2024-11175. Air Plan Approval; Pennsylvania; Attainment Plan for the Indiana Nonattainment Area for the 2010 1-Hour Sulfur Dioxide National Ambient Air Quality Standard

AGENCY:

Environmental Protection Agency (EPA).

ACTION:

Proposed rule.

SUMMARY:

The Environmental Protection Agency (EPA) is proposing to approve a state implementation plan (SIP) revision submitted by the Commonwealth of Pennsylvania (Pennsylvania). This revision pertains to the attainment plan for the Indiana, Pennsylvania (PA) nonattainment area for the 2010 1-Hour Sulfur Dioxide (SO₂) national ambient air quality standard (NAAQS). This action is being taken under the Clean Air Act (CAA).

DATES:

Written comments must be received on or before July 8, 2024.

ADDRESSES:

Submit your comments, identified by Docket ID No. EPA-R03-OAR-2024-0024 at www.regulations.gov, or via email to [e-mail]. For comments submitted at Regulations.gov, follow the online instructions for submitting comments. Once submitted, comments cannot be edited or removed from Regulations.gov. For either manner of submission, the EPA may publish any comment received to its public docket. Do not submit electronically any information you consider to be confidential business information (CBI) or other information whose disclosure is restricted by statute. Multimedia submissions (audio, video, etc.) must be accompanied by a written comment. The written comment is considered the official comment and should include discussion of all points you wish to make. The EPA will generally not consider comments or comment contents located outside of the primary submission ( i.e., on the web, cloud, or other file sharing system). For additional submission methods, please contact the person identified in the For Further Information Contact section. For the full EPA public comment policy, information about CBI or multimedia submissions, and general guidance on making effective comments, please visit www.epa.gov/​dockets/​commenting-epa-dockets.

FOR FURTHER INFORMATION CONTACT:

Megan Goold, Planning & Implementation Branch (3AD30), Air & Radiation Division, U.S. Environmental Protection Agency, Region III, 1600 John F Kennedy Boulevard, Philadelphia, Pennsylvania 19103. The telephone number is [phone]. Ms. Goold can also be reached via electronic mail at [e-mail].

SUPPLEMENTARY INFORMATION:

On October 5, 2023, the Pennsylvania Department of Environmental Protection (PADEP) submitted a revision to its SIP to demonstrate attainment of the 2010 SO₂ NAAQS in the Indiana, PA nonattainment area. This plan includes Pennsylvania's attainment demonstration and other attainment plan elements required under the CAA, including the requirement for meeting reasonable further progress (RFP) toward attainment of the NAAQS, reasonably available control measures and reasonably available control technology (RACM/RACT), enforceable emission limitations and control measures, and contingency measures. Notably, the submission does not contain information regarding the required emissions inventory or the state's Nonattainment New Source Review (NNSR) program, as these were previously approved by the EPA (87 FR 50778, August 18, 2022).

I. Background

On June 22, 2010, the EPA published a new 1-hour primary SO₂ NAAQS of 75 parts per billion (ppb) at 40 CFR 50.17(a), which is met at an ambient air quality monitoring site when the 3-year average of the annual 99th percentile of daily maximum 1-hour average concentrations does not exceed 75 ppb, as determined in accordance with 40 Code of Federal Regulations (CFR) part 50 appendix T (75 FR 35520, June 22, 2010). Under CAA section 107(d)(1), the EPA is required to designate areas as “nonattainment,” “attainment,” or “unclassifiable” within two years of establishing a new or revising an existing standard. As part of this process, states must submit recommendations for area designations and boundaries to the EPA within one year of the effective date of the standard. Effective on October 4, 2013,^[1] the Indiana Area (which encompasses Indiana County, and Plumcreek Township, South Bend Township and Eldertown Borough of Armstrong County) was designated as nonattainment for the 2010 SO₂ NAAQS for an area that encompasses the primary SO₂ emitting sources: the Keystone Generating Station (Keystone), Conemaugh Generating Station (Conemaugh), Homer City Generating Station (Homer City), and Seward Generating Station (Seward) (hereafter referred to as “the Indiana, PA NAA”). The October 4, 2013, final designation triggered a requirement for Pennsylvania to submit by April 4, 2015 (within 18 months per CAA section 191(a)), a SIP revision with an attainment plan for how the Indiana, PA NAA would attain the 2010 SO₂ NAAQS as expeditiously as practicable, but no later than October 4, 2018, (five years from the designation per CAA section 192(a)) in accordance with CAA sections 110(a), 172(c) and 191-192.

For a number of areas, including the Indiana, PA NAA, the EPA published a March 18, 2016 Finding of Failure to Submit, with an effective date of April 18, 2016, finding that Pennsylvania and other pertinent states had failed to submit the required SO₂ attainment plan by this submittal deadline. (see 81 FR 14736, March 18, 2016). This finding initiated a deadline under CAA section 179(a) for the potential imposition of new source review and highway funding sanctions. However, as a result of Pennsylvania's October 11, 2017 submittal (hereafter referred to as “the 2017 SIP submittal”), and the EPA's subsequent October 13, 2017 letter to Pennsylvania finding the submittal complete, the CAA section 179(a) sanctions were not imposed. Additionally, under CAA section 110(c), the March 18, 2016, finding triggered a requirement that the EPA promulgate a Federal implementation plan (FIP) within two years of the effective date of the finding unless, by that time, the state has made the necessary complete submittal and the EPA has approved the submittal as meeting applicable requirements. The EPA took final action approving this attainment plan on October 19, 2020 (85 FR 66240, October 19, 2020), which removed the FIP obligation.

On December 18, 2020, the Sierra Club, Clean Air Council, and Citizens for Pennsylvania's Future filed a petition for judicial review with the U.S. Court of Appeals for the Third Circuit, challenging that final approval.^[2] On April 5, 2021, the EPA filed a motion for voluntary remand without vacatur of its approval of the Indiana, PA SO₂ attainment plan.

On August 17, 2021, the U.S. Court of Appeals for the Third Circuit granted the EPA's request for remand without vacatur of the final approval of Pennsylvania's SO₂ attainment plan for the Indiana, PA NAA, and required that the EPA take final action in response to the remand no later than one year from the date of the court's order.

