uncertainty

2.2 Levels of Sophistication of Air Quality Analyses and Models

2.3 Availability of Models

3.0  Preferred and Alternative Air Quality Models

3.1 Preferred Models

3.1.1 Discussion

3.1.2 Requirements

3.2 Alternative Models

3.2.1 Discussion

3.2.2 Requirements

3.3 EPA's Model Clearinghouse

4.0 Models for Carbon Monoxide, Lead, Sulfur Dioxide, Nitrogen Dioxide and Primary Particulate Matter

4.1 Discussion

4.2 Requirements

4.2.1 Screening Models and Techniques

4.2.1.1 AERSCREEN

4.2.1.2 CTSCREEN

4.2.1.3 Screening in Complex Terrain

4.2.2 Refined Models

4.2.2.1 AERMOD

4.2.2.2 CTDMPLUS

4.2.2.3 OCD

4.2.3 Pollutant Specific Modeling Requirements

4.2.3.1 Models for Carbon Monoxide

4.2.3.2 Models for Lead

4.2.3.3 Models for Sulfur Dioxide

4.2.3.4 Models for Nitrogen Dioxide

4.2.3.5 Models for PM_2.5

4.2.3.6 Models for PM₁₀

5.0 Models for Ozone and Secondarily Formed Particulate Matter

5.1 Discussion

5.2 Recommendations

5.3 Recommended Models and Approaches for Ozone

5.3.1 Models for NAAQS Attainment Demonstrations and Multi-Source Air Quality Assessments

5.3.2 Models for Single-Source Air Quality Assessments

5.4 Recommended Models and Approaches for Secondarily Formed PM_2.5

5.4.1 Models for NAAQS Attainment Demonstrations and Multi-Source Air Quality Assessments

5.4.2 Models for Single-Source Air Quality Assessments

6.0 Modeling for Air Quality Related Values and Other Governmental Programs

6.1 Discussion

6.2 Air Quality Related Values

6.2.1 Visibility

6.2.1.1 Models for Estimating Near-Field Visibility Impairment

6.2.1.2 Models for Estimating Visibility Impairment for Long-Range Transport

6.2.2 Models for Estimating Deposition Impacts

6.3 Modeling Guidance for Other Governmental Programs

7.0 General Modeling Considerations

7.1 Discussion

7.2 Recommendations

7.2.1 All sources

7.2.1.1 Dispersion Coefficients

7.2.1.2 Complex Winds

7.2.1.3 Gravitational Settling and Deposition

7.2.2 Stationary Sources

7.2.2.1 Good Engineering Practice Stack Height

7.2.2.2 Plume Rise

7.2.3 Mobile Sources

8.0 Model Input Data

8.1 Modeling Domain

8.1.1 Discussion

8.1.2 Requirements

8.2 Source Data

8.2.1 Discussion

8.2.2 Requirements

8.3 Background Concentrations

8.3.1 Discussion

8.3.2 Recommendations for Isolated Single Source

8.3.3 Recommendations for Multi-Source Areas

8.4 Meteorological Input Data

8.4.1 Discussion

8.4.2 Recommendations and Requirements

8.4.3 National Weather Service Data

8.4.3.1 Discussion

8.4.3.2 Recommendations

8.4.4 Site-specific data

8.4.4.1 Discussion

8.4.4.2 Recommendations

8.4.5 Prognostic meteorological data

8.4.5.1 Discussion

8.4.5.2 Recommendations

8.4.6 Treatment of Near-Calms and Calms

8.4.6.1 Discussion

8.4.6.2 Recommendations

9.0 Regulatory Application of Models

9.1 Discussion

9.2 Recommendations

9.2.1 Modeling Protocol

9.2.2 Design Concentration and Receptor Sites

9.2.3 NAAQS and PSD Increments Compliance Demonstrations for New or Modified Sources

9.2.3.1 Considerations in Developing Emissions Limits

9.2.4 Use of Measured Data in Lieu of Model Estimates

10.0 References

Appendix A to Appendix W of Part 51—Summaries of Preferred Air Quality Models

List of Tables

Table No.	Title
8-1	Point Source Model Emission Input for SIP Revisions of Inert Pollutants.
8-2	Point Source Model Emission Input for NAAQS Compliance in PSD Demonstrations.

1.0 Introduction

a. The Guideline recommends air quality modeling techniques that should be applied to State Implementation Plan (SIP) submittals and revisions, to New Source Review (NSR), including new or modifying sources under Prevention of Significant Deterioration (PSD),^1 2 3 conformity analyses,⁴ and other air quality assessments required under EPA regulation. Applicable only to criteria air pollutants, the Guideline is intended for use by the EPA Regional Offices in judging the adequacy of modeling analyses performed by the EPA, by state, local, and tribal permitting authorities, and by industry. It is appropriate for use by other federal government agencies and by state, local, and tribal agencies with air quality and land management responsibilities. The Guideline serves to identify, for all interested parties, those modeling techniques and databases that the EPA considers acceptable. The Guideline is not intended to be a compendium of modeling techniques. Rather, it should serve as a common measure of acceptable technical analysis when supported by sound scientific judgment.

b. Air quality measurements ⁵ are routinely used to characterize ambient concentrations of criteria pollutants throughout the nation but are rarely sufficient for characterizing the ambient impacts of individual sources or demonstrating adequacy of emissions limits for an existing source due to limitations in spatial and temporal coverage of ambient monitoring networks. The impacts of new sources that do not yet exist and modifications to existing sources that have yet to be implemented can only be determined through modeling. Thus, models have become a primary analytical tool in most air quality assessments. Air quality measurements can be used in a complementary manner to air quality models, with due regard for the strengths and weaknesses of both analysis techniques, and are particularly useful in assessing the accuracy of model estimates.

c. It would be advantageous to categorize the various regulatory programs and to apply a designated model to each proposed source needing analysis under a given program. However, the diversity of the nation's topography and climate, and variations in source configurations and operating characteristics dictate against a strict modeling “cookbook.” There is no one model capable of properly addressing all conceivable situations even within a broad category such as point sources. Meteorological phenomena associated with threats to air quality standards are rarely amenable to a single mathematical treatment; thus, case-by-case analysis and judgment are frequently required. As modeling efforts become more complex, it is increasingly important that they be directed by highly competent individuals with a broad range of experience and knowledge in air quality meteorology. Further, they should be coordinated closely with specialists in emissions characteristics, air monitoring and data processing. The judgment of experienced meteorologists, atmospheric scientists, and analysts is essential.

d. The model that most accurately estimates concentrations in the area of interest is always sought. However, it is clear from the needs expressed by the EPA Regional Offices, by state, local, and tribal agencies, by many industries and trade associations, and also by the deliberations of Congress that consistency in the selection and application of models and databases should also be sought, even in case-by-case analyses. Consistency ensures that air quality control agencies and the general public have a common basis for estimating pollutant concentrations, assessing control strategies, and specifying emissions limits. Such consistency is not, however, promoted at the expense of model and database accuracy. The Guideline provides a consistent basis for selection of the most accurate models and databases for use in air quality assessments.

e. Recommendations are made in the Guideline concerning air quality models and techniques, model evaluation procedures, and model input databases and related requirements. The guidance provided here should be followed in air quality analyses relative to SIPs, NSR, and in supporting analyses required by the EPA and by state, local, and tribal permitting authorities. Specific models are identified for particular applications. The EPA may approve the use of an alternative model or technique that can be demonstrated to be more appropriate than those recommended in the Guideline. In all cases, the model or technique applied to a given situation should be the one that provides the most accurate representation of atmospheric transport, dispersion, and chemical transformations in the area of interest. However, to ensure consistency, deviations from the Guideline should be carefully documented as part of the public record and fully supported by the appropriate reviewing authority, as discussed later.

f. From time to time, situations arise requiring clarification of the intent of the guidance on a specific topic. Periodic workshops are held with EPA headquarters, EPA Regional Office, and state, local, and tribal agency modeling representatives to ensure consistency in modeling guidance and to promote the use of more accurate air quality models, techniques, and databases. The workshops serve to provide further explanations of Guideline requirements to the EPA Regional Offices and workshop materials are issued with this clarifying information. In addition, findings from ongoing research programs, new model development, or results from model evaluations and applications are continuously evaluated. Based on this information, changes in the applicable guidance may be indicated and appropriate revisions to the Guideline may be considered.

g. All changes to the Guideline must follow rulemaking requirements since the Guideline is codified in appendix W to 40 Code of Federal Regulations (CFR) part 51. The EPA will promulgate proposed and final rules in the Federal Register to amend this appendix. The EPA utilizes the existing procedures under CAA section 320 that requires EPA to conduct a Conference on Air Quality Modeling at least every 3 years. These modeling conferences are intended to develop standardized air quality modeling procedures and form the basis for associated revisions to this Guideline in support of the EPA's continuing effort to prescribe with “reasonable particularity” air quality models and meteorological and emission databases suitable for modeling National Ambient Air Quality Standards (NAAQS) ⁶ and PSD increments (CAA 320, 42 U.S.C. 7620). Ample opportunity for public comment will be provided for each proposed change and public hearings scheduled.

h. A wide range of topics on modeling and databases are discussed in the Guideline. Section 2 gives an overview of models and their suitability for use in regulatory applications. Section 3 provides specific guidance on the determination of preferred air quality models and on the selection of alternative models or techniques. Sections 4 through 6 provide recommendations on modeling techniques for assessing criteria pollutant impacts from single and multiple sources with specific modeling requirements for selected regulatory applications. Section 7 discusses general considerations common to many modeling analyses for stationary and mobile sources. Section 8 makes recommendations for data inputs to models including source, background air quality, and meteorological data. Section 9 summarizes how estimates and measurements of air quality are used in assessing source impact and in evaluating control strategies.

i. Appendix W to 40 CFR part 51 contains an appendix: Appendix A. Thus, when reference is made to “appendix A” in this document, it refers to appendix A to appendix W to 40 CFR part 51. Appendix A contains summaries of refined air quality models that are “preferred” for particular applications; both EPA models and models developed by others are included.

2.0 Overview of Model Use

a. Increasing reliance has been placed on concentration estimates from air quality models as the primary basis for regulatory decisions concerning source permits and emission control requirements. In many situations, such as review of a proposed new source, no practical alternative exists. Before attempting to implement the guidance contained in this document, the reader should be aware of certain general information concerning air quality models and their evaluation and use. Such information is provided in this section.

2.1 Suitability of Models

a. The extent to which a specific air quality model is suitable for the assessment of source impacts depends upon several factors. These include: (1) The topographic and meteorological complexities of the area; (2) the detail and accuracy of the input databases, i.e., emissions inventory, meteorological data, and air quality data; (3) the manner in which complexities of atmospheric processes are handled in the model; (4) the technical competence of those undertaking such simulation modeling; and (5) the resources available to apply the model. Any of these factors can have a significant influence on the overall model performance, which must be thoroughly evaluated to determine the suitability of an air quality model to a particular application or range of applications.

b. Air quality models are most accurate and reliable in areas that have gradual transitions of land use and topography. Meteorological conditions in these areas are spatially uniform such that observations are broadly representative and air quality model projections are not further complicated by a heterogeneous environment. Areas subject to major topographic influences experience meteorological complexities that are often difficult to measure and simulate. Models with adequate performance are available for increasingly complex environments. However, they are resource intensive and frequently require site-specific observations and formulations. Such complexities and the related challenges for the air quality simulation should be considered when selecting the most appropriate air quality model for an application.

c. Appropriate model input data should be available before an attempt is made to evaluate or apply an air quality model. Assuming the data are adequate, the greater the detail with which a model considers the spatial and temporal variations in meteorological conditions and permit-enforceable emissions, the greater the ability to evaluate the source impact and to distinguish the effects of various control strategies.

d. There are three types of models that have historically been used in the regulatory demonstrations applicable in the Guideline, each having strengths and weaknesses that lend themselves to particular regulatory applications.

i. Gaussian plume models use a “steady-state” approximation, which assumes that over the model time step, the emissions, meteorology and other model inputs, are constant throughout the model domain, resulting in a resolved plume with the emissions distributed throughout the plume according to a Gaussian distribution. This formulation allows Gaussian models to estimate near-field impacts of a limited number of sources at a relatively high resolution, with temporal scales of an hour and spatial scales of meters. However, this formulation allows for only relatively inert pollutants, with very limited considerations of transformation and removal (e.g., deposition), and further limits the domain for which the model may be used. Thus, Gaussian models may not be appropriate if model inputs are changing sharply over the model time step or within the desired model domain or if more advanced considerations of chemistry are needed.

ii. Lagrangian puff models, on the other hand, are non-steady-state, and assume that model input conditions are changing over the model domain and model time step. Lagrangian models can also be used to determine near and far-field impacts from a limited number of sources at a high resolution. Traditionally, Lagrangian models have been used for relatively inert pollutants, with slightly more complex considerations of removal than Gaussian models. Some Lagrangian models treat in-plume gas and particulate chemistry. However, these models require time and space varying concentration fields of oxidants and, in the case of fine particulate matter (PM_2.5), neutralizing agents, such as ammonia. Reliable background fields are critical for applications involving secondary pollutant formation because secondary impacts generally occur when in-plume precursors mix and react with species in the background atmosphere.^7 8 These oxidant and neutralizing agents are not routinely measured, but can be generated with a three-dimensional photochemical grid model.

iii. Photochemical grid models are three-dimensional Eulerian grid-based models that treat chemical and physical processes in each grid cell and use diffusion and transport processes to move chemical species between grid cells.⁹ Eulerian models assume that emissions are spread evenly throughout each model grid cell. Typically, Eulerian models have difficulty with fine scale resolution of individual plumes. However, these types of models can be appropriately applied for assessment of near-field and regional scale reactive pollutant impacts from specific sources ^{7 10 11 12} or all sources.^13 14 15 Photochemical gird models simulate a more realistic environment for chemical transformation,^7 12 but simulations can be more resource intensive than Lagrangian or Gaussian plume models.

e. Competent and experienced meteorologists, atmospheric scientists, and analysts are an essential prerequisite to the successful application of air quality models. The need for such specialists is critical when the more sophisticated models are used or the area being investigated has complicated meteorological or topographic features. It is important to note that a model applied improperly or with inappropriate data can lead to serious misjudgments regarding the source impact or the effectiveness of a control strategy.

f. The resource demands generated by use of air quality models vary widely depending on the specific application. The resources required may be important factors in the selection and use of a model or technique for a specific analysis. These resources depend on the nature of the model and its complexity, the detail of the databases, the difficulty of the application, the amount and level of expertise required, and the costs of manpower and computational facilities.

2.1.1 Model Accuracy and Uncertainty

a. The formulation and application of air quality models are accompanied by several sources of uncertainty. “Irreducible” uncertainty stems from the “unknown” conditions, which may not be explicitly accounted for in the model (e.g., the turbulent velocity field). Thus, there are likely to be deviations from the observed concentrations in individual events due to variations in the unknown conditions. “Reducible” uncertainties ¹⁶ are caused by: (1) Uncertainties in the “known” input conditions (e.g., emission characteristics and meteorological data); (2) errors in the measured concentrations; and (3) inadequate model physics and formulation.

b. Evaluations of model accuracy should focus on the reducible uncertainty associated with physics and the formulation of the model. The accuracy of the model is normally determined by an evaluation procedure which involves the comparison of model concentration estimates with measured air quality data.¹⁷ The statement of model accuracy is based on statistical tests or performance measures such as bias, noise, correlation, etc.^18 19

c. Since the 1980's, the EPA has worked with the modeling community to encourage development of standardized model evaluation methods and the development of continually improved methods for the characterization of model performance.^{16 18 20 21 22} There is general consensus on what should be considered in the evaluation of air quality models; namely, quality assurance planning, documentation and scrutiny should be consistent with the intended use and should include:

• Scientific peer review;
• Supportive analyses (diagnostic evaluations, code verification, sensitivity analyses);
• Diagnostic and performance evaluations with data obtained in trial locations; and
• Statistical performance evaluations in the circumstances of the intended applications.

Performance evaluations and diagnostic evaluations assess different qualities of how well a model is performing, and both are needed to establish credibility within the client and scientific community.

d. Performance evaluations allow the EPA and model users to determine the relative performance of a model in comparison with alternative modeling systems. Diagnostic evaluations allow determination of a model capability to simulate individual processes that affect the results, and usually employ smaller spatial/temporal scale date sets (e.g., field studies). Diagnostic evaluations enable the EPA and model users to build confidence that model predictions are accurate for the right reasons. However, the objective comparison of modeled concentrations with observed field data provides only a partial means for assessing model performance. Due to the limited supply of evaluation datasets, there are practical limits in assessing model performance. For this reason, the conclusions reached in the science peer reviews and the supportive analyses have particular relevance in deciding whether a model will be useful for its intended purposes.

2.2 Levels of Sophistication of Air Quality Analyses and Models

a. It is desirable to begin an air quality analysis by using simplified or conservative methods (or both) followed, as appropriate, by more complex and refined methods. The purpose of this approach is to streamline the process and sufficiently address regulatory requirements by eliminating the need of more detailed modeling when it is not necessary in a specific regulatory application. For example, in the context of a PSD permit application, a simplified or conservative analysis may be sufficient where it shows the proposed construction clearly will not cause or contribute to ambient concentrations in excess of either the NAAQS or the PSD increments.^2 3

b. There are two general levels of sophistication of air quality models. The first level consists of screening models that provide conservative modeled estimates of the air quality impact of a specific source or source category based on simplified assumptions of the model inputs (e.g., preset, worst-case meteorological conditions). In the case of a PSD assessment, if a screening model indicates that the concentration contributed by the source could cause or contribute to a violation of any NAAQS or PSD increment, then the second level of more sophisticated models should be applied.

c. The second level consists of refined models that provide more detailed treatment of physical and chemical atmospheric processes, require more detailed and precise input data, and provide spatially and temporally resolved concentration estimates. As a result they provide a more sophisticated and, at least theoretically, a more accurate estimate of source impact and the effectiveness of control strategies.

d. There are situations where a screening model or a refined model is not available such that screening and refined modeling are not viable options to determine source-specific air quality impacts. In such situations, a screening technique or reduced-form model may be viable options for estimating source impacts.

i. Screening techniques are differentiated from a screening model in that screening techniques are approaches that make simplified and conservative assumptions about the physical and chemical atmospheric processes important to determining source impacts while screening models make assumptions about conservative inputs to a specific model. The complexity of screening techniques ranges from simplified assumptions of chemistry applied to refined or screening model output to sophisticated approximations of the chemistry applied within a refined model.

ii. Reduced-form models are computationally efficient simulation tools for characterizing the pollutant response to specific types of emission reductions for a particular geographic area or background environmental conditions that reflect underlying atmospheric science of a refined model but reduce the computational resources of running a complex, numerical air quality model such as a photochemical grid model.

In such situations, an attempt should be made to acquire or improve the necessary databases and to develop appropriate analytical techniques, but the screening technique or reduced-form model may be sufficient in conducting regulatory modeling applications when applied in consultation with the EPA Regional Office.

e. Consistent with the general principle described in paragraph 2.2(a), the EPA may establish a demonstration tool or method as a sufficient means for a user or applicant to make a demonstration required by regulation, either by itself or as part of a modeling demonstration. To be used for such regulatory purposes, such a tool or method must be reflected in a codified regulation or have a well-documented technical basis and reasoning that is contained or incorporated in the record of the regulatory decision in which it is applied.

2.3 Availability of Models

a. For most of the screening and refined models discussed in the Guideline, codes, associated documentation and other useful information are publicly available for download from the EPA's Support Center for Regulatory Atmospheric Modeling (SCRAM) Web site at http://www.epa.gov/​ttn/​scram. This is a Web site with which air quality modelers should become familiar and regularly visit for important model updates and additional clarifications and revisions to modeling guidance documents that are applicable to EPA programs and regulations. Codes and documentation may also available from the National Technical Information Service (NTIS), http://www.ntis.gov, and, when available, is referenced with the appropriate NTIS accession number.

3.0 Preferred and Alternative Air Quality Models

a. This section specifies the approach to be taken in determining preferred models for use in regulatory air quality programs. The status of models developed by the EPA, as well as those submitted to the EPA for review and possible inclusion in this Guideline, is discussed in this section. The section also provides the criteria and process for obtaining EPA approval for use of alternative models for individual cases in situations where the preferred models are not applicable or available. Additional sources of relevant modeling information are the EPA's Model Clearinghouse ²³ (section 3.3), EPA modeling conferences, periodic Regional, State, and Local Modelers' Workshops, and the EPA's SCRAM Web site (section 2.3).

b. When approval is required for a specific modeling technique or analytical procedure in this Guideline, we refer to the “appropriate reviewing authority.” Many states and some local agencies administer NSR and PSD permitting under programs approved into SIPs. In some EPA regions, federal authority to administer NSR and PSD permitting and related activities has been delegated to state or local agencies. In these cases, such agencies “stand in the shoes” of the respective EPA regions. Therefore, depending on the circumstances, the appropriate reviewing authority may be an EPA Regional Office, a state, local, or tribal agency, or perhaps the Federal Land Manager (FLM). In some cases, the Guideline requires review and approval of the use of an alternative model by the EPA Regional Office (sometimes stated as “Regional Administrator”). For all approvals of alternative models or techniques, the EPA Regional Office will coordinate and shall seek concurrence with the EPA's Model Clearinghouse. If there is any question as to the appropriate reviewing authority, you should contact the EPA Regional Office modeling contact (http://www.epa.gov/​ttn/​scram/​guidance_​cont_​regions.htm), whose jurisdiction generally includes the physical location of the source in question and its expected impacts.

c. In all regulatory analyses, early discussions among the EPA Regional Office staff, state, local, and tribal agency staff, industry representatives, and where appropriate, the FLM, are invaluable and are strongly encouraged. Prior to the actual analyses, agreement on the databases to be used, modeling techniques to be applied, and the overall technical approach helps avoid misunderstandings concerning the final results and may reduce the later need for additional analyses. The preparation of a written modeling protocol that is vetted with the appropriate reviewing authority helps to keep misunderstandings and resource expenditures at a minimum.

d. The identification of preferred models in this Guideline should not be construed as a determination that the preferred models identified here are to be permanently used to the exclusion of all others or that they are the only models available for relating emissions to air quality. The model that most accurately estimates concentrations in the area of interest is always sought. However, designation of specific preferred models is needed to promote consistency in model selection and application.

3.1 Preferred Models

3.1.1 Discussion

a. The EPA has developed some models suitable for regulatory application, while other models have been submitted by private developers for possible inclusion in the Guideline. Refined models that are preferred and required by the EPA for particular applications have undergone the necessary peer scientific reviews ^24 25 and model performance evaluation exercises ^26 27 that include statistical measures of model performance in comparison with measured air quality data as described in section 2.1.1.

b. An American Society for Testing and Materials (ASTM) reference ²⁸ provides a general philosophy for developing and implementing advanced statistical evaluations of atmospheric dispersion models, and provides an example statistical technique to illustrate the application of this philosophy. Consistent with this approach, the EPA has determined and applied a specific evaluation protocol that provides a statistical technique for evaluating model performance for predicting peak concentration values, as might be observed at individual monitoring locations.²⁹

c. When a single model is found to perform better than others, it is recommended for application as a preferred model and listed in appendix A. If no one model is found to clearly perform better through the evaluation exercise, then the preferred model listed in appendix A may be selected on the basis of other factors such as past use, public familiarity, resource requirements, and availability. Accordingly, the models listed in appendix A meet these conditions:

i. The model must be written in a common programming language, and the executable(s) must run on a common computer platform.

ii. The model must be documented in a user's guide or model formulation report which identifies the mathematics of the model, data requirements and program operating characteristics at a level of detail comparable to that available for other recommended models in appendix A.

iii. The model must be accompanied by a complete test dataset including input parameters and output results. The test data must be packaged with the model in computer-readable form.

iv. The model must be useful to typical users, e.g., state air agencies, for specific air quality control problems. Such users should be able to operate the computer program(s) from available documentation.

v. The model documentation must include a robust comparison with air quality data (and/or tracer measurements) or with other well- established analytical techniques.

vi. The developer must be willing to make the model and source code available to users at reasonable cost or make them available for public access through the Internet or National Technical Information Service. The model and its code cannot be proprietary.

d. The EPA's process of establishing a preferred model includes a determination of technical merit, in accordance with the above six items including the practicality of the model for use in ongoing regulatory programs. Each model will also be subjected to a performance evaluation for an appropriate database and to a peer scientific review. Models for wide use (not just an isolated case) that are found to perform better will be proposed for inclusion as preferred models in future Guideline revisions.

e. No further evaluation of a preferred model is required for a particular application if the EPA requirements for regulatory use specified for the model in the Guideline are followed. Alternative models to those listed in appendix A should generally be compared with measured air quality data when they are used for regulatory applications consistent with recommendations in section 3.2.

3.1.2 Requirements

a. Appendix A identifies refined models that are preferred for use in regulatory applications. If a model is required for a particular application, the user must select a model from appendix A or follow procedures in section 3.2.2 for use of an alternative model or technique. Preferred models may be used without a formal demonstration of applicability as long as they are used as indicated in each model summary in appendix A. Further recommendations for the application of preferred models to specific source applications are found in subsequent sections of the Guideline.

b. If changes are made to a preferred model without affecting the modeled concentrations, the preferred status of the model is unchanged. Examples of modifications that do not affect concentrations are those made to enable use of a different computer platform or those that only affect the format or averaging time of the model results. The integration of a graphical user interface (GUI) to facilitate setting up the model inputs and/or analyzing the model results without otherwise altering the model kernel is another example of a modification that does not affect concentrations. However, when any changes are made, the Regional Administrator must require a test case example to demonstrate that the modeled concentration are not affected.

c. A preferred model must be operated with the options listed in appendix A for its intended regulatory application. If other options are exercised, the model is no longer “preferred.” Any other modification to a preferred model that would result in a change in the concentration estimates likewise alters its status so that it is no longer a preferred model. Use of the modified model must then be justified as an alternative model on a case-by-case basis to the appropriate reviewing authority and approved by the Regional Administrator.

d. Where the EPA has not identified a preferred model for a particular pollutant or situation, the EPA may establish a multi-tiered approach for making a demonstration required under PSD or another CAA program. The initial tier or tiers may involve use of demonstration tools, screening models, screening techniques, or reduced-form models; while the last tier may involve the use of demonstration tools, refinded models or techniques, or alternative models approved under section 3.2.

3.2 Alternative Models

3.2.1 Discussion

a. Selection of the best model or techniques for each individual air quality analysis is always encouraged, but the selection should be done in a consistent manner. A simple listing of models in this Guideline cannot alone achieve that consistency nor can it necessarily provide the best model for all possible situations. As discussed in section 3.1.1, the EPA has determined and applied a specific evaluation protocol that provides a statistical technique for evaluating model performance for predicting peak concentration values, as might be observed at individual monitoring locations.²⁹ This protocol is available to assist in developing a consistent approach when justifying the use of other-than-preferred models recommended in the Guideline (i.e., alternative models). The procedures in this protocol provide a general framework for objective decision-making on the acceptability of an alternative model for a given regulatory application. These objective procedures may be used for conducting both the technical evaluation of the model and the field test or performance evaluation.

b. This subsection discusses the use of alternate models and defines three situations when alternative models may be used. This subsection also provides a procedure for implementing 40 CFR 51.166(l)(2) in PSD permitting. This provision requires written approval of the Administrator for any modification or substitution of an applicable model. An applicable model for purposes of 40 CFR 51.166(l) is a preferred model in appendix A to the Guideline. Approval to use an alternative model under section 3.2 of the Guideline qualifies as approval for the modification or substitution of a model under 40 CFR 51.166(l)(2). The Regional Administrators are delegated authority to issue such approvals under section 3.2 of the Guideline, provided that such approval is issued after consultation with EPA's Model Clearinghouse and formally documented in a concurrence memorandum from EPA's Model Clearinghouse which demonstrates that the requirements within section 3.2 for use of an alternative model have been met.

