2024-25575. Air Plan Approval; Arizona; Yuma 2015 8-Hour Ozone Nonattainment Area; Redesignation Request and Maintenance Plan

Table 1—Yuma Supersite Monitor (AQS ID No. 04-027-8011) Design Value for the 2015 Ozone NAAQS From 2008-2022

[Parts per million (ppm)] ^a

Year	Design value
2010	0.073
2011	0.073
2012	0.077
2013	0.076
2014	0.077
2015	0.076
2016	0.074
2017	0.072
2018	0.071
2019	0.071
2020	0.068
2021	0.067
2022	0.068
2023	0.070
a  Source: AQS, Design Value Report, dated May 1, 2024.

Consequently, based upon three complete, quality assured and certified data from 2021-2023, the EPA proposes to determine that the Yuma area has attained and continues to attain the 2015 ozone NAAQS.

B. The Area Must Have a Fully Approved SIP Meeting the Requirements Applicable for the Purposes of Redesignation Under Section 110 and Part D of the CAA

Sections 107(d)(3)(E)(ii) and (v) of the CAA require the EPA to determine that the area has a fully approved applicable SIP under CAA section 110(k) that meets all applicable requirements under section 110 and part D for the purposes of redesignation. We interpret the references to the “applicable implementation plan” and “applicable requirements” in section 107(d)(3)(E)(ii) and in 107(d)(3)(E)(v), respectively, to mean that a SIP must be fully approved only with respect to requirements that are applicable for purposes of redesignation. The CAA section 110 and part D requirements that are linked to a particular nonattainment area's designation and classification (except those directly related to attainment) are the relevant measures to evaluate in reviewing a redesignation request. Requirements that apply regardless of the designation of an area of a State are not applicable requirements for the purpose of redesignation, and the State will remain subject to these requirements after the nonattainment area is redesignated to attainment.^[43] The EPA may rely on prior SIP approvals in ( print page 87833) approving a redesignation request ^[44] as well as any additional measure or element it may approve in conjunction with a redesignation action.^[45]

1. State Implementation Plan Requirements Under Section 110

The general SIP elements and requirements set forth in CAA section 110(a)(2) include, but are not limited to, the following: (1) submittal of a SIP that has been adopted by the State after reasonable public notice and hearing; (2) provisions for establishment and operation of appropriate procedures needed to monitor ambient air quality; (3) implementation of a source permitting program; (4) provisions for the implementation of part C requirements for prevention of significant deterioration (PSD); (5) provisions for the implementation of part D requirements for nonattainment new source review permit programs; (6) provisions for air pollution modeling; and (7) provisions for public and local agency participation in planning and emission control rule development.

On numerous occasions, Arizona has submitted, and the EPA has approved provisions addressing the basic CAA section 110 provisions. The Arizona SIP contains enforceable emissions limitations, requires monitoring, compiling, and analyzing of ambient air quality data; requires preconstruction review of new or modified stationary sources; provides for adequate funding, staff, and associated resources necessary to implement its requirements; and provides the necessary assurances that the State maintains responsibility for ensuring that the CAA requirements are satisfied in the event that local or regional agencies are unable to meet their CAA obligations. Most recently, on September 4, 2024, the EPA finalized a partial approval and partial disapproval of SIP revisions submitted by the State of Arizona with respect to the requirements of CAA section 110(a)(2) for the 2015 ozone NAAQS.^[46] Specifically, the EPA partially disapproved the SIP revisions for CAA requirements in sections 110(a)(2)(C) (program for enforcement of control measures and regulation of new stationary sources), 110(a)(2)(D)(i)(II) (interference with maintenance of the NAAQS in any other State, or “prong 3”), 110(a)(2)(D)(ii) (interstate pollution abatement, CAA section 126), and 110(a)(2)(J) (PSD and visibility protection). The deficiencies are adequately addressed by existing Federal implementation plans.

In conclusion, we find that there are no outstanding or disapproved applicable SIP submittals that prevent redesignation of the Yuma area for the 2015 ozone NAAQS. Therefore, we propose to conclude that the State has met all SIP requirements for the Yuma area that are applicable for purposes of redesignation under section 110 of the CAA.