On August 18, 2022, the EPA revised and corrected its prior full approval action (85 FR 66240, October 19, 2020) without further submission from Pennsylvania (effective September 19, 2022) (87 FR 50778, August 18, 2022). Specifically, the EPA retained the approval of the emissions inventory and NNSR program requirements, and disapproved the attainment demonstration, RACM/RACT requirements, RFP requirements, and contingency measures (hereafter referred to as the “2022 Partial Approval/Partial Disapproval”) (87 FR 50778, August 18, 2022). The partial disapproval action initiated a sanctions clock under CAA section 179, providing for emission offset sanctions for new sources if EPA has not fully approved a revised attainment plan within 18 months (March 19, 2024) after final partial disapproval, and providing for highway funding sanctions if the EPA has not fully approved a revised plan within 6 months thereafter (September 19, 2024). The sanctions clock can be stopped only if the conditions of the EPA's regulations at 40 CFR 52.31 are met. Also, under CAA section 110(c), the partial disapproval action initiated an obligation for EPA to promulgate a FIP within two years unless Pennsylvania has submitted, and EPA has fully approved, a plan addressing the disapproved attainment planning requirements.

On October 5, 2023, Pennsylvania submitted a 2023 SO₂ Attainment Plan SIP Revision for the Indiana, PA NAA (hereafter referred to as the “2023 SIP submittal”). The 2023 SIP submittal addresses the requirements of CAA sections 172(c), 191 and 192 and the disapproved attainment planning requirements in the EPA's 2022 Partial Approval/Partial Disapproval. Specifically, this SIP revision contains a modified attainment demonstration using dispersion modeling, evaluates sources for RACT/RACM purposes, gives an RFP explanation, and provides for contingency measures, and includes revised emissions limitations and control measures.

Nonattainment area SO₂ SIPs must meet the applicable requirements of the CAA, specifically CAA sections 110, 172, 191 and 192. The EPA's regulations governing nonattainment area SIPs are set forth at 40 CFR part 51, with specific procedural requirements and control strategy requirements residing at subparts F and G, respectively. Soon after Congress enacted the 1990 amendments to the CAA, the EPA issued comprehensive guidance on SIPs in a document entitled the “General Preamble for the Implementation of Title I of the Clean Air Act Amendments of 1990,” published in the Federal Register at 57 FR 13498 (April 16, 1992) (General Preamble). Among other things, the General Preamble addressed SO₂ SIPs and fundamental principles for SIP control strategies. Id. at 13545-49, 13567-68. On April 23, 2014, the EPA issued guidance and recommendations for meeting the statutory requirements in SO₂ SIPs addressing the 2010 primary NAAQS, in a document entitled, “Guidance for 1-Hour SO₂ Nonattainment Area SIP Submissions” (hereafter referred to as “2014 SO₂ Nonattainment Guidance”).^[3] In the 2014 SO₂ Nonattainment Guidance, the EPA described the statutory requirements for a complete nonattainment area SIP, which include an accurate emissions inventory of current emissions for all sources of SO₂ within the nonattainment area; an attainment demonstration; enforceable emissions limitations and control measures; demonstration of RFP; implementation of RACM (including RACT); nonattainment new source review; and adequate contingency measures for the affected area.

For the EPA to fully approve a SIP as meeting the requirements of CAA sections 110, 172, 191, and 192 and the EPA's regulations at 40 CFR part 51, the SIP for the affected area needs to demonstrate to the EPA's satisfaction that each of the aforementioned requirements have been met. Under CAA sections 110(l) and 193, the EPA may not approve a SIP that would interfere with any applicable requirement concerning NAAQS attainment and RFP, or any other applicable requirement, and no requirement in effect before November 15, 1990 (or required to be adopted by an order, settlement, agreement, or plan in effect before November 15, 1990), in any area which is a nonattainment area for any air pollutant, may be modified in any manner unless it ensures equivalent or greater emission reductions of such air pollutant.

CAA section 172(c)(1) directs states with areas designated as nonattainment to demonstrate that the submitted plan provides for attainment of the NAAQS. 40 CFR part 51, subpart G further delineates the control strategy requirements that SIPs must meet, and the EPA has long required that all SIPs and control strategies reflect the four fundamental principles of quantification, enforceability, replicability, and accountability. See General Preamble, at 13567-68. SO₂ attainment plans must consist of two components: (1) emission limits and other control; measures that assure implementation of permanent, enforceable and necessary emission controls, and (2) a modeling analysis which meets the requirements of 40 CFR part 51, appendix W and demonstrates that these emission limits and control measures provide for timely attainment of the primary SO₂ NAAQS as expeditiously as practicable, but by no later than the attainment date for the affected area. In all cases, the emission limits and control measures must be accompanied by appropriate methods and conditions to determine compliance with the respective emission limits and control measures, and must be quantifiable ( i.e., a specific amount of emission reduction can be ascribed to the measures), fully enforceable (specifying clear, unambiguous and measurable requirements for which compliance can be practicably determined), replicable (the procedures for determining compliance are sufficiently specific and non-subjective so that two independent entities applying the procedures would obtain the same result), and accountable (source-specific limits must be permanent and must reflect the assumptions used in the SIP demonstrations).

The EPA's 2014 SO₂ Nonattainment Guidance recommends that the emission limits established for the attainment demonstration be expressed as short-term average limits ( e.g., addressing emissions averaged over one or three hours), but also describes the option to utilize emission limits with longer averaging times of up to 30 days so long as the state meets various suggested criteria. See 2014 SO₂ Nonattainment Guidance, pp. 22 to 39. The guidance recommends that—should states and sources utilize longer averaging times—the longer-term average limit should be set at an adjusted level that reflects a stringency comparable to the 1-hour average limit at the critical emission value (CEV) shown to provide for attainment that the plan otherwise would have set.

The 2014 SO₂ Nonattainment Guidance provides an extensive discussion of the EPA's rationale for concluding that appropriately set, comparably stringent limitations based on averaging times as long as 30 days can be found to provide for attainment of the 2010 SO₂ NAAQS. In evaluating this option, the EPA considered the nature of the standard, conducted detailed analyses of the impact of 30-day average limits on the prospects for attaining the standard, and carefully reviewed how best to achieve an appropriate balance among the various factors that warrant consideration in judging whether a state's plan provides for attainment. Id. at pp. 22-39, and Appendices B, C, and D.

As specified in 40 CFR 50.17(b), the 1-hour primary SO₂ NAAQS is met at an ambient air quality monitoring site when the 3-year average of the annual 99th percentile of daily maximum 1-hour average concentrations is less than or equal to 75 ppb. In a year with 365 days of valid monitoring data, the 99th percentile would be the fourth highest daily maximum 1-hour value. The 2010 SO₂ NAAQS, including this form of determining compliance with the standard, was upheld by the U.S. Court of Appeals for the District of Columbia Circuit in Nat'l Envt'l Dev. Ass'n's Clean Air Project v. EPA, 686 F.3d 803 (D.C. Cir. 2012). Because the standard has this form, a single hourly exceedance of the 75 ppb NAAQS level does not by itself result in a violation of the standard. Instead, at issue is whether a source operating in compliance with a properly set longer-term average could cause multiple hourly exceedances over multiple days in a year, and if so, the resulting frequency and magnitude of such exceedances, and in particular, whether the EPA can have reasonable confidence that a properly set longer-term average limit will provide that the 3-year average of annual fourth highest daily maximum hourly values will be at or below 75 ppb. A synopsis of how the EPA evaluates whether such plans “provide for attainment,” based on modeling of projected allowable emissions and in light of the SO₂ NAAQS' form for determining attainment at monitoring sites, follows.