3.2.2 Requirements

a. Determination of acceptability of an alternative model is an EPA Regional Office responsibility in consultation with EPA's Model Clearinghouse as discussed in paragraphs 3.0(b) and 3.2.1(b). Where the Regional Administrator finds that an alternative model is more appropriate than a preferred model, that model may be used subject to the approval of the EPA Regional Office based on the requirements of this subsection. This finding will normally result from a determination that (1) a preferred air quality model is not appropriate for the particular application; or (2) a more appropriate model or technique is available and applicable.

b. An alternative model shall be evaluated from both a theoretical and a performance perspective before it is selected for use. There are three separate conditions under which such a model may be approved for use:

1. If a demonstration can be made that the model produces concentration estimates equivalent to the estimates obtained using a preferred model;

2. If a statistical performance evaluation has been conducted using measured air quality data and the results of that evaluation indicate the alternative model performs better for the given application than a comparable model in appendix A; or

3. If there is no preferred model.

Any one of these three separate conditions may justify use of an alternative model. Some known alternative models that are applicable for selected situations are listed on the EPA's SCRAM Web site (section 2.3). However, inclusion there does not confer any unique status relative to other alternative models that are being or will be developed in the future.

c. Equivalency, condition (1) in paragraph (b) of this subsection, is established by demonstrating that the maximum or highest, second highest concentrations are within+/− 2 percent of the estimates obtained from the preferred model. The option to show equivalency is intended as a simple demonstration of acceptability for an alternative model that is so nearly identical (or contains options that can make it identical) to a preferred model that it can be treated for practical purposes as the preferred model. However, notwithstanding this demonstration, models that are not equivalent may be used when one of the two other conditions described in paragraphs (d) and (e) of this subsection are satisfied.

d. For condition (2) in paragraph (b) of this subsection, established statistical performance evaluation procedures and techniques ^28 29 for determining the acceptability of a model for an individual case based on superior performance should be followed, as appropriate. Preparation and implementation of an evaluation protocol which is acceptable to both control agencies and regulated industry is an important element in such an evaluation.

e. Finally, for condition (3) in paragraph (b) of this subsection, an alternative model or technique may be approved for use provided that:

i. The model or technique has received a scientific peer review;

ii. The model or technique can be demonstrated to be applicable to the problem on a theoretical basis;

iii. The databases which are necessary to perform the analysis are available and adequate;

iv. Appropriate performance evaluations of the model or technique have shown that the model or technique is not inappropriately biased for regulatory application; ^[a] and

v. A protocol on methods and procedures to be followed has been established.

f. To formally document that the requirements of section 3.2 for use of an alternative model are satisfied for a particular application or range of applications, a memorandum will be prepared by the EPA's Model Clearinghouse through a consultative process with the Region Office.

3.3 EPA's Model Clearinghouse

a. The Regional Administrator has the authority to select models that are appropriate for use in a given situation. However, there is a need for assistance and guidance in the selection process so that fairness, consistency, and transparency in modeling decisions are fostered among the EPA Regional Offices and the state, local, and tribal agencies. To satisfy that need, the EPA established the Model Clearinghouse ²³ to serve a central role of coordination and collaboration between EPA headquarters and the EPA Regional Offices. Additionally, the EPA holds periodic workshops with EPA headquarters, EPA Regional Office, and state, local, and tribal agency modeling representatives.

b. The EPA Regional Office should always be consulted for information and guidance concerning modeling methods and interpretations of modeling guidance, and to ensure that the air quality model user has available the latest most up-to-date policy and procedures. As appropriate, the EPA Regional Office may also request assistance from the EPA's Model Clearinghouse on other applications of models, analytical techniques, or databases or to clarify interpretation of the Guideline or related modeling guidance.

c. The EPA Regional Office will coordinate with the EPA's Model Clearinghouse after an initial evaluation and decision has been developed concerning the application of an alternative model. The acceptability and formal approval process for an alternative model is described in section 3.2.

4.0 Models for Carbon Monoxide, Lead, Sulfur Dioxide, Nitrogen Dioxide and Primary Particulate Matter

4.1 Discussion

a. This section identifies modeling approaches generally used in the air quality impact analysis of sources that emit the criteria pollutants carbon monoxide (CO), lead, sulfur dioxide (SO₂), nitrogen dioxide (NO₂), and primary particulates (PM_2.5 and PM₁₀).

b. The guidance in this section is specific to the application of the Gaussian plume models identified in appendix A. Gaussian plume models assume that emissions and meteorology are in a steady-state, which is typically based on an hourly time step. This approach results in a plume that has an hourly-averaged distribution of emission mass according to a Gaussian curve through the plume. Though Gaussian steady-state models conserve the mass of the primary pollutant throughout the plume, they can still take into account a limited consideration of first-order removal processes (e.g., wet and dry deposition) and limited chemical conversion (e.g., OH oxidation).

c. Due to the steady-state assumption, Gaussian plume models are generally considered applicable to distances less than 50 km, beyond which, modeled predictions of plume impact are likely conservative. The locations of these impacts are expected to be unreliable due to changes in meteorology that are likely to occur during the travel time.

d. The applicability of Gaussian plume models may vary depending on the topography of the modeling domain, i.e., simple or complex. Simple terrain, as used here, is considered to be an area where terrain features are all lower in elevation than the top of the stack of the source(s) in question. Complex terrain is defined as terrain exceeding the height of the stack being modeled.

e. Gaussian models determine source impacts at discrete locations (receptors) for each meteorological and emission scenario, and generally attempt to estimate concentrations at specific sites that represent an ensemble average of numerous repetitions of the same “event.” Uncertainties in model estimates are driven by this formulation, and as noted in section 2.1.1, evaluations of model accuracy should focus on the reducible uncertainty associated with physics and the formulation of the model. The “irreducible” uncertainty associated with Gaussian plume models may be responsible for variation in concentrations of as much as +/− 50 percent.³⁰ “Reducible” uncertainties ¹⁶ can be on a similar scale. For example, Pasquill ³¹ estimates that, apart from data input errors, maximum ground-level concentrations at a given hour for a point source in flat terrain could be in error by 50 percent due to these uncertainties. Errors of 5 to 10 degrees in the measured wind direction can result in concentration errors of 20 to 70 percent for a particular time and location, depending on stability and station location. Such uncertainties do not indicate that an estimated concentration does not occur, only that the precise time and locations are in doubt. Composite errors in highest estimated concentrations of 10 to 40 percent are found to be typical.^32 33 However, estimates of concentrations paired in time and space with observed concentrations are less certain.

f. Model evaluations and inter-comparisons should take these aspects of uncertainty into account. For a regulatory application of a model, the emphasis of model evaluations is generally placed on the highest modeled impacts. Thus, the Cox-Tikvart model evaluation approach, which compares the highest modeled impacts on several timescales, is recommended for comparisons of models and measurements and model inter-comparisons. The approach includes bootstrap techniques to determine the significance of various modeled predictions and increases the robustness of such comparisons when the number of available measurements are limited.^34 35 Because of the uncertainty in paired modeled and observed concentrations, any attempts at calibration of models based on these comparisons is of questionable benefit and shall not be done.

4.2 Requirements

a. For NAAQS compliance demonstrations under PSD, use of the screening and preferred models for the pollutants listed in this subsection shall be limited to the near-field at a nominal distance of 50 km or less. Near-field application is consistent with capabilities of Gaussian plume models and, based on the EPA's assessment, is sufficient to address whether a source will cause or contribution to ambient concentrations in excess to a NAAQS. In most cases, maximum source impacts of inert pollutant are anticipated to occur within 10 to 20 km from the source. Therefore, the EPA does not consider a long-range transport assessment beyond 50 km necessary for these pollutants.³⁶

b. For assessment of PSD increments within the near-field nominal distance of 50 km or less, use of the screening and preferred models for the pollutants listed in this subsection shall be limited to the same screening and preferred models approved for NAAQS compliance demonstrations.

c. To determine if a Class I PSD increment analyses may be necessary beyond 50 km (i.e., long-range transport assessment), the following screening approach shall be used to determine if a significant impact will occur with particular focus on Class I areas that may be threatened at such distances.

i. Based on application in the near-field of the appropriate screening and/or preferred model, determine the significance of the ambient impacts at or about 50 km from the new or modifying source. If this initial step indicates there may be significant ambient impacts at that distance or such near-field assessment is not available, then further assessment is necessary.

ii. For assessment of Class I significance of ambient impacts and cumulative increment analyses, there is not a preferred model or screening approach for distances beyond 50 km. Thus, the EPA Regional Office shall be consulted in determining the appropriate and agreed upon modeling approach to conduct the second level assessment. Typically a Lagrangian model may be the type of model used for this second level assessment, but applicants shall reach agreed upon approaches (models and modeling parameters) on a case-by-case basis. When Lagrangian models are used in this manner, they shall not include plume-depleting reactions, such that model estimates are considered conservative, as is generally appropriate for screening assessments.

d. In those limited situations where a cumulative increment analysis beyond 50 km is necessary, the selection and use of an alternative model shall occur in agreement with the appropriate reviewing authority (paragraph 3.0(b)) and approval by the EPA Regional Office based on the requirements of paragraph 3.2.2(e).

4.2.1 Screening Models and Techniques

a. Where a preliminary or conservative estimate is desired, point source screening techniques are an acceptable approach to air quality analyses.

b. As discussed in paragraph 2.2(a), screening models or techniques are designed to provide a conservative estimate of concentrations. The screening models used in most applications are the screening versions of the preferred models for refined applications. The two screening models, AERSCREEN ^37 38 and CTSCREEN, are screening versions of AERMOD (American Meteorological Society (AMS)/EPA Regulatory Model) and CTDMPLUS (Complex Terrain Dispersion Model Plus Algorithms for Unstable Situations), respectively. AERSCREEN is the preferred screening model for most applications in all types of terrain and for applications involving building downwash. For those applications in complex terrain where the application involves a well-defined hill or ridge, CTSCREEN ³⁹ can be used.

c. Although AERSCREEN and CTSCREEN are designed to address a single-source scenario, there are approaches that can be used on a case-by-case basis to address multi-source situations using screening meteorology or other conservative model assumptions. However, the appropriate reviewing authority (paragraph 3.0(b)) shall be consulted, and concurrence obtained, on the protocol for modeling multiple sources with AERSCREEN or CTSCREEN to ensure that the worst case is identified and assessed.

d. As discussed in section 4.2.3.4, there are also screening techniques built into AERMOD that use simplified or limited chemistry assumptions for determining the partitioning of NO and NO₂ for NO₂ modeling. These screening techniques are part of the EPA's preferred modeling approach for NO₂ and do not need to be approved as an alternative model. However, as with other screening models and techniques, their usage shall occur in agreement with the appropriate reviewing authority (paragraph 3.0(b)).

e. All screening models and techniques shall be configured to appropriately address the site and problem at hand. Close attention must be paid to whether the area should be classified urban or rural in accordance with section 7.2.1.1. The climatology of the area must be studied to help define the worst-case meteorological conditions. Agreement shall be reached between the model user and the appropriate reviewing authority (paragraph 3.0(b)) on the choice of the screening model or technique for each analysis, on the input data and model settings, and the appropriate metric for satisfying regulatory requirements.

4.2.1.1 AERSCREEN

a. Released in 2011, AERSCREEN is the EPA's recommended screening model for simple and complex terrain for single sources including point sources, area sources, horizontal stacks, capped stacks, and flares. AERSCREEN runs AERMOD in a screening mode and consists of two main components: (1) The MAKEMET program which generates a site-specific matrix of meteorological conditions for input into the AERMOD model; and (2) the AERSCREEN command-prompt interface.

b. The MAKEMET program generates a matrix of meteorological conditions, in the form of AERMOD-ready surface and profile files, based on user-specified surface characteristics, ambient temperatures, minimum wind speed, and anemometer height. The meteorological matrix is generated based on looping through a range of wind speeds, cloud covers, ambient temperatures, solar elevation angles, and convective velocity scales (w*, for convective conditions only) based on user-specified surface characteristics (Z_o, Bo, r). For unstable cases, the convective mixing height (Z_ic) is calculated based on w*, and the mechanical mixing height (Z_im) is calculated for unstable and stable conditions based on the friction velocity, u*.

c. For applications involving simple or complex terrain, AERSCREEN interfaces with AERMAP. AERSCREEN also interfaces with BIPPRM to provide the necessary building parameters for applications involving building downwash using the PRIME downwash algorithm. AERSCREEN generates inputs to AERMOD via MAKEMET, AERMAP, and BPIPPRM and invokes AERMOD in a screening mode. The screening mode of AERMOD forces the AERMOD model calculations to represent values for the plume centerline, regardless of the source-receptor-wind direction orientation. The maximum concentration output from AERSCREEN represents a worst-case 1-hour concentration. Averaging-time scaling factors of 0.9 for 3-hour, 0.7 for 8-hour, 0.40 for 24-hour, and 0.08 for annual concentration averages are applied internally by AERSCREEN to the highest 1-hour concentration calculated by the model for non-area type sources. For area type source concentrations for averaging times greater than one hour, the concentrations are equal to the 1-hour estimates.^37 40

4.2.1.2 CTSCREEN

a. CTSCREEN ^39 41 can be used to obtain conservative, yet realistic, worst-case estimates for receptors located on terrain above stack height. CTSCREEN accounts for the three-dimensional nature of plume and terrain interaction and requires detailed terrain data representative of the modeling domain. The terrain data must be digitized in the same manner as for CTDMPLUS and a terrain processor is available.⁴² CTSCREEN is designed to execute a fixed matrix of meteorological values for wind speed (u), standard deviation of horizontal and vertical wind speeds (σv, σw), vertical potential temperature gradient (dθ/dz), friction velocity (u*), Monin-Obukhov length (L), mixing height (z_i) as a function of terrain height, and wind directions for both neutral/stable conditions and unstable convective conditions. The maximum concentration output from CTSCREEN represents a worst-case 1-hour concentration. Time-scaling factors of 0.7 for 3-hour, 0.15 for 24-hour and 0.03 for annual concentration averages are applied internally by CTSCREEN to the highest 1-hour concentration calculated by the model.

4.2.1.3 Screening in Complex Terrain

a. For applications utilizing AERSCREEN, AERSCREEN automatically generates a polar-grid receptor network with spacing determined by the maximum distance to model. If the application warrants a different receptor network than that generated by AERSCREEN, it may be necessary to run AERMOD in screening mode with a user-defined network. For CTSCREEN applications or AERMOD in screening mode outside of AERSCREEN, placement of receptors requires very careful attention when modeling in complex terrain. Often the highest concentrations are predicted to occur under very stable conditions, when the plume is near, or impinges on, the terrain. The plume under such conditions may be quite narrow in the vertical, so that even relatively small changes in a receptor's location may substantially affect the predicted concentration. Receptors within about a kilometer of the source may be even more sensitive to location. Thus, a dense array of receptors may be required in some cases.

b. For applications involving AERSCREEN, AERSCREEN interfaces with AERMAP to generate the receptor elevations. For applications involving CTSCREEN, digitized contour data must be preprocessed ⁴² to provide hill shape parameters in suitable input format. The user then supplies receptors either through an interactive program that is part of the model or directly, by using a text editor; using both methods to select receptors will generally be necessary to assure that the maximum concentrations are estimated by either model. In cases where a terrain feature may “appear to the plume” as smaller, multiple hills, it may be necessary to model the terrain both as a single feature and as multiple hills to determine design concentrations.

c. Other screening techniques may be acceptable for complex terrain cases where established procedures ⁴³ are used. The user is encouraged to confer with the appropriate reviewing authority (paragraph 3.0(b)) if any unresolvable problems are encountered, e.g., applicability, meteorological data, receptor siting, or terrain contour processing issues.

4.2.2 Refined Models

a. A brief description of each preferred model for refined applications is found in appendix A. Also listed in that appendix are availability, the model input requirements, the standard options that shall be selected when running the program, and output options.

4.2.2.1 AERMOD

a. For a wide range of regulatory applications in all types of terrain, and for aerodynamic building downwash, the recommended model is AERMOD.^44 45 The AERMOD regulatory modeling system consists of the AERMOD dispersion model, the AERMET meteorological processor, and the AERMAP terrain processor. AERMOD is a steady-state Gaussian plume model applicable to directly emitted air pollutants that employs best state-of-practice parameterizations for characterizing the meteorological influences and dispersion. Differentiation of simple versus complex terrain is unnecessary with AERMOD. In complex terrain, AERMOD employs the well-known dividing-streamline concept in a simplified simulation of the effects of plume-terrain interactions.

b. The AERMOD modeling system has been extensively evaluated across a wide range of scenarios based on numerous field studies, including tall stacks in flat and complex terrain settings, sources subject to building downwash influences, and low-level non-buoyant sources.²⁷ These evaluations included several long-term field studies associated with operating plants as well as several intensive tracer studies. Based on these evaluations, AERMOD has shown consistently good performance, with “errors” in predicted vs. observed peak concentrations, based on the Robust Highest Concentration (RHC) metric, consistently within the range of 10 to 40 percent cited in paragraph 4.1(g).

c. AERMOD incorporates the Plume Rise Model Enhancements (PRIME) algorithm to account for enhanced plume growth and restricted plume rise for plumes affected by building wake effects.⁴⁶ The PRIME algorithm accounts for entrainment of plume mass into the cavity recirculation region, including re-entrainment of plume mass into the wake region beyond the cavity.

d. AERMOD incorporates the Buoyant Line and Point Source (BLP) Dispersion model to account for buoyant plume rise from line sources. The BLP option within AERMOD utilizes the standard meteorological inputs provided by the AERMET meteorological processor.

e. The state-of-the-science for modeling atmospheric deposition is evolving and new modeling techniques are continually being assessed and their results are being compared with observations. Consequently, while deposition treatment is available in AERMOD, the approach taken for any purpose shall be coordinated with the appropriate reviewing authority (paragraph 3.0(b)).

4.2.2.2 CTDMPLUS

a. If the modeling application involves an elevated point source with a well-defined hill or ridge and a detailed dispersion analysis of the spatial pattern of plume impacts is of interest, CTDMPLUS is available. CTDMPLUS provides greater resolution of concentrations about the contour of the hill feature than does AERMOD through a different plume-terrain interaction algorithm.

4.2.2.3 OCD

a. If the modeling application involves determining the impact of offshore emissions from point, area, or line sources on the air quality of coastal regions, the recommended model is the OCD (Offshore and Coastal Dispersion) Model. OCD is a straight-line Gaussian model that incorporates overwater plume transport and dispersion as well as changes that occur as the plume crosses the shoreline. OCD is also applicable for situations that involve platform building downwash.

4.2.3 Pollutant Specific Modeling Requirements

4.2.3.1 Models for Carbon Monoxide

a. Models for assessing the impact of CO emissions are needed to meet NSR requirements, including PSD, to address compliance with the CO NAAQS and to determine localized impacts from transportations projects. Examples include evaluating effects of point sources, congested roadway intersections, and highways, as well as the cumulative effect of numerous sources of CO in an urban area.

b. The general modeling recommendations and requirements for screening models in section 4.2.1 and refined models in section 4.2.2 shall be applied for CO modeling. Given the relatively low CO background concentrations, screening techniques are likely to be adequate in most cases. However, since the screening model specified in section 4.2.1 (AERSCREEN) can only handle one source at a time, a section 4.2.2 model may be used with screening meteorology (e.g., generated with MAKEMET) to conduct screening assessments of CO projects involving more than one source (e.g., roadway hotspot assessments).⁴⁷

4.2.3.2 Models for Lead

a. In January 1999 (40 CFR part 58, appendix D), the EPA gave notice that concern about ambient lead impacts was being shifted away from roadways and toward a focus on stationary point sources. Thus, models for assessing the impact of lead emissions are needed to meet NSR requirements, including PSD, to address compliance with the lead NAAQS and for SIP attainment demonstrations. The EPA has also issued guidance on siting ambient monitors in the vicinity of stationary point sources.⁴⁸ For lead, the SIP should contain an air quality analysis to determine the maximum rolling 3-month average lead concentration resulting from major lead point sources, such as smelters, gasoline additive plants, etc. The EPA has developed a post-processor to calculate rolling 3-month average concentrations from model output.⁴⁹ General guidance for lead SIP development is also available.⁵⁰

b. For major lead point sources, such as smelters, which contribute fugitive emissions and for which deposition is important, professional judgment should be used, and there shall be coordination with the appropriate reviewing authority (paragraph 3.0(b)). For most applications, the general requirements for screening and refined models of section 4.2.1 and 4.2.2 are applicable to lead modeling.

4.2.3.3 Models for Sulfur Dioxide

a. Models for SO₂ are needed to meet NSR requirements, including PSD, to address compliance with the SO₂ NAAQS and PSD increments, for SIP attainment demonstrations,⁵¹ and for characterizing current air quality via modeling.⁵² SO₂ is one of a group of highly reactive gasses known as “oxides of sulfur” with largest emissions sources being fossil fuel combustion at power plants and other industrial facilities.

b. Given the relatively inert nature of SO₂ on the short-term time scales of interest (i.e., 1-hour) and the sources of SO₂ (i.e., stationary point sources), the general modeling requirements for screening models in section 4.2.1 and refined models in section 4.2.2 are applicable for SO₂ modeling applications. For urban areas, AERMOD automatically invokes a half-life of 4 hours ⁵³ to SO_2. Therefore, care must be taken when determining whether a source is urban or rural (see section 7.2.1.1 for urban/rural determination methodology).

4.2.3.4 Models for Nitrogen Dioxide

a. Models for assessing the impact of sources on ambient NO₂ concentrations are needed to meet NSR requirements, including PSD, to address compliance with the NO₂ NAAQS and PSD increments. Impact of an individual source on ambient NO₂ depends, in part, on the chemical environment into which the source's plume is to be emitted. This is due to the fact that NO₂ sources co-emit NO along with NO₂ and any emitted NO may react with ambient ozone to convert to additional NO₂ downwind. Thus, comprehensive modeling of NO₂ would need to consider the ratio of emitted NO and NO₂, the ambient levels of ozone and subsequent reactions between ozone and NO, and the photolysis of NO₂ to NO.

b. Due to the complexity of NO₂ modeling, a multi-tiered approach is required to obtain hourly and annual average estimates of NO₂.⁵⁴ Since these methods are considered screening, their usage shall occur in agreement with the appropriate reviewing authority (paragraph 3.0(b)). Additionally, since screening techniques are conservative by their nature, there are limitations to how these options can be used. Specifically, negative emissions should not be modeled because decreases in concentrations would be overestimated. Each tiered approach (see Figure 4-1) accounts for increasing complex considerations of NO₂ chemistry and is described in paragraphs b through d of this subsection. The tiers of NO₂ modeling include:

i. A first-tier (most conservative) “full” conversion approach;

ii. A second-tier approach that assumes ambient equilibrium between NO and NO₂; and

iii. A third-tier consisting of several detailed screening techniques that account for ambient ozone and the relative amount of NO and NO₂ emitted from a source.

c. For Tier 1, use an appropriate section 4.2.2 refined model to estimate nitrogen oxides (NO_X) concentrations and assume a total conversion of NO to NO₂. If the resulting design concentrations exceed the NAAQS or PSD increments for NO₂, proceed to Tier 2.

d. For Tier 2, multiply the Tier 1 result(s) by the Ambient Ratio Method 2 (ARM2), which provides estimates of representative equilibrium ratios of NO₂/NO_X value based ambient levels of NO₂ and NO_X derived from national data from the EPA's Air Quality System (AQS).⁵⁵ The national default for ARM2 will include a minimum NO₂/NO_X ratio of 0.5 and a maximum ratio of 0.9. The reviewing agency may establish alternative default minimum NO₂/NO_X values based on the source's in-stack emissions ratios, with alternative minimum values reflecting the source's in-stack NO₂/NO_X ratios. Preferably, alternative default NO₂/NO_X values should be based on source-specific data which satisfies all quality assurance procedures that ensure data accuracy for both NO₂ and NO_X within the typical range of measured values. However, alternate information may be used to justify a source's anticipated NO₂/NO_X in-stack ratios, such as manufacturer test data, state or local agency guidance, peer-reviewed literature, the EPA's NO₂/NO_X ratio database.

e. For Tier 3, a detailed screening technique shall be applied on a case-by-case basis. Because of the additional input data requirements and complexities associated with the Tier 3 options, their usage shall occur in consultation with the EPA Regional Office in addition to the appropriate reviewing authority. The Ozone Limiting Method (OLM) ⁵⁶ and the Plume Volume Molar Ratio Method (PVMRM) ⁵⁷ are two detailed screening techniques that may be used for most sources. These two techniques use an appropriate section 4.2.2 model to estimate NO_X concentrations and then estimate the conversion of primary NO emissions to NO₂ based on the ambient levels of ozone and the plume characteristics. OLM only accounts for NO₂ formation based on the ambient levels of ozone while PVMRM also accommodates distance-dependent conversion ratios based on ambient ozone. Both PVMRM and OLM require that ambient ozone concentrations be provided on an hourly basis and explicit specification of the speciation of the NO₂/NO_X in-stack ratios. PVMRM works best for relatively isolated and elevated point source modeling while OLM works best for large groups of sources, area sources, and near-surface releases, including road-way sources.

f. Alternative models or techniques may be considered on a case-by-case basis and their usage shall be approved by the EPA Regional Office (section 3.2). Such techniques should consider individual quantities of NO and NO₂ emissions, atmospheric transport and dispersion, and atmospheric transformation of NO to NO₂. Dispersion models that account for more explicit photochemistry may also be applied to estimate ambient impacts of NO_X sources.