2. State Implementation Plan Requirements Under Part D

Part D of the CAA establishes the plan requirements for nonattainment areas. Section 172(c) in subpart 1 of part D sets forth the basic requirements of air quality plans for States with nonattainment areas that are required to submit plans on a schedule pursuant to CAA section 172(b). Subpart 2 of part D, which includes section 182 of the CAA, establishes specific requirements for ozone nonattainment areas depending on the areas' nonattainment classifications. The Yuma area is classified as Marginal nonattainment for the 2015 ozone NAAQS, and thus, the area is subject to the subpart 1 requirements contained in CAA sections 172(c) and 176, as well as CAA section 182(c). A thorough discussion of the requirements contained in sections 172(c) and 182(c) can be found in the “General Preamble for Implementation of Title I of the Clean Air Act Amendments of 1990.” ^[47]

CAA requirements for ozone nonattainment areas are cumulative in that “Extreme” areas must also meet the applicable requirements for the four lesser classifications: Marginal, “Moderate,” “Serious,” and “Severe.” Because the Yuma area was classified as Marginal nonattainment for ozone, the area is only required to meet the applicable Marginal ozone requirements.^[48] Since its classification as a Marginal nonattainment area, the State has adopted and the EPA has approved into the Arizona SIP the following Marginal area requirements for the Yuma area with respect to the 2015 ozone NAAQS: emissions inventories,^[49] emissions statements,^[50] and nonattainment new source review programs.^[51]

In conclusion, we find that the Yuma area has a fully approved SIP under CAA section 110(k) that satisfies the criterion for redesignation under sections 107(d)(3)(E)(ii) and (v) of the CAA.

C. The Area Must Show That the Improvement in Air Quality Is Due to Permanent and Enforceable Emissions Reductions

To approve a redesignation to attainment, section 107(d)(3)(E)(iii) of the CAA requires the EPA to determine that the improvement in air quality is due to emissions reductions that are permanent and enforceable, and that the improvement results from the implementation of the applicable SIP, applicable Federal air pollution control regulations, and other permanent and enforceable regulations. Under this criterion, a State must be able to reasonably attribute the improvement in air quality to permanent and enforceable emissions reductions. Attainment resulting from temporary reductions in emissions ( e.g., reduced production or shutdown due to temporary adverse economic conditions) or unusually favorable meteorology would not qualify as an air quality improvement due to permanent and enforceable emissions reductions.^[52]

ADEQ found that the Yuma area has demonstrated that the observed air quality improvements with respect to the 2015 ozone NAAQS are due to enforceable emissions reductions through the implementation of Federal vehicle standards, engine standards, and rules for boilers, reciprocating internal combustion engines, and electrical utility generation units. ADEQ found that, within the Yuma area, Federal regulations have been the primary measures contributing to permanent and enforceable emissions reductions, leading to attainment of the NAAQS.^[53]

The Federal motor vehicle program and Federal fuel standards for sulfur content in diesel have contributed to attainment of the 2015 ozone NAAQS in the Yuma area by reducing VOC and NO_X .^[54] Federal tier 2 and 3 motor vehicle standards implemented from 2004 to 2014 helped to reduce on-road mobile source VOC and NO_X emissions in the Yuma area through increasingly stringent emissions standards. Federal sulfur content standards for diesel fuel were implemented in conjunction with the Federal motor vehicle program standards. Sulfur occurs naturally in gasoline and interferes with the operation of catalytic converters on vehicles, which results in higher NO_X ( print page 87834) emissions. Lower sulfur content fuel has reduced sulfur dioxide (SO₂) emissions and allowed pollution control equipment to operate more effectively to reduce emissions of other pollutants as well.

