SUPPLEMENTARY INFORMATION:

Throughout this document, “we,” “us,” and “our” refer to the EPA.

Table of Contents

I. Regulatory Context and Background

A. PM₁₀ NAAQS and West Pinal Area Designation and Classification

B. CAA NNSR Requirements for PM₁₀ Nonattainment Area SIPs Under CAA Section 189(e)

C. The EPA's Previous Actions on the ADEQ's NNSR Program for PM₁₀ for the West Pinal PM₁₀ Nonattainment Area

II. The ADEQ's 2023 SIP Submission Addressing the Requirements of CAA Section 189(e) for the ADEQ's PM₁₀ NNSR Program in the West Pinal PM₁₀ Nonattainment Area

III. Evaluation of PM₁₀ Precursors in the West Pinal PM₁₀ Nonattainment Area for the ADEQ's NNSR Program Under CAA Section 189(e) and 40 CFR 51.165(a)(10)

A. The EPA's PM₁₀ Precursor Evaluation and Determination

B. Determination Regarding the ADEQ's NNSR Program for PM₁₀ in the West Pinal PM₁₀ Nonattainment Area

IV. Proposed Action

V. Statutory and Executive Order Reviews

I. Regulatory Context and Background

A. PM₁₀ NAAQS and West Pinal Area Designation and Classification

The EPA sets the NAAQS for certain ambient air pollutants at levels required to protect human health and the environment. Particulate matter with an aerodynamic diameter less than or equal to a nominal ten micrometers, or PM₁₀, is one of these ambient air pollutants for which the EPA has established health-based standards. On July 1, 1987, the EPA promulgated two primary standards for PM₁₀ : a 24-hour standard of 150 micrograms per cubic meter (µg/m³ ); and an annual PM₁₀ standard of 50 µg/m³ . The EPA also promulgated secondary PM₁₀ standards that were identical to the primary standards. 52 FR 24634 (July 1, 1987). Because they are identical, we refer to the primary and secondary standards using the singular term, NAAQS. Effective December 18, 2006, the EPA revoked the annual PM₁₀ NAAQS but retained the 24-hour PM₁₀ NAAQS. 71 FR 61144 (October 17, 2006).

Effective July 2, 2012, the EPA designated a portion of state lands in Pinal County, Arizona, (“West Pinal”) as nonattainment for the 1987 p.m.₁₀ NAAQS.^[1] West Pinal is located in central Arizona, southeast of the Phoenix metropolitan area and northwest of the city of Tucson. Pinal County covers 5,365 square miles and has two distinct western and eastern regions with different characteristics relevant to pollution formation. The West Pinal PM₁₀ nonattainment area is located within the western region.

As a result of this nonattainment designation, EPA initially classified West Pinal as a “Moderate” PM₁₀ nonattainment area.^[2] Pursuant to CAA section 188(b)(2), the EPA later determined that the West Pinal nonattainment area had not attained the 1987 24-hour PM₁₀ NAAQS by the attainment date for the area, which resulted in the area's reclassification as a “Serious” PM₁₀ nonattainment area, effective July 24, 2020.^[3]

B. CAA NNSR Requirements for PM₁₀ Nonattainment Area SIPs Under CAA Section 189(e)

The Act's preconstruction permitting requirements for stationary sources, also called New Source Review (NSR), are found in title I of the Act and in the EPA's implementing regulations addressing the SIP requirements for state NSR programs at 40 CFR part 51, subpart I. The EPA's designation of West Pinal as a nonattainment area for the 1987 PM₁₀ NAAQS triggered the requirement for the ADEQ to develop and submit an NNSR program to the EPA for SIP approval for that nonattainment area pursuant to CAA sections 172(c)(5) and 173 and 40 CFR 51.160-51.165, which must satisfy the NNSR requirements applicable to PM₁₀ nonattainment areas.

Among other CAA requirements applicable to PM₁₀ nonattainment areas, section 189(e) of the Act requires that the control requirements for major stationary sources of direct PM₁₀ also apply to major stationary sources of PM₁₀ precursors, except where the EPA determines that such sources do not ( printed page 43981) contribute significantly to PM₁₀ levels that exceed the standard in the area. The EPA's regulations implementing state NNSR program requirements likewise provide that the state NNSR program requirements at 40 CFR 51.165 that are applicable to major stationary sources and major modifications of PM₁₀ must also apply to major stationary sources and major modifications of PM₁₀ precursors, except where the EPA determines that such sources do not contribute significantly to PM₁₀ levels that exceed the PM₁₀ NAAQS in the area. 40 CFR 51.165(a)(10).