4.2.3.5 Models for PM_2.5

a. The PM_2.5 NAAQS, promulgated on July 18, 1997, includes particles with an aerodynamic diameter nominally less than or equal to 2.5 micrometers. PM_2.5 is a mixture consisting of several diverse components⁵⁸. Ambient PM_2.5 generally consists of two components, the primary component, emitted directly from a source, and the secondary component, which is formed in the atmosphere from other pollutants emitted from the source. Models for PM_2.5 are needed to meet NSR requirements, including PSD, to address compliance with the PM_2.5 NAAQS and PSD increments and for SIP attainment demonstrations.

b. For NSR, including PSD, modeling assessments, the refined methods in section 4.2.2 are required for modeling the primary component of PM_2.5, while the methods in section 5.4 are recommended for addressing the secondary component of PM_2.5. Guidance for PSD assessments is available for determining the best approach to handling sources of primary and secondary PM_2.5.⁵⁹

c. For SIP attainment demonstrations and regional haze reasonable progress goal analyses, effects of a control strategy on PM_2.5 are estimated from the sum of the effects on the primary and secondary components composing PM_2.5. Model users should refer to section 5.4.1 and associated SIP modeling guidance ⁶⁰ for further details concerning appropriate modeling approaches.

d. The general modeling requirements for the refined models discussed in section 4.2.2 should be applied for PM_2.5 hot-spot modeling for mobile sources. Specific guidance is available for analyzing direct PM_2.5 impacts from highways, terminals, and other projects.⁶¹

4.2.3.6 Models for PM₁₀

a. The NAAQS for PM₁₀ was promulgated on July 1, 1987. The EPA promulgated regulations for PSD increment measured as PM₁₀ in a document published on June 3, 1993. Models for PM₁₀ are needed to meet NSR requirements, including PSD, to address compliance with the PM₁₀ NAAQS and PSD increments and for SIP attainment demonstrations.

b. For most sources, the general modeling requirements for screening models in section 4.2.1 and refined models in section 4.2.2 shall be applied for PM₁₀ modeling. In cases where the particle size and its effect on ambient concentrations need to be considered, particle deposition may be used in on a case-by-case basis and their usage shall be approved by the EPA Regional Office (section 3.2). A SIP development guide ⁶² is also available to assist in PM₁₀ analyses and control strategy development.

c. Fugitive dust usually refers to dust put into the atmosphere by the wind blowing over plowed fields, dirt roads or desert or sandy areas with little or no vegetation. Fugitive emissions include the emissions resulting from the industrial process that are not captured and vented through a stack but may be released from various locations within the complex. In some unique cases, a model developed specifically for the situation may be needed. Due to the difficult nature of characterizing and modeling fugitive dust and fugitive emissions, the proposed procedure shall be determined in consultation with the appropriate reviewing authority (paragraph 3.0(b)) for each specific situation before the modeling exercise is begun. Re-entrained dust is created by vehicles driving over dirt roads (e.g., haul roads) and dust-covered roads typically found in arid areas. Such sources can be characterized as line, area or volume sources.^61 63 Emission rates may be based on site-specific data or values from the general literature.

d. Under certain conditions, recommended dispersion models may not be suitable to appropriately address the nature of ambient PM₁₀. In these circumstances, the alternative modeling approach shall be approved by the EPA Regional Office (section 3.2).

e. The general modeling requirements for the refined models discussed in section 4.2.2 should be applied for PM₁₀ hot-spot modeling for mobile sources. Specific guidance is available for analyzing direct PM₁₀ impacts from highways, terminals, and other projects.⁶¹

5.0 Models for Ozone and Secondarily Formed Particulate Matter

5.1 Discussion

a. Air pollutants formed through chemical reactions in the atmosphere are referred to as secondary pollutants. For example, ground-level ozone and a portion of particulate matter with aerodynamic diameter less than 2.5 μ m (PM_2.5 or fine PM) are secondary pollutants formed through photochemical reactions. Ozone and secondarily formed particulate matter are closely related to each other in that they share common sources of emissions or are formed in the atmosphere from chemical reactions with similar precursors.

b. Ozone formation is driven by emissions of NO_X and volatile organic compounds (VOCs). Ozone formation is a complicated nonlinear process that requires favorable meteorological conditions in addition to VOC and NO_X emissions. Sometimes complex terrain features also contribute to the build-up of precursors and subsequent ozone formation or destruction.

c. PM_2.5 can be either primary (i.e., emitted directly from sources) or secondary in nature. The fraction of PM_2.5 which is primary versus secondary varies by location and season. In the United States, PM_2.5 is dominated by a variety of chemical species or components of atmospheric particles, such as ammonium sulfate, ammonium nitrate, organic carbon (OC) mass, elemental carbon (EC), and other soil compounds and oxidized metals. PM_2.5sulfate, nitrate, and ammonium ions are predominantly the result of chemical reactions of the oxidized products of sulfur dioxide (SO₂) and NO_X emissions with direct ammonia (NH₃) emissions.⁶⁴

d. Modeled strategies designed to reduce ozone or PM_2.5 levels typically need to consider the chemical coupling between these pollutants. Control measures reducing ozone and PM_2.5 precursor emissions may not lead to proportional reductions in ozone and PM_2.5. This coupling is important in understanding processes that control the levels of both pollutants. Thus, when feasible, it is important to use models that take into account the chemical coupling between ozone and PM_2.5. In addition, using such a multi-pollutant modeling system can reduce the resource burden associated with applying and evaluating separate models for each pollutant and promotes consistency among the strategies themselves.

e. PM_2.5 is a mixture consisting of several diverse chemical species or components of atmospheric particles. Because chemical and physical properties and origins of each component differ, it may be appropriate to use either a single model capable of addressing several of the important components or to model primary and secondary components using different models. Effects of a control strategy on PM_2.5 is estimated from the sum of the effects on the specific components composing PM_2.5.

5.2 Recommendations

a. Chemical transformations can play an important role in defining the concentrations and properties of certain air pollutants. Models that take into account chemical reactions and physical processes of various pollutants (including precursors) are needed for determining the current state of air quality, as well as predicting and projecting the future evolution of these pollutants. It is important that a modeling system provide a realistic representation of chemical and physical processes leading to secondary pollutant formation and removal from the atmosphere.

b. Chemical transport models treat atmospheric chemical and physical processes such as deposition and motion. There are two types of chemical transport models, Eulerian (grid based) and Lagrangian. These types of models are differentiated from each other by their frame of reference. Eulerian models are based on a fixed frame of reference and Lagrangian models use a frame of reference that moves with parcels of air between the source and receptor point.⁹ Photochemical grid models are three-dimensional Eulerian grid-based models that treat chemical and physical processes in each grid cell and use diffusion and transport processes to move chemical species between grid cells. These types of models are appropriate for assessment of near-field and regional scale reactive pollutant impacts from specific sources^{7 10 11 12} or all sources.^13 14 15 In some limited cases, the secondary processes can be treated with a box model, potentially in combination with a number of other modeling techniques and/or analyses to treat individual source sectors.

c. Regardless of the modeling system used to estimate secondary impacts of ozone and/or PM_2.5, model results should be compared to observation data to generate confidence that the modeling system is representative of the local and regional air quality. For ozone related projects, model estimates of ozone should be compared with observations in both time and space. For PM_2.5, model estimates of speciated PM_2.5 components (such as sulfate ion, nitrate ion, etc.) should be compared with observations in both time and space.⁶⁵

d. Model performance metrics comparing observations and predictions are often used to summarize model performance. These metrics include mean bias, mean error, fractional bias, fractional error, and correlation coefficient.⁶⁵ There are no specific levels of any model performance metric that indicate “acceptable” model performance. The EPA's preferred approach for providing context about model performance is to compare model performance metrics with similar contemporary applications.^60 65 Because model application purpose and scope vary, model users should consult with the appropriate reviewing authority (paragraph 3.0(b)) to determine what model performance elements should be emphasized and presented to provide confidence in the regulatory model application.

e. There is no preferred modeling system or technique for estimating ozone or secondary PM_2.5 for specific source impacts or to assess impacts from multiple sources. For assessing secondary pollutant impacts from single sources, the degree of complexity required to assess potential impacts varies depending on the nature of the source, its emissions, and the background environment. The EPA recommends a two-tiered approach where the first tier consists of using existing technically credible and appropriate relationships between emissions and impacts developed from previous modeling that is deemed sufficient for evaluating a source's impacts. The second tier consists of more sophisticated case-specific modeling analyses. The appropriate tier for a given application should be selected in consultation with the appropriate reviewing authority (paragraph 3.0(b)) and be consistent with EPA guidance.⁶⁶

5.3 Recommended Models and Approaches for Ozone

a. Models that estimate ozone concentrations are needed to guide the choice of strategies for the purposes of a nonattainment area demonstrating future year attainment of the ozone NAAQS. Additionally, models that estimate ozone concentrations are needed to assess impacts from specific sources or source complexes to satisfy requirements for NSR, including PSD, and other regulatory programs. Other purposes for ozone modeling include estimating the impacts of specific events on air quality, ozone deposition impacts, and planning for areas that may be attaining the ozone NAAQS.

5.3.1 Models for NAAQS Attainment Demonstrations and Multi-Source Air Quality Assessments

a. Simulation of ozone formation and transport is a complex exercise. Control agencies with jurisdiction over areas with ozone problems should use photochemical grid models to evaluate the relationship between precursor species and ozone. Use of photochemical grid models is the recommended means for identifying control strategies needed to address high ozone concentrations in such areas. Judgment on the suitability of a model for a given application should consider factors that include use of the model in an attainment test, development of emissions and meteorological inputs to the model, and choice of episodes to model. Guidance on the use of models and other analyses for demonstrating attainment of the air quality goals for ozone is available.⁶⁰ Users should consult with the appropriate reviewing authority (paragraph 3.0(b)) to ensure the most current modeling guidance is applied.

5.3.2 Models for Single-Source Air Quality Assessments

a. Depending on the magnitude of emissions, estimating the impact of an individual source's emissions of NO_X and VOC on ozone concentrations is necessary for obtaining a permit. The simulation of ozone formation and transport requires realistic treatment of atmospheric chemistry and deposition. Models should be applied which integrate chemical and physical processes important in the formation, decay, and transport of ozone and important precursor species (e.g., Lagrangian and photochemical grid models). Photochemical grid models are primarily designed to characterize precursor emissions and impacts from a wide variety of sources over a large geographic area but can also be used to assess the impacts from specific sources.^7 11 12

b. The first tier of assessment for ozone impacts involves those situations where existing technical information is available (e.g., results from existing photochemical grid modeling, published empirical estimates of source specific impacts, or reduced-form models) in combination with other supportive information and analysis for the purposes of estimating secondary impacts from a particular source. The existing technical information should provide a credible and representative estimate of the secondary impacts from the project source. The appropriate reviewing authority (paragraph 3.0(b)) and appropriate EPA guidance ⁶⁶ should be consulted to determine what types of assessments may be appropriate on a case-by-case basis.

c. The second tier of assessment for ozone impacts involves those situations where existing technical information is not available such that chemical transport models (e.g., photochemical grid models) should be used to address single-source impacts. Special considerations are needed when using these models to evaluate the ozone impact from an individual source. Guidance on the use of models and other analyses for demonstrating the impacts of single sources for ozone is available.⁶⁶ This document provides a more detailed discussion of the appropriate approaches to obtaining estimates of ozone impacts from a single source. Model users should use the latest version of this guidance in consultation with the appropriate reviewing authority (paragraph 3.0(b)) to determine the most suitable single-source ozone modeling approach on a case-by-case basis.

5.4 Recommended Models and Approaches for Secondarily Formed PM _2.5

a. Models are needed to guide the choice of strategies to address an observed PM_2.5 problem in an area not attaining the PM_2.5 NAAQS. Additionally, models are needed to assess PM_2.5 impacts from specific sources or industrial source complexes to satisfy requirements for NSR, including PSD, and other regulatory programs. Other purposes for PM_2.5 modeling include estimating the impacts of specific events on air quality, visibility, deposition impacts, and planning for areas that may be attaining the PM_2.5 NAAQS.

5.4.1 Models for NAAQS Attainment Demonstrations and Multi-Source Air Quality Assessments

a. Models for PM_2.5 are needed to assess the adequacy of a proposed strategy for meeting the annual and/or 24-hour PM_2.5 NAAQS. Modeling primary and secondary PM_2.5 can be a multi-faceted and complex problem, especially for secondary components of PM_2.5 such as sulfates and nitrates. Control agencies with jurisdiction over areas with secondary PM_2.5 problems should use models which integrate chemical and physical processes important in the formation, decay, and transport of these species (e.g., photochemical grid models). Suitability of a modeling approach or mix of modeling approaches for a given application requires technical judgment as well as professional experience in choice of models, use of the model(s) in an attainment test, development of emissions and meteorological inputs to the model, and selection of days to model. Guidance on the use of models and other analyses for demonstrating attainment of the air quality goals for PM_2.5 is available.^59 60 Users should consult with the appropriate reviewing authority (paragraph 3.0(b)) to ensure the most current modeling guidance is applied.

5.4.2 Models for Single-Source Air Quality Assessments

a. Depending on the magnitude of emissions, estimating the impact of an individual source's emissions on secondary particulate matter concentrations is necessary for obtaining a permit. Primary PM_2.5 components shall be simulated using AERMOD (see section 4.2.2). The simulation of secondary particulate matter formation and transport is a complex exercise requiring realistic treatment of atmospheric chemistry and deposition. Models should be applied which integrate chemical and physical processes important in the formation, decay, and transport of these species (e.g., Lagrangian and photochemical grid models). Photochemical grid models are primarily designed to characterize precursor emissions and impacts from a wide variety of sources over a large geographic area and can also be used to assess the impacts from specific sources.^7 10

b. The first tier of assessment for secondary PM_2.5 impacts involves those situations where existing technical information is available (e.g., results from existing photochemical grid modeling, published empirical estimates of source specific impacts, or reduced-form models) in combination with other supportive information and analysis for the purposes of estimating secondary impacts from a particular source. The existing technical information should provide a credible and representative estimate of the secondary impacts from the project source. The appropriate reviewing authority (paragraph 3.0(b)) and appropriate EPA guidance ⁶⁶ should be consulted to determine what types of assessments may be appropriate on a case-by-case basis.

c. The second tier of assessment for secondary PM_2.5 impacts involves those situations where existing technical information is not available such that chemical transport models (e.g., photochemical grid models) should be used for assessments of single-source impacts. Special considerations are needed when using these models to evaluate the secondary particulate matter impact from an individual source. Guidance on the use of models and other analyses for demonstrating the impacts of single sources for secondary PM_2.5 is available.⁶⁶ This document provides a more detailed discussion of the appropriate approaches to obtaining estimates of secondary particulate matter concentrations from a single source. Model users should use the latest version of this guidance in consultation with the appropriate reviewing authority (paragraph 3.0(b)) to determine the most suitable single-source modeling approach for secondary PM_2.5 on a case-by-case basis.

6.0 Modeling for Air Quality Related Values and Other Governmental Programs

6.1 Discussion

a. Other federal agencies have also developed specific modeling approaches for their own regulatory or other requirements. Although such regulatory requirements and guidance have come about because of EPA rules or standards, the implementation of such regulations and the use of the modeling techniques is under the jurisdiction of the agency issuing the guidance or directive. This section covers such situations with reference to those guidance documents, when they are available.

b. When using the model recommended or discussed in the Guideline in support of programmatic requirements not specifically covered by EPA regulations, the model user should consult the appropriate federal or state agency to ensure the proper application and use of the models and/or techniques. Other federal agencies have developed specific modeling approaches for their own regulatory or other requirements. Most of the programs have, or will have when fully developed, separate guidance documents that cover the program and a discussion of the tools that are needed. The following paragraphs reference those guidance documents, when they are available. No attempt has been made to provide a comprehensive discussion of each topic since the reference documents were designed to do that.

6.2 Air Quality Related Values

a. The 1997 CAA Amendments give FLMs an “affirmative responsibility” to protect the natural and cultural resources of Class I areas from the adverse impacts of air pollution and to provide the appropriate procedures and analysis techniques. The Act identifies the FLM as the Secretary of the department, or their designee, with authority over these lands. Mandatory Federal Class I areas are defined in the CAA as international parks, national parks over 6,000 acres and wilderness areas and memorial parks over 5,000 acres, established as of 1977. The FLMs are also concerned with the protection of resources in federally managed Class II areas because of other statutory mandates to protect these areas.

b. The FLM agency responsibilities include the review of air quality permit applications from proposed new or modified major pollution sources that may affect these Class I areas to determine if emissions from a proposed or modified source will cause or contribute to adverse impacts on air quality related values (AQRVs) of a Class I area and making recommendations to the FLM. AQRVs are resources identified by the FLM agencies, which have the potential to be affected by air pollution. These resources may include visibility, scenic, cultural, physical, or ecological resources for a particular area. The FLM agencies take into account the particular resources and AQRVs that would be affected; the frequency and magnitude of any potential impacts; and the direct, indirect, and cumulative effects of any potential impacts in making their recommendations.

c. While the AQRV notification and impact analysis requirements are outlined in the PSD regulations at 40 CFR 51.166(p) and 40 CFR 52.21(p), determination of appropriate analytical methods and metrics for AQRV's are determined by the FLM agencies and are published in guidance external to the general recommendations of this paragraph.

d. To develop greater consistency in the application of air quality models to assess potential AQRV impacts in both Class I areas and protected Class II areas, the FLM agencies have developed the Federal Land Managers' Air Quality Related Values Work Group Phase I Report (FLAG) ⁶⁷. FLAG focuses upon specific technical and policy issues associated with visibility impairment, effects of pollutant deposition on soils and surface waters, and ozone effects on vegetation. Model users should consult the latest version of the FLAG report for current modeling guidance and with affected FLM agency representatives for any application specific guidance which is beyond the scope of the Guideline.

6.2.1 Visibility

a. Visibility in important natural areas (e.g., Federal Class I areas) is protected under a number of provisions of the CAA, including sections 169A and 169B (addressing impacts primarily from existing sources) and section 165 (new source review). Visibility impairment is caused by light scattering and light absorption associated with particles and gases in the atmosphere. In most areas of the country, light scattering by PM_2.5 is the most significant component of visibility impairment. The key components of PM_2.5 contributing to visibility impairment include sulfates, nitrates, organic carbon, elemental carbon, and crustal material.⁶⁷

b. Visibility regulations (40 CFR 51.300 through 51.309) require state, local, and tribal agencies to mitigate current and prevent future visibility impairment in any of the 156 mandatory Federal Class I areas where visibility is considered an important attribute. In 1999, the EPA issued revisions to the regulations to address visibility impairment in the form of regional haze, which is caused by numerous, diverse sources (e.g., stationary, mobile, and area sources) located across a broad region (40 CFR 51.308 through 51.309). The state of relevant scientific knowledge has expanded significantly since the 1997 CAA Amendments. A number of studies and reports ^68 69 have concluded that long-range transport (e.g., up to hundreds of kilometers) of fine particulate matter plays a significant role in visibility impairment across the country. CAA section 169A requires states to develop SIPs containing long-term strategies for remedying existing and preventing future visibility impairment in the 156 mandatory Class I Federal areas, where visibility is considered an important attribute. In order to develop long-term strategies to address regional haze, many state, local, and tribal agencies will need to conduct regional-scale modeling of fine particulate concentrations and associated visibility impairment.

c. The FLAG visibility modeling recommendations are divided into two distinct sections to address different requirements for (1) near field modeling where plumes or layers are compared against a viewing background and (2) distant/multi-source modeling for plumes and aggregations of plumes that affect the general appearance of a scene.⁶⁷ The recommendations separately address visibility assessments for sources proposing to locate relatively near and at farther distances from these areas.⁶⁷

6.2.1.1 Models for Estimating Near-Field Visibility Impairment

a. To calculate the potential impact of a plume of specified emissions for specific transport and dispersion conditions (“plume blight”) for source-receptor distances less than 50 km, a screening model and guidance are available.^67 70 If a more comprehensive analysis is necessary, a refined model should be selected. The model selection, procedures, and analyses should be determined in consultation with the appropriate reviewing authority (paragraph 3.0(b)) and the affected FLM(s).

6.2.1.2 Models for Estimating Visibility Impairment for Long-Range Transport

a. Chemical transformations can play an important role in defining the concentrations and properties of certain air pollutants. Models that take into account chemical reactions and physical processes of various pollutants (including precursors) are needed for determining the current state of air quality, as well as predicting and projecting the future evolution of these pollutants. It is important that a modeling system provide a realistic representation of chemical and physical processes leading to secondary pollutant formation and removal from the atmosphere.

b. Chemical transport models treat atmospheric chemical and physical processes such as deposition and motion. There are two types of chemical transport models, Eulerian (grid based) and Lagrangian. These types of models are differentiated from each other by their frame of reference. Eulerian models are based on a fixed frame of reference and Lagrangian models use a frame of reference that moves with parcels of air between the source and receptor point.⁹ Photochemical grid models are three-dimensional Eulerian grid-based models that treat chemical and physical processes in each grid cell and use diffusion and transport processes to move chemical species between grid cells.⁹ These types of models are appropriate for assessment of near-field and regional scale reactive pollutant impacts from specific sources ^{7 10 11 12} or all sources.^13 14 15

c. Development of the requisite meteorological and emissions databases necessary for use of photochemical grid models to estimate AQRVs should conform to recommendations in section 8 and those outlined in the EPA's Modeling Guidance for Demonstrating Attainment of Air Quality Goals for Ozone, PM_2.5, and Regional Haze.⁶⁰ Demonstration of the adequacy of prognostic meteorological fields can be established through appropriate diagnostic and statistical performance evaluations consistent with recommendations provided in the appropriate guidance.⁶⁰ Model users should consult the latest version of this guidance and with the appropriate reviewing authority (paragraph 3.0(b)) for any application specific guidance which is beyond the scope of this subsection.

6.2.2 Models for Estimating Deposition Impacts

a. For many Class I areas, AQRVs have been identified that are sensitive to atmospheric deposition of air pollutants. Emissions of NO_X, sulfur oxides, NH₃, mercury, and secondary pollutants such as ozone and particulate matter affect components of ecosystems. In sensitive ecosystems, these compounds can acidify soils and surface waters, add nutrients that change biodiversity, and affect the ecosystem services provided by forests and natural areas.⁶⁷ To address the relationship between deposition and ecosystem effects the FLM agencies have developed estimates of critical loads. A critical load is defined as “A quantitative estimate of an exposure to one or more pollutants below which significant harmful effects on specified sensitive elements of the environment do not occur according to present knowledge.” ⁷¹

b. The FLM deposition modeling recommendations are divided into two distinct sections to address different requirements for (1) near field modeling, and (2) distant/multi-source modeling for cumulative effects. The recommendations separately address deposition assessments for sources proposing to locate relatively near and at farther distances from these areas.⁶⁷ Where the source and receptors are not in close proximity, chemical transport (e.g., photochemical grid) models generally should be applied for an assessment of deposition impacts due to one or a small group of sources. Over these distances chemical and physical transformations can change atmospheric residence time due to different propensity for deposition to the surface of different forms of nitrate and sulfate. Users should consult the latest version of the FLAG report ⁶⁷ and relevant FLM representatives for guidance on the use of models for deposition. Where source and receptors are in close proximity, users should contact the appropriate FLM for application specific guidance.

6.3 Modeling Guidance for Other Governmental Programs

a. Dispersion and photochemical grid modeling need to be conducted to ensure that individual and cumulative offshore oil and gas exploration, development, and production plans and activities do not significantly affect the air quality of any state as required under the Outer Continental Shelf Lands Act (OCSLA). Air quality modeling requires various input datasets, including emissions sources, meteorology, and pre-existing pollutant concentrations. For sources under the reviewing authority of the Department of Interior, Bureau of Ocean Energy Management (BOEM), guidance for the development of all necessary Outer Continental Shelf (OCS) air quality modeling inputs and appropriate model selection and application is available from the BOEMS's Web site: http://www.boem.gov/​Environmental-Stewardship/​Environmental-Studies/​Gulf-of-Mexico-Region/​Approved-Air-Quality-Models-for-the-GOMR.aspx.

b. The Federal Aviation Administration (FAA) is the appropriate reviewing authority for air quality assessments of primary pollutant impacts at airports and air bases. Air quality application for this purpose is intended for estimating the collective impact of changes in aircraft operations, point source, and mobile source emissions at airports on pollutant concentrations. The latest version of the Aviation Environmental Design Tool (AEDT), is developed and is supported by the FAA, and is appropriate for air quality assessment of primary pollutant impacts at airports or air bases. AEDT has adopted AERMOD for treating dispersion. Application of AEDT is intended for estimating the collective impact of changes in aircraft operations, point source, and mobile source emissions on pollutant concentrations. It is not intended for PSD, SIP, or other regulatory air quality analyses of point or mobile sources at or peripheral to airport property that are unrelated to airport operations. The latest version of AEDT may be obtained from FAA at its Web site: https://aedt.faa.gov.

7.0 General Modeling Considerations

7.1 Discussion

a. This section contains recommendations concerning a number of different issues not explicitly covered in other sections of the Guideline. The topics covered here are not specific to any one program or modeling area but are common to dispersion modeling analyses for criteria pollutants.

7.2 Recommendations

7.2.1 All Sources

7.2.1.1 Dispersion Coefficients

a. For any dispersion modeling exercise, the urban or rural determination of a source is critical in determining the boundary layer characteristics that affect the model's prediction of downwind concentrations. Historically, steady-state Gaussian plume models used in most applications have employed dispersion coefficients based on Pasquill-Gifford ⁷² in rural areas and McElroy- Pooler ⁷³ in urban areas. These coefficients are still incorporated in the BLP and OCD models. However, the AERMOD model incorporates a more up-to-date characterization of the atmospheric boundary layer using continuous functions of parameterized horizontal and vertical turbulence based on Monin-Obukhov similarity (scaling) relationships.⁴⁴ Another key feature of AERMOD's formulation is the option to use directly observed variables of the boundary layer to parameterize dispersion.^44 45

b. The selection of rural or urban dispersion coefficients in a specific application should follow one of the procedures suggested by Irwin ⁷⁴ to determine whether the character of an area is primarily urban or rural:

i. Land Use Procedure: (1) Classify the land use within the total area, A_o, circumscribed by a 3km radius circle about the source using the meteorological land use typing scheme proposed by Auer; ⁷⁵ (2) if land use types I1, I2, C1, R2, and R3 account for 50 percent or more of A_o, use urban dispersion coefficients; otherwise, use appropriate rural dispersion coefficients.

ii. Population Density Procedure: (1) Compute the average population density, p̄ per square kilometer with A_o as defined above; (2) If p̄ is greater than 750 people/km², use urban dispersion coefficients; otherwise use appropriate rural dispersion coefficients. (Of the two methods, the land use procedure is considered more definitive.)

c. Population density should be used with caution and generally not be applied to highly industrialized areas where the population density may be low and thus a rural classification would be indicated. However, the area is likely to be sufficiently built-up so that the urban land use criteria would be satisfied. Therefore, in this case, the classification should be “urban” and urban dispersion parameters should be used.

d. For applications of AERMOD in urban areas, under either the Land Use Procedure or the Population Density Procedure, the user needs to estimate the population of the urban area affecting the modeling domain because the urban influence in AERMOD is scaled based on a user-specified population. For non-population oriented urban areas, or areas influenced by both population and industrial activity, the user will need to estimate an equivalent population to adequately account for the combined effects of industrialized areas and populated areas within the modeling domain. Selection of the appropriate population for these applications should be determined in consultation with the appropriate reviewing authority (paragraph 3.0(b)) and the latest version of the AERMOD Implementation Guide.⁷⁶

e. It should be noted that AERMOD allows for modeling rural and urban sources in a single model run. For analyses of whole urban complexes, the entire area should be modeled as an urban region if most of the sources are located in areas classified as urban. For tall stacks located within or adjacent to small or moderate sized urban areas, the stack height or effective plume height may extend above the urban boundary layer and, therefore, may be more appropriately modeled using rural coefficients. Model users should consult with the appropriate reviewing authority (paragraph 3.0(b)) when evaluating this situation and the latest version of the AERMOD Implementation Guide.⁷⁶

f. Buoyancy-induced dispersion (BID), as identified by Pasquill,⁷⁷ is included in the preferred models and should be used where buoyant sources, e.g., those involving fuel combustion, are involved.

7.2.1.2 Complex Winds

a. Inhomogeneous local winds. In many parts of the United States, the ground is neither flat nor is the ground cover (or land use) uniform. These geographical variations can generate local winds and circulations, and modify the prevailing ambient winds and circulations. Geographic effects are most apparent when the ambient winds are light or calm.⁷⁸ In general these geographically induced wind circulation effects are named after the source location of the winds, e.g., lake and sea breezes, and mountain and valley winds. In very rugged hilly or mountainous terrain, along coastlines, or near large land use variations, the characterization of the winds is a balance of various forces, such that the assumptions of steady-state straight-line transport both in time and space are inappropriate. In such cases, a model should be chosen to fully treat the time and space variations of meteorology effects on transport and dispersion. The setup and application of such a model should be determined in consultation with the appropriate reviewing authority (paragraph 3.0(b)) consistent with limitations of paragraph 3.2.2(e). The meteorological input data requirements for developing the time and space varying three-dimensional winds and dispersion meteorology for these situations are discussed in paragraph 8.4.1.2(c). Examples of inhomogeneous winds include, but are not limited to, situations described in the following paragraphs:

i. Inversion breakup fumigation. Inversion breakup fumigation occurs when a plume (or multiple plumes) is emitted into a stable layer of air and that layer is subsequently mixed to the ground through convective transfer of heat from the surface or because of advection to less stable surroundings. Fumigation may cause excessively high concentrations but is usually rather short- lived at a given receptor. There are no recommended refined techniques to model this phenomenon. There are, however, screening procedures ⁴⁰ that may be used to approximate the concentrations. Considerable care should be exercised in using the results obtained from the screening techniques.

ii. Shoreline fumigation. Fumigation can be an important phenomenon on and near the shoreline of bodies of water. This can affect both individual plumes and area-wide emissions. When fumigation conditions are expected to occur from a source or sources with tall stacks located on or just inland of a shoreline, this should be addressed in the air quality modeling analysis. EPA has evaluated several coastal fumigation models, and the evaluation results of these models are available for their possible application on a case-by-case basis when air quality estimates under shoreline fumigation conditions are needed.⁷⁹ Selection of the appropriate model for applications where shoreline fumigation is of concern should be determined in consultation with the appropriate reviewing authority (paragraph 3.0(b)).

iii. Stagnation. Stagnation conditions are characterized by calm or very low wind speeds, and variable wind directions. These stagnant meteorological conditions may persist for several hours to several days. During stagnation conditions, the dispersion of air pollutants, especially those from low- level emissions sources, tends to be minimized, potentially leading to relatively high ground-level concentrations. If point sources are of interest, users should note the guidance provided in paragraph (a) of this subsection. Selection of the appropriate model for applications where stagnation is of concern should be determined in consultation with the appropriate reviewing authority (paragraph 3.0(b)).