For SO₂ attainment plans based on 1-hour emission limits, the standard approach is to conduct modeling using fixed 1-hour emission rates. The maximum modeled emission rate that results in attainment is labeled the “critical emissions value” (CEV). The modeling process for identifying this CEV inherently considers the numerous variables that affect ambient concentrations of SO₂, such as meteorological data, background concentrations, and topography. In the standard approach, the state would then provide for attainment by setting a continuously applicable 1-hour emission limit for each stationary SO₂ source at this CEV.

The EPA recognizes that some sources have highly variable emissions, for example due to variations in fuel sulfur content and operating rate, that can make it extremely difficult, even with a well-designed control strategy, to ensure in practice that emissions for any given hour do not exceed the CEV. The EPA also acknowledges the concern that longer-term emission limits can allow short periods with emissions above the CEV, which, if coincident with meteorological conditions conducive to high SO₂ concentrations, could in turn create the possibility of an hourly NAAQS exceedance occurring on a day when an exceedance would not have occurred if emissions were continuously controlled at the level corresponding to the CEV. However, for several reasons, EPA believes that the approach recommended in its guidance document suitably addresses this concern.

First, from a practical perspective, the EPA expects the actual emission profile of a source subject to an appropriately set longer-term average limit to be similar to the emission profile of a source subject to an analogous 1-hour average limit. The EPA expects this similarity because it has recommended that the longer-term average limit be set at a level that is comparably stringent to the otherwise applicable 1-hour limit (reflecting a downward adjustment from the CEV) and that takes the source's emissions profile (and inherent level of emissions variability) into account. As a result, the EPA expects either form of emission limit to yield comparable air quality.

Second, from a more theoretical perspective, the EPA has compared the likely air quality with a source having maximum allowable emissions under an appropriately set longer-term limit, to the likely air quality with the source having maximum allowable emissions under the comparable 1-hour limit. In this comparison, in the 1-hour average limit scenario, the source is presumed at all times to emit at the CEV, and in the longer-term average limit scenario, the source is presumed occasionally to emit more than the CEV, but on average, and presumably at most times, to emit well below the CEV. In an “average year,” ^[4] compliance with the 1-hour limit is expected to result in three exceedance days ( i.e., three days with maximum hourly values above 75 ppb) and a fourth day with a maximum hourly value at 75 ppb. By comparison, with the source complying with a longer-term limit, it is possible that additional hourly exceedances would occur that would not occur in the 1-hour limit scenario (if emissions exceed the CEV at times when meteorology is conducive to poor air quality). However, this comparison must also factor in the likelihood that exceedances that would be expected in the 1-hour limit scenario would not occur in the longer-term limit scenario. This result arises because the longer-term limit requires lower emissions most of the time (because the limit is set below the CEV), so a source complying with an appropriately set longer-term limit is likely to have lower emissions at critical times than would be the case if the source were emitting as allowed with a 1-hour limit.

To illustrate this point, the EPA conducted a statistical analysis using a range of scenarios using actual plant data. The analysis is described in appendix B of EPA's 2014 SO₂ Nonattainment Guidance. Based on the analysis described in the 2014 SO₂ Nonattainment Guidance, the EPA expects that an emission profile with maximum allowable emissions under an appropriately set, comparably stringent 30-day average limit is likely to have the net effect of having a lower number of hourly exceedances and better air quality than an emission profile with maximum allowable emissions under a 1-hour emission limit at the CEV. This result provides a compelling policy rationale for allowing the use of a longer averaging period, in appropriate circumstances where the facts indicate this result can be expected to occur.

The 2014 SO₂ Nonattainment Guidance offers specific recommendations for determining an appropriate longer-term average limit. The recommended method starts with determination of the 1-hour emission limit that would provide for attainment ( i.e., the CEV), and applies an adjustment factor to determine the (lower) level of the longer-term average emission limit that would be estimated to have a stringency comparable to the otherwise necessary 1-hour emission limit. This method uses a database of continuous emission data reflecting the type of control that the source will be using to comply with the SIP emission limits, which (if compliance requires new controls) may require use of an emission database from another source. The recommended method involves using these data to compute a complete set of emission averages, computed according to the averaging time and averaging procedures of the prospective emission limitation ( i.e., using 1-hour historical emission values from the emissions database to calculate 30-day average emission values). In this recommended method, the ratio of the 99th percentile among these long-term averages to the 99th percentile of the 1-hour values represents an adjustment factor that may be multiplied to the candidate 1-hour emission limit (CEV) to determine a longer-term average emission limit that may be considered comparably stringent.^[5]

The 2014 SO₂ Nonattainment Guidance also addresses a variety of related topics, including the potential utility of setting supplemental emission limits, such as mass-based limits or work practice requirements for the operation of SO₂ control equipment, to reduce the likelihood and/or magnitude of elevated emission levels that might occur under the longer-term emission rate limit.

Preferred air quality models for use in regulatory applications are described in appendix A of the EPA's Guideline on Air Quality Models (40 CFR part 51, appendix W). In 2005, the EPA promulgated AERMOD as the Agency's preferred near-field dispersion modeling for a wide range of regulatory applications addressing stationary sources (for example in estimating SO₂ concentrations) in all types of terrain based on extensive developmental and performance evaluation. Supplemental guidance on modeling for purposes of demonstrating attainment of the SO₂ standard is provided in appendix A to the 2014 SO₂ Nonattainment Guidance. Appendix A provides extensive guidance on the modeling domain, the source inputs, assorted types of meteorological data, and background concentrations. Consistency with the recommendations in this guidance is generally necessary for the attainment demonstration to offer adequately reliable assurance that the plan provides for attainment.