C. The EPA's Previous Actions on the ADEQ's NNSR Program for PM₁₀ for the West Pinal PM₁₀ Nonattainment Area

In November 2015, the EPA issued a final rule taking action on a comprehensive 2012 NSR SIP submittal from the ADEQ that constituted a major update to the ADEQ's NSR program (“2015 NSR action”). 80 FR 67319 (Nov. 2, 2015). The EPA determined that this SIP submittal met most NSR requirements and significantly strengthened ADEQ's NSR program. However, the EPA also identified deficiencies with various aspects of the SIP submittal, and therefore the EPA's 2015 NSR action was a limited approval and limited disapproval of the ADEQ's NNSR program for all areas within the ADEQ's jurisdiction, including but not limited to the West Pinal PM₁₀ nonattainment area, based on the program not meeting certain NNSR program requirements. 80 FR 67319, 67321, 67332.

The EPA's 2015 NSR action also included additional actions, including a separate limited approval of the ADEQ's NNSR program for PM₁₀ only with respect to the West Pinal PM₁₀ nonattainment area, based on the requirements of section 189(e) of the Act, without an accompanying limited disapproval. 80 FR 67319, 67332; see also 80 FR 14044, 14058, 14060 (Mar. 18, 2015) (proposed rulemaking for the 2015 NSR action).^[4] The EPA explained the basis for this limited approval specific to the West Pinal PM₁₀ nonattainment area in its proposal for the 2015 NSR action.

In the proposal, the EPA observed that CAA section 189(e) requires that the ADEQ's NNSR program for PM₁₀ nonattainment areas apply to major stationary sources of PM₁₀ precursors unless the EPA determines that such sources do not contribute significantly to PM₁₀ levels, which exceed the standard in the area. The EPA noted that while the ADEQ's NSR SIP submittal generally included NNSR requirements for PM₁₀ nonattainment areas, such as the PM₁₀ significant emissions rate, the regulation of PM₁₀ condensable emissions, and the applicable emissions offset requirements, the submittal contained a number of deficiencies. The EPA explained that as a result of these deficiencies it was proposing a limited approval and limited disapproval of the ADEQ's NNSR submittal.

More importantly for the purposes of this proposed action, the EPA explained that separate and aside from the issues identified in the EPA's proposed limited approval and limited approval of ADEQ's NNSR submittal, the EPA was not prepared at that time to grant full approval to the ADEQ's NSR SIP submittal as to the PM₁₀ NNSR program requirements specifically for the West Pinal nonattainment area in light of the requirements in CAA section 189(e). The EPA explained that the evaluation of which PM₁₀ precursors need to be controlled to achieve the PM₁₀ standard in a particular area is typically conducted in the context of the state's preparation and the EPA's review of an area's attainment plan. The EPA further explained that it did not have such a plan before it for West Pinal at that time and therefore did not have the state's precursor analysis, and that it could not fully approve, as complying with the CAA, a NNSR SIP submission that did not address PM₁₀ precursors. 80 FR 14044, 14058.

The EPA further noted in the proposal that while the ADEQ's submittal may not contain all of the elements necessary to satisfy the CAA NNSR requirements, the revisions to the ADEQ's NNSR program represented a considerable strengthening of the Arizona SIP, which, until that time, did not address NSR requirements for PM₁₀ at all. The EPA therefore proposed a limited approval of the PM₁₀ NNSR provisions in the ADEQ's NSR submittal as they apply to the West Pinal PM₁₀ nonattainment area, recognizing the need to address the requirements of CAA section 189(e). The EPA indicated that once the ADEQ submitted a new PM₁₀ attainment plan for the West Pinal PM₁₀ nonattainment area, the EPA would consider whether a limited disapproval should also be proposed based on CAA section 189(e). 80 FR 14044, 14058. The EPA received no comments on our proposed limited approval for the ADEQ's NNSR program for PM₁₀ for the West Pinal PM₁₀ nonattainment area based on CAA section 189(e), and we finalized this portion of our action as proposed. See 80 FR 67319, 67332 and n.29.