7.2.1.3 Gravitational Settling and Deposition

a. Gravitational settling and deposition may be directly included in a model if either is a significant factor. When particulate matter sources can be quantified and settling and dry deposition are problems, professional judgment should be used, and there should be coordination with the appropriate reviewing authority (paragraph 3.0(b)). AERMOD contains algorithms for dry and wet deposition of gases and particles.⁸⁰ For other Gaussian plume models, an “infinite half-life” may be used for estimates of particle concentrations when only exponential decay terms are used for treating settling and deposition. Lagrangian models have varying degrees of complexity for dealing with settling and deposition and the selection of a parameterization for such should be included in the approval process for selecting a Lagrangian model. Eulerian grid models tend to have explicit parameterizations for gravitational settling and deposition as well as wet deposition parameters already included as part of the chemistry scheme.

7.2.2 Stationary Sources

7.2.2.1 Good Engineering Practice Stack Height

a. The use of stack height credit in excess of Good Engineering Practice (GEP) stack height or credit resulting from any other dispersion technique is prohibited in the development of emissions limits by 40 CFR 51.118 and 40 CFR 51.164. The definition of GEP stack height and dispersion technique are contained in 40 CFR 51.100. Methods and procedures for making the appropriate stack height calculations, determining stack height credits and an example of applying those techniques are found in several references,^{81 82 83 84} which provide a great deal of additional information for evaluating and describing building cavity and wake effects.

b. If stacks for new or existing major sources are found to be less than the height defined by the EPA's refined formula for determining GEP height, then air quality impacts associated with cavity or wake effects due to the nearby building structures should be determined. The EPA refined formula height is defined as H + 1.5L.⁸³ Since the definition of GEP stack height defines excessive concentrations as a maximum ground-level concentration due in whole or in part to downwash of at least 40 percent in excess of the maximum concentration without downwash, the potential air quality impacts associated with cavity and wake effects should also be considered for stacks that equal or exceed the EPA formula height for GEP. The AERSCREEN model can be used to obtain screening estimates of potential downwash influences, based on the PRIME downwash algorithm incorporated in the AERMOD model. If more refined concentration estimates are required, the recommended steady-state plume dispersion model in section 4.2.2, AERMOD, should be used.

7.2.2.2 Plume Rise

a. The plume rise methods of Briggs ^85 86 are incorporated in many of the preferred models and are recommended for use in many modeling applications. In AERMOD,^44 45 for the stable boundary layer, plume rise is estimated using an iterative approach, similar to that in the CTDMPLUS model. In the convective boundary layer, plume rise is superposed on the displacements by random convective velocities.⁸⁷ In AERMOD, plume rise is computed using the methods of Briggs except cases involving building downwash, in which a numerical solution of the mass, energy, and momentum conservation laws is performed.⁸⁸ No explicit provisions in these models are made for multistack plume rise enhancement or the handling of such special plumes as flares; these problems should be considered on a case-by-case basis.

b. Gradual plume rise is generally recommended where its use is appropriate: (1) In AERMOD; (2) in complex terrain screening procedures to determine close-in impacts and (3) when calculating the effects of building wakes. The building wake algorithm in AERMOD incorporates and exercises the thermodynamically based gradual plume rise calculations as described in paragraph (a) of this subsection. If the building wake is calculated to affect the plume for any hour, gradual plume rise is also used in downwind dispersion calculations to the distance of final plume rise, after which final plume rise is used. Plumes captured by the near wake are re-emitted to the far wake as a ground-level volume source.

c. Stack tip downwash generally occurs with poorly constructed stacks and when the ratio of the stack exit velocity to wind speed is small. An algorithm developed by Briggs ⁸⁶ is the recommended technique for this situation and is used in preferred models for point sources.

7.2.3 Mobile Sources

a. Emissions of primary pollutants from mobile sources can be modeled with an appropriate model identified in section 4.2. Screening of mobile sources can be accomplished by using screening meteorology, such as that generated by the MAKEMET component of AERSCREEN, which can generate a range of meteorological scenarios using site-specific characteristics, such as albedo, Bowen ratio, and surface roughness. Maximum hourly concentrations computed from screening runs can be converted to longer averaging periods using the scaling ratios specific in the AERSCREEN User's Guide.³⁷

b. Mobile sources can be modeled in AERMOD as either line (i.e., elongated area) sources or as a series of volume sources. However, since mobile source modeling usually includes an analysis of very near-source impacts (e.g., hot-spot modeling, which can include receptors within 5-10 meters of the roadway), the results can be highly sensitive to the characterization of the mobile emissions. When modeling roadway links, such as highway and arterial links, the EPA recommends that line/area sources instead of volume sources be used whenever possible, as it is easier to characterize them correctly. Important characteristics for both line/area and volume sources include the plume release height, source width, and initial dispersion characteristics, which should also take into account the impact of traffic-induced turbulence, which can cause roadway sources to have larger initial dimensions than might normally be used for representing line sources.

c. The EPA's quantitative PM hot-spot guidance ⁶¹ and Haul Road Workgroup Final Report ⁶³ provide guidance on the appropriate characterization of mobile sources as a function of the roadway and vehicle characteristics. The EPA's quantitative PM hot-spot guidance includes important considerations and should be consulted when modeling roadway links. Line or area sources are recommended for mobile sources. However, if volume sources are used, it is particularly important to insure that roadway emissions are appropriately spaced when using volume source so that the emissions field is uniform across the roadway. Additionally, receptor placement is particularly important for volume sources, which have “exclusion zones”, where concentrations are not calculated for receptors located “within” the volume sources, i.e., less than 2.15 times the initial lateral dispersion coefficient from the center of the volume.⁶¹ Placing receptors in these “exclusion zones” will result in underestimates of roadway impacts.

8.0 Model Input Data

a. Databases and related procedures for estimating input parameters are an integral part of the modeling process. The most appropriate input data available should always be selected for use in modeling analyses. Modeled concentrations can vary widely depending on the source data or meteorological data used. This section attempts to minimize the uncertainty associated with database selection and use by identifying requirements for input data used in modeling. More specific data requirements and the format required for the individual models are described in detail in the users' guide and/or associated documentation for each model.

8.1 Modeling Domain

8.1.1 Discussion

a. The modeling domain is the geographic area for which the required air quality analyses for the NAAQS and PSD increments are conducted.

8.1.2 Requirements

a. For a NAAQS or PSD increment assessment, the modeling domain or project's impact area shall include all locations where the emissions of a pollutant from the new or modifying source(s) may cause a significant ambient impact. This impact area is defined as an area with a radius extending from the new or modifying source to: (1) The most distant point source where air quality modeling predicts a significant ambient impact will occur, or (2) the nominal 50 km distance considered applicable for Gaussian dispersion models, whichever is less. The required air quality analysis shall be carried out within this geographical area with characterization of source impacts, nearby source impacts, and background concentrations, as recommended later in this section.

b. For SIP attainment demonstrations for ozone and PM_2.5, or regional haze reasonable progress goal analyses, the modeling domain is determined by the nature of the problem being modeled and the spatial scale of the emissions which impact the nonattainment or Class I area(s). The modeling domain shall be designed so that all major upwind source areas that influence the downwind nonattainment area are included in addition to all monitor locations that are currently or recently violating the NAAQS or close to violating the NAAQS in the nonattainment area. Similarly, all Class I areas to be evaluated in a regional haze modeling application shall be included and sufficiently distant from the edge of the modeling domain. Guidance on the determination of the appropriate modeling domain for photochemical grid models in demonstrating attainment of these air quality goals is available.⁶⁰ Users should consult the latest version of this guidance for the most current modeling guidance and with the appropriate reviewing authority (paragraph 3.0(b)) for any application specific guidance which is beyond the scope of this section.

8.2 Source Data

8.2.1 Discussion

a. Sources of pollutants can be classified as point, line, area, and volume sources. Point sources are defined in terms of size and may vary between regulatory programs. The line sources most frequently considered are roadways and streets along which there are well-defined movements of motor vehicles. They may also be lines of roof vents or stacks, such as in aluminum refineries. Area and volume sources are often collections of a multitude of minor sources with individually small emissions that are impractical to consider as separate point or line sources. Large area sources are typically treated as a grid network of square areas, with pollutant emissions distributed uniformly within each grid square. Generally, input data requirements for air quality models necessitate the use of metric units. As necessary, any English units common to engineering applications should be appropriately converted to metric.

b. For point sources, there are many source characteristics and operating conditions that may be needed to appropriately model the facility. For example, the plant layout (e.g., location of stacks and buildings), stack parameters (e.g., height and diameter), boiler size and type, potential operating conditions, and pollution control equipment parameters. Such details are required inputs to air quality models and are needed to determine maximum potential impacts.

c. Modeling mobile emissions from streets and highways requires data on the road layout, including the width of each traveled lane, the number of lanes, and the width of the median strip. Additionally, traffic patterns should be taken into account (e.g., daily cycles of rush hour, differences in weekday and weekend traffic volumes, and changes in the distribution of heavy-duty trucks and light-duty passenger vehicles), as these patterns will affect the types and amounts of pollutant emissions allocated to each lane, and the height of emissions.

d. Emission factors can be determined through source specific testing and measurements (e.g., stack test data) from existing sources or provided from a manufacturing association or vendor. Additionally, emissions factors for a variety of source types are compiled in an EPA publication commonly known as AP-42.⁸⁹ AP-42 also provides an indication of the quality and amount of data on which many of the factors are based. Other information concerning emissions is available in EPA publications relating to specific source categories. The appropriate reviewing authority (paragraph 3.0(b)) should be consulted to determine appropriate source definitions and for guidance concerning the determination of emissions from and techniques for modeling the various source types.

8.2.2 Requirements

a. For SIP attainment demonstrations for the purpose of projecting future year NAAQS attainment for ozone, PM_2.5, and regional haze reasonable progress goal analyses, emissions which reflect actual emissions during the base modeling year time period should be input to models for base year modeling. Emissions projections to future years should account for key variables such as growth due to increased or decreased activity, expected emissions controls due to regulations, settlement agreements or consent decrees, fuel switches, and any other relevant information. Guidance on emissions estimation techniques (including future year projections) for SIP attainment demonstrations is available.^60 90

b. For the purpose of SIP revisions for stationary point sources, the regulatory modeling of inert pollutants shall use the emissions input data shown in Table 8-1 for short-term and long-term NAAQS. To demonstrate compliance and/or establish the appropriate SIP emissions limits, Table 8-1 generally provides for the use of “allowable” emissions in the regulatory dispersion modeling of the stationary point source(s) of interest. In such modeling, these source(s) should be modeled sequentially with these loads for every hour of the year. As part of a cumulative impact analysis, Table 8-1 allows for the model user to account for actual operations in developing the emissions inputs for dispersion modeling of nearby sources, while other sources are best represented by air quality monitoring data. Consultation with the appropriate reviewing authority (paragraph 3.0(b)) is advisable on the establishment of the appropriate emissions inputs for regulatory modeling applications with respect to SIP revisions for stationary point sources.

c. For the purposes of demonstrating NAAQS compliance in a PSD assessment, the regulatory modeling of inert pollutants shall use the emissions input data shown in Table 8-2 for short and long-term NAAQS. The new or modifying stationary point source shall be modeled with “allowable” emission in the regulatory dispersion modeling. As part of a cumulative impact analysis, Table 8-2 allows for the model user to account for actual operations in developing the emissions inputs for dispersion modeling of nearby sources, while other sources are best represented by air quality monitoring data. For purposes of situations involving emissions trading refer to current EPA policy and guidance to establish input data. Consultation with the appropriate reviewing authority (paragraph 3.0(b)) is advisable on the establishment of the appropriate emissions inputs for regulatory modeling applications with respect to PSD assessments for a proposed new or modifying source.

d. For stationary source applications, changes in operating conditions that affect the physical emission parameters (e.g., release height, initial plume volume, and exit velocity) shall be considered to ensure that maximum potential impacts are appropriately determined in the assessment. For example, the load or operating condition for point sources that causes maximum ground-level concentrations shall be established. As a minimum, the source should be modeled using the design capacity (100 percent load). If a source operates at greater than design capacity for periods that could result in violations of the NAAQS or PSD increment, this load should be modeled. Where the source operates at substantially less than design capacity, and the changes in the stack parameters associated with the operating conditions could lead to higher ground level concentrations, loads such as 50 percent and 75 percent of capacity should also be modeled. Malfunctions which may result in excess emissions are not considered to be a normal operating condition. They generally should not be considered in determining allowable emissions. However, if the excess emissions are the result of poor maintenance, careless operation, or other preventable conditions, it may be necessary to consider them in determining source impact. A range of operating conditions should be considered in screening analyses; the load causing the highest concentration, in addition to the design load, should be included in refined modeling.

e. Emissions from mobile sources also have physical and temporal characteristics that should be appropriately accounted for. For example, an appropriate emissions model shall be used to determine emissions profiles. Such emissions should include speciation specific for the vehicle types used on the roadway (e.g., light duty and heavy duty trucks) and subsequent parameterizations of the physical emissions characteristics (e.g., release height) should reflect those emissions sources. For long-term standards, annual average emissions may be appropriate, but for short-term standards, discrete temporal representation of emissions should be used (e.g., variations in weekday and weekend traffic or the diurnal rush-hour profile typical of many cities). Detailed information and data requirements for modeling mobile sources of pollution are provided in the user's manuals for each of the models applicable to mobile sources.^61 63

Table 8-1—Point Source Model Emission Input for SIP Revisions of Inert Pollutants ¹

Averaging time	Emissions limit (lb/MMBtu) 2	×	Operating level (lb/MMBtu) 2	×	Operating factor (e.g., hr/yr. hr/day)
Stationary Point Source(s) Subject to SIP Emissions Limit(s) Evaluation for Compliance With Ambient Standards
(Including Areawide Demonstrations)
Annual & quarterly	Maximum allowable emission limit or federal enforceable permit limit		Actual or design capacity (whichever is greater), or federally permit enforceable permit condition		Actual operating factor averaged over the most recent 2 years.3
Short term (≤24 hours)	Maximum allowable emission limit or federally enforceable permit limit		Actual or design capacity (whichever is greater), or federally enforceable permit condition.4		Continuous operation, i.e., all hours of each time period under consideration (for all hours of the meteorological database).5
Nearby Source(s).6
Annual & quarterly	Maximum allowable emission limit or federal enforceable permit limit.5		Annual level when actually operating, averaged over the most recent 2 years.3		Actual operating factor averaged over the most recent 2 years.3 8
Short term (≤24 hours)	Maximum allowable emission limit or federal enforceable permit limit.6		Temporally representative level when actually operating, reflective of the most recent 2 years.3 7		Continuous operation, i.e., all hours of each time period under consideration (for all hours of the meteorological database).5
Other Source(s) 8 9
The ambient impacts from Non-nearby or Other Sources (e.g., natural sources, minor sources and, distant major source and unidentified sources) can be represented by air quality monitoring data unless adequate data do not exist.
1 For purposes of emissions trading, NSR, or PSD, other model input criteria may apply. See Section 8.2 for more information regarding attainment demonstrations of primary PM2.5.
2 Terminology applicable to fuel burning sources; analogous terminology (e.g., lb/throughput) may be used for other types of sources.
3 Unless it is determined that this period is not representative.
4 Operating levels such as 50 percent and 75 percent of capacity should also be modeled to determine the load causing the highest concentration.
5 If operation does not occur for all hours of the time period of consideration (e.g., 3 or 24-hours) and the source operation is constrained by a federally enforceable permit condition, an appropriate adjustment to the modeled emission rate may be made (e.g., if operation is only 8 a.m. to 4 p.m. each day, only these hours will be modeled with emissions from the source. Modeled emissions should not be averaged across non-operating
6 See Section 8.3.3.
7 Temporally representative operating level could be based on Continuous Emissions Monitoring (CEM) data or other information and should be determined through consultation with the appropriate reviewing authority (Paragraph 3.0(b)).
8 For those permitted sources not in operation or that have not established an appropriate factor, continuous operation (i.e., 8760) should be used.
9 See Section 8.3.2.

Table 8-2—Point Source Model Emission Input for NAAQS Compliance in PSD Demonstrations

Averaging time	Emissions limit (lb/MMBtu) 1	×	Operating level (lb/MMBtu) 2	×	Operating factor (e.g., hr/yr. hr/day)
Proposed Major New or Modified Source
Annual & quarterly	Maximum allowable emission limit or federal enforceable permit limit		Design capacity or federally enforceable permit condition.		Continuous operation (i.e., 8760 hours).2
Short term (≤24 hours)	Maximum allowable emission limit or federal enforceable permit limit		Design capacity or federally enforceable permit condition.3		Continuous operation, i.e., all hours of each time period under consideration (for all hours of the meteorological database).2
Nearby Source(s) 4 5
Annual & quarterly	Maximum allowable emission limit or federal enforceable permit limit.5		Annual level when actually operating, averaged over the most recent 2 years.6		Actual operating factor averaged over the most recent 2 years.6 8
Short term (≤24 hours)	Maximum allowable emission limit or federal enforceable permit limit.5		Annual level when actually operating, averaged over the most recent 2 years.6 7		Continuous operation, i.e., all hours of each time period under consideration (for all hours of the meteorological database).2
Other Source(s) 5 9
The ambient impacts from Non-nearby or Other Sources (e.g., natural sources, minor sources and,distant major sources, and unidentified sources) can be represented by air quality monitoring data unless adequate data do not exist.
1 Terminology applicable to fuel burning sources; analogous terminology (e.g., lb/throughput) may be used for other types of sources.
2 If operation does not occur for all hours of the time period of consideration (e.g., 3 or 24-hours) and the source operation is constrained by a federally enforceable permit condition, an appropriate adjustment to the modeled emission rate may be made (e.g., if operation is only 8 a.m. to 4 p.m. each day, only these hours will be modeled with emissions from the source. Modeled emissions should not be averaged across non-operating
3 Operating levels such as 50 percent and 75 percent of capacity should also be modeled to determine the load causing the highest concentration.
4 Includes existing facility to which modification is proposed if the emissions from the existing facility will not be affected by the modification. Otherwise use the same parameters as for major modification.
5 See Section 8.3.3.
6 Unless it is determined that this period is not representative.
7 Temporally representative operating level could be based on Continuous Emissions Monitoring (CEM) data or other information and should be determined through consultation with the appropriate reviewing authority (Paragraph 3.0(b)).
8 For those permitted sources not in operation or that have not established an appropriate factor, continuous operation (i.e., 8760) should be used.
9 See Section 8.3.2.

8.3 Background Concentrations

8.3.1 Discussion

a. Background concentrations are essential in constructing the design concentration, or total air quality concentration, as part of a cumulative impact analysis for NAAQS and PSD increments (section 9.2.4). Background air quality should not include the ambient impacts of the project source under consideration. Instead, it should include:

i. Nearby sources: These are individual sources in the vicinity of the source(s) under consideration for emissions limits that are not adequately represented by ambient monitoring data. Typically, sources that cause a significant concentration gradient in the vicinity of the source(s) under consideration for emissions limits are not adequately represented by background ambient monitoring. The ambient contributions from these nearby sources are thereby accounted for by explicitly modeling their emissions (section 8.2).

ii. Other sources: That portion of the background attributable to natural sources, other unidentified sources in the vicinity of the project, and regional transport contributions from more distant sources (domestic and international). The ambient contributions from these sources are typically accounted for through use of ambient monitoring data or, in some cases, regional-scale photochemical grid modeling results.

b. The monitoring network used for developing background concentrations is expected to conform to the same quality assurance and other requirements as those networks established for PSD purposes.⁹¹ Accordingly, the air quality monitoring data should be of sufficient completeness and follow appropriate data validation procedures. These data should be adequately representative of the area to inform calculation of the design concentration for comparison to the applicable NAAQS (section 9.2.2)

c. For photochemical grid modeling conducted in SIP attainment demonstrations for ozone, PM_2.5 and regional haze, the emissions from nearby and other sources are included as model inputs and fully accounted for in the modeling application and predicted concentrations. The concept of adding individual components to develop a design concentration, therefore, do not apply in these SIP applications. However, such modeling results may then be appropriate for consideration in characterizing background concentrations for other regulatory applications. Also, as noted in section 5, this modeling approach does provide for an appropriate atmospheric environment to assess single-sources impacts for ozone and secondary PM_2.5.

d. For PSD assessments in general and SIP attainment demonstrations for inert pollutants, the development of the appropriate background concentration for a cumulative impact analysis involves proper accounting of each contribution to the design concentration and will depend upon whether the project area's situation consists of either an isolated single source(s) or a multitude of sources.

8.3.2 Recommendations for Isolated Single Source

a. In areas with an isolated source(s), determining the appropriate background concentration should focus on characterization of contributions from all other sources through adequately representative ambient monitoring data.

b. The EPA recommends use of the most recent quality assured air quality monitoring data collected in the vicinity of the source to determine the background concentration for the averaging times of concern. In most cases, the EPA recommends using data from the monitor closest to and upwind of the project area. If several monitors are available, preference should be given to the monitor with the most similar characteristics as the project area. If there are no monitors located in the vicinity of the new or modify source, a “regional site” may be used to determine background concentrations. A regional site is one that is located away from the area of interest but is impacted by similar or adequately representative sources.

c. Many of the challenges related to cumulative impact analyses arise in the context of defining the appropriate metric to characterize background concentrations from ambient monitoring data and determining the appropriate method for combining this monitor-based background contribution to the modeled impact of the project and other nearby sources. For many cases, the best starting point would be use of the current design value for the applicable NAAQS as a uniform monitored background contribution across the project area. However, there are cases in which the current design value may not be appropriate. Such cases include but are not limited to:

i. For situations involving a modifying source where the existing facility is determined to impact the ambient monitor, the background concentration at each monitor can be determined by excluding values when the source in question is impacting the monitor. In such cases, monitoring sites inside a 90° sector downwind of the source may be used to determine the area of impact.

ii. There may be other circumstances which would necessitate modifications to the ambient data record. Such cases could include removal of data from specific days or hours when a monitor is being impacted activities that are not typical or expected to occur again in the future (e.g., construction, roadway repairs, forest fires, or unusual agricultural activities). There may also be cases where scaling (multiplying the monitored concentrations with a scaling factor) or adjusting (adding or subtracting a constant value the monitored concentrations) of data from specific days or hours. Such adjustments would make the monitored background concentrations more temporally and/or spatially representative of area around the new or modifying source for the purposes of the regulator assessment.

iii. For short-term standards, the diurnal or seasonal patterns of the air quality monitoring data may differ significantly from the patterns associated with the modeled concentrations. When this occurs, it may be appropriate to pair the air quality monitoring data in a temporal manner that reflects these patterns (e.g., pairing by season and/or hour of day).⁹²

iv. For situations where monitored air quality concentrations vary across the modeling domain, it may be appropriate to consider air quality monitoring data from multiple monitors within the project area.

d. Determination of the appropriate background concentrations should be consistent with appropriate EPA modeling guidance ^59 92 and justified in the modeling protocol that is vetted with the appropriate reviewing authority (paragraph 3.0(b)).

e. Considering the spatial and temporal variability throughout a typical modeling domain on an hourly basis and the complexities and limitations of hourly observations from the ambient monitoring network, the EPA does not recommend hourly or daily pairing of monitored background and modeled concentrations except in rare cases of relatively isolated sources where the available monitor can be shown to be representative of the ambient concentration levels in the areas of maximum impact from the proposed new source. The implicit assumption underlying hourly pairing is that the background monitored levels for each hour are spatially uniform and that the monitored values are fully representative of background levels at each receptor for each hour. Such an assumption clearly ignores the many factors that contribute to the temporal and spatial variability of ambient concentrations across a typical modeling domain on an hourly basis. In most cases, the seasonal (or quarterly) pairing of monitored and modeled concentrations should sufficiently address situations to which the impacts from modeled emissions are not temporally correlated with background monitored levels.

f. In those cases where adequately representative monitoring data to characterize background concentrations are not available, it may be appropriate to use results from a regional-scale photochemical grid model or other representative model application as background concentrations consistent with the considerations discussed above and in consultation with the appropriate reviewing authority (paragraph 3.0(b)).

8.3.3 Recommendations for Multi-Source Areas

a. In multi-source areas, determining the appropriate background concentration involves: (1) identification and characterization of contributions from nearby sources through explicit modeling, and (2) characterization of contributions from other sources through adequately representative ambient monitoring data. A key point here is the interconnectedness of each component in that the question of which nearby sources to include in the cumulative modeling is inextricably linked to the question of what the ambient monitoring data represents within the project area.

b. Nearby sources: All sources in the vicinity of the source(s) under consideration for emissions limits that are not adequately represented by ambient monitoring data should be explicitly modeled. Since an ambient monitor is limited to characterizing air quality at a fixed location, sources that causes a significant concentration gradient in the vicinity of the source(s) under consideration for emissions limits are not likely to be adequately characterized by the monitored data due to the high degree of variability of the source's impact.

i. The pattern of concentration gradients can vary significantly based on the averaging period being assessed. In general, concentration gradients will be smaller and more spatially uniform for annual averages than for short-term averages, especially for hourly averages. The spatial distribution of annual impacts around a source will often have a single peak downwind of the source based on the prevailing wind direction, except in cases where terrain or other geographic effects are important. By contrast, the spatial distribution of peak short-term impacts will typically show several localized concentration peaks with more significant gradient.

ii. Concentration gradients associated with a particular source will generally be largest between that source's location and the distance to the maximum ground-level concentrations from that source. Beyond the maximum impact distance, concentration gradients will generally be much smaller and more spatially uniform. Thus, the magnitude of a concentration gradient will be greatest in the proximity of the source and will generally not be significant at distances greater than 10 times the height of the stack(s) at that source without consideration of terrain influences.

iii. The number of nearby sources to be explicitly modeled in the air quality analysis is expected to be few except in unusual situations. In most cases, the few nearby sources will be located within 10 to 20 km from the source(s) under consideration. Owing to both the uniqueness of each modeling situation and the large number of variables involved in identifying nearby sources, no attempt is made here to comprehensively define a “significant concentration gradient.” Rather, identification of nearby sources calls for the exercise of professional judgement by the appropriate reviewing authority (paragraph 3.0(b)). This guidance is not intended to alter the exercise of that judgement or to comprehensively prescribe which sources should be included as nearby sources.

c. For cumulative impact analyses of short-term and annual ambient standards, the nearby sources as well as the project source(s) must be evaluated using an appropriate appendix A model or approved alternative model with the emission input data shown in Table 8-1 or 8-2.

i. When modeling a nearby source that does not have a permit and the emissions limits contained in the SIP for a particular source category is greater than the emissions possible given the source's maximum physical capacity to emit, the “maximum allowable emissions limit” for such a nearby source may be calculated as the emissions rate representative of the nearby source's maximum physical capacity to emit, considering its design specifications and allowable fuels and process materials. However, the burden is on the permit applicant to sufficiently document what the maximum physical capacity to emit is for such a nearby source.

ii. It is appropriate to model nearby sources only during those times when they, by their nature, operate at the same time as the primary source(s). Accordingly, it is not necessary to model impacts of a nearby source that does not, by its nature, operate at the same time as the primary source, regardless of an identified significant concentration gradient from the nearby source. The burden is on the permit applicant to adequately justify the exclusion of nearby sources to the satisfaction of the appropriate reviewing authority (paragraph 3.0(b)). The following examples illustrate two cases in which a nearby source may be shown not to operate at the same time as the primary source(s) being modeled: (1) Seasonal sources (only used during certain seasons of the year). Such sources would not be modeled as nearby sources during times in which they do not operate; and (2) Emergency backup generators, to the extent that they do not operate simultaneously with the sources that they back up. Such emergency equipment would not be modeled as nearby sources.

d. Other sources. That portion of the background attributable to all other sources (e.g., natural sources, minor and distance major sources) should be accounted for through use of ambient monitoring data and determined by the procedures found in section 8.3.2 in keeping with eliminating or reducing the source-oriented impacts from nearby sources to avoid potential double-counting of modeled and monitored contributions.