Attainment demonstrations for the 2010 1-hour primary SO₂ NAAQS must demonstrate future attainment and maintenance of the NAAQS in the entire area designated as nonattainment ( i.e., not just at the violating monitor) by using air quality dispersion modeling ( see appendix W to 40 CFR part 51) to show that the mix of sources and enforceable control measures and emission rates in an identified area will not lead to a violation of the SO₂ NAAQS. For a short-term ( i.e., 1-hour) standard, the EPA believes that dispersion modeling, using allowable emissions and addressing stationary sources in the affected area (and in some cases those sources located outside the nonattainment area which may affect attainment in the area) is technically appropriate, efficient, and effective in demonstrating attainment in nonattainment areas because it takes into consideration combinations of meteorological and emission source operating conditions that may contribute to peak ground-level concentrations of SO₂.

The meteorological data used in the analysis should generally be processed with the most recent version of AERMET. AERMET is a meteorological data preprocessor that incorporates air dispersion based on planetary boundary layer turbulence structure and scaling concepts. Estimated concentrations should include ambient background concentrations, should follow the form of the standard, and should be calculated as described in section 2.6.1.2 of the August 23, 2010, clarification memo on “Applicability of Appendix W Modeling Guidance for the 1-hour SO₂ National Ambient Air Quality Standard” (U.S. EPA, 2010).

II. Summary of SIP Revision and EPA Analysis

Pennsylvania's 2023 SIP submittal contained an attainment demonstration that located, identified, and quantified sources of emissions contributing to violations of the 2010 SO₂ NAAQS in the Indiana, PA NAA; a determination that the control strategy for the primary SO₂ sources (Keystone, Conemaugh, Homer City, and Seward) constitutes RACM/RACT; requirements for RFP toward attaining the SO₂ NAAQS in the Indiana, PA NAA; contingency measures; and the request that emission limitations and compliance parameters for Keystone, Conemaugh, and Seward be incorporated into the SIP.^[6] The EPA disapproved these elements of PADEP's 2017 SIP submittal because they were based on longer-term averaging SO₂ limits for Keystone and Seward that EPA could not approve. Those particular longer-term averaging limits were unsupportable because PADEP's modeling and analysis fell short of demonstrating that the longer-term limits were comparably stringent to the 1-hour CEV and that such limits would provide for attainment under worst-case emission scenarios, unlike the approach set forth in the 2014 SO₂ Nonattainment Guidance. But PADEP's 2023 SIP submittal includes appropriate modeling and revised longer-term averaging emission limits for Keystone, Conemaugh, and Seward that are comparably stringent to the 1-hour CEV for each facility. Therefore, the 2023 SIP submittal's attainment plan elements, the effectiveness of which are dependent upon correct longer-term averaging emission limits, are similarly approvable. The EPA already determined that Pennsylvania satisfied the emissions inventory and NNSR requirements and approved those elements of the attainment plan into Pennsylvania's SIP as stated in the 2022 Partial Approval/Partial Disapproval of Pennsylvania's 2017 submittal (87 FR 50778, August 18, 2022).

A. Attainment Demonstration-Air Quality Modeling

The SO₂ attainment demonstration provides air quality dispersion modeling analyses to demonstrate that control strategies chosen to reduce SO₂ source emissions will bring the Indiana, PA NAA into attainment. The modeling analyses, conducted pursuant to recommendations outlined in appendix W to 40 CFR part 51 (EPA's Modeling Guidance), are used to assess the control strategy for a nonattainment area and establish emission limits that will provide for attainment. The analysis requires five years of meteorological data to simulate the dispersion of pollutant plumes from multiple point, area, or volume sources across the averaging times of interest.^[7] The modeling demonstration typically also relies on maximum allowable emissions from sources in the nonattainment area. Though the actual emissions are likely to be below the allowable emissions, sources have the ability to run at higher production rates or optimize controls such that emissions approach the allowable emissions limits. A modeling analysis that provides for attainment under all scenarios of operation for each source must therefore consider the worst-case scenario of both the meteorology ( e.g., predominant wind directions, stagnation, etc.) and the maximum allowable emissions.

Air dispersion modeling served as the basis for developing SO₂ emission limits that provide for attainment of the 2010 SO₂ NAAQS throughout the Indiana, PA NAA. PADEP's air dispersion modeling methodology is fully described in appendix A of the state submittal, the Air Dispersion Modeling Technical Support Document.

PADEP's air dispersion modeling utilized the AERMOD v22112 and its associated preprocessors, the building downwash preprocessor (BPIPPRM) v04274, the AERMOD terrain preprocessor (AERMAP) v18081, and the AERMOD meteorological preprocessor (AERMET) v22112.

The modeling analysis included the following SO₂ sources in the NAA: (1) Keystone's SO₂ emission sources include two coal-fired boilers (Unit 1 & Unit 2 or Source ID 031 & 032). The SO₂ emissions vent from each source to the atmosphere through separate flues within a common stack, which was characterized in AERMOD as a point source; (2) Homer City's SO₂ emission sources include three coal-fired boilers (Unit 1, Unit 2 & Unit 3 or Source ID 031, 032 & 033). The SO₂ emissions vent from each source to the atmosphere through separate stacks, which were each characterized in AERMOD as a point source; (3) Conemaugh's SO₂ emission sources include two coal-fired boilers (Unit 1 & Unit 2 or Source ID 031 & 032). The SO₂ emissions vent from each source to the atmosphere through separate flues within a common stack, which was characterized in AERMOD as a point source; and (4) Seward's SO₂ emission sources include two refuse coal-fired boilers (Unit 1 & Unit 2 or Source ID 034 & 035). The SO₂ emissions vent from each source to the atmosphere through a common stack, which was characterized in AERMOD as a point source.

PADEP modeled three domains with three meteorological data sets. Domain 1, the Armstrong County portion of the Indiana, PA NAA, included SO₂ emissions data from Keystone and Homer City in AERMOD. Domain 2, the Indiana County portion of the Indiana, PA NAA, included SO₂ emissions data from all four power plant facilities in AERMOD. The air dispersion modeling in Domains 1 and 2 utilized representative meteorological datasets from the Johnstown—Cambria County Airport (KJST) meteorological site. The KJST meteorological dataset consists of a 5-year period of hourly records from January 1, 2011, through December 31, 2015, consistent with the meteorological data period that was utilized in the air dispersion modeling for PADEP's 2017 SIP submittal for the Indiana, PA NAA. Additionally, a second KJST meteorological dataset was utilized, which consists of a more recent 5-year period of hourly records from January 1, 2017, through December 31, 2021.

Domain 3, the portion of Indiana County near Conemaugh and Seward, included SO₂ emissions data from those two plants. PADEP used data from the Conemaugh-Seward meteorological site to represent atmospheric conditions in the vicinity of Conemaugh and Seward. A 1-year (September 1, 2015-August 31, 2016) Conemaugh-Seward meteorological dataset was utilized with AERMOD.