Federal nonroad and stationary standards also contributed to attainment of the 2015 ozone NAAQS. Nonroad spark-ignition and recreational engine standards implemented between 2003 and 2020 have contributed to a 72 percent reduction in VOCs and an 80 percent reduction in NO_X .^[55] The boiler and reciprocating internal combustion engines national emissions standards for hazardous air pollutants (NESHAPs), as well as the utility mercury and air toxics standards (MATS) and new source performance standards (NSPS) rules for electricity generating units also have contributed to VOC and NO_X reductions in the Yuma area.^[56] Taken together, these Federal programs contributed to VOC and NO_X emissions reductions in the Yuma area.

With respect to the connection between the emissions reductions and the improvement in air quality, we also conclude that the air quality improvement in the Yuma area is due to permanent and enforceable emissions reductions and not the result of temporary or impermanent factors such as local economic downturn, temporary emissions reductions, or unusual or extreme weather patterns. Our conclusion is based on the observation that the ozone design value for the Yuma area has been below 0.070 ppm, the level of the 2015 ozone NAAQS, since 2020.^[57] In sum, ambient ozone concentrations in the Yuma area have been consistently below the NAAQS since 2020 and have not been subject to large swings and disparate observations that might result from a sudden facility closure or an extreme weather pattern.

In conclusion, we find that the improvement in ambient air quality in the Yuma area is due to permanent and enforceable reductions in emissions of VOC and NO_X, resulting from control measures such as (1) Federal vehicle standards and (2) stationary source control measures. Therefore, we propose to find that Arizona has satisfied the criterion for redesignation set forth at CAA section 107(d)(3)(E)(iii).

D. The Area Must Have a Fully Approved Maintenance Plan Under CAA Section 175A

Section 107(d)(3)(E)(iv) of the CAA requires, as a pre-condition to being redesignated from nonattainment to attainment, that the Administrator has fully approved a maintenance plan for the area as meeting the requirements of section 175A of the Act.

Section 175A of the CAA sets forth the elements of a maintenance plan for areas seeking redesignation from nonattainment to attainment. The maintenance plan is a SIP revision that provides for maintenance of the relevant NAAQS in the area for at least 10 years after redesignation. The Calcagni memo, dated September 4, 1992, provides additional guidance on the required content of a maintenance plan.

A maintenance plan should address the following five areas: the attainment emissions inventory, maintenance demonstration, monitoring network, verification of continued attainment, and a contingency plan. The attainment emissions inventory identifies the emissions level in the area that is sufficient to attain the 2015 ozone NAAQS based on emissions during a three-year period which had no monitored violations. To demonstrate maintenance of the 2015 ozone NAAQS, ADEQ has projected the attainment emissions inventory through the first maintenance period and ensured that it was less than or equal to the inventory at the time of attainment. A maintenance plan must also include provisions for continued operation of an appropriate air quality monitoring network. The State must show how it will track and verify the progress of the maintenance plan. Finally, the maintenance plan must include a list of potential contingency measures which ensure prompt correction of any violation of the 2015 ozone NAAQS. Based on our review and evaluation of the Yuma Maintenance Plan, as described in the following sections, we are proposing to approve the Plan as meeting the requirements of CAA section 175A.

1. Attainment Inventories and Projected Future Inventories

A maintenance plan should include an “attainment emissions inventory” of ozone precursors in the area to identify a level of emissions sufficient to attain the NAAQS.^[58] The attainment emissions inventory should be consistent with the EPA's most recent guidance on emissions inventories for nonattainment areas available at the time it was developed and should represent emissions during the timeframe associated with the ambient air quality monitoring data showing attainment of the NAAQS. The attainment inventory will generally be the actual inventory during the time period that the area attained the standard. The EPA has provided guidance for developing ozone emissions inventories in “Emissions Inventory Guidance for Implementation of Ozone and Particulate Matter National Ambient Air Quality Standards (NAAQS) and Regional Haze Regulations”.^[59]

ADEQ selected 2020 as the base year for the attainment inventory in the Yuma Maintenance Plan. The Yuma area attained the 2015 ozone NAAQS in 2020, and therefore, the emissions inventory from 2020 represents emissions levels consistent with continued attainment ( i.e., maintenance) of the NAAQS. We consider the selection of the 2020 base year inventory to be appropriate given that it was the most recent emissions inventory associated with the reporting schedule required under the Air Emissions Reporting Requirements rule at the time of development of the Plan.