8.4 Meteorological Input Data

8.4.1 Discussion

a. This subsection covers meteorological input data for use in dispersion modeling for regulatory applications and is separate from recommendations made for photochemical grid modeling. Recommendations for meteorological data for photochemical grid modeling applications are outlined in the latest version of EPA's Guidance on the Use of Models and Other Analyses for Demonstrating Attainment of Air Quality Goals for Ozone, PM_2.5, and Regional Haze ⁹³. In cases where Lagrangian models are applied for regulatory purposes, appropriate meteorological inputs should be determined in consultation with the appropriate reviewing authority (paragraph 3.0(b)).

b. The meteorological data used as input to a dispersion model should be selected on the basis of spatial and climatological (temporal) representativeness as well as the ability of the individual parameters selected to characterize the transport and dispersion conditions in the area of concern. The representativeness of the measured data is dependent on numerous factors including but not limited to: (1) The proximity of the meteorological monitoring site to the area under consideration; (2) The complexity of the terrain; (3) The exposure of the meteorological monitoring site; and (4) The period of time during which data are collected. The spatial representativeness of the data can be adversely affected by large distances between the source and receptors of interest and the complex topographic characteristics of the area. Temporal representativeness is a function of the year-to-year variations in weather conditions. Where appropriate, data representativeness should be viewed in terms of the appropriateness of the data for constructing realistic boundary layer profiles and, where applicable, three-dimensional meteorological fields, as described in paragraphs (c) and (d) of this subsection.

c. The meteorological data should be adequately representative and may be site-specific data, data from a nearby National Weather Service (NWS) or comparable station, or prognostic meteorological data. The implementation of ASOS (automated surface observing stations) in recent years should not preclude the use of NWS-ASOS data if such a station is determined to be representative of the modeled area.⁹⁴

d. Model input data are normally obtained either from the NWS or as part of a site-specific measurement program. State climatology offices, local universities, FAA, military stations, industry and pollution control agencies may also be sources of such data. In specific cases, prognostic meteorological data may be appropriate for use and obtained from similar sources. Some recommendations and requirements for the use of each type of data are included in this subsection.

8.4.2 Recommendations and Requirements

a. AERMET ⁹⁵ shall be used to preprocess all meteorological data, be it observed or prognostic, for use with AERMOD in regulatory applications. The AERMINUTE ⁹⁶ processor, in most cases, should be used to process 1-minute ASOS wind data for input into AERMET when processing NWS ASOS sites in AERMET. When processing prognostic meteorological data for AERMOD, the Mesoscale Model Interface Program (MMIF) ⁹³ should be used to process data for input into AERMET. Other methods of processing prognostic meteorological data for input into AERMET should be approved by the appropriate reviewing authority. Additionally, the following meteorological preprocessors are recommended by the EPA: PCRAMMET ⁹⁷, MPRM ⁹⁸, and METPRO ⁹⁹. PCRAMMET is the recommended meteorological data preprocessor for use in applications of OCD employing hourly NWS data. MPRM is the recommended meteorological data preprocessor for applications of OCD employing site-specific meteorological data. METPRO is the recommended meteorological data preprocessor for use with CTDMPLUS.¹⁰⁰

b. Regulatory application of AERMOD necessitates careful consideration of the meteorological data for input to AERMET. Data representativeness, in the case of AERMOD, means utilizing data of an appropriate type for constructing realistic boundary layer profiles. Of particular importance is the requirement that all meteorological data used as input to AERMOD should be adequately representative of the transport and dispersion within the analysis domain. Where surface conditions vary significantly over the analysis domain, the emphasis in assessing representativeness should be given to adequate characterization of transport and dispersion between the source(s) of concern and areas where maximum design concentrations are anticipated to occur. The EPA recommends that the surface characteristics input to AERMET should be representative of the land cover in the vicinity of the meteorological data, i.e., the location of the meteorological tower for measured data or the representative grid cell for prognostic data. Therefore, the model user should apply the latest version AERSURFACE ^101 102, where applicable, for determining surface characteristics when processing measured meteorological data through AERMET. In areas where it is not possible to use AERSURFACE output, surface characteristics can determined using techniques that apply the same analysis as AERSURFACE. In the case of prognostic meteorological data, the surface characteristics associated with the prognostic meteorological model output for the representative grid cell should be used.^103 104 Furthermore, since the spatial scope of each variable could be different, representativeness should be judged for each variable separately. For example, for a variable such as wind direction, the data should ideally be collected near plume height to be adequately representative, especially for sources located in complex terrain. Whereas, for a variable such as temperature, data from a station several kilometers away from the source may be considered to be adequately representative. More information about meteorological data, representativeness, and surface characteristics can be found in the AERMOD Implementation Guide ⁷⁶.

c. Regulatory application of CTDMPLUS requires the input of multi-level measurements of wind speed, direction, temperature, and turbulence from an appropriately sited meteorological tower. The measurements should be obtained up to the representative plume height(s) of interest. Plume heights of interest can be determined by use of screening procedures such as CTSCREEN.

d. Regulatory application of OCD requires meteorological data over land and over water. The over land or surface data processed through PCRAMMET ⁹⁷ which provides hourly stability class, wind direction and speed, ambient temperature, and mixing height are required. Data over water requires hourly mixing height, relative humidity, air temperature, and water surface temperature. Missing winds are substituted with the surface winds. Vertical wind direction shear, vertical temperature gradient, and turbulence intensities are optional.

e. The model user should acquire enough meteorological data to ensure that worst-case meteorological conditions are adequately represented in the model results. The use of 5 years of adequately representative NWS meteorological data, at least 1 year of site-specific, or at least 3 years of prognostic meteorological data are required. If 1 year or more, up to 5 years, of site-specific data is available, these data are preferred for use in air quality analyses. Such data should have been subjected to quality assurance procedures as described in section 8.4.4.2.

f. Objective analysis in meteorological modeling is to improve meteorological analyses (the “first guess field”) used as initial conditions for prognostic meteorological models by incorporating information from meteorological observations. Direct and indirect (using remote sensing techniques) observations of temperature, humidity, and wind from surface and radiosonde reports are commonly employed to improve these analysis fields. For LRT applications, it is recommended that objective analysis procedures using direct and indirect meteorological observations be employed in preparing input fields to produce prognostic meteorological datasets. The length of record of observations should conform to recommendations outlined in paragraph 8.4.2(e) for prognostic meteorological model datasets.

8.4.3 National Weather Service Data

8.4.3.1 Discussion

a. The NWS meteorological data are routinely available and familiar to most model users. Although the NWS does not provide direct measurements of all the needed dispersion model input variables, methods have been developed and successfully used to translate the basic NWS data to the needed model input. Site-specific measurements of model input parameters have been made for many modeling studies, and those methods and techniques are becoming more widely applied, especially in situations such as complex terrain applications, where available NWS data are not adequately representative. However, there are many modeling applications where NWS data are adequately representative, and the applications still rely heavily on the NWS data.

b. Many models use the standard hourly weather observations available from the National Centers for Environmental Information (NCEI) ^[b] . These observations are then preprocessed before they can be used in the models. Prior to the advent of ASOS in the early 1990's, the “hourly” weather observation was a human observer-based observation reflecting a single 2-minute average generally taken about 10 minutes before the hour. However, beginning with January 2000 for first-order stations and March 2005 for all stations, NCEI has archived the rolling 2-minute average winds at every minute for ASOS sites. The AERMINUTE processor ⁹⁶ was developed to reduce calm and missing hours by taking advantage of the availability of the 1-minute ASOS wind data to calculate full hourly average winds to replace standard hourly observations and reduce the number of calm and missing winds in AERMET processing.

8.4.3.2 Recommendations

a. The preferred models listed in appendix A all accept as input the NWS meteorological data preprocessed into model compatible form. If NWS data are judged to be adequately representative for a specific modeling application, they may be used. NEIS makes available surface ^105 106 and upper air ¹⁰⁷ meteorological data online and in CD-ROM format. Upper air data are also available at the Earth System Research Laboratory Global Systems Divisions Web site (http://esrl.noaa.gov/​gsd).

b. Although most NWS wind measurements are made at a standard height of 10 meters, the actual anemometer height should be used as input to the preferred meteorological processor and model.

c. Standard hourly NWS wind directions are reported to the nearest 10 degrees. A specific set of randomly generated numbers has been developed for use with the preferred EPA models and should be used with standard NWS data to ensure a lack of bias in wind direction assignments within the models.

d. Beginning with year 2000, NCDC began archiving 2-minute winds, reported every minute for NWS ASOS sites. The AERMINUTE processor was developed to read those winds and calculate hourly average winds for input into AERMET. When such data are available for the NWS ASOS site being processed, the AERMINUTE processor should be used in most cases to calculate hourly average wind speed and direction when processing NWS ASOS data for input to AERMOD.⁹⁴

e. Data from universities, FAA, military stations, industry and pollution control agencies may be used if such data are equivalent in accuracy and detail (e.g., siting criteria, frequency of observations, data completeness, etc.) to the NWS data, they are judged to be adequately representative for the particular application and have undergone quality assurance checks.

f. After valid data retrieval requirements have been met,¹⁰⁸ large number of hours in the record having missing data should be treated according to an established data substitution protocol provided that adequately representative alternative data are available. Data substitution guidance is provided in section 5.3 of reference 108. If no representative alternative data are available for substitution, the absent data should be coded as missing using missing data codes appropriate to the applicable meteorological pre-processor. Appropriate model options for treating missing data, if available in the model, should be employed.

8.4.4 Site-Specific data

8.4.4.1 Discussion

a. Spatial or geographical representativeness is best achieved by collection of all of the needed model input data in close proximity to the actual site of the source(s). Site-specific measured data are therefore preferred as model input, provided that appropriate instrumentation and quality assurance procedures are followed and that the data collected are adequately representative (free from inappropriate local or microscale influences) and compatible with the input requirements of the model to be used. It should be noted that, while site-specific measurements are frequently made “on-property” (i.e., on the source's premises), acquisition of adequately representative site-specific data does not preclude collection of data from a location off property. Conversely, collection of meteorological data on a source's property does not of itself guarantee adequate representativeness. For help in determining representativeness of site-specific measurements, technical guidance ¹⁰⁸ is available. Site-specific data should always be reviewed for representativeness and adequacy by an experienced meteorologist, atmospheric scientist, or other qualified scientist.

8.4.4.2 Recommendations

a. The EPA guidance ¹⁰⁸ provides recommendations on the collection and use of site-specific meteorological data. Recommendations on characteristics, siting, and exposure of meteorological instruments and on data recording, processing, completeness requirements, reporting, and archiving are also included. This publication should be used as a supplement to other limited guidance on these subjects.^{5 91 109 110} Detailed information on quality assurance is also available.¹¹¹ As a minimum, site-specific measurements of ambient air temperature, transport wind speed and direction, and the variables necessary to estimate atmospheric dispersion should be available in meteorological datasets to be used in modeling. Care should be taken to ensure that meteorological instruments are located to provide an adequately representative characterization of pollutant transport between sources and receptors of interest. The appropriate reviewing authority (paragraph 3.0(b)) is available to help determine the appropriateness of the measurement locations.

b. All processed site-specific data should be in the form of hourly averages for input into the dispersion model. These data include surface wind speed, transport direction, dilution wind speed, and turbulence measurements σ_A and σ_E (for use in stability determinations and direct input into the dispersion model). The hourly average turbulence measurements should be the square root of the arithmetic average of the 15-minute average variances (square of σ_A or σ_E).

c. Missing data substitution. After valid data retrieval requirements have been met,¹⁰⁸ hours in the record having missing data should be treated according to an established data substitution protocol provided that adequately representative alternative data are available. Such protocols are usually part of the approved monitoring program plan. Data substitution guidance is provided in section 5.3 of reference 108. If no representative alternative data are available for substitution, the absent data should be coded as missing using missing data codes appropriate to the applicable meteorological pre-processor. Appropriate model options for treating missing data, if available in the model, should be employed.

d. Solar radiation measurements. Total solar radiation or net radiation should be measured with a reliable pyranometer or net radiometer, sited and operated in accordance with established site-specific meteorological guidance.^108 111

e. Temperature measurements. Temperature measurements should be made at standard shelter height (2m) in accordance with established site-specific meteorological guidance.¹⁰⁸

f. Temperature difference measurements. Temperature difference (DT) measurements should be obtained using matched thermometers or a reliable thermocouple system to achieve adequate accuracy. Siting, probe placement, and operation of DT systems should be based on guidance found in Chapter 3 of reference 108 and such guidance should be followed when obtaining vertical temperature gradient data. AERMET may employ the Bulk Richardson scheme, which requires measurements of temperature difference, in lieu of cloud cover or insolation data. To ensure correct application and acceptance, AERMOD users should consult with the appropriate reviewing authority (paragraph 3.0(b)) before using the Bulk Richardson scheme for their analysis.

g. Wind measurements. For simulation of plume rise and dispersion of a plume emitted from a stack, characterization of the wind profile up through the layer in which the plume disperses is desirable. This is especially important in complex terrain and/or complex wind situations where wind measurements at heights up to hundreds of meters above stack base may be required in some circumstances. For tall stacks when site-specific data are needed, these winds have been obtained traditionally using meteorological sensors mounted on tall towers. A feasible alternative to tall towers is the use of meteorological remote sensing instruments (e.g., acoustic sounders or radar wind profilers) to provide winds aloft, coupled with 10-meter towers to provide the near-surface winds. Note that when site-specific wind measurements are used, AERMOD, at a minimum, requires wind observations at a height above ground between seven times the local surface roughness height and 100 meters. (For additional requirements for AERMOD and CTDMPLUS, see appendix A.) Specifications for wind measuring instruments and systems are contained in reference 108.

h. Turbulence. There are several dispersion models that are capable of using direct measurements of turbulence (wind fluctuations) in the characterization of the vertical and lateral dispersion (e.g., CTDMPLUS, AERMOD). For specific requirements for CTDMPLUS, AERMOD, see appendix A. For technical guidance on measurement and processing of turbulence parameters, see reference 108. When turbulence data are used in this manner to directly characterize the vertical and lateral dispersion, the averaging time for the turbulence measurements should be 1 hour. However, since AERMOD incorporates an algorithm to account for horizontal plume meander under low wind conditions, the methodology outlined in paragraph 8.4.4.2(b) should be used to calculate hourly averages of σ_θ, based on four 15-minuite values, to minimize “double counting” of plume spread associated with meander. The calculation of hourly σ_θ discussed above is automatically applied within AERMET when sub-hourly data are processed. There are other dispersion models that employ P-G stability categories for the characterization of the vertical and lateral dispersion. Methods for using site-specific turbulence data for the characterization of P-G stability categories are discussed in reference 108. When turbulence data are used in this manner to determine the P-G stability category, the averaging time for the turbulence measurements should be 15 minutes, with hourly averaged values based on methodology in paragraph 8.4.4.2(b).

i. Stability categories. For dispersion models that employ P-G stability categories for the characterization of the vertical and lateral dispersion, the P-G stability categories, as originally defined, couple near-surface measurements of wind speed with subjectively determined insolation assessments based on hourly cloud cover and ceiling height observations. The wind speed measurements are made at or near 10m. The insolation rate is typically assessed using observations of cloud cover and ceiling height based on criteria outlined by Turner.⁷² It is recommended that the P-G stability category be estimated using the Turner method with site-specific wind speed measured at or near 10m and representative cloud cover and ceiling height. Implementation of the Turner method, as well as considerations in determining representativeness of cloud cover and ceiling height in cases for which site-specific cloud observations are unavailable, may be found in section 6 of reference 108. In the absence of requisite data to implement the Turner method, the solar radiation/delta-T (SRDT) method or wind fluctuation statistics (i.e., the σ_E and σ_A methods) may be used.

j. The SRDT method, described in section 6.4.4.2 of reference 108, is modified slightly from that published from earlier work ¹¹² and has been evaluated with three site-specific databases.¹¹³ The two methods of stability classification which use wind fluctuation statistics, the σ_E and σ_A methods, are also described in detail in section 6.4.4 of reference 108 (note applicable tables in section 6). For additional information on the wind fluctuation methods, several references are available.^{114 115 116 117}

8.4.5 Prognostic Meteorological Data

8.4.5.1 Discussion

a. For some modeling applications, there may not be a representative NWS or comparable meteorological station available (e.g., complex terrain), and it may be cost prohibitive or infeasible to collect adequately representative site-specific data. For these cases, it may be necessary to use prognostic meteorological data in a regulatory modeling application.

b. The EPA has developed a processor, the MMIF (Mesoscale Model Interface Program) to process MM5 (Mesoscale Model 5) or WRF (Weather Research and Forecasting) model data for input into various models including AERMOD. MMIF can process data for input into AERMET or AERMOD for a single grid cell or multiple grid cells. MMIF output has been found to compare favorably against observed data (site-specific or NWS).¹¹⁸ Specific guidance on processing MMIF for AERMOD can be found in reference 104. When using MMIF to process prognostic data for regulatory applications, the data should be processed to generate AERMET inputs and the data subsequently processed through AERMET for input into AERMOD. If an alternative method of processing data for input into AERMET is used, it must be approved by the appropriate reviewing authority (paragraph 3.0(b)).

8.4.5.2 Recommendations

a. Prognostic model evaluation. Appropriate effort should be devoted to the process of evaluating the prognostic meteorological data. The modeling data should be compared to NWS observational data in an effort to show that the data are accurately replicating the observed meteorological conditions of the time periods modeled. An operational evaluation of the modeling data for all model years (i.e., statistical, graphical) should be completed.⁹³ The use of output from prognostic mesoscale meteorological models is contingent upon the concurrence with the appropriate reviewing authority (paragraph 3.0(b)) that the data are of acceptable quality, which can be demonstrated through statistical comparisons with meteorological observations aloft and at the surface at several appropriate locations.⁹³

b. Representativeness. When processing MMIF data for use with AERMOD, the grid cell used for the dispersion modeling should be adequately spatially representative of the analysis domain. In most cases, this may be the grid cell containing the emission source of interest. Since the dispersion modeling may involve multiple sources and the domain may cover several grid cells, depending on grid resolution of the prognostic model, professional judgement may be needed to select the appropriate grid cell to use. In such cases, the selected grid cell should be adequately representative of the entire domain.

c. Grid resolution. The grid resolution of the prognostic meteorological data should be considered and evaluated appropriately, particularly for projects involving complex terrain. The operational evaluation of the modeling data should consider whether a finer grid resolution is needed to ensure that the data are representative. The use of output from prognostic mesoscale meteorological models is contingent upon the concurrence with the appropriate reviewing authority (paragraph 3.0(b)) that the data are of acceptable quality.

8.4.6 Treatment of Near-Calms and Calms

8.4.6.1 Discussion

a. Treatment of calm or light and variable wind poses a special problem in modeling applications since steady-state Gaussian plume models assume that concentration is inversely proportional to wind speed, depending on model formulations. Procedures have been developed to prevent the occurrence of overly conservative concentration estimates during periods of calms. These procedures acknowledge that a steady-state Gaussian plume model does not apply during calm conditions, and that our knowledge of wind patterns and plume behavior during these conditions does not, at present, permit the development of a better technique. Therefore, the procedures disregard hours which are identified as calm. The hour is treated as missing and a convention for handling missing hours is recommended. With the advent of the AERMINUTE processor, when processing NWS ASOS data, the inclusion of hourly averaged winds from AERMINUTE will, in some instances, dramatically reduce the number of calm and missing hours, especially when the ASOS wind are derived from a sonic anemometer. To alleviate concerns about low winds, especially those introduced with AERMINUTE, the EPA implemented a wind speed threshold in AERMET for use with ASOS derived winds.⁹⁶ Winds below the threshold will be treated as calms.

b. AERMOD, while fundamentally a steady-state Gaussian plume model, contains algorithms for dealing with low wind speed (near calm) conditions. As a result, AERMOD can produce model estimates for conditions when the wind speed may be less than1 m/s, but still greater than the instrument threshold. Required input to AERMET for site-specific data, the meteorological processor for AERMOD, includes a threshold wind speed and a reference wind speed. The threshold wind speed is typically the threshold of the instrument used to collect the wind speed data. The reference wind speed is selected by the model as the lowest level of non-missing wind speed and direction data where the speed is greater than the wind speed threshold, and the height of the measurement is between seven times the local surface roughness and 100 meters. If the only valid observation of the reference wind speed between these heights is less than the threshold, the hour is considered calm, and no concentration is calculated. None of the observed wind speeds in a measured wind profile that are less than the threshold speed are used in construction of the modeled wind speed profile in AERMOD.

8.4.6.2 Recommendations

a. Hourly concentrations calculated with steady-state Gaussian plume models using calms should not be considered valid; the wind and concentration estimates for these hours should be disregarded and considered to be missing. Critical concentrations for3-, 8-, and 24-hour averages should be calculated by dividing the sum of the hourly concentrations for the period by the number of valid or non-missing hours. If the total number of valid hours is less than 18 for 24-hour averages, less than 6 for 8-hour averages or less than 3 for 3-hour averages, the total concentration should be divided by 18 for the 24-hour average, 6 for the 8-hour average and 3 for the 3-hour average. For annual averages, the sum of all valid hourly concentrations is divided by the number of non-calm hours during the year. AERMOD has been coded to implement these instructions. For hours that are calm or missing, the AERMOD hourly concentrations will be zero. For other models listed in appendix A, a post-processor computer program, CALMPRO ¹¹⁹ has been prepared, is available on the EPA's SCRAM Web site (section 2.3), and should be used.

b. Stagnant conditions that include extended periods of calms often produce high concentrations over wide areas for relatively long averaging periods. The standard steady-state Gaussian plume models are often not applicable to such situations. When stagnation conditions are of concern, other modeling techniques should be considered on a case-by-case basis (see also section 7.2.1.2).

c. When used in steady-state Gaussian plume models, measured site-specific wind speeds of less than 1 m/s but higher than the response threshold of the instrument should be input as 1 m/s; the corresponding wind direction should also be input. Wind observations below the response threshold of the instrument should be set to zero, with the input file in ASCII format. For input to AERMOD, no adjustment should be made to the site-specific wind data. For NWS ASOS data, especially data using the 1-minute ASOS winds, a wind speed threshold option is allowed with a recommended speed of 0.5 m/s. ⁹⁴ When using prognostic data processed by MMIF, a 0.5 m/s threshold is also invoked by MMIF for input into AERMET. Observations with wind speeds less than the threshold are considered calm, and no concentration is calculated. In all cases involving steady-state Gaussian plume models, calm hours should be treated as missing, and concentrations should be calculated as in paragraph (a) of this subsection.

9.0 Regulatory Application of Models

9.1 Discussion

a. Standardized procedures are valuable in the review of air quality modeling and data analyses conducted to support SIP submittals and revisions, NSR, including PSD, or other EPA requirements to ensure consistency in their regulatory application. This section recommends procedures specific to NSR, including PSD, that facilitate some degree of standardization while at the same time allowing the flexibility needed to assure the technically best analysis for each regulatory application. For SIP attainment demonstrations, refer to the appropriate EPA guidance ^51 60 for the recommended procedures.

b. Air quality model estimates, especially with the support of measured air quality data, are the preferred basis for air quality demonstrations. A number of actions have been taken to ensure that the best air quality model is used correctly for each regulatory application and that it is not arbitrarily imposed.

• First, the Guideline clearly recommends that the most appropriate model be used in each case. Preferred models are identified, based on a number of factors, for many uses.
• Second, the preferred models have been subjected to a systematic performance evaluation and a peer scientific review. Statistical performance measures, including measures of difference (or residuals) such as bias, variance of difference and gross variability of the difference, and measures of correlation such as time, space, and time and space combined as described in section 2.1.1, were generally followed.
• Third, more specific information has been provided for considering the incorporation of new models into the Guideline (section 3.1) and the Guideline contains procedures for justifying the case-by-case use of alternative models and obtaining EPA approval (section 3.2).

The Guideline, therefore, provides objective methods that allow a determination to be made as to what air quality model or technique is most appropriate for a particular application.

c. Air quality modeling is the preferred basis for air quality demonstrations. Nevertheless, there are rare circumstances where the performance of the preferred air quality model may be shown to be less than reasonably acceptable or where no preferred air quality model, screening model or technique, or alternative model are suitable for the situation. In these unique instances, there is the possibility of assuring compliance and establishing emissions limits for an existing source solely on the basis of observed air quality data in lieu of an air quality modeling analysis. Comprehensive air quality monitoring in the vicinity of the existing source with proposed modifications will be necessary in these cases. The same attention should be given to the detailed analyses of the air quality data as would be applied to a model performance evaluation.

d. The current levels and forms of the NAAQS for the six criteria pollutants can be found on the EPA's NAAQS Web site at http://www.epa.gov/​air/​criteria.html. Under the CAA, the NAAQS are subjected to extensive review every 5 years and the standards, including the level and the form, may be revised as part of that review. The criteria pollutants have either long-term (annual or quarterly) and/or short-term (24-hour or less) forms that are not to be exceeded more than a certain frequency over a period of time (e.g., no exceedance on a rolling 3-month average, no more than once per year, or no more than once per year averaged over 3 years), are averaged over a period of time (e.g., an annual mean or an annual mean averaged over 3 years), or are some percentile that is averaged over a period of time (e.g., annual 99th or 98th percentile averaged over 3 years). The 3-year period for ambient monitoring design values does not dictate the length of the data periods recommended for modeling (i.e., 5 years of NWS meteorological data, at least 1 year of site-specific, or at least 3 years of prognostic meteorological data).

e. This section discusses general recommendations on the regulatory application of models for the purposes of NSR, including PSD permitting, and particularly for estimating design concentration(s), appropriately comparing these estimates to NAAQS and PSD increment, and developing emissions limits. Lastly, this section provides the criteria necessary for considering use of analysis based on measured ambient data in lieu of modeling as the sole basis for demonstrating compliance with NAAQS and PSD increments.

9.2 Recommendations

9.2.1 Modeling Protocol

a. Every effort should be made by the appropriate reviewing authority (paragraph 3.0(b)) to meet with all parties involved in either a SIP submission or revision or a PSD permit application prior to the start of any work on such a project. During this meeting, a protocol should be established between the preparing and reviewing parties to define the procedures to be followed, the data to be collected, the model to be used, and the analysis of the source and concentration data to be performed. An example of the content for such an effort is contained in the Air Quality Analysis Checklist posted on the EPA's SCRAM Web site (section 2.3). This checklist suggests the appropriate level of detail to assess the air quality resulting from the proposed action. Special cases may require additional data collection or analysis and this should be determined and agreed upon at this pre-application meeting. The protocol should be written and agreed upon by the parties concerned, although it is not intended that this protocol be a binding, formal legal document. Changes in such a protocol or deviations from the protocol are often necessary as the data collection and analysis progresses. However, the protocol establishes a common understanding of how the demonstration required to meet regulatory requirements will be made.

9.2.2 Design Concentration and Receptor Sites

a. Under the PSD permitting program, an air quality analysis for criteria pollutants is required to demonstrate that emissions from the construction or operation of a proposed new source or modification will not cause or contribute to a violation of the NAAQS or PSD increments.

i. For a NAAQS assessment, the design concentration is the combination of the appropriate background concentration (section 8.3) with the estimated modeled impact of the source. The NAAQS design concentration is then compared to the applicable NAAQS.

ii. For a PSD increment assessment, the design concentration includes impacts after the appropriate baseline date from all increment consuming and increment expanding sources. The PSD increment design concentration is then compared to the applicable PSD increment.

b. The specific form of the NAAQS for the pollutant(s) of concern will also influence how the background and modeled data should be combined for appropriate comparison with the respective NAAQS in such a modeling demonstration. Given the potential for revision of the form of the NAAQS and the complexities of combining background and modeled data, specific details on this process can be found in applicable modeling guidance available on the EPA's SCRAM Web site (section 2.3). Modeled concentrations should not be rounded before comparing the resulting design concentration to the NAAQS or PSD increments. Ambient monitoring and dispersion modeling address different issues and needs relative to each aspect of the overall air quality assessment.

c. The PSD increments for criteria pollutants are listed in 40 CFR 52.21(c) and 40 CFR 51.166(c). For short-term increments, these maximum allowable increases in pollutant concentrations may be exceeded once per year at each site, while the annual increment may not be exceeded. The highest, second-highest increase in estimated concentrations for the short-term averages as determined by a model should be less than or equal to the permitted increment. The modeled annual averages should not exceed the increment.

d. Receptor sites for refined dispersion modeling should be located within the modeling domain (section 8.1). In designing a receptor network, the emphasis should be placed on receptor density and location, not total number of receptors. Typically, the density of receptor sites should be progressively more resolved near the new or modifying source, areas of interest, and areas with the highest concentrations with sufficient detail to determine where possible violations of a NAAQS or PSD increment are most likely to occur. The placement of receptor sites should be determined on a case-by-case basis, taking into consideration the source characteristics, topography, climatology, and monitor sites. Locations of particular importance include: (1) The area of maximum impact of the point source; (2) the area of maximum impact of nearby sources; and (3) the area where all sources combine to cause maximum impact. Depending on the complexities of the source and the environment to which the source is located, a dense array of receptors may be required in some cases. In order to avoid unreasonably large computer runs due to an excessively large array of receptors, it is often desirable to model the area twice. The first model run would use a moderate number of receptors more resolved nearby the new or modifying source and over areas of interest. The second model run would modify the receptor network from the first model run with a denser array of receptors in areas showing potential for high concentrations and possible violations, as indicated by the results of the first model run. Accordingly, the EPA neither anticipates nor encourages that numerous iterations of modeling runs be made to continually refine the receptor network.