Background SO₂ was represented in AERMOD by temporally varying (by season and hour-of-day), 99th-percentile concentrations that were derived from data measured at the Allegheny County Health Department's South Fayette monitor (Site ID: 42-003-0067) for the 3-year period, 2019-2021.

AERMOD was used to determine the CEVs for Conemaugh, Keystone, and Seward where the modeled 1-hour emission rates demonstrate attainment of the 2010 1-hour SO₂ NAAQS. The SO₂ emission rates for Homer City were based on the unit 1, unit 2, and unit 3 combined mass-based SO₂ emission limits established in Plan Approval 32-00055H,^[8] which authorized the installation of Novel Integrated Desulfurization (NID) systems, often referred to as Dry Flue Gas Desulphurization (FGD) systems on unit 1 and unit 2. This 1-hour SO₂ limit was based on air dispersion modeling that demonstrated attainment of the 2010 1-hour SO₂ NAAQS. The CEV rates used in the demonstration analysis for each of the four sources are summarized in Table 1, in this document. The modeled emission rate in grams per second (g/s) was converted to pounds per hour (lbs/hr), which is the CEV.^[9]

Table 1—Critical Emission Values (CEV) From Indiana, PA SIP Modeling Demonstration

Facility	Modeled rate (g/s)	CEV limit (lbs/hr)
Conemaugh Generating Station	398.02731	3,159
Homer City Generating Station, Unit 1	195.29672	1,550
Homer City Generating Station, Unit 2	195.29672	1,550
Homer City Generating Station, Unit 3	410.75310	3,260
Keystone Generating Station	1,224.44741	9,718
Seward Generating Station	482.57189	3,830

Using the EPA conversion factor for the SO₂ NAAQS, the maximum 1-hour CEV model run design values for Domain 1 (196.00 μg/m^[3] ), Domain 2 (187.51 μg/m^[3] ) and Domain 3 (195.99 μg/m^[3] ) of the Indiana Area are less than 75 ppb.^[10] EPA has reviewed the modeling that Pennsylvania submitted to support the attainment demonstration for the Indiana Area and has determined that the AERMOD modeling is consistent with CAA requirements, appendix W to 40 CFR part 51, and EPA's 2014 SO₂ Guidance for SO₂ attainment demonstration modeling. Unlike the 2017 SIP submittal which the EPA partially disapproved in 2022 (87 FR 50778, August 18, 2022), PADEP's 2023 SIP submittal used an appropriate analysis to show that the modeled 1-hour CEV and longer-term emission limits were comparably stringent. In doing so, the 2023 SIP submission followed EPA guidance to develop an adjustment factor to convert the modeled 1-hour CEV to the comparably stringent longer-term emission limit. The 2023 SIP submission appropriately developed the adjustment factor by comparing the 99th percentile of historic hourly emissions to the 99th percentile of the longer-term averaged emissions of the same dataset to develop the longer-term emission limits. Conversely, the 2017 SIP submittal developed longer-term limits based on a novel modeling approach with a different variability metric without appropriate justification—PADEP had not demonstrated that the longer-term emission limits would provide for attainment under worst-case scenarios permissible under the limits. (87 FR 15166 at 15171-74, March 17, 2022).

EPA's review supports PADEP's modeling methodology and conclusions. More information about EPA's review of PADEP's attainment demonstration and modeling can be found in EPA's March 2024 “Technical Support Document the Critical Emissions Value Modeling Analysis for the Indiana, PA 1-Hour SO₂ Nonattainment Area” under Docket ID No. EPA-R03-OAR-2024-0024 and online at www.regulations.gov.

1. Longer-Term Emission Limits

The 2017 SIP submittal established longer-term average SO₂ limits for Keystone, Conemaugh, and Seward, and a 1-hour SO₂ limit for Homer City. As described above, the limits in the 2017 submittal for Keystone and Seward were based on a novel modeling approach and an analysis that did not demonstrate that the longer-term emission limits were comparably stringent to the 1-hour CEV. (87 FR 15166 at 15171-74, March 17, 2022). EPA thus disapproved the longer-term average SO₂ limits for Keystone and Seward as not properly characterizing maximally possible emissions. (87 FR 15166 at 15173, March 17, 2022). Nonetheless, the EPA retained the limits as SIP strengthening in its partial approval and partial disapproval. (87 FR 15166 at 15176, March 17, 2022).

PADEP's 2023 SIP submittal established revised longer-term average SO₂ emission limits for Keystone, Conemaugh, and Seward facilities and retained the 1-hour SO₂ emission limit previously established and approved for the Homer City facility.^[11] PADEP's 2023 SIP submittal established comparably stringent limits because PADEP used the ratio of the 99th percentile values of the hourly and longer-term emission rates as the adjustment factors for calculating the longer-term limits.^[12] The revised longer-term emission limits were calculated from the 1-hour SO₂ CEVs using adjustment factors that correspond to the averaging periods already established in emission limits for each facility ( i.e., Seward's emission limit uses a 30-operating day averaging period, Keystone uses a 24-hour block averaging period, and Conemaugh uses a 3-hour block averaging period). The adjustment factors which are used for deriving longer-term emission limits that are as comparably stringent as the 1-hour SO₂ CEVs were calculated in accordance with the EPA's 2014 SO₂ Nonattainment Guidance. All and only operating hours with measured values were used in the calculations.^[13] PADEP utilized four years of stable operations hourly emissions data from 2018-2022. In accordance with the 2014 SO₂ Nonattainment Guidance's recommendation to use data from years with stable operations, data from March through September of 2020, during which operations at Keystone and Conemaugh shifted toward low-load conditions as a result of the COVID-19 pandemic, were excluded from adjustment factor calculations for both stations. To have a complete four calendar years' worth of data, data from March through September of 2022 were used as replacement for the March through September of 2020 data. The calculation of the adjustment factors is described in detail in appendix B of the state submittal and was based on the data reduction criteria and average emission rate calculation established for demonstrating compliance with the longer-term emission limits. For example, Seward's 30-operating day rolling average is the average of all the hourly emission data, using only hours during which fuel is combusted from the preceding 30 operating days. An operating day is defined as a 24-hour period between 12midnight and the following 12 midnight during which any fuel is combusted at any time. This compliance approach is the same as the calculations and definitions used in developing the adjustment factor for this source.

The 1-hour SO₂ CEVs, the adjustment factors, the longer-term SO₂ emission limits, and the averaging periods for the three other facilities are summarized in Table 2, in this document.