Table 2 presents the VOC and NO_X emissions estimates contained in the Yuma Maintenance Plan for 2020 and presents the Plan's projected emissions inventories of ozone precursors for an interim year (2030) and the maintenance plan's horizon year (2037).^[60]

( print page 87835)

Table 2—Yuma Area 2020 and Projected 2030 and 2037 VOC and NO _X Ozone Season Day Emissions

[Pounds per day] ^a

Emissions source	2020		2030		2037
	VOC	NO X	VOC	NO X	VOC	NO X
Point	104	1,662	1,384	4,628	1,384	4,628
Nonpoint	5,754	1,410	6,281	1,556	6,738	1,660
Mobile—Nonroad	1,792	1,620	1,720	923	1,754	782
Mobile—On-road	8,939	5,412	4,908	2,747	4,222	2,390
Total b	16,589	10,104	14,293	9,854	14,098	9,460
a  Source: Yuma Maintenance Plan, 43.
b  Emissions have been added to the Mobile—On-road emissions source category to account for safety margins added to proposed motor vehicle emissions budget. See the Yuma Maintenance Plan, page 50.

The data shown in Table 3 in this document is based on the EPA's 2020 national emissions inventory (NEI). The inventory addresses point sources,^[61] nonpoint sources,^[62] nonroad mobile sources,^[63] and on-road mobile sources. Appendix A to the Yuma Maintenance Plan contains source-specific descriptions of emissions calculation procedures and sources of input data.

The EPA has reviewed the emissions inventory submitted by ADEQ and proposes to conclude that the Plan's inventory is based on reasonable assumptions and methodologies, and that the inventory is comprehensive, current, accurate, and consistent with applicable CAA provisions and the Calcagni memo. Therefore, we are proposing that the inventory is acceptable for use in demonstrating maintenance of the 2015 ozone NAAQS.

2. Maintenance Demonstration

Section 175A(a) of the CAA requires that the maintenance plan “provide for the maintenance of the national primary ambient air quality standard for such air pollutant in the area concerned for at least 10 years after the redesignation.” A State may generally demonstrate maintenance of the NAAQS by either showing that future emissions of a pollutant or its precursors will not exceed the level of the attainment inventory, or by conducting modeling that shows that the future mix of sources and emissions rates will not cause a violation of the NAAQS.^[64] Assumptions concerning emissions rates in maintenance demonstrations should generally reflect permanent, enforceable measures.^[65] Therefore, the analysis should assume that sources are operating at permitted levels (or historic peak levels), unless evidence is presented that such an assumption is unrealistic.^[66]

The Yuma Maintenance Plan demonstrates maintenance of the 2015 ozone NAAQS for a ten-year period beginning in 2027. ADEQ used the 2020 NEI and the Motor Vehicle Emission Simulator (MOVES) version 3.1 as the baseline to develop growth factors for point, nonpoint, mobile on-road, and mobile nonroad sources. For point sources, ADEQ used permitted potential to emit (PTE) levels of emissions, except for the Arizona Public Service (APS) Yucca Power Plant. ADEQ argues that, for the APS Yucca Power Plant, permitted PTE is calculated by assuming the facility would burn fuel oil at maximum capacity, overestimating emissions from the facility. ADEQ argues that the facility runs primarily on natural gas and burns little fuel oil each year.^[67] Therefore, ADEQ used historic high emissions from 2018 for the baseline. We agree that this method for estimating emissions from APS Yucca Power Plant is consistent with EPA guidance. For nonpoint sources, ADEQ used the 2020 NEI held constant, or multiplied by a projection factor based on population.^[68] ADEQ used MOVES 3.1 to model future on-road emissions, using Arizona Department of Transportation vehicle registration data and MOVES defaults.^[69] ADEQ used the EPA defaults for Yuma County and applied a population allocation factor to apportion the county-wide nonroad mobile source emissions to the Yuma nonattainment area. ADEQ also used MOVES to model future nonroad mobile source emissions.