9.2.3 NAAQS and PSD Increments Compliance Demonstrations for New or Modified Sources

a. As described in this subsection, the recommended procedure for conducting either a NAAQS or PSD increment assessment under PSD permitting is a multi-stage approach that includes the following two stages:

i. The first stage is referred to as a single-source impact analysis, since only the new or modifying source is considered in the analysis. There are two possible levels of detail in conducting a single-source impact analysis with the model user beginning with use of a screening model and proceeding to use of a refined model as necessary.

ii. The second stage is referred to as a cumulative impact analysis, since it takes into account all sources affecting the air quality in an area. In addition to the project source impact, it includes consideration of background, which includes contributions from natural, nearby, and unknown sources.

b. Each stage involves increasing complexity and details, as required to fully demonstrate a new or modifying source will not cause of contribution to a violation of any NAAQS or PSD increment. As such, starting with a single-source impact analysis may alleviate the need for a more time consuming and comprehensive cumulative modeling analysis.

c. The single-source impact analysis, or first stage of an air quality analysis, begins by determining the potential of a proposed new or modifying source to cause or contribute to a NAAQS or PSD increment violation. In certain circumstances, a screening model or technique may be used instead of the preferred model because it will provide estimated worst-case ambient impacts from the proposed new or modifying source. If these worst case ambient concentration estimates indicate that there will not be a significant impact, then the analysis is sufficient for the required demonstration under PSD. If the ambient concentration estimates indicate that significant impacts may occur, then the use of a refined model to estimate the source's impact should be pursued. The refined modeling analysis should use a model or technique consistent with the Guideline (either a preferred model or technique or an alternative model or technique) and follow the requirements and recommendations for model inputs outlined in section 8. If the estimated ambient concentrations indicate that there will not be a significant impact, then the analysis is generally sufficient to demonstrate that the source will not cause or contribute to an exceedance. However, if the concentration estimates from the refined modeling analysis indicate that significant impacts may occur, then a cumulative impact analysis should be undertaken. The receptors that indicate the location of significant impacts should be used to define the modeling domain for use in the cumulative impact analysis (section 8.2.2).

d. The cumulative impact analysis, or the second stage of an air quality analysis, should be conducted with the same refined model or technique to characterize the project source and then include the appropriate background concentrations (section 8.3). The resulting design concentrations are used to determine whether the source will cause or contribute to a NAAQS or PSD increment violation. This determination should be based on: (1) The appropriate design concentration for each applicable NAAQS (and averaging period); and (2) the significance of the source's contribution, in a temporal and spatial sense, to any modeled violation, i.e., where and when the predicted design concentration is greater than the NAAQS. For PSD increment, the cumulative impact analysis should also consider the amount of the air quality increment that has already been consumed by other sources, or, conversely, whether increment has expanded relative to the baseline concentration. Therefore, the applicant should model the existing or permitted nearby increment-consuming and increment-expanding sources, rather than using past modeling analyses of those sources as part of background concentration. This would permit the use of newly acquired data or improved modeling techniques if such data and/or techniques have become available since the last source was permitted.

9.2.3.1 Considerations in Developing Emissions Limits

a. Emissions limits and resulting control requirements should be established to provide for compliance with each applicable NAAQS (and averaging period) and PSD increment. It is possible that multiple emissions limits will be required for a source to demonstrate compliance with several criteria pollutants (and averaging periods) and PSD increments. Case-by-case determinations must be made as to the appropriate form of the limits, i.e., whether the emissions limits restrict the emission factor (e.g., limiting lb/MMBTU), the emission rate (e.g., lb/hr), or both. The appropriate reviewing authority (paragraph 3.0(b)) and appropriate EPA guidance should be consulted to determine the appropriate emissions limits on a case-by-case basis.

9.2.4 Use of Measured Data in Lieu of Model Estimates

a. As described throughout the Guideline, modeling is the preferred method for demonstrating compliance with the NAAQS and PSD increments and for determining the most appropriate emissions limits for new and existing sources. When a preferred model or adequately justified and approved alternative model is available, model results, including the appropriate background, are sufficient for air quality demonstrations and establishing emissions limits, if necessary. In instances when the modeling technique available is only a screening technique, the addition of air quality monitoring data to the analysis may lend credence to the model results. However, air quality monitoring data alone will normally not be acceptable as the sole basis for demonstrating compliance with the NAAQS and PSD increments or for determining emissions limits.

b. There may be rare circumstances where the performance of the preferred air quality model will be shown to be less than reasonably acceptable when compared with air quality monitoring data measured in the vicinity of an existing source. Additionally, there may not be an applicable preferred air quality model, screening technique, or justifiable alternative model suitable for the situation. In these unique instances, there may be the possibility of establishing emissions limits and demonstrating compliance with the NAAQS and PSD increments solely on the basis of analysis of observed air quality data in lieu of an air quality modeling analysis. However, only in the case of a modification to an existing source should air quality monitoring data alone be a basis for determining adequate emissions limits or for demonstration that the modification will not cause or contribute to a violation of any NAAQS or PSD increment.

c. The following items should be considered prior to the acceptance of an analysis of measured air quality data as the sole basis for an air quality demonstration or determining an emissions limit:

i. Does a monitoring network exist for the pollutants and averaging times of concern in the vicinity of the existing source?

ii. Has the monitoring network been designed to locate points of maximum concentration?

iii. Do the monitoring network and the data reduction and storage procedures meet EPA monitoring and quality assurance requirements?

iv. Do the dataset and the analysis allow impact of the most important individual sources to be identified if more than one source or emission point is involved?

v. Is at least one full year of valid ambient data available?

vi. Can it be demonstrated through the comparison of monitored data with model results that available air quality models and techniques are not applicable?

c. Comprehensive air quality monitoring in the area affected by the existing source with proposed modifications will be necessary in these cases. Additional meteorological monitoring may also be necessary. The appropriate number of air quality and meteorological monitors from a scientific and technical standpoint is a function of the situation being considered. The source configuration, terrain configuration, and meteorological variations all have an impact on number and optimal placement of monitors. Decisions on the monitoring network appropriate for this type of analysis can only be made on a case-by-case basis.

d. Sources should obtain approval from the appropriate reviewing authority (paragraph 3.0(b)) and the EPA Regional Office for the monitoring network prior to the start of monitoring. A monitoring protocol agreed to by all parties involved is necessary to assure that ambient data are collected in a consistent and appropriate manner. The design of the network, the number, type, and location of the monitors, the sampling period, averaging time as well as the need for meteorological monitoring or the use of mobile sampling or plume tracking techniques, should all be specified in the protocol and agreed upon prior to start-up of the network.

e. Given the uniqueness and complexities of these rare circumstances, the procedures can only be established on a case-by-case basis for analyzing the source's emissions data and the measured air quality monitoring data and for projecting with a reasoned basis the air quality impact of a proposed modification to an existing source in order to demonstrate that emissions from the construction or operation of the modification will not cause or contribute to a violation of the applicable NAAQS and PSD increment, and to determine adequate emissions limits. The same attention should be given to the detailed analyses of the air quality data as would be applied to a comprehensive model performance evaluation. In some cases, the monitoring data collected for use in the performance evaluation of preferred air quality models, screening technique, or existing alternative models may help inform the development of a suitable new alternative model. Early coordination with the appropriate reviewing authority (paragraph 3.0(b)) and the EPA Regional Office is fundamental with respect to any potential use of measured data in lieu of model estimates.

10.0 References

1. Code of Federal Regulations; Title 40 (Protection of Environment); Part 51; Sections 51.112, 51.117, 51.150, 51.160.

2. Environmental Protection Agency, 1990. New Source Review Workshop Manual: Prevention of Significant Deterioration and Nonattainment Area Permitting (Draft). Office of Air Quality Planning & Standards, Research Triangle Park, NC. http://www.epa.gov/​nsr.

3. Code of Federal Regulations; Title 40 (Protection of Environment); Part 51; Sections 51.166 and 52.21.

4. Code of Federal Regulations; Title 40 (Protection of Environment); Part 93; Sections 93.116, 93.123, and 93.150.

5. Code of Federal Regulations; Title 40 (Protection of Environment); Part 58 (Ambient Air Quality Surveillance).

6. Code of Federal Regulations; Title 40 (Protection of Environment); Part 50 (National Primary and Secondary Ambient Air Quality Standards).

7. Baker, K.R., Kelly, J.T., 2014. Single source impacts estimated with photochemical model source sensitivity and apportionment approaches. Atmospheric Environment, 96: 266-274.

8. ENVIRON, 2012. Evaluation of Chemical Dispersion Models using Atmospheric Plume Measurements from Field Experiments. ENVIRON International, Corp., Novato, CA. Prepared under contract No. EP-D-07-102 for U.S. Environmental Protection Agency, Research Triangle Park, NC. http://www.epa.gov/​ttn/​scram/​reports/​Plume_​Eval_​Final_​Sep_​2012v5.pdf.

9. McMurry, P.H., Shepherd, M.F., Vickery, J.S., 2004. Particulate matter science for policy makers: A NARSTO assessment. Cambridge University Press.

10. Baker, K.R., Foley, K.M., 2011. A nonlinear regression model estimating single source concentrations of primary and secondarily formed PM_2.5. Atmospheric Environment, 45: 3758-3767.

11. Bergin, M.S., Russell, A.G., Odman, M.T., Cohan, D.S., Chameldes, W.L., 2008. Single-Source Impact Analysis Using Three-Dimensional Air Quality Models. Journal of the Air & Waste Management Association, 58: 1351-1359.

12. Zhou, W., Cohan, D.S., Pinder, R.W., Neuman, J.A., Holloway, J.S., Peischl, J., Ryerson, T.B., Nowak, J.B., Flocke, F., Zheng, W.G., 2012. Observation and modeling of the evolution of Texas power plant plumes. Atmospheric Chemistry and Physics, 12: 455-468.

13. Chen, J., Lu, J., Avise, J.C., DaMassa, J.A., Kleeman, M.J., Kaduwela, A.P., 2014. Seasonal modeling of PM 2.5 in California's San Joaquin Valley. Atmospheric Environment, 92: 182-190.

14. Russell, A.G., 2008. EPA Supersites program-related emissions-based particulate matter modeling: initial applications and advances. Journal of the Air & Waste Management Association, 58: 289-302.

15. Tesche, T., Morris, R., Tonnesen, G., McNally, D., Boylan, J., Brewer, P., 2006. CMAQ/CAMx annual 2002 performance evaluation over the eastern US. Atmospheric Environment, 40: 4906-4919.

16. Fox, D.G., 1984. Uncertainty in air quality modeling. Bulletin of the American Meteorological Society, 65(1): 27-36.

17. Bowne, NE., 1981. Validation and Performance Criteria for Air Quality Models. Appendix F in Air Quality Modeling and the Clean Air Act: Recommendations to EPA on Dispersion Modeling for Regulatory Applications. American Meteorological Society, Boston, MA; pp. 159-171. (Docket No. A-80-46, II-A-106).

18. Fox, D.G., 1981. Judging Air Quality Model Performance. Bulletin of the American Meteorological Society, 62(5): 599-609.

19. Simon, H., Baker, K.R., Phillips, S., 2012. Compilation and interpretation of photochemical model performance statistics published between 2006 and 2012. Atmospheric Environment, 61: 124-139.

20. Burton, C.S., 1981. The Role of Atmospheric Models in Regulatory Decision-Making: Summary Report. Systems Applications, Inc., San Rafael, CA. Prepared under contract No. 68-01-5845 for U.S. Environmental Protection Agency, Research Triangle Park, NC. (Docket No. A-80-46, II-M-6).

21. “Ten years of Harmonisation activities: Past, present and future” at http://www.dmu.dk/​AtmosphericEnvironment/​Harmoni/​Conferences/​Belgirate/​BelgiratePapers.asp.

22. Weil, Sykes, and Venkatram, 1992. Evaluating Air-Quality Models: Review and Outlook. Journal of Applied Meteorology, 31: 1121-1145.

23. Environmental Protection Agency, 1988. Model Clearinghouse: Operational Plan (Revised). Staff Report. Office of Air Quality Planning & Standards, Research Triangle Park, NC. (Docket No. A-88-04, II-J-1)

24. American Meteorological Society, 1983. Synthesis of the Rural Model Reviews. Publication No. EPA-600/3-83-108. Office of Research & Development, Research Triangle Park, NC. (NTIS No. PB 84-121037)

25. Hanna, S., M. Garrison and B. Turner, 1998. AERMOD Peer Review report. Prepared by SAI, Inc. under EPA Contract No. 68-D6-0064/1-14 for Environmental Protection Agency, Research Triangle Park, NC. 12pp. & appendices. (Docket No. A-99-05, II-A-6)

26. Scire, J.S. and L.L. Schulman, 1981. Evaluation of the BLP and ISC Models with SF6 Tracer Data and SO₂ Measurements at Aluminum Reduction Plants. APCA Specialty Conference on Dispersion Modeling for Complex Sources, St. Louis, MO.

27. Environmental Protection Agency, 2003. AERMOD: Latest Features and Evaluation Results. Publication No. EPA-454/R-03-003. Office of Air Quality Planning & Standards, Research Triangle Park, NC. http://www.epa.gov/​ttn/​scram/​7thconf/​aermod/​aermod_​mep.pdf.

28. ASTM D6589: Standard Guide for Statistical Evaluation of Atmospheric Dispersion Model Performance. (2010).

29. Environmental Protection Agency, 1992. Protocol for Determining the Best Performing Model. Publication No. EPA-454/R-92-025. Office of Air Quality Planning & Standards, Research Triangle Park, NC. http://www.epa.gov/​ttn/​scram/​guidance/​guide/​modleval.zip. (NTIS No. PB 93-226082)

30. Hanna, S.R., 1982. Natural Variability of Observed Hourly SO₂ and CO Concentrations in St. Louis. Atmospheric Environment, 16(6): 1435-1440.

31. Pasquill, F., 1974. Atmospheric Diffusion, 2nd Edition. John Wiley and Sons, New York, NY; 479pp.

32. Rhoads, R.G., 1981. Accuracy of Air Quality Models. Staff Report. Office of Air Quality Planning & Standards, Research Triangle Park, NC. (Docket No. A-80-46, II-G-6)

33. Hanna, S.R., 1993. Uncertainties in air quality model predictions. Boundary-Layer Meteorology, 62: 3-20.

34. Hanna, S.R., 1989. Confidence limits for air quality model evaluations, as estimated by bootstrap and jackknife resampling methods. Atmospheric Environment, 23(6): 1385-1398.

35. Cox, W.M. and J.A. Tikvart, 1990. A statistical procedure for determining the best performing air quality simulation model. Atmospheric Environment, 24A(9): 2387-2395.

36. Environmental Protection Agency, 2015. Technical Support Document (TSD) for AERMOD-Based Assessments of Long-Range Transport Impacts for Primary Pollutants. Publication No. EPA-454/B-15-003. Office of Air Quality Planning & Standards, Research Triangle Park, NC.

37. Environmental Protection Agency, 2015: AERSCREEN User's Guide. Publication No. EPA-454-/B-15-005. Office of Air Quality Planning & Standards, Research Triangle Park, NC. http://www.epa.gov/​ttn/​scram/​dispersion_​screening.htm#aerscreen.

38. Environmental Protection Agency, 2011: AERSCREEN Released as the EPA Recommended Screening Model. Tyler Fox Memorandum dated April 11, 2011, Office of Air Quality Planning & Standards, Research Triangle Park, NC. http://www.epa.gov/​ttn/​scram/​20110411_​AERSCREEN_​Release_​Memo.pdf .

39. Perry, S.G., D.J. Burns and A.J. Cimorelli, 1990. User's Guide to CTDMPLUS: Volume 2. The Screening Mode (CTSCREEN). Publication No. EPA-600/8-90-087. U.S. Environmental Protection Agency, Research Triangle Park, NC. http://www.epa.gov/​ttn/​scram/​dispersion_​prefrec.htm#ctdmplus. (NTIS No. PB 91-136564)

40. Environmental Protection Agency, 1992. Screening Procedures for Estimating the Air Quality Impact of Stationary Sources, Revised. Publication No. EPA-454/R-92-019. Office of Air Quality Planning & Standards, Research Triangle Park, NC. (NTIS No. PB 93-219095)

41. Burns, D.J., S.G. Perry and A.J. Cimorelli, 1991. An Advanced Screening Model for Complex Terrain Applications. Paper presented at the 7th Joint Conference on Applications of Air Pollution Meteorology (cosponsored by the American Meteorological Society and the Air & Waste Management Association), January 13-18, 1991, New Orleans, LA.

42. Mills, M.T., R.J. Paine, E.A. Insley and B.A. Egan, 1987. The Complex Terrain Dispersion Model Terrain Preprocessor System—User's Guide and Program Description. Publication No. EPA-600/8-88-003. U.S. Environmental Protection Agency, Research Triangle Park, NC. (NTIS No. PB 88-162094)

43. Environmental Research and Technology, 1987. User's Guide to the Rough Terrain Diffusion Model (RTDM), Rev. 3.20. ERT Document No. P-D535-585. Environmental Research and Technology, Inc., Concord, MA. (NTIS No. PB 88-171467)

44. Environmental Protection Agency, 2004. AERMOD: Description of Model Formulation. Publication No. EPA-454/R-03-004. Office of Air Quality Planning & Standards, Research Triangle Park, NC; September 2004. http://www.epa.gov/​ttn/​scram/​7thconf/​aermod/​aermod_​mfd.pdf.

45. Cimorelli, A. et al., 2005. AERMOD: A Dispersion Model for Industrial Source Applications. Part I: General Model Formulation and Boundary Layer Characterization. Journal of Applied Meteorology, 44(5): 682-693.

46. L.L. Schulman, D.G. Strimaitis and J.S. Scire, 2002. Development and evaluation of the PRIME plume rise and building downwash model. Journal of the Air & Waste Management Association, 50: 378-390.

47. Environmental Protection Agency, 2015, Technical Support Document (TSD) for Replacement of CALINE3 with AERMOD for Transportation Related Air Quality Analyses. Publication No. EPA-454/B-15-002. Office of Air Quality Planning & Standards, Research Triangle Park, NC.

48. Environmental Protection Agency, 1997. Guidance for Siting Ambient Air Monitors around Stationary Lead Sources. Publication No. EPA-454/R-92-009R. Office of Air Quality Planning & Standards, Research Triangle Park, NC. (NTIS No. PB 97-208094)

49. LEADPOST processor: http://www.epa.gov/​ttn/​scram/​models/​aermod/​leadpost.zip.

50. Environmental Protection Agency, 1993. Lead Guideline Document. Publication No. EPA-452/R-93-009. Office of Air Quality Planning & Standards, Research Triangle Park, NC. (NTIS No. PB 94-111846)

51. U.S. EPA, 2014: Guidance for 1-Hour SO₂ Nonattaiment Area SIP Submissions. Stephen D. Page Memorandum dated April 23, 2011, Office of Air Quality Planning & Standards, Research Triangle Park, NC. http://www.epa.gov/​airquality/​sulfurdioxide/​pdfs/​20140423guidance.pdf.

52. U.S. EPA, 2013: SO₂ NAAQS Designations Modeling Technical Assistance Document. Office of Air Quality Planning & Standards, Research Triangle Park, NC. http://www.epa.gov/​airquality/​sulfurdioxide/​pdfs/​SO2ModelingTAD.pdf.

53. Turner, D.B., 1964. A Diffusion Model for an Urban Area. Journal of Applied Meteorology, 3(1):83-91.

54. Environmental Protection Agency, 2015. Technical Support Document (TSD) for NO₂-Related AERMOD Options and Modifications, Publication No. EPA-454/B-15-004. Office of Air Quality Planning & Standards, Research Triangle Park, NC.

55. Podrez, M. 2015. An Update to the Ambient Ratio Method for 1-h NO₂ Air Quality Standards Dispersion Modeling. Atmospheric Environment, 103: 163-170.

56. Cole, H.S. and J.E. Summerhays, 1979. A Review of Techniques Available for Estimation of Short-Term NO₂ Concentrations. Journal of the Air Pollution Control Association, 29(8): 812-817.

57. Hanrahan, P.L., 1999. The Polar Volume Polar Ratio Method for Determining NO₂/NO_X Ratios in Modeling—Part I: Methodology. Journal of the Air & Waste Management Association, 49: 1324-1331.

58. Environmental Protection Agency, 2004. The Particle Pollution Report, Publication No. EPA-454-R-04-002. Office of Air Quality Planning & Standards, Research Triangle Park, NC. http://www.epa.gov/​airtrends/​aqtrnd04/​pmreport03/​report_​2405.pdf#page=​1.

59. Environmental Protection Agency, 2014. Guidance for PM_2.5 Modeling. May 20, 2014, Publication No. EPA-454/B-14-001. Office of Air Quality Planning & Standards, Research Triangle Park, NC. http://www.epa.gov/​ttn/​scram/​guidance/​guide/​Guidance_​for_​PM25_​Permit_​Modeling.pdf.

60. Environmental Protection Agency, 2014. Modeling Guidance for Demonstrating Attainment of Air Quality Goals for Ozone, PM_2.5, and Regional Haze. Office of Air Quality Planning & Standards, Research Triangle Park, NC. http://www.epa.gov/​ttn/​scram/​guidance/​guide/​Draft_​O3-PM-RH_​Modeling_​Guidance-2014.pdf.

61. Environmental Protection Agency, 2013. Transportation Conformity Guidance for Quantitative Hot-Spot Analyses in PM_2.5 and PM₁₀ Nonattainment and Maintenance Areas. Publication No. EPA-420-B-053. Office of Transportation and Air Quality, Ann Arbor, MI. http://www.epa.gov/​otaq/​stateresources/​transconf/​policy/​420b13053-sec.pdf.

62. Environmental Protection Agency, 1987. PM₁₀ SIP Development Guideline. Publication No. EPA-450/2-86-001. Office of Air Quality Planning & Standards, Research Triangle Park, NC. (NTIS No. PB 87-206488)

63. Environmental Protection Agency, 2012. Memorandum: Haul Road Workgroup Final Report Submission to EPA-OAQPS. March 2, 2012. Office of Air Quality Planning & Standards, Research Triangle Park, NC. http://www.epa.gov/​ttn/​scram/​reports/​Haul_​Road_​Workgroup-Final_​Report_​Package-20120302.pdf .

64. Seinfeld, J.H., Pandis, S.N., 2012. Atmospheric chemistry and physics: from air pollution to climate change. John Wiley & Sons.

65. Simon, H., Baker, K.R., Phillips, S., 2012. Compilation and interpretation of photochemical model performance statistics published between 2006 and 2012. Atmospheric Environment, 61, 124-139.

66. Environmental Protection Agency, 2015. Guidance on the use of models for assessing the impacts from single sources on secondarily formed pollutants ozone and PM_2.5. Publication No. EPA 454/P-15-001. Office of Air Quality Planning & Standards, Research Triangle Park, NC.

67. U.S. Department of the Interior, 2010. Federal Land Managers' Air Quality Related Values Work Group (FLAG) Phase I Report—Revised 2010. http://www.nature.nps.gov/​air/​pubs/​pdf/​flag/​FLAG_​2010.pdf. Natural Resource Report NPS/NPRC/NRR-2010/232.

68. National Acid Precipitation Assessment Program (NAPAP), 1991. Acid Deposition: State of Science and Technology. Volume III Terrestrial, Materials, Health and Visibility Effects. Report 24, Visibility: Existing and Historical Conditions—Causes and Effects. Edited by Patricia M. Irving. Washington, DC 129pp.

69. National Research Council, 1993. Protecting Visibility in National Parks and Wilderness Areas. National Academy Press, Washington, DC 446pp.

70. U.S. Environmental Protection Agency, 1992. Workbook for plume visual impact screening and analysis (revised). Publication No. EPA-454/R-92-023. Office of Air Quality Planning & Standards, Research Triangle Park, NC (NTIS No. PB 93-223592).

71. Nilsson, J., Grennfelt, P., Ministerråd, N., 1988. Critical Loads for Sulphur and Nitrogen: Report from a Workshop Held at Skokloster, Sweden, 19-24 March, 1988. Nordic Council of Ministers.

72. Turner, D.B., 1969. Workbook of Atmospheric Dispersion Estimates. PHS Publication No. 999-AP-26. U.S. Department of Health, Education and Welfare, Public Health Service, Cincinnati, OH. (NTIS No. PB-191482)

73. McElroy, J.L. and F. Pooler, Jr., 1968. St. Louis Dispersion Study, Volume II— Analysis. National Air Pollution Control Administration Publication No. AP-53, U.S. Department of Health, Education and Welfare, Public Health Service, Arlington, VA. (NTIS No. PB-190255)

74. Irwin, J.S., 1978. Proposed Criteria for Selection of Urban Versus Rural Dispersion Coefficients. (Draft Staff Report). Meteorology and Assessment Division, U.S. Environmental Protection Agency, Research Triangle Park, NC. (Docket No. A-80-46, II-B-8)

75. Auer, Jr., A.H., 1978. Correlation of Land Use and Cover with Meteorological Anomalies. Journal of Applied Meteorology, 17(5): 636-643.

76. Environmental Protection Agency, 2015. AERMOD Implementation Guide. U.S. Office of Air Quality Planning & Standards, Research Triangle Park, NC. http://www.epa.gov/​ttn/​scram/​dispersion_​prefrec.htm#aermod.

77. Pasquill, F., 1976. Atmospheric Dispersion Parameters in Gaussian Plume Modeling, Part II. Possible Requirements for Change in the Turner Workbook Values. Publication No. EPA-600/4-76-030b. Office of Research & Development, Research Triangle Park, NC. (NTIS No. PB-258036/3BA)

78. Stull, R.B., 1988. An Introduction to Boundary Layer Meteorology. Kluwer Academic Publishers, Boston, MA. 666pp.

79. Environmental Protection Agency, 1987. Analysis and Evaluation of Statistical Coastal Fumigation Models. Publication No. EPA-450/4-87-002. Office of Air Quality Planning & Standards, Research Triangle Park, NC. (NTIS No. PB 87-175519)

80. Wesely, M.L, P.V. Doskey, and J.D. Shannon, 2002: Deposition Parameterizations for the Industrial Source Complex (ISC3) Model. Draft ANL report ANL/ER/TRB01/003, DOE/xxnnnn, Argonne National Laboratory, Argonne, Illinois 60439.