Table 2—Sources in Indiana, PA NAA With Longer-Term SO ₂ Emission Limits

Source	1-Hour CEV (lbs/hr)	Adjustment factor	Longer-term limit (lbs/hr)	Averaging period
Keystone	9,718	0.857	8,328	24-hr block.
Conemaugh	3,159	0.975	3,080	3-hour block.
Seward	3,830	0.756	2,895	30-operating day rolling.

Additionally, PADEP implemented a supplemental measure to control any potential hourly emissions spikes at Seward station. Seward shall inject limestone into Source ID 034 and Source ID 035 during initial firing each time Source ID 034 and Source ID 035 are operated to reduce the magnitude and frequency of SO₂ emission spikes in accordance with good air pollution control practices.

The EPA reviewed PADEP's adjustment factor calculations, including the selected years of emissions data and the exclusion of March through September of 2020 due to the COVID pandemic and the claim that the operation of Keystone and Conemaugh was not considered stable during that time period. The EPA notes that removing this period of time, and adding the period of March through September of 2022, produced similar adjustment factors as would have been calculated without replacing the data. The EPA reviewed the justification provided by PADEP regarding this issue and concludes that PADEP properly characterized the hourly load impact of the COVID pandemic in the data ( i.e., shift from high load to low load operation during this time), and properly included data where stable operation of the sources was verified. PADEP followed the EPA's 2014 SO₂ Nonattainment Guidance in developing the comparably stringent longer-term limits for Seward, Conemaugh and Keystone. The EPA is proposing to approve the longer-term emission limits described above as being comparably stringent to the 1-hour CEV for Seward, Conemaugh and Keystone, and as correcting the deficiencies of the 2017 submittal previously identified in 2022 by removing the previously approved (and retained as SIP strengthening) longer-term averaging SO₂ limits for Keystone, Conemaugh, and Seward as described in the RACM/RACT section that follows.

B. RACM/RACT and Enforceable Emission Limitations

Section 172(c)(1) of the CAA requires states to adopt and submit all RACM, including RACT, as needed to attain the standards as expeditiously as practicable. Section 172(c)(6) requires the SIP to contain enforceable emission limits and control measures necessary to provide for timely attainment of the standard.

Pennsylvania's submittal discusses that the main SO₂ emitting sources at Conemaugh, Homer City, Keystone, and Seward are all equipped with FGD systems (wet limestone scrubbers, dry FGD, or in-furnace limestone injection systems) to reduce SO₂ emissions. Table 3, in this document, lists the control technology at each of the main SO₂ emitting sources at each facility.

Table 3—Control Technology at the Four Major SO ₂ Sources in the Indiana Area

Facility	Unit	SO 2 control	Control installation date
Conemaugh	031—Main Boiler 1	Wet limestone scrubber	~1994
	031—Main Boiler 2	Wet limestone scrubber	~1995
Homer City	031—Boiler 1	Dry FGD	11/18/2015
	032—Boiler 2	Dry FGD	5/23/2016
	033—Boiler 3	Wet limestone scrubber	~2002
Keystone	031—Boiler 1	Wet limestone scrubber	9/24/2009
	032—Boiler 2	Wet limestone scrubber	11/22/2009
Seward	034—CFB Boiler 1	In-furnace limestone injection	~2004
	035—CFB Boiler 2	In-furnace limestone injection	~2004

With these controls installed, Pennsylvania's submittal discusses facility-specific control measures, namely SO₂ emission limits for Homer City, Conemaugh, Seward and Keystone. Homer City has a 1-hour averaging period emission limit which was previously in its existing Title V Operating Permit (TVOP). The 1-hour SO₂ CEV is equivalent to the 1-hour SO₂ emission limit in the current TVOP #32-00055.

PADEP issued Consent Order and Agreements (COAs) with both Keystone and Conemaugh on August 15, 2023, as well as Seward on August 17, 2023 (2023 COAs), which established new emission limits that were demonstrated to provide for attainment in the Indiana, PA Area. PADEP has asked the EPA to incorporate into the SIP the following updated combination of SO₂ emission limits for these three facilities (as well as the compliance strategies listed in the unredacted portion of the COAs found in appendix C of the state submittal):

• Keystone—Remove 9,600 lbs/hr on a 24-hour (daily) block average and replace with 8,328 lbs/hr combined based on a 24-hour block average for Boiler 1 & Boiler 2 (Source IDs 031 & 032).
• Seward—Remove 3,038.4 lbs/hr and replace with 2,895 lbs/hr combined based on a 30-day operating hours average rolling by one day for Source IDs 034 & 035. Remove 13,308 tpy and replace with 12,680 tpy combined for Source IDs 034 & 035.^[14] Add the requirement to inject limestone into Source ID 034 and Source ID 035 during initial firing each time Source ID 034 and Source ID 035 are operated to reduce the magnitude and frequency of SO₂ emission spikes in accordance with good air pollution control practices.

• Conemaugh—Add 3,080 lbs/hr combined on a 3-hour block average for Units 1 & 2 (Source IDs 031 & 032).^[15]

These emissions limits and associated compliance parameters will be federally enforceable upon the EPA's approval of the SIP. The EPA retained Homer City's 1-hour SO₂ emission limit as a SIP strengthening measure in its 2022 Partial Approval/Partial Disapproval and now proposes to retain that limit as part of PADEP's attainment demonstration here.

The emission limits described here have been shown to provide for attainment of the NAAQS, and thus the EPA is proposing to determine that these are emissions limitations as defined under CAA section 302(k) that are necessary and appropriate to meet the applicable requirements of the CAA under CAA section 110(a)(2)(A), including that the state's plan satisfies requirements for RACM/RACT under CAA section 172(c)(1) and includes enforceable emission limitations as may be necessary and appropriate to provide for attainment of the NAAQS under CAA section 172(c)(6). The EPA is also proposing to determine that the removal of Keystone and Seward's previously SIP-approved SO₂ emission limits (87 FR 50778, August 18, 2022) and replacement with the new SO₂ emission limits listed in Table 2 of this document does not pose an issue with respect to CAA section 110(l) or 193.^[16]

C. Reasonable Further Progress (RFP)

Section 172 of the CAA requires Pennsylvania's attainment plan to provide for RFP toward attainment. The relationship between SO₂ and sources is more directly quantifiable as compared to other NAAQS pollutants, and there is usually a single step between pre-control nonattainment and post-control attainment. Therefore, for SO₂ SIPs, which address a small number of affected sources, requiring expeditious compliance with attainment emission limits can address the RFP requirement. To be approved by the EPA under CAA section 192(a), attainment plans need to provide for future attainment of the NAAQS as expeditiously as practicable, but no later than 5 years from the effective date of the area's designation as nonattainment. For areas designated nonattainment effective October 4, 2013, attainment plans were required to contain demonstrations that the area would attain as expeditiously as practicable, but no later than October 4, 2018.