Following the 2015 NSR action, the ADEQ submitted, and the EPA acted on, a number of subsequent NSR SIP submittals, both to address the deficiencies that formed the bases for the EPA's limited disapproval of the ADEQ's NSR program in the 2015 NSR action and to make other updates to its NSR program. Among these actions, in a 2018 rulemaking, the EPA reviewed and approved revisions to the ADEQ's NNSR program into the Arizona SIP that were intended to address the deficiencies with the ADEQ's NNSR program that had been identified by the EPA in its 2015 limited disapproval of the ADEQ's NNSR rules. See 83 FR 19631 (May 4, 2018) (the “2018 Major NSR action”). The EPA determined in the 2018 Major NSR action that the ADEQ's revised NNSR rules addressed the deficiencies in the ADEQ's NNSR program under Part D of the CAA that had formed the basis for its limited disapproval of the NNSR program in the 2015 NSR action for PM₁₀ and other nonattainment pollutants.^[5] The issue of whether PM₁₀ precursors must be regulated by the ADEQ under the NNSR program in the West Pinal PM₁₀ nonattainment area pursuant to CAA section 189(e) and 40 CFR 51.165(a)(10) was not a basis for the EPA's limited disapproval in its 2015 NSR action, and the ADEQ SIP submittal that the EPA acted on in the 2018 Major NSR action did not address this issue; therefore, the EPA did not address the issue as part of the 2018 Major NSR action.^[6]

II. The ADEQ's 2023 SIP Submission Addressing the Requirements of CAA Section 189(e) for the ADEQ's PM 10 NNSR Program in the West Pinal PM 10 Nonattainment Area

On December 15, 2023, the ADEQ submitted the “2023 Five Percent Particulate Plan for PM-10 for the West Pinal County Nonattainment Area” (“2023 PM₁₀ Plan”) as a revision to the Arizona SIP. The ADEQ is the ( printed page 43982) governor's designee for submitting official revisions of the Arizona SIP to the EPA. The EPA reviewed this December 2023 SIP submission for completeness, and we issued a letter dated June 14, 2024, in which it determined that the SIP revision met the completeness criteria in 40 CFR part 51, appendix V.^[7]

The 2023 PM₁₀ Plan includes Chapter 3, an “Assessment of Air Quality Conditions,” which discusses PM₁₀ formation in the West Pinal PM₁₀ nonattainment area and summarizes the State's PM₁₀ precursor evaluation and determination that precursors do not contribute significantly to PM₁₀ levels that exceed the NAAQS in the West Pinal PM₁₀ nonattainment area. In addition, Chapter 9 of the 2023 PM₁₀ Plan, Appendix A, Exhibit 1, Attachment A, includes a “Weight of Evidence Demonstration that Particulate Matter Precursors Do Not Significantly Contribute to PM-10 Exceedances in the West Pinal County PM-10 Nonattainment Area.”

The information described above that is included in this plan submittal supplements the information contained in the 2012 NSR SIP submittal. It also provides a basis on which the EPA may now make the necessary determination, for purposes of the ADEQ's NNSR program for PM₁₀, pursuant to CAA section 189(e) and 40 CFR 51.165(a)(10). The information allows the EPA to determine whether major stationary sources and major modifications of PM₁₀ precursors contribute significantly to PM₁₀ levels that exceed the PM₁₀ NAAQS in the West Pinal PM₁₀ nonattainment area, and therefore must be regulated as part of the PM₁₀ NNSR program for West Pinal.

III. Evaluation of PM 10 Precursors in the West Pinal PM 10 Nonattainment Area for the ADEQ's NNSR Program Under CAA Section 189(e) and 40 CFR 51.165(a)(10)

A. The EPA's PM₁₀ Precursor Evaluation and Determination

This proposed action is focused on the EPA's determination concerning whether the ADEQ's NNSR program for PM₁₀ for the West Pinal PM₁₀ nonattainment area satisfies the requirements of section 189(e) and 40 CFR 51.165(a)(10).

Pursuant to CAA section 189(e) and 40 CFR 51.165(a)(10), for purposes of the ADEQ's NNSR program regulating PM₁₀, the EPA analyzed whether major stationary sources and major modifications of PM₁₀ precursors contribute significantly to PM₁₀ levels that exceed the PM₁₀ NAAQS in the West Pinal PM₁₀ nonattainment area. An EPA determination that such sources and modifications do not contribute significantly means that the ADEQ's NNSR program for PM₁₀ for this nonattainment area is consistent with CAA section 189(e) and 40 CFR 51.165(a)(10) without regulating PM₁₀ precursors.

The State's submission provided three lines of evidence to support its conclusion that PM₁₀ precursors do not significantly contribute to PM₁₀ exceedances in the West Pinal PM₁₀ nonattainment area: (1) an evaluation of co-located PM_2.5 /PM₁₀ ratios and concentrations on PM₁₀ exceedance days in the nonattainment area, (2) a chemical characterization of coarse and fine fraction particulate matter in the nonattainment area on PM₁₀ exceedance days, and (3) photochemical modeling of the impact of the precursors on PM_2.5 concentrations in the nonattainment area.