81. Environmental Protection Agency, 1981. Guideline for Use of Fluid Modeling to Determine Good Engineering Practice (GEP) Stack Height. Publication No. EPA-450/4-81-003. Office of Air Quality Planning & Standards, Research Triangle Park, NC. (NTIS No. PB 82-145327)

82. Lawson, Jr., R.E. and W.H. Snyder, 1983. Determination of Good Engineering Practice Stack Height: A Demonstration Study for a Power Plant. Publication No. EPA-600/3-83-024. Office of Research & Development, Research Triangle Park, NC. (NTIS No. PB 83-207407)

83. Environmental Protection Agency, 1985. Guideline for Determination of Good Engineering Practice Stack Height (Technical Support Document for the Stack Height Regulations), Revised. Publication No. EPA-450/4-80-023R. Office of Air Quality Planning & Standards, Research Triangle Park, NC. http://www.epa.gov/​ttn/​scram/​guidance/​guide/​gep.pdf. (NTIS No. PB 85-225241)

84. Snyder, W.H. and R.E. Lawson, Jr., 1985. Fluid Modeling Demonstration of Good Engineering-Practice Stack Height in Complex Terrain. Publication No. EPA-600/3-85-022. Office of Research & Development, Research Triangle Park, NC. (NTIS No. PB 85-203107)

85. Briggs, G.A., 1975. Plume Rise Predictions. Chapter 3 in Lectures on Air Pollution and Environmental Impact Analyses. American Meteorological Society, Boston, MA; pp. 59-111.

86. Hanna, S.R., G.A. Briggs and R.P. Hosker, Jr., 1982. Plume Rise. Chapter 2 in Handbook on Atmospheric Diffusion. Technical Information Center, U.S. Department of Energy, Washington, DC; pp. 11-24. DOE/TIC-11223 (DE 82002045)

87. Weil, J.C., L.A. Corio and R.P. Brower, 1997. A PDF dispersion model for buoyant plumes in the convective boundary layer. Journal of Applied Meteorology, 36: 982-1003.

88. L.L. Schulman, D.G. Strimaitis and J.S. Scire, 2002. Development and evaluation of the PRIME plume rise and building downwash model. Journal of the Air & Waste Management Association, 50: 378-390.

89. Environmental Protection Agency, 1995. Compilation of Air Pollutant Emission Factors, Volume I: Stationary Point and Area Sources (Fifth Edition, AP-42: GPO Stock No. 055-000-00500-1), and Supplements A- D; Volume II: Mobile Sources (Fifth Edition). Office of Air Quality Planning & Standards, Research Triangle Park, NC. Volume I can be downloaded from EPA's Web site at http://www.epa.gov/​ttn/​chief/​ap42.html; Volume II can be downloaded from http://www.epa.gov/​omswww/​ap42.htm .

90. Environmental Protection Agency, 2014. Draft Emissions Inventory Guidance for Implementation of Ozone and Particulate Matter National Ambient Air Quality Standards (NAAQS) and Regional Haze Regulations. Office of Air Quality Planning & Standards, Research Triangle Park, NC. http://www.epa.gov/​ttn/​chief/​eidocs/​eiguid/​2014revisedeiguidance.pdf.

91. Environmental Protection Agency, 1987. Ambient Air Monitoring Guidelines for Prevention of Significant Deterioration (PSD). Publication No. EPA-450/4-87-007. Office of Air Quality Planning & Standards, Research Triangle Park, NC. (NTIS No. PB 90-168030)

92. Environmental Protection Agency, 2011. Additional Clarification Regarding Application of Appendix W Modeling Guidance for the 1-hour NO₂ National Ambient Air Quality Standard. Office of Air Quality Planning & Standards, Research Triangle Park, NC. http://www.epa.gov/​ttn/​scram/​guidance/​clarification/​Additional_​Clarifications_​AppendixW_​Hourly-NO2-NAAQS_​FINAL_​03-01-2011.pdf.

93. Environmental Protection Agency, 2014. Modeling Guidance For Demonstrating Attainment of Air Quality Goals for Ozone, PM_2.5, and Regional Haze (Draft). U.S. Office of Air Quality Planning & Standards, Research Triangle Park, NC. http://www.epa.gov/​ttn/​scram/​guidance/​guide/​Draft_​O3-PM-RH_​Modeling_​Guidance-2014.pdf.

94. Environmental Protection Agency, 2013. Memorandum: Use of ASOS meteorological data in AERMOD dispersion modeling. March 8, 2013. Office of Air Quality Planning & Standards, Research Triangle Park, NC. http://www.epa.gov/​ttn/​scram/​guidance/​clarification/​20130308_​Met_​Data_​Clarification.pdf.

95. Environmental Protection Agency, 2004. User's Guide for the AERMOD Meteorological Preprocessor (AERMET). Publication No. EPA-454/B-03-002. Office of Air Quality Planning & Standards, Research Triangle Park, NC. http://www.epa.gov/​ttn/​scram/​metobsdata_​procaccprogs.htm#aermet.

96. U.S Environmental Protection Agency. AERMINUTE User's Guide. Office of Air Quality Planning & Standards, Research Triangle Park, NC. http://www.epa.gov/​ttn/​scram/​metobsdata_​procaccprogs.htm#aermet.

97. Environmental Protection Agency, 1993. PCRAMMET User's Guide. Publication No. EPA-454/R-96-001. Office of Air Quality Planning & Standards, Research Triangle Park, NC. (NTIS No. PB 97-147912)

98. Environmental Protection Agency, 1996. Meteorological Processor for Regulatory Models (MPRM). Publication No. EPA-454/R-96-002. Office of Air Quality Planning & Standards, Research Triangle Park, NC. (NTIS No. PB 96-180518)

99. Paine, R.J., 1987. User's Guide to the CTDM Meteorological Preprocessor Program. Publication No. EPA-600/8-88-004. Office of Research & Development, Research Triangle Park, NC. (NTIS No. PB-88-162102)

100. Perry, S.G., D.J. Burns, L.H. Adams, R.J. Paine, M.G. Dennis, M.T. Mills, D.G. Strimaitis, R.J. Yamartino and E.M. Insley, 1989. User's Guide to the Complex Terrain Dispersion Model Plus Algorithms for Unstable Situations (CTDMPLUS). Volume 1: Model Descriptions and User Instructions. EPA Publication No. EPA-600/8-89-041. Environmental Protection Agency, Research Triangle Park, NC. http://www.epa.gov/​ttn/​scram/​dispersion_​prefrec.htm#ctdmplus. (NTIS No. PB 89-181424)

101. Environmental Protection Agency, 2008. AERSURFACE User's Guide. Publication No. EPA-454/B-08-001. Office of Air Quality Planning & Standards, Research Triangle Park, NC. http://www.epa.gov/​ttn/​scram/​dispersion_​related.htm#aersurface.

102. Wesson, K., R. Brode, W. Peters, and C. Tillerson, 2006. AERMOD Sensitivity to the Choice of Surface Characteristics. A&WMA presentation.

103. Environ, 2014. The Mesocale Model Interface Program (MMIF) Version 3.1 User's Manual.

104. Environmental Protection Agency, 2015 Guidance on the Use of the Mesoscale Model Interface Program (MMIF) for AERMOD Applications. Publication No. EPA-454/B-15-001. Office of Air Quality Planning & Standards, Research Triangle Park, NC.

105. Solar and Meteorological Surface Observation Network, 1961-1990; 3-volume CD-ROM. Version 1.0, September 1993. Produced jointly by National Climatic Data Center and National Renewable Energy Laboratory. Can be ordered from NOAA National Data Center's Web site at http://www.nndc.noaa.gov.

106. Hourly United States Weather Observations, 1990-1995 (CD-ROM). October 1997. Produced jointly by National Climatic Data Center and Environmental Protection Agency. Can be ordered from NOAA National Data Center's Web site at http://www.ncdc.noaa.gov.

107. Radiosonde Data of North America, 1946-1996; 4-volume CD-ROM. August1996. Produced jointly by Forecast Systems laboratory and National Climatic Data Center. Can be ordered from NOAA National Data Center's Web site at http://lwf.ncdc.noaa.gov/​oa/​ncdc.html.

108. Environmental Protection Agency, 2000. Meteorological Monitoring Guidance for Regulatory Modeling Applications. Publication No. EPA-454/R-99-005. Office of Air Quality Planning & Standards, Research Triangle Park, NC. (NTIS No. PB 2001-103606)

109. ASTM D5527: Standard Practice for Measuring Surface Winds and Temperature by Acoustic Means. (2011)

110. ASTM D5741: Standard Practice for Characterizing Surface Wind Using Wind Vane and Rotating Anemometer. (2011)

111. Environmental Protection Agency, 1995. Quality Assurance for Air Pollution Measurement Systems, Volume IV— Meteorological Measurements. Publication No. EPA600/R-94/038d. Office of Air Quality Planning & Standards, Research Triangle Park, NC. Note: For copies of this handbook, you may make inquiry to ORD Publications, 26 West Martin Luther King Dr., Cincinnati, OH 45268.

112. Bowen, B.M., J.M. Dewart and A.I. Chen, 1983. Stability Class Determination: A Comparison for One Site. Proceedings, Sixth Symposium on Turbulence and Diffusion. American Meteorological Society, Boston, MA; pp. 211-214. (Docket No. A-92-65, II-A-7)

113. Environmental Protection Agency, 1993. An Evaluation of a Solar Radiation/Delta-T (SRDT) Method for Estimating Pasquill-Gifford (P-G) Stability Categories. Publication No. EPA-454/R-93-055. Office of Air Quality Planning & Standards, Research Triangle Park, NC. (NTIS No. PB 94-113958)

114. Irwin, J.S., 1980. Dispersion Estimate Suggestion #8: Estimation of Pasquill Stability Categories. Office of Air Quality Planning & Standards, Research Triangle Park, NC. (Docket No. A-80-46, II-B-10)

115. Mitchell, Jr., A.E. and K.O. Timbre, 1979. Atmospheric Stability Class from Horizontal Wind Fluctuation. Presented at 72nd Annual Meeting of Air Pollution Control Association, Cincinnati, OH; June 24-29, 1979. (Docket No. A-80-46, II-P-9)

116. Smedman—Hogstrom, A. and V. Hogstrom, 1978. A Practical Method for Determining Wind Frequency Distributions for the Lowest 200m from Routine Meteorological Data. Journal of Applied Meteorology, 17(7): 942-954.

117. Smith, T.B. and S.M. Howard, 1972. Methodology for Treating Diffusivity. MRI 72 FR-1030. Meteorology Research, Inc., Altadena, CA. (Docket No. A-80-46, II-P-8)

118. Environmental Protection Agency, 2015. Evaluation of Prognostic Meteorological Data in AERMOD Applications. Publication No. EPA-454/R-15-004. Office of Air Quality Planning & Standards, Research Triangle Park, NC.

119. Environmental Protection Agency, 1984. Calms Processor (CALMPRO) User's Guide. Publication No. EPA-901/9-84-001. Office Of Air Quality Planning & Standards, Region I, Boston, MA. (NTIS No. PB 84-229467)

Appendix A to Appendix W of Part 51—Summaries of Preferred Air Quality Models

Table of Contents

A.0  Introduction and Availability

A.1  AERMOD (AMS/EPA Regulatory Model)

A.2  CTDMPLUS (Complex Terrain Dispersion Model Plus Algorithms for Unstable Situations)

A.3  OCD (Offshore and Coastal Dispersion Model)

A.0  Introduction and Availability

(1) This appendix summarizes key features of refined air quality models preferred for specific regulatory applications. For each model, information is provided on availability, approximate cost (where applicable), regulatory use, data input, output format and options, simulation of atmospheric physics, and accuracy. These models may be used without a formal demonstration of applicability provided they satisfy the recommendations for regulatory use; not all options in the models are necessarily recommended for regulatory use.

(2) Many of these models have been subjected to a performance evaluation using comparisons with observed air quality data. Where possible, several of the models contained herein have been subjected to evaluation exercises, including (1) statistical performance tests recommended by the American Meteorological Society and (2) peer scientific reviews. The models in this appendix have been selected on the basis of the results of the model evaluations, experience with previous use, familiarity of the model to various air quality programs, and the costs and resource requirements for use.

(3) Codes and documentation for all models listed in this appendix are available from the EPA's Support Center for Regulatory Air Models (SCRAM) Web site at http://www.epa.gov/​ttn/​scram. Codes and documentation may also available from the National Technical Information Service (NTIS), http://www.ntis.gov, and, when available, is referenced with the appropriate NTIS accession number.

A.1  AERMOD (AMS/EPA Regulatory Model)

References

Environmental Protection Agency, 2004. AERMOD: Description of Model Formulation. Publication No. EPA-454/R-03-004. Office of Air Quality Planning & Standards, Research Triangle Park, NC; September 2004. http://www.epa.gov/​ttn/​scram/​7thconf/​aermod/​aermod_​mfd.pdf.

Cimorelli, A., et al., 2005. AERMOD: A Dispersion Model for Industrial Source Applications. Part I: General Model Formulation and Boundary Layer Characterization. Journal of Applied Meteorology, 44(5): 682-693.

Perry, S. et al., 2005. AERMOD: A Dispersion Model for Industrial Source Applications. Part II: Model Performance against 17 Field Study Databases. Journal of Applied Meteorology, 44(5): 694-708.

Environmental Protection Agency, 2004. User's Guide for the AMS/EPA Regulatory Model—AERMOD. Publication No. EPA-454/B-03-001. Office of Air Quality Planning & Standards, Research Triangle Park, NC; September 2004. http://www.epa.gov/​ttn/​scram/​dispersion_​prefrec.htm#aermod.

Environmental Protection Agency, 2004. User's Guide for the AERMOD Meteorological Preprocessor (AERMET). Publication No. EPA-454/B-03-002. Office of Air Quality Planning & Standards, Research Triangle Park, NC; November 2004. http://www.epa.gov/​ttn/​scram/​metobsdata_​procaccprogs.htm#aermet.

User's Guide for the AERMOD Terrain Preprocessor (AERMAP). Publication No. EPA-454/B-03-003. Office of Air Quality Planning & Standards, Research Triangle Park, NC; October 2004. http://www.epa.gov/​ttn/​scram/​dispersion_​related.htm#aermap.

Schulman, L. L., D.G. Strimaitis and J.S. Scire, 2000. Development and evaluation of the PRIME plume rise and building downwash model. Journal of the Air and Waste Management Association, 50: 378-390.

Schulman, L. L., and Joseph S. Scire, 1980. Buoyant Line and Point Source (BLP) Dispersion Model User's Guide. Document P-7304B. Environmental Research and Technology, Inc., Concord, MA. (NTIS No. PB 81-164642).

Availability

The model codes and associated documentation are available on EPA's SCRAM Web site (paragraph A.0(3)).

Abstract

AERMOD is a steady-state plume dispersion model for assessment of pollutant concentrations from a variety of sources. AERMOD simulates transport and dispersion from multiple point, area, or volume sources based on an up-to-date characterization of the atmospheric boundary layer. Sources may be located in rural or urban areas, and receptors may be located in simple or complex terrain. AERMOD accounts for building wake effects (i.e., plume downwash) based on the PRIME building downwash algorithms. The model employs hourly sequential preprocessed meteorological data to estimate concentrations for averaging times from 1-hour to 1-year (also multiple years). AERMOD can be used to estimate the concentrations of nonreactive pollutants from highway traffic. AERMOD also handles unique modeling problems associated with aluminum reduction plants, and other industrial sources where plume rise and downwash effects from stationary buoyant line sources are important. AERMOD is designed to operate in concert with two pre-processor codes: AERMET processes meteorological data for input to AERMOD, and AERMAP processes terrain elevation data and generates receptor and hill height information for input to AERMOD.

a. Recommendations for Regulatory Use

(1) AERMOD is appropriate for the following applications:

• Point, volume, and area sources;
• Buoyant, elevated line sources (e.g., aluminum reduction plants);
• Mobile (line) sources;
• Surface, near-surface, and elevated releases;
• Rural or urban areas;
• Simple and complex terrain;
• Transport distances over which steady- state assumptions are appropriate, up to 50km;
• 1-hour to annual averaging times; and
• Continuous toxic air emissions.

(2) For regulatory applications of AERMOD, the regulatory default option should be set, i.e., the parameter DFAULT should be employed in the MODELOPT record in the Control Pathway. The DFAULT option requires the use of terrain elevation data, stack-tip downwash, sequential date checking, and does not permit the use of the model in the SCREEN mode. In the regulatory default mode, pollutant half-life or decay options are not employed, except in the case of an urban source of sulfur dioxide where a four-hour half-life is applied. Terrain elevation data from the U.S. Geological Survey 7.5-Minute Digital Elevation Model (DEM) or equivalent (approx. 30-meter resolution) should be used in all applications. Starting in 2011, data from the National Elevation Dataset (NED, http://ned.usgs.gov) can also be used in AERMOD, which includes a range of resolutions, ranging from 1-m to 2 arc seconds and such high resolution would always be preferred. In some cases, exceptions of the terrain data requirement may be made in consultation with the appropriate reviewing authority (paragraph 3.0(b)).

b. Input Requirements

(1) Source data: Required input includes source type, location, emission rate, stack height, stack inside diameter, stack gas exit velocity, stack gas temperature, area and volume source dimensions, and source elevation. Building dimensions and variable emission rates are optional. Buoyant line sources require coordinates of the end points of the line, release height, emission rate, average line source width, average building width, average spacing between buildings, and average line source buoyancy parameter. For mobile sources, traffic volume; emission factor, source height, and mixing zone width are needed.

(2) Meteorological data: The AERMET meteorological preprocessor requires input of surface characteristics, including surface roughness (zo), Bowen ratio, and albedo, as well as, hourly observations of wind speed between 7zo and 100m (reference wind speed measurement from which a vertical profile can be developed), wind direction, cloud cover, and temperature between zo and 100m (reference temperature measurement from which a vertical profile can be developed). Meteorological data can be in the form of observed data or prognostic modeled data as discussed in paragraph 8.4.1(d). Surface characteristics may be varied by wind sector and by season or month. When using observed meteorological data, a morning sounding (in National Weather Service format) from a representative upper air station is required. Latitude, longitude, and time zone of the surface, site-specific (if applicable) and upper air meteorological stations are required. The wind speed starting threshold is also required in AERMET for applications involving site-specific data). When using prognostic data, modeled profiles of temperature and winds are input into AERMET. These can be hourly or a time that represents a morning sounding. Additionally, measured profiles of wind, temperature, vertical and lateral turbulence may be required in certain applications (e.g., in complex terrain) to adequately represent the meteorology affecting plume transport and dispersion. Optionally, measurements of solar, or net radiation may be input to AERMET. Two files are produced by the AERMET meteorological preprocessor for input to the AERMOD dispersion model. When using observed data, the surface file contains observed and calculated surface variables, one record per hour. For applications with multi-level site-specific meteorological data, the profile contains the observations made at each level of the meteorological tower (or remote sensor). When using prognostic data, the surface file contains surface variables calculated by the prognostic model and AERMET. The profile file contains the observations made at each level of a meteorological tower (or remote sensor), the one-level observations taken from other representative data (e.g., National Weather Service surface observations), one record per level per hour, or in the case of prognostic data, the prognostic modeled values of temperature and winds at user-specified levels.

(i) Data used as input to AERMET should possess an adequate degree of representativeness to insure that the wind, temperature and turbulence profiles derived by AERMOD are both laterally and vertically representative of the source area. The adequacy of input data should be judged independently for each variable. The values for surface roughness, Bowen ratio, and albedo should reflect the surface characteristics in the vicinity of the meteorological tower or representative grid cell when using prognostic data, and should be adequately representative of the modeling domain. Finally, the primary atmospheric input variables including wind speed and direction, ambient temperature, cloud cover, and a morning upper air sounding should also be adequately representative of the source area, when using observed data.

(ii) For recommendations regarding the length of meteorological record needed to perform a regulatory analysis with AERMOD, see section 8.4.2.

(3) Receptor data: Receptor coordinates, elevations, height above ground, and hill height scales are produced by the AERMAP terrain preprocessor for input to AERMOD. Discrete receptors and/or multiple receptor grids, Cartesian and/or polar, may be employed in AERMOD. AERMAP requires input of DEM terrain data produced by the U.S. Geological Survey (USGS), or other equivalent data. AERMAP can be used optionally to estimate source elevations.

c. Output

Printed output options include input information, high concentration summary tables by receptor for user-specified averaging periods, maximum concentration summary tables, and concurrent values summarized by receptor for each day processed. Optional output files can be generated for: A listing of occurrences of exceedances of user-specified threshold value; a listing of concurrent (raw) results at each receptor for each hour modeled, suitable for post-processing; a listing of design values that can be imported into graphics software for plotting contours; a listing of results suitable for NAAQS analyses including NAAQS exceedances and culpability analyses; an unformatted listing of raw results above a threshold value with a special structure for use with the TOXX model component of TOXST; a listing of concentrations by rank (e.g., for use in quantile-quantile plots); and, a listing of concentrations, including arc-maximum normalized concentrations, suitable for model evaluation studies.

d. Type of Model

AERMOD is a steady-state plume model, using Gaussian distributions in the vertical and horizontal for stable conditions, and in the horizontal for convective conditions. The vertical concentration distribution for convective conditions results from an assumed bi-Gaussian probability density function of the vertical velocity.

e. Pollutant Types

AERMOD is applicable to primary pollutants and continuous releases of toxic and hazardous waste pollutants. Chemical transformation is treated by simple exponential decay.

f. Source-Receptor Relationships

AERMOD applies user-specified locations for sources and receptors. Actual separation between each source-receptor pair is used. Source and receptor elevations are user input or are determined by AERMAP using USGS DEM terrain data. Receptors may be located at user-specified heights above ground level.

g. Plume Behavior

(1) In the convective boundary layer (CBL), the transport and dispersion of a plume is characterized as the superposition of three modeled plumes: The direct plume (from the stack), the indirect plume, and the penetrated plume, where the indirect plume accounts for the lofting of a buoyant plume near the top of the boundary layer, and the penetrated plume accounts for the portion of a plume that, due to its buoyancy, penetrates above the mixed layer, but can disperse downward and re-enter the mixed layer. In the CBL, plume rise is superposed on the displacements by random convective velocities (Weil et al., 1997).

(2) In the stable boundary layer, plume rise is estimated using an iterative approach to account for height-dependent lapse rates, similar to that in the CTDMPLUS model (see A.2 in this appendix).

(3) Stack-tip downwash and buoyancy induced dispersion effects are modeled. Building wake effects are simulated for stacks subject to building downwash using the methods contained in the PRIME downwash algorithms (Schulman, et al., 2000). For plume rise affected by the presence of a building, the PRIME downwash algorithm uses a numerical solution of the mass, energy and momentum conservation laws (Zhang and Ghoniem, 1993). Streamline deflection and the position of the stack relative to the building affect plume trajectory and dispersion. Enhanced dispersion is based on the approach of Weil (1996). Plume mass captured by the cavity is well-mixed within the cavity. The captured plume mass is re-emitted to the far wake as a volume source.

(4) For elevated terrain, AERMOD incorporates the concept of the critical dividing streamline height, in which flow below this height remains horizontal, and flow above this height tends to rise up and over terrain (Snyder et al., 1985). Plume concentration estimates are the weighted sum of these two limiting plume states. However, consistent with the steady-state assumption of uniform horizontal wind direction over the modeling domain, straight-line plume trajectories are assumed, with adjustment in the plume/receptor geometry used to account for the terrain effects.

h. Horizontal Winds

Vertical profiles of wind are calculated for each hour based on measurements and surface-layer similarity (scaling) relationships. At a given height above ground, for a given hour, winds are assumed constant over the modeling domain. The effect of the vertical variation in horizontal wind speed on dispersion is accounted for through simple averaging over the plume depth.

i. Vertical Wind Speed

In convective conditions, the effects of random vertical updraft and downdraft velocities are simulated with a bi-Gaussian probability density function. In both convective and stable conditions, the mean vertical wind speed is assumed equal to zero.

j. Horizontal Dispersion

Gaussian horizontal dispersion coefficients are estimated as continuous functions of the parameterized (or measured) ambient lateral turbulence and also account for buoyancy- induced and building wake-induced turbulence. Vertical profiles of lateral turbulence are developed from measurements and similarity (scaling) relationships. Effective turbulence values are determined from the portion of the vertical profile of lateral turbulence between the plume height and the receptor height. The effective lateral turbulence is then used to estimate horizontal dispersion.

k. Vertical Dispersion

In the stable boundary layer, Gaussian vertical dispersion coefficients are estimated as continuous functions of parameterized vertical turbulence. In the convective boundary layer, vertical dispersion is characterized by a bi-Gaussian probability density function, and is also estimated as a continuous function of parameterized vertical turbulence. Vertical turbulence profiles are developed from measurements and similarity (scaling) relationships. These turbulence profiles account for both convective and mechanical turbulence. Effective turbulence values are determined from the portion of the vertical profile of vertical turbulence between the plume height and the receptor height. The effective vertical turbulence is then used to estimate vertical dispersion.

l. Chemical Transformation

Chemical transformations are generally not treated by AERMOD. However, AERMOD does contain an option to treat chemical transformation using simple exponential decay, although this option is typically not used in regulatory applications, except for sources of sulfur dioxide in urban areas. Either a decay coefficient or a half-life is input by the user. Note also that the Plume Volume Molar Ratio Method and the Ozone Limiting Method (section 4.2.3.4) and for point-source NO₂ analyses are available.

m. Physical Removal

AERMOD can be used to treat dry and wet deposition for both gases and particles.

n. Evaluation Studies

American Petroleum Institute, 1998. Evaluation of State of the Science of Air Quality Dispersion Model, Scientific Evaluation, prepared by Woodward-Clyde Consultants, Lexington, Massachusetts, for American Petroleum Institute, Washington, DC, 20005-4070.

Brode, R.W., 2002. Implementation and Evaluation of PRIME in AERMOD. Preprints of the 12th Joint Conference on Applications of Air Pollution Meteorology, May 20-24, 2002; American Meteorological Society, Boston, MA.

Brode, R.W., 2004. Implementation and Evaluation of Bulk Richardson Number Scheme in AERMOD. 13th Joint Conference on Applications of Air Pollution Meteorology, August 23-26, 2004; American Meteorological Society, Boston, MA.

Environmental Protection Agency, 2003. AERMOD: Latest Features and Evaluation Results. Publication No. EPA-454/R-03-003. Office of Air Quality Planning & Standards, Research Triangle Park, NC. http://www.epa.gov/​ttn/​scram/​7thconf/​aermod/​aermod_​mep.pdf.

Heist, D., et al, 2013. Estimating near-road pollutant dispersion: A model inter-comparison. Transportation Research Part D: Transport and Environment, 25: pp 93-105.

A.2  CTDMPLUS (Complex Terrain Dispersion Model Plus Algorithms for Unstable Situations)

References

Perry, S.G., D.J. Burns, L.H. Adams, R.J. Paine, M.G. Dennis, M.T. Mills, D.G. Strimaitis, R.J. Yamartino and E.M. Insley, 1989. User's Guide to the Complex Terrain Dispersion Model Plus Algorithms for Unstable Situations (CTDMPLUS). Volume 1: Model Descriptions and User Instructions. EPA Publication No. EPA-600/8-89-041. Environmental Protection Agency, Research Triangle Park, NC. http://www.epa.gov/​ttn/​scram/​dispersion_​prefrec.htm#ctdmplus. (NTIS No. PB 89-181424)

Perry, S.G., 1992. CTDMPLUS: A Dispersion Model for Sources near Complex Topography. Part I: Technical Formulations. Journal of Applied Meteorology, 31(7): 633-645.

Availability

The model codes and associated documentation are available on the EPA's SCRAM Web site (paragraph A.0(3)).

Abstract

CTDMPLUS is a refined point source Gaussian air quality model for use in all stability conditions for complex terrain applications. The model contains, in its entirety, the technology of CTDM for stable and neutral conditions. However, CTDMPLUS can also simulate daytime, unstable conditions, and has a number of additional capabilities for improved user friendliness. Its use of meteorological data and terrain information is different from other EPA models; considerable detail for both types of input data is required and is supplied by preprocessors specifically designed for CTDMPLUS. CTDMPLUS requires the parameterization of individual hill shapes using the terrain preprocessor and the association of each model receptor with a particular hill.

a. Recommendation for Regulatory Use

CTDMPLUS is appropriate for the following applications:

• Elevated point sources;
• Terrain elevations above stack top;
• Rural or urban areas;
• Transport distances less than 50 kilometers; and
• 1-hour to annual averaging times when used with a post-processor program such as CHAVG.

b. Input Requirements

(1) Source data: For each source, user supplies source location, height, stack diameter, stack exit velocity, stack exit temperature, and emission rate; if variable emissions are appropriate, the user supplies hourly values for emission rate, stack exit velocity, and stack exit temperature.