Table 2 compares the VOC and NO_X emissions estimated for the Yuma area for 2020 with those projected for 2030 and 2037 by source category. The projected VOC and NO_X emissions show that VOC and NO_X emissions are projected to remain below the attainment levels throughout the 10-year period following redesignation.

3. Monitoring Network

Once an area has been redesignated, the State should continue to operate an appropriate air quality monitoring network in accordance with 40 CFR part 58 to verify the attainment status of the area.^[70] Data collected by the monitoring network are also needed to determine if the contingency provisions of the maintenance plan are triggered. ADEQ currently operates one ozone monitor (the Yuma Supersite monitor) in the Yuma area.

In the Yuma Maintenance Plan ^[71] ADEQ indicates its intention to continue operation of an air quality monitoring network to verify continued attainment of the 2015 ozone NAAQS.^[72] We approved ADEQ's State and Local Air Monitoring Stations (SLAMS) air quality network in their annual monitoring network plan for year 2023 ( print page 87836) on October 30, 2023, prior to ADEQ's submittal of the Yuma Maintenance Plan.^[73] We find ADEQ's commitment for continued ambient ozone monitoring set forth in the Yuma Maintenance Plan to be acceptable for the purpose of verifying continued maintenance in the Yuma area.

4. Verification of Continued Attainment

Each State should ensure that it has the legal authority to implement and enforce all measures necessary to maintain the NAAQS.^[74]

ADEQ has the legal authority to implement and enforce the requirements of the Yuma Maintenance Plan under title 49 of the Arizona Revised Statutes, “The Environment.” This includes the authority to adopt, implement, and enforce any emissions control contingency measures determined to be necessary to correct 2015 ozone NAAQS violations. To verify continued attainment, ADEQ committed to the continued operation of an ozone monitoring network that meets the EPA air quality surveillance requirements. In addition, ADEQ committed to track the progress of the maintenance plan through continued development and submission of periodic emissions inventories to the EPA and review of the inventory to determine whether changes could affect maintenance of the 2015 ozone NAAQS. These methods are sufficient for the purpose of verifying continued attainment.

5. Contingency Provisions

Section 175A(d) of the CAA requires that maintenance plans contain contingency provisions, as the EPA deems necessary, to promptly correct any violations of the NAAQS that occur after redesignation of the area. Such provisions must include a requirement that the State will implement all measures with respect to the control of the air pollutant concerned that were contained in the SIP prior to the area being redesignated to attainment. These contingency provisions are distinguished from contingency measures required for nonattainment areas under CAA section 172(c)(9) in that they are not required to be fully adopted measures that will take effect without further action by the State for the maintenance plan to be approved. The contingency provisions of a maintenance plan are, however, an enforceable part of the SIP and should ensure that contingency measures are adopted expeditiously once the Plan's contingency provisions are triggered by a specified event. Thus, a State should identify the specific indicators or triggers that will be used to determine when the contingency measures need to be implemented. Next, the maintenance plan should clearly identify the measures to be adopted, include a schedule and procedure for adoption and implementation of the measures, and contain a specific timeline for action by a State.

As required by section 175A of the CAA, ADEQ has adopted a contingency plan to address possible future ozone air quality problems.^[75] ADEQ identifies two triggers: a warning level response and action level response. The warning level response would be triggered if an annual 4th high monitored concentration is above the level of the 2015 ozone NAAQS. This would initiate an analysis to determine whether the high ozone concentrations indicate a trend towards high ozone levels. The study would include an assessment of meteorological conditions and an assessment of whether actual emissions have deviated significantly from the emissions projections contained in the maintenance plan, along with an evaluation of which source sectors are responsible for emissions increases. The action level response would be triggered if a certified design value exceeds the level of the 2015 ozone NAAQS. Within 18 months of the certified design value exceedance, ADEQ commits to identify and implement necessary control measures.