The four sources in the Indiana, PA NAA were subject to federally enforceable SO₂ emissions limits since the EPA's initial approval of the 2017 SIP submittal on October 19, 2020 (85 FR 66240, October 19, 2020). After the 2022 Partial Approval/Partial Disapproval of the 2017 SIP submittal, those emission limits remained in the SIP as SIP strengthening measures. The appropriate SO₂ limits were already established for Homer City effective February 28, 2017 in the state permit, and remain the same as they were since being incorporated into the SIP effective November 18, 2020. For the remaining three sources, Keystone, Seward, and Conemaugh, due to the timeline of events, it was not practical for Pennsylvania to have a compliance schedule which provided for attainment no later than 5 years from the area's designation of nonattainment ( i.e., October 4, 2018). However, in response to the EPA's 2022 partial disapproval of its SIP for the Indiana, PA NAA, Pennsylvania acted quickly in establishing new emission limits which provide for attainment in the Indiana, PA NAA as expeditiously as practicable with this 2023 SIP submittal. Through the aforementioned COAs dated August 15, 2023 for Keystone and Conemaugh and August 17, 2023 for Seward, the new limits were effective immediately after the date of each of the 2023 COAs. The EPA asserts that PADEP established the emission limits as expeditiously as practicable to provide for attainment in the Indiana, PA NAA and to remedy the EPA's 2022 partial disapproval, and therefore the EPA proposes to find that Pennsylvania's plan provides for RFP, based on the proposed determination that the revised emissions limitations as defined under CAA section 302(k) are necessary and appropriate to meet the applicable requirements of the CAA, including the RFP requirement of CAA section 172(c)(2).

D. Contingency Measures

Section 172 of the CAA requires that attainment plans include additional measures, called contingency measures, which will take effect if an area fails to meet RFP or fails to attain the standard by the attainment date. The EPA's 2014 SO₂ Nonattainment Guidance describes special features of SO₂ planning that influence the suitability of alternative means of addressing the requirement in CAA section 172(c)(9) for contingency measures for SO₂ . That is, SO₂ control measures are based on what is directly and quantifiably necessary to attain the SO₂ NAAQS, and consequently, an area that implements such control measures would be unlikely to fail to attain the NAAQS.^[17] Therefore, an appropriate means of satisfying the contingency measures requirement is for the state to have a comprehensive enforcement program that identifies sources of violations of the SO₂ NAAQS and for the state to undertake aggressive follow-up for compliance and enforcement. Pennsylvania's plan provides for satisfying the contingency measure requirement in this manner for the nonattainment area. PADEP has a comprehensive compliance and enforcement program to identify sources of violations of the 2010 1-hour SO₂ NAAQS and can undertake aggressive follow-up for compliance and enforcement including the ability to enact a COA in a timely manner (section 4(27) of the Pennsylvania Air Pollution Control Act, 35 P.S. section 4004(27)). The EPA is proposing to approve the emissions limits from the 2023 SIP submittal as enforceable limitations under CAA section 302(k) which are necessary and appropriate to provide for attainment of the standard in the Indiana, PA NAA and meet the requirements of the CAA, including sections 110(a)(2)(A), 172(c)(1), 172(c)(2), and 172(c)(6). Consequently, the EPA is proposing to find that PA's comprehensive enforcement program for such necessary and appropriate emission limitations is an appropriate contingency measure for this area and meets the requirement of CAA section 172(c)(9).

III. Proposed Action

The EPA is proposing to approve Pennsylvania's SIP revision submitted to the EPA on October 5, 2023, for the purpose of attaining the 2010 1-hour SO₂ NAAQS for the Indiana, PA NAA. Specifically, the EPA is proposing to approve the following elements of this SO₂ attainment plan: Pennsylvania's attainment demonstration for the nonattainment area, RACT/RACM and emission limitations, RFP plan, and contingency measures. The EPA previously approved Pennsylvania's attainment plan requirements regarding nonattainment area Emissions Inventory and NNSR.

The EPA is proposing to conclude that the modeling and comparably stringent longer-term emission limits in Pennsylvania's plan adequately demonstrate that the control requirements in the COAs provide for attainment in the area. This attainment plan also properly addresses requirements for RACT/RACM and emission limitations, RFP, and contingency measures because the plan now includes emission limits that provide for attainment. Thus, the EPA is proposing to determine that Pennsylvania's Indiana Area SO₂ attainment plan meets the applicable requirements of CAA sections 172, 191, and 192. The EPA is taking public comments for thirty days following the publication of this proposed action in the Federal Register . The EPA will take these comments into consideration in our final action.

IV. Incorporation by Reference

In accordance with requirements of 1 CFR 51.5, the EPA is proposing to incorporate by reference the SO₂ emission limits and compliance parameters established in (the unredacted portions of) the COAs for Seward, Conemaugh and Keystone facilities as discussed in section II. of this document. The EPA has made, and will continue to make, these materials generally available through https://www.regulations.gov and at the EPA Region III Office (please contact the person identified in the FOR FURTHER INFORMATION CONTACT section of this preamble for more information).

V. Statutory and Executive Order Reviews

Under the CAA, the Administrator is required to approve a SIP submission that complies with the provisions of the CAA and applicable Federal regulations. 42 U.S.C. 7410(k); 40 CFR 52.02(a). Thus, in reviewing SIP submissions, the EPA's role is to approve state choices, provided that they meet the criteria of the CAA. Accordingly, this action merely approves state law as meeting Federal requirements and does not impose additional requirements beyond those imposed by state law. For that reason, this proposed action:

• Is not a significant regulatory action subject to review by the Office of Management and Budget under Executive Orders 12866 (58 FR 51735, October 4, 1993) and 13563 (76 FR 3821, January 21, 2011);
• Does not impose an information collection burden under the provisions of the Paperwork Reduction Act (44 U.S.C. 3501 et seq.);
• Is certified as not having a significant economic impact on a substantial number of small entities under the Regulatory Flexibility Act (5 U.S.C. 601 et seq.);
• Does not contain any unfunded mandate or significantly or uniquely affect small governments, as described in the Unfunded Mandates Reform Act of 1995 (Pub. L. 104-4);
• Does not have federalism implications as specified inExecutive Order 13132 (64 FR 43255, August 10, 1999);
• Is not an economically significant regulatory action based on health or safety risks subject toExecutive Order 13045 (62 FR 19885, April 23, 1997);
• Is not a significant regulatory action subject toExecutive Order 13211 (66 FR 28355, May 22, 2001); and
• Is not subject to requirements of Section 12(d) of the National Technology Transfer and Advancement Act of 1995 (15 U.S.C. 272 note) because application of those requirements would be inconsistent with the Clean Air Act.