(2) Meteorological data: For applications of CTDMPLUS, multiple level (typically three or more) measurements of wind speed and direction, temperature and turbulence (wind fluctuation statistics) are required to create the basic meteorological data file (“PROFILE”). Such measurements should be obtained up to the representative plume height(s) of interest (i.e., the plume height(s) under those conditions important to the determination of the design concentration). The representative plume height(s) of interest should be determined using an appropriate complex terrain screening procedure (e.g., CTSCREEN) and should be documented in the monitoring/modeling protocol. The necessary meteorological measurements should be obtained from an appropriately sited meteorological tower augmented by SODAR and/or RASS if the representative plume height(s) of interest is above the levels represented by the tower measurements. Meteorological preprocessors then create a SURFACE data file (hourly values of mixed layer heights, surface friction velocity, Monin-Obukhov length and surface roughness length) and a RAWINsonde data file (upper air measurements of pressure, temperature, wind direction, and wind speed).

(3) Receptor data: Receptor names (up to 400) and coordinates, and hill number (each receptor must have a hill number assigned).

(4) Terrain data: User inputs digitized contour information to the terrain preprocessor which creates the TERRAIN data file (for up to 25 hills).

c. Output

(1) When CTDMPLUS is run, it produces a concentration file, in either binary or text format (user's choice), and a list file containing a verification of model inputs, i.e.,

• Input meteorological data from “SURFACE” and “PROFILE”,
• Stack data for each source,
• Terrain information,
• Receptor information, and
• Source-receptor location (line printer map).

(2) In addition, if the case-study option is selected, the listing includes:

• Meteorological variables at plume height,
• Geometrical relationships between the source and the hill, and
• Plume characteristics at each receptor, i.e.,

—Distance in along-flow and cross flow direction

—Effective plume-receptor height difference

—Effective σy & σz values, both flat terrain and hill induced (the difference shows the effect of the hill)

—Concentration components due to WRAP, LIFT and FLAT

(3) If the user selects the TOPN option, a summary table of the top four concentrations at each receptor is given. If the ISOR option is selected, a source contribution table for every hour will be printed.

(4) A separate output file of predicted (1-hour only) concentrations (“CONC”) is written if the user chooses this option. Three forms of output are possible:

(i) A binary file of concentrations, one value for each receptor in the hourly sequence as run;

(ii) A text file of concentrations, one value for each receptor in the hourly sequence as run; or

(iii) A text file as described above, but with a listing of receptor information (names, positions, hill number) at the beginning of the file.

(5) Hourly information provided to these files besides the concentrations themselves includes the year, month, day, and hour information as well as the receptor number with the highest concentration.

d. Type of Model

CTDMPLUS is a refined steady-state, point source plume model for use in all stability conditions for complex terrain applications.

e. Pollutant Types

CTDMPLUS may be used to model non- reactive, primary pollutants.

f. Source-Receptor Relationship

Up to 40 point sources, 400 receptors and 25 hills may be used. Receptors and sources are allowed at any location. Hill slopes are assumed not to exceed 15°, so that the linearized equation of motion for Boussinesq flow are applicable. Receptors upwind of the impingement point, or those associated with any of the hills in the modeling domain, require separate treatment.

g. Plume Behavior

(1) As in CTDM, the basic plume rise algorithms are based on Briggs' (1975) recommendations.

(2) A central feature of CTDMPLUS for neutral/stable conditions is its use of a critical dividing-streamline height (H_c) to separate the flow in the vicinity of a hill into two separate layers. The plume component in the upper layer has sufficient kinetic energy to pass over the top of the hill while streamlines in the lower portion are constrained to flow in a horizontal plane around the hill. Two separate components of CTDMPLUS compute ground-level concentrations resulting from plume material in each of these flows.

(3) The model calculates on an hourly (or appropriate steady averaging period) basis how the plume trajectory (and, in stable/neutral conditions, the shape) is deformed by each hill. Hourly profiles of wind and temperature measurements are used by CTDMPLUS to compute plume rise, plume penetration (a formulation is included to handle penetration into elevated stable layers, based on Briggs (1984)), convective scaling parameters, the value of H_c, and the Froude number above H_c.

h. Horizontal Winds

CTDMPLUS does not simulate calm meteorological conditions. Both scalar and vector wind speed observations can be read by the model. If vector wind speed is unavailable, it is calculated from the scalar wind speed. The assignment of wind speed (either vector or scalar) at plume height is done by either:

• Interpolating between observations above and below the plume height, or
• Extrapolating (within the surface layer) from the nearest measurement height to the plume height.

i. Vertical Wind Speed

Vertical flow is treated for the plume component above the critical dividing streamline height (H_c); see “Plume Behavior.”

j. Horizontal Dispersion

Horizontal dispersion for stable/neutral conditions is related to the turbulence velocity scale for lateral fluctuations, σv, for which a minimum value of 0.2 m/s is used. Convective scaling formulations are used to estimate horizontal dispersion for unstable conditions.

k. Vertical Dispersion

Direct estimates of vertical dispersion for stable/neutral conditions are based on observed vertical turbulence intensity, e.g., σw (standard deviation of the vertical velocity fluctuation). In simulating unstable (convective) conditions, CTDMPLUS relies on a skewed, bi-Gaussian probability density function (pdf) description of the vertical velocities to estimate the vertical distribution of pollutant concentration.

l. Chemical Transformation

Chemical transformation is not treated by CTDMPLUS.

m. Physical Removal

Physical removal is not treated by CTDMPLUS (complete reflection at the ground/hill surface is assumed).

n. Evaluation Studies

Burns, D.J., L.H. Adams and S.G. Perry, 1990. Testing and Evaluation of the CTDMPLUS Dispersion Model: Daytime Convective Conditions. Environmental Protection Agency, Research Triangle Park, NC.

Paumier, J.O., S.G. Perry and D.J. Burns, 1990. An Analysis of CTDMPLUS Model Predictions with the Lovett Power Plant Data Base. Environmental Protection Agency, Research Triangle Park, NC.

Paumier, J.O., S.G. Perry and D.J. Burns, 1992. CTDMPLUS: A Dispersion Model for Sources near Complex Topography. Part II: Performance Characteristics. Journal of Applied Meteorology, 31(7): 646-660.

A.3 OCD (Offshore and Coastal Dispersion Model)

Reference

DiCristofaro, D.C. and S.R. Hanna, 1989. OCD: The Offshore and Coastal Dispersion Model, Version 4. Volume I: User's Guide, and Volume II: Appendices. Sigma Research Corporation, Westford, MA. http://www.epa.gov/​ttn/​scram/​dispersion_​prefrec.htm#ocd. (NTIS Nos. PB 93-144384 and PB 93-144392)

Availability

The model codes and associated documentation are available on EPA's SCRAM Web site (paragraph A.0(3)). Official contact at Minerals Management Service: Mr. Dirk Herkhof, Parkway Atrium Building, 381 Elden Street, Herndon, VA 20170, Phone: [phone].

Abstract

(1) OCD is a straight-line Gaussian model developed to determine the impact of offshore emissions from point, area or line sources on the air quality of coastal regions. OCD incorporates overwater plume transport and dispersion as well as changes that occur as the plume crosses the shoreline. Hourly meteorological data are needed from both offshore and onshore locations. These include water surface temperature, overwater air temperature, mixing height, and relative humidity.

(2) Some of the key features include platform building downwash, partial plume penetration into elevated inversions, direct use of turbulence intensities for plume dispersion, interaction with the overland internal boundary layer, and continuous shoreline fumigation.

a. Recommendations for Regulatory Use

OCD has been recommended for use by the Minerals Management Service for emissions located on the Outer Continental Shelf (50 FR 12248; 28 March 1985). OCD is applicable for overwater sources where onshore receptors are below the lowest source height. Where onshore receptors are above the lowest source height, offshore plume transport and dispersion may be modeled on a case-by-case basis in consultation with the appropriate reviewing authority (paragraph 3.0(b)).

b. Input Requirements

(1) Source data: Point, area or line source location, pollutant emission rate, building height, stack height, stack gas temperature, stack inside diameter, stack gas exit velocity, stack angle from vertical, elevation of stack base above water surface and gridded specification of the land/water surfaces. As an option, emission rate, stack gas exit velocity and temperature can be varied hourly.

(2) Meteorological data (over water): Wind direction, wind speed, mixing height, relative humidity, air temperature, water surface temperature, vertical wind direction shear (optional), vertical temperature gradient (optional), turbulence intensities (optional).

(3) Meteorological data:

Over land: Surface weather data from a preprocessor such as PCRAMMET which provides hourly stability class, wind direction, wind speed, ambient temperature, and mixing height are required.

Over water: Hourly values for mixing height, relative humidity, air temperature, and water surface temperature are required; if wind speed/direction are missing, values over land will be used (if available); vertical wind direction shear, vertical temperature gradient, and turbulence intensities are optional.

(4) Receptor data: Location, height above local ground-level, ground-level elevation above the water surface.

c. Output

(1) All input options, specification of sources, receptors and land/water map including locations of sources and receptors.

(2) Summary tables of five highest concentrations at each receptor for each averaging period, and average concentration for entire run period at each receptor.

(3) Optional case study printout with hourly plume and receptor characteristics. Optional table of annual impact assessment from non-permanent activities.

(4) Concentration output files can be used by ANALYSIS postprocessor to produce the highest concentrations for each receptor, the cumulative frequency distributions for each receptor, the tabulation of all concentrations exceeding a given threshold, and the manipulation of hourly concentration files.

d. Type of Model

OCD is a Gaussian plume model constructed on the framework of the MPTER model.

e. Pollutant Types

OCD may be used to model primary pollutants. Settling and deposition are not treated.

f. Source-Receptor Relationship

(1) Up to 250 point sources, 5 area sources, or 1 line source and 180 receptors may be used.

(2) Receptors and sources are allowed at any location.

(3) The coastal configuration is determined by a grid of up to 3600 rectangles. Each element of the grid is designated as either land or water to identify the coastline.

g. Plume Behavior

(1) As in ISC, the basic plume rise algorithms are based on Briggs' recommendations.

(2) Momentum rise includes consideration of the stack angle from the vertical.

(3) The effect of drilling platforms, ships, or any overwater obstructions near the source are used to decrease plume rise using a revised platform downwash algorithm based on laboratory experiments.

(4) Partial plume penetration of elevated inversions is included using the suggestions of Briggs (1975) and Weil and Brower (1984).

(5) Continuous shoreline fumigation is parameterized using the Turner method where complete vertical mixing through the thermal internal boundary layer (TIBL) occurs as soon as the plume intercepts the TIBL.

h. Horizontal Winds

(1) Constant, uniform wind is assumed for each hour.

(2) Overwater wind speed can be estimated from overland wind speed using relationship of Hsu (1981).

(3) Wind speed profiles are estimated using similarity theory (Businger, 1973). Surface layer fluxes for these formulas are calculated from bulk aerodynamic methods.

i. Vertical Wind Speed

Vertical wind speed is assumed equal to zero.

j. Horizontal Dispersion

(1) Lateral turbulence intensity is recommended as a direct estimate of horizontal dispersion. If lateral turbulence intensity is not available, it is estimated from boundary layer theory. For wind speeds less than 8 m/s, lateral turbulence intensity is assumed inversely proportional to wind speed.

(2) Horizontal dispersion may be enhanced because of obstructions near the source. A virtual source technique is used to simulate the initial plume dilution due to downwash.

(3) Formulas recommended by Pasquill (1976) are used to calculate buoyant plume enhancement and wind direction shear enhancement.

(4) At the water/land interface, the change to overland dispersion rates is modeled using a virtual source. The overland dispersion rates can be calculated from either lateral turbulence intensity or Pasquill-Gifford curves. The change is implemented where the plume intercepts the rising internal boundary layer.

k. Vertical Dispersion

(1) Observed vertical turbulence intensity is not recommended as a direct estimate of vertical dispersion. Turbulence intensity should be estimated from boundary layer theory as default in the model. For very stable conditions, vertical dispersion is also a function of lapse rate.

(2) Vertical dispersion may be enhanced because of obstructions near the source. A virtual source technique is used to simulate the initial plume dilution due to downwash.

(3) Formulas recommended by Pasquill (1976) are used to calculate buoyant plume enhancement.

(4) At the water/land interface, the change to overland dispersion rates is modeled using a virtual source. The overland dispersion rates can be calculated from either vertical turbulence intensity or the Pasquill-Gifford coefficients. The change is implemented where the plume intercepts the rising internal boundary layer.

l. Chemical Transformation

Chemical transformations are treated using exponential decay. Different rates can be specified by month and by day or night.

m. Physical Removal

Physical removal is also treated using exponential decay.

n. Evaluation Studies

DiCristofaro, D.C. and S.R. Hanna, 1989. OCD: The Offshore and Coastal Dispersion Model. Volume I: User's Guide. Sigma Research Corporation, Westford, MA.

Hanna, S.R., L.L. Schulman, R.J. Paine and J.E. Pleim, 1984. The Offshore and Coastal Dispersion (OCD) Model User's Guide, Revised. OCS Study, MMS 84-0069. Environmental Research & Technology, Inc., Concord, MA. (NTIS No. PB 86-159803).

Hanna, S.R., L.L. Schulman, R.J. Paine, J.E. Pleim and M. Baer, 1985. Development and Evaluation of the Offshore and Coastal Dispersion (OCD) Model. Journal of the Air Pollution Control Association, 35: 1039-1047.

Hanna, S.R. and D.C. DiCristofaro, 1988. Development and Evaluation of the OCD/API Model. Final Report, API Pub. 4461, American Petroleum Institute, Washington, DC.

Footnotes

1.  U.S. EPA, 2010. Applicability of Appendix W Modeling Guidance for the 1-hour NO₂ NAAQS. Tyler Fox Memorandum dated June 28, 2010, Office of Air Quality Planning & Standards, Research Triangle Park, North Carolina 27711. http://www.epa.gov/​ttn/​scram/​guidance/​clarification/​ClarificationMemo_​AppendixW_​Hourly-NO2-NAAQS_​FINAL_​06-28-2010.pdf.

2.  U.S. EPA, 2010. Applicability of Appendix W Modeling Guidance for the 1-hour SO₂ NAAQS. Tyler Fox Memorandum dated August 23, 2010, Office of Air Quality Planning & Standards, Research Triangle Park, North Carolina 27711. http://www.epa.gov/​ttn/​scram/​guidance/​clarification/​ClarificationMemo_​AppendixW_​Hourly-SO2-NAAQS_​FINAL_​08-23-2010.pdf.

3.  U.S. EPA, 2010. Guidance Concerning the Implementation of the 1-hour SO₂ NAAQS for the Prevention of Significant Deterioration Program. Stephen D. Page, Memorandum dated August 23, 2010, Office of Air Quality Planning & Standards, Research Triangle Park, North Carolina 27711. http://www.epa.gov/​region07/​air/​nsr/​nsrmemos/​appwso2.pdf.

4.  U.S. EPA, 2010. Guidance Concerning the Implementation of the 1-hour NO₂ NAAQS for the Prevention of Significant Deterioration Program. Stephen D. Page, Memorandum dated June 29, 2010, Office of Air Quality Planning & Standards, Research Triangle Park, North Carolina 27711. http://www.epa.gov/​ttn/​scram/​guidance/​clarification/​ClarificationMemo_​AppendixW_​Hourly-SO2-NAAQS_​FINAL_​08-23-2010.pdf.

5.  AECOM, 2010. AERMOD low wind speed evaluation study results, http://mycommittees.api.org/​rasa/​amp/​Modeling%20Documents/​AECOM%202009%20Low%20Wind%20Speed%20Evaluation%20Study%20Report.pdf.

6.  U.S. EPA, 2012. Gina McCarthy Letter to Robert Ukeiley dated January 4, 2012, Washington, DC 20460. http://www.epa.gov/​ttn/​scram/​10thmodconf/​review_​material/​Sierra_​Club_​Petition_​OAR-11-002-1093.pdf.

7.  U.S. EPA, 2014. Guidance for PM_2.5 Modeling. May 20, 2014, EPA-454/B-14-001. Office of Air Quality Planning & Standards, Research Triangle Park, NC. http://www.epa.gov/​ttn/​scram/​guidance/​guide/​Guidance_​for_​PM25_​Permit_​Modeling.pdf.

8.  Phase 1 of the IWAQM effort focused on review of respective regional modeling programs, development of an organizational framework, and formulating reasonable objectives and plans that were presented to EPA management for support and commitment. Phase 2 of the IWAQM process continued this work and largely concluded in 1998 with the publication of the Interagency Workgroup on Air Quality Modeling (IWAQM) Phase 2 Summary Report and Recommendations for Modeling Long Range Transport Impacts (EPA-454/R-98-019) (USEPA, 1998). This report provided a series of recommendations concerning the application of the CALPUFF model for use in regulatory long-range transport (LRT) modeling that supported the revisions in 2003 to the Guideline.

9.  U.S. EPA, 1993. “Proposal for Calculating Plume Rise for Stacks with Horizontal Releases or Rain Caps for Cookson Pigment, Newark, New Jersey”, Memorandum from Joseph A. Tikvart, U.S. EPA/OAQPS, Research Triangle Park, NC. July 9, 1993. http://www.epa.gov/​ttn/​scram/​guidance/​mch/​new_​mch/​R1076_​TIKVART_​9_​JUL_​93.pdf.

10.  Chu, S.H. and E.L. Meyer, 1991. Use of Ambient Ratios to Estimate Impact of NO_X Sources on Annual NO2 Concentrations. Proceedings, 84th Annual Meeting & Exhibition of the Air & Waste Management Association, Vancouver, B.C.; 16-21 June 1991. (16pp.) (Docket No. A-92-65, II-A-9).

11.  Podrez, M. 2015. An Update to the Ambient Ratio Method for 1-h NO2 Air Quality Standards Dispersion Modeling. Atmospheric Environment, 103: 163-170.

12.  Cole, H.S. and J.E. Summerhays, 1979. A Review of Techniques Available for Estimation of Short-Term NO2 Concentrations. Journal of the Air Pollution Control Association, 29(8): 812-817.

13.  Hanrahan, P.L., 1999. The Polar Volume Polar Ratio Method for Determining NO2/NOX Ratios in Modeling—Part I: Methodology. Journal of the Air & Waste Management Association, 49: 1324-1331.

14.  Benson, Paul E., 1979. CALINE3—A Versatile Dispersion Model for Predicting Air Pollutant Levels Near Highways and Arterial Streets. Interim Report, Report Number FHWA/CA/TL-79/23. Federal Highway Administration, Washington, DC (NTIS No. PB 80-220841).

15.  Cimorelli, A. et al., 2005. AERMOD: A Dispersion Model for Industrial Source Applications. Part I: General Model Formulation and Boundary Layer Characterization. Journal of Applied Meteorology, 44(5): 682-693.

16.  Heist, D., V. Isakov, S. Perry, M. Snyder, A. Venkatram, C. Hood, J. Stocker, D. Carruthers, S. Arunachalam, AND C. Owen. Estimating near-road pollutant dispersion: a model inter-comparison. Transportation Research Part D: Transport and Environment. Elsevier BV, AMSTERDAM, Netherlands, 25:93-105, (2013).

17.  U.S. EPA, 2013, Transportation Conformity Guidance for Quantitative Hot-Spot Analyses in PM_2.5 and PM₁₀ Nonattainment and Maintenance Areas. Publication No. EPA-420-B-13-053, Office of Transportation and Air Quality, Ann Arbor, MI. http://www.epa.gov/​otaq/​stateresources/​transconf/​policy/​420b13053-sec.pdf.

18.  U.S. EPA, 2012. Gina McCarthy Letter to Robert Ukeiley dated January 4, 2012, Washington, DC 20460. http://www.epa.gov/​ttn/​scram/​10thmodconf/​review_​material/​Sierra_​Club_​Petition_​OAR-11-002-1093.pdf.

19.  McMurry, P.H., Shepherd, M.F., Vickery, J.S., 2004. Particulate matter science for policy makers: A NARSTO assessment. Cambridge University Press.

20.  Baker, K.R., Kelly, J.T., 2014. Single source impacts estimated with photochemical model source sensitivity and apportionment approaches. Atmospheric Environment, 96: 266-274.

21.  ENVIRON, 2012. Evaluation of chemical dispersion models using atmospheric plume measurements from field experiments, EPA Contract No: EP-D-07-102. September 2012. 06-20443M6. http://www.epa.gov/​ttn/​scram/​reports/​Plume_​Eval_​Final_​Sep_​2012v5.pdf.

22.  Zhou, W., Cohan, D.S., Pinder, R.W., Neuman, J.A., Holloway, J.S., Peischl, J., Ryerson, T.B., Nowak, J.B., Flocke, F., Zheng, W.G., 2012. Observation and modeling of the evolution of Texas power plant plumes. Atmospheric Chemistry and Physics, 12: 455-468.

23.  ENVIRON, 2014. User's Guide Comprehensive Air Quality Model with Extensions version 6, http://www.camx.com. ENVIRON International Corporation, Novato.

24.  Byun, D., Schere, K.L., 2006. Review of the governing equations, computational algorithms, and other components of the models-3 Community Multiscale Air Quality (CMAQ) modeling system. Applied Mechanics Reviews, 59: 51-77.

25.  Chen, J., Lu, J., Avise, J.C., DaMassa, J.A., Kleeman, M.J., Kaduwela, A.P., 2014. Seasonal modeling of PM 2.5 in California's San Joaquin Valley. Atmospheric Environment, 92: 182-190.

26.  Civerolo, K., Hogrefe, C., Zalewsky, E., Hao, W., Sistla, G., Lynn, B., Rosenzweig, C., Kinney, P.L., 2010. Evaluation of an 18-year CMAQ simulation: Seasonal variations and long-term temporal changes in sulfate and nitrate. Atmospheric Environment, 44: 3745-3752.

27.  Russell, A.G., 2008. EPA Supersites program-related emissions-based particulate matter modeling: initial applications and advances. Journal of the Air & Waste Management Association, 58: 289-302.

28.  Tesche, T., Morris, R., Tonnesen, G., McNally, D., Boylan, J., Brewer, P., 2006. CMAQ/CAMx annual 2002 performance evaluation over the eastern US. Atmospheric Environment, 40: 4906-4919.

29.  Cai, C., Kelly, J.T., Avise, J.C., Kaduwela, A.P., Stockwell, W.R., 2011. Photochemical modeling in California with two chemical mechanisms: model intercomparison and response to emission reductions. Journal of the Air & Waste Management Association, 61: 559-572.

30.  Hogrefe, C., Hao, W., Zalewsky, E., Ku, J.-Y., Lynn, B., Rosenzweig, C., Schultz, M., Rast, S., Newchurch, M., Wang, L., 2011. An analysis of long-term regional-scale ozone simulations over the Northeastern United States: variability and trends. Atmospheric Chemistry and Physics, 11: 567-582.

31.  U.S. EPA, 2015. Guidance on the use of models for assessing the impacts from single sources on secondarily formed pollutants ozone and PM_2.5. Publication No. EPA 454/P-15-001. Office of Air Quality Planning & Standards, Research Triangle Park, North Carolina 27711.

32.  U.S. EPA, 2015. “Proposed Approach for Demonstrating PM_2.5 PSD Compliance”, Memorandum to Docket No. EPA-HQ-OAR-2015-0310 by Tyler J Fox, U.S. EPA/OAQPS, Research Triangle Park, NC. June 30, 2015.

33.  U.S. EPA, 2015. “Proposed Approach for Demonstrating Ozone PSD Compliance”, Memorandum to Docket No. EPA-HQ-OAR-2015-0310 by Tyler J Fox, U.S. EPA/OAQPS, Research Triangle Park, NC. June 30, 2015.

34.  Baker, K.R., Foley, K.M., 2011. A nonlinear regression model estimating single source concentrations of primary and secondarily formed PM_2.5. Atmospheric Environment, 45: 3758-3767.

35.  Bergin, M.S., Russell, A.G., Odman, M.T., Cohan, D.S., Chameldes, W.L., 2008. Single-Source Impact Analysis Using Three-Dimensional Air Quality Models. Journal of the Air & Waste Management Association, 58: 1351-1359.

36.  Baker, K.R., Kelly, J.T., Fox, T., 2013. Estimating second pollutant impacts from single sources (control #27). http://aqmodels.awma.org/​conference-proceedings.

37.  U.S. EPA, 2009. Reassessment of the Interagency Workgroup on Air Quality Modeling (IWAQM) Phase 2 Summary Report; Revisions to Phase 2 Recommendations. Draft. Office of Air Quality Planning & Standards, Research Triangle Park, North Carolina 27711. http://www.epa.gov/​ttn/​scram/​guidance/​reports/​Draft_​IWAQM_​Reassessment_​052709.pdf.

38.  U.S. EPA, 2015. “Summary of CALPUFF Ownership Since 2003 Promulgation”, Memorandum to Docket No. EPA-HQ-OAR-2015-0310 by Tyler J Fox, U.S. EPA/OAQPS, Research Triangle Park, NC. June 30, 2015.

39.  U.S. EPA, 2015. Interagency Workgroup on Air Quality Modeling Phase 3 Summary Report: Near-Field Single Source Secondary Impacts. Publication No. EPA 454/P-15-002. Office of Air Quality Planning & Standards, Research Triangle Park, North Carolina 27711.

40.  U.S. EPA, 2015. Interagency Workgroup on Air Quality Modeling Phase 3 Summary Report: Long Range Transport and Air Quality Related Values. Publication No. EPA 454/P-15-003. Office of Air Quality Planning & Standards, Research Triangle Park, North Carolina 27711.

41.  ENVIRON, 2012. Documentation of the Evaluation of CALPUFF and Other Long Range Transport Models using Tracer Field Experiment Data, EPA Contract No: EP-D-07-102. February 2012. 06-20443M4. http://www.epa.gov/​ttn/​scram/​reports/​EPA-454_​R-12-003.pdf.

42.  Hegarty, J., Draxler, R.R., Stein, A.F., Brioude, J., Mountain, M., Eluszkiewicz, J., Nehrkorn, T., Ngan, F., Andrews, A., 2013. Evaluation of Lagrangian particle dispersion models with measurements from controlled tracer releases. Journal of Applied Meteorology and Climatology, 52: 2623-2637.

43.  See 70 FR 39104, 39122-23 (July 6, 2005).

44.  U.S. EPA, 2011. Additional Clarification Regarding Applicability of Appendix W Modeling Guidance for the 1-hour NO2 NAAQS. Tyler Fox Memorandum dated March 1, 2011, Office of Air Quality Planning & Standards, Research Triangle Park, North Carolina 27711. http://www.epa.gov/​ttn/​scram/​guidance/​clarification/​NO2_​Clarification_​Memo-20140930.pdf.

45.  U.S. EPA, 2014. Clarification on the Use of AERMOD Dispersion Modeling for Demonstrating Compliance with the NO₂ National Ambient Air Quality Standard. R. Chris Owen and Roger Brode Memorandum dated September 30, 2014, Office of Air Quality Planning & Standards, Research Triangle Park, North Carolina 27711. http://www.epa.gov/​ttn/​scram/​guidance/​clarification/​NO2_​Clarification_​Memo-20140930.pdf.

46.  U.S. EPA, 1990. New Source Review Workshop Manual: Prevention of Significant Deterioration and Nonattainment Area Permitting (Draft). Office of Air Quality Planning & Standards, Research Triangle Park, North Carolina 27711. http://www.epa.gov/​nsr.

47.  U.S. EPA, 2015. Technical Support Document (TSD) for Replacement of CALINE3 with AERMOD for Transportation Related Air Quality Analyses. Publication No. EPA-454/B-15-002. Office of Air Quality Planning & Standards, Research Triangle Park, NC.

a.  For PSD and other applications that use the model results in an absolute sense, the model should not be biased toward underestimates. Alternatively, for ozone and PM_2.5 SIP attainment demonstrations and other applications that use the model results in a relative sense, the model should note be biased toward overestimates.

b.  Formerly the National Climatic Data Center (NCDC).