Potential contingency measures listed in the maintenance plan are those emissions controls or other measures that ADEQ may choose to adopt and implement in response to the contingency trigger. The contingency plan lists the following potential contingency provisions that will be considered for adoption and implementation by the applicable State agency, while the Plan indicates that the list is not considered to be exhaustive:

• Anti-idling control program for mobile sources, targeting diesel vehicles;
• Diesel exhaust retrofits;
• Traffic flow improvements;
• Park and ride facilities; and
• Rideshare/carpool program.

Upon our review of the Plan, we find that the contingency provisions in the Yuma Maintenance Plan clearly identify specific contingency measures, contain sufficient tracking and triggering mechanisms to determine when contingency provisions are needed, contain an adequate description of the process of recommending and implementing contingency measures, and contain specific timelines for action. Thus, we conclude that the contingency provisions of the Yuma Maintenance Plan are adequate to ensure prompt correction of a violation and therefore comply with section 175A(d) of the Act.

6. Motor Vehicle Emissions Budgets

Section 176(c) of the CAA requires Federal actions in nonattainment and maintenance areas to conform to the SIP's goals of eliminating or reducing the severity and number of violations of the NAAQS and achieving expeditious attainment of the standards. Conformity to the SIP's goals means that such actions will not: (1) cause or contribute to violations of the NAAQS, (2) worsen the severity of an existing violation, (3) or delay timely attainment of any NAAQS or any interim milestone.

Actions involving Federal Highway Administration (FHWA) or Federal Transit Administration (FTA) funding or approval are subject to the EPA's transportation conformity rule, codified in 40 CFR part 93, subpart A. Under this rule, regional transit authorities in nonattainment and maintenance areas coordinate with State and local air quality and transportation agencies, the EPA, FHWA, and FTA to demonstrate that an area's regional transportation plans and transportation improvement programs conform to the applicable SIP. This demonstration is typically done by showing that estimated emissions from existing and planned highway and transit systems are less than or equal to the motor vehicle emissions budgets (“budgets”) contained in submitted or approved control strategy SIPs and maintenance plans.^[76]

These control strategy SIPs and maintenance plans typically set budgets for criteria pollutants and/or their precursors to address pollution from cars and trucks. Budgets are generally established for specific years and specific pollutants or precursors. Maintenance plan submittals should identify budgets for transportation related VOC and NO_X emissions in the last year of the maintenance period.

For budgets in a maintenance plan to be approvable, they must meet, at a minimum, the EPA's adequacy criteria.^[77] To meet these requirements, ( print page 87837) the budgets must be consistent, when considered with emissions from all other sources, with maintenance of the NAAQS and reflect all the motor vehicle control measures relied upon for the maintenance demonstration. The EPA's process for determining adequacy of a budget consists of three basic steps: (1) providing public notification of a SIP submission; (2) providing the public the opportunity to comment on the budgets during a public comment period; and (3) making a finding of adequacy. The process for determining the adequacy of a submitted budget is codified at 40 CFR 93.118(f). The EPA can notify the public by either posting an announcement that the EPA has received SIP budgets on the EPA's adequacy website,^[78] or via a Federal Register notice of proposed rulemaking when the EPA reviews the adequacy of a maintenance plan budget simultaneously with its review and action on the SIP submittal itself.^[79]

The Yuma Maintenance Plan contains VOC and NO_X budgets for 2020, 2030, and 2037. Any and all comments on the approvability of the budgets should be submitted during the comment period stated in the Dates section of this document.

The EPA proposes to approve the 2020, 2030, and 2037 budgets in the Yuma Maintenance Plan for transportation conformity purposes. If the EPA issues a final action to approve the budgets, these budgets must be used in future transportation conformity determinations for the Yuma area for the 2015 ozone standard.

The new budgets, if approved in the final rulemaking, will be effective on the date of publication of the EPA's final rulemaking in the Federal Register . The applicable VOC and NO_X budgets for the Yuma nonattainment area are defined in Table 3.

Table 3—Proposed Motor Vehicle Emissions Budgets for the Yuma Area

[Pounds per ozone season day]

Budget year	VOC	NO X
2020	8,939	5,412
2030	4,908	2,747
2037	4,222	2,390
Source: Yuma Maintenance Plan, Table 7-1.