Executive Order 12898 (Federal Actions to Address Environmental Justice in Minority Populations and Low-Income Populations, 59 FR 7629, February 16, 1994) directs Federal agencies to identify and address “disproportionately high and adverse human health or environmental effects” of their actions on minority populations and low-income populations to the greatest extent practicable and permitted by law. The EPA defines environmental justice (EJ) as “the fair treatment and meaningful involvement of all people regardless of race, color, national origin, or income with respect to the development, implementation, and enforcement of environmental laws, regulations, and policies.” The EPA further defines the term fair treatment to mean that “no group of people should bear a disproportionate burden of environmental harms and risks, including those resulting from the negative environmental consequences of industrial, governmental, and commercial operations or programs and policies.” PADEP did not evaluate environmental justice considerations as part of its SIP submittal; the CAA and applicable implementing regulations neither prohibit nor require such an evaluation. The EPA did not perform an EJ analysis and did not consider EJ in this action. Due to the nature of the action being taken here, this action is expected to have a neutral to positive impact on the air quality of the affected area. Consideration of EJ is not required as part of this action, and there is no information in the record inconsistent with the stated goal of E.O. 12898 of achieving environmental justice for people of color, low-income populations, and Indigenous peoples.

In addition, this proposed rulemaking, approval of Pennsylvania's Indiana Area SO₂ attainment plan, does not have tribal implications as specified by Executive Order 13175 (65 FR 67249, November 9, 2000), because the SIP is not approved to apply in Indian country located in the State, and EPA notes that it will not impose substantial direct costs on tribal governments or preempt tribal law.

List of Subjects in 40 CFR Part 52

• Environmental protection
• Air pollution control
• Incorporation by reference
• Intergovernmental relations
• Reporting and recordkeeping requirements
• Sulfur oxides

Adam Ortiz,

Regional Administrator, Region III.

Footnotes

1.  78 FR 47191 (August 5, 2013).

2.   Sierra Club, et al. v. EPA, Case No. 20-3568 (3d Cir.).

3.   www.epa.gov/​sites/​default/​files/​2016-06/​documents/​20140423guidance_​nonattainment_​sip.pdf.

4.  An “average year” is used to mean a year with average air quality. While 40 CFR part 50, appendix T, provides for averaging three years of annual 99th percentile daily maximum hourly values ( e.g., the fourth highest maximum daily hourly concentration in a year with 365 days with valid data), this discussion and an example below uses a single “average year” in order to simplify the illustration of relevant principles.

5.  For example, if the CEV is 1,000 pounds of SO₂ per hour, and a suitable adjustment factor is determined to be 70 percent, the recommended longer-term average limit would be 700 pounds per hour.

6.  SO₂ emission limits for Homer City that were used in the attainment modeling were already approved into the SIP. (87 FR 50778, August 18, 2022).

7.  The period of meteorological data needed for an air-quality analysis is described in section 8.4.2(e) of Appendix W: “The use of 5 years of adequately representative [National Weather Service] or comparable meteorological data, at least 1 year of site-specific, or at least 3 years of prognostic meteorological data, are required.”

8.  Plan Approval 32-00055H was issued on April 2, 2012, and modified on April 4, 2013, by the DEP.

9.  Based on the National Institute of Standards and Technology conversion: 1 pound = 453.59237 grams.

10.  The SO₂ NAAQS level is expressed in ppb but AERMOD gives results in μg/m3. . The conversion factor for SO₂ (at the standard conditions applied in the ambient SO₂ reference method) is 1 ppb = approximately 2.619 μg/m3. . See Pennsylvania's SO₂ Round 3 Designations Proposed Technical Support Document at www.epa.gov/​sites/​production/​files/​2017-08/​documents/​35_​pa_​so2_​rd3-final.pdf.

11.  While at the time of publication, the evidence suggests that Homer City's three units have ceased operations, EPA's approval of this attainment plan is independent of Homer City's ceasing operations. Emissions data indicates that Units 1 and 2 last emitted on March 24, 2023, and December 11, 2022, and Unit 3 on May 17, 2023. However, as the EPA is not aware of PADEP rescinding Homer City's operating permits,-Homer City ceasing operations does not guarantee that the units are permanently and enforceably shutdown. Importantly, PADEP's 2023 SIP submittal and the accompanying attainment demonstration, which the EPA is proposing to approve, properly accounted for Homer City's continued operation. To be clear, the EPA's proposed approval of this attainment plan is based on Homer City's possible continued operation.

12.  Conemaugh's 3-hour block average emission limits in PADEP's October 11, 2017 submission for each individual unit was roughly in line with the CEV modeled limit and the ratio from appendix C in EPA's 2014 SO₂ Guidance. (87 FR 15166 at 15175, March 17, 2022). Nonetheless, in its 2023 SIP submittal PADEP included a combined 3-hour block average emission limit using the 99th percentile ratio to develop the adjustment factor to calculate Conemaugh's 3-hour block combined averaging SO₂ limit.

13.  Substituted values and nonoperating hours were not used in the calculations.

14.  The new annual limit is calculated to be consistent with the new 30-day limit, and is considered a supplemental limit.

15.  Conemaugh's new 3,080 lbs/hr combined 3-hour block average limit for Units 1 and 2 is in addition to the emission limits retained as SIP strengthening in the 2022 Partial Approval/Partial Disapproval. Specifically, this action does not remove the 1,656 lbs/hr emission limit on a 3-hour block average for Units 1 and 2 individually.

16.  Under CAA sections 110(l) and 193, the EPA may not approve a SIP that would interfere with any applicable requirement concerning NAAQS attainment and RFP, or any other applicable requirement, and no requirement in effect before November 15, 1990 (or required to be adopted by an order, settlement, agreement, or plan in effect before November 15, 1990), in any area which is a nonattainment area for any air pollutant, may be modified in any manner unless it ensures equivalent or greater emission reductions of such air pollutant. The newly established emissions limit for Keystone of 8,328 on 24-hour block period is more stringent that the previously SIP-approved emission limit of 9,600 lb/hr on a 24-hour block period, and the newly established emission limits for Seward of 2,895 lb/hr on a 30-operating day rolling average is more stringent that the previously SIP-approved emission limit of 3,084.4 lb/hr on a 30-operating day rolling average.

17.  See 75 FR 35520 at 35576 (June 22, 2010) and the 2014 SO₂ Nonattainment Guidance.