In its analysis of monitoring data from co-located PM_2.5 and PM₁₀ monitors, the State examined the ratio of PM_2.5 to PM₁₀ concentrations on exceedance days from 2016 to 2020. Because secondarily formed PM is generally smaller, falling into the PM_2.5 category, where the ratio of PM_2.5 to PM₁₀ is low, this is generally indicative of wind-blown or other fugitive dust sources. The data in the submitted analysis show that most exceedance days correspond to low PM_2.5 to PM₁₀ ratios, and the State has provided plausible justifications for those days where the ratio is somewhat higher.

In its analysis of the chemical characterization of coarse and fine fraction PM, the State shows that the concentration of secondary inorganic particulate matter is low, suggesting that ammonia (NH₃), sulfur dioxide (SO₂), and oxides of nitrogen (NO_X) are unlikely to contribute to PM₁₀ exceedances. The State also analyzed organic aerosols in captured PM and compared these data to chemically analyzed soil samples, reaching the conclusion that most of the organic portion of PM₁₀ was due to primary emissions. The submitted analysis also suggests that precursor contributions may have dropped in the last fifteen years, as activities leading to emissions of precursors in the nonattainment area have decreased during that time.

In its analysis of photochemical modeling, the State estimated precursor pollutant contributions to PM_2.5 using the Comprehensive Air Quality Model with Extensions (CAM_X) model. Anthropogenic emissions of NH₃, NO_X, SO₂, and volatile organic compounds (VOC) from the 2017 National Emissions Inventory were reduced by 70 percent in the modeling to determine the sensitivity of PM_2.5 concentrations to these precursor emissions. The modeling results show reduction of 0.5 µg/m³ or less when each precursor pollutant was reduced by 70 percent.

The EPA has evaluated the precursor demonstration submitted by the State and proposes to conclude that these lines of evidence, when taken together, sufficiently support the conclusion that precursor pollutants to PM₁₀ do not significantly contribute to PM₁₀ exceedances in the West Pinal PM₁₀ nonattainment area. Our Technical Support Document (TSD), which is included in the docket for this action, provides a more detailed discussion of our analysis supporting this determination.

B. Determination Regarding the ADEQ's NNSR Program for PM₁₀ in the West Pinal PM₁₀ Nonattainment Area

As described in section III.A. of this document, the EPA is proposing to determine that major stationary sources and major modifications of PM₁₀ precursors do not contribute significantly to PM₁₀ levels that exceed the PM₁₀ NAAQS in the West Pinal PM₁₀ nonattainment area for purposes of the ADEQ's NNSR program, and therefore the ADEQ's NNSR program for PM₁₀ for the West Pinal PM₁₀ nonattainment area is consistent with CAA section 189(e) and 40 CFR 51.165(a)(10) without regulating PM₁₀ precursors.

As discussed in section I.C. of this document, the EPA has previously reviewed and fully approved the ADEQ's NNSR program for all pollutants, including PM₁₀, in all areas within the ADEQ's jurisdiction, with the ( printed page 43983) exception of the ADEQ's NNSR program for PM₁₀ in the West Pinal PM₁₀ nonattainment area, for which the EPA issued a limited approval in its 2015 NSR action specifically based on the need for the EPA to make a determination regarding the program's consistency with CAA section 189(e) and 40 CFR 51.165(a)(10) for this area. In these prior actions on the ADEQ's NSR SIP submittals, the EPA determined that the ADEQ's NNSR program for PM₁₀ met all relevant CAA requirements in all areas, other than the requirements of CAA section 189(e) and 40 CFR 51.165(a)(10) in the West Pinal PM₁₀ nonattainment area.

The EPA's determination in this action that the ADEQ's NNSR program for PM₁₀ for the West Pinal PM₁₀ nonattainment area is consistent with CAA section 189(e) and 40 CFR 51.165(a)(10) resolves the issue that led the EPA to issue a limited approval of the 2012 comprehensive NSR submittal for the ADEQ's NNSR program for PM₁₀ for the West Pinal PM₁₀ nonattainment area in our 2015 NSR action, which is the only remaining issue to be resolved from the 2012 submittal regarding the full approvability of the ADEQ's NNSR program for PM₁₀ in West Pinal. Therefore, the EPA is now proposing full approval of the ADEQ's NNSR program for PM₁₀ for the West Pinal PM₁₀ nonattainment area based specifically on our determination that the program is consistent with the requirements of CAA section 189(e) and 40 CFR 51.165(a)(10). If finalized, this proposed approval would address the only remaining element from the 2012 submittal that was not acted upon in the 2015 NSR action.

IV. Proposed Action

For the reasons discussed in this document, as authorized in section 110(k)(3) of the Act, the EPA proposes to fully approve the ADEQ's NNSR program for PM₁₀ for the West Pinal PM₁₀ nonattainment area because we have determined that it fulfills the relevant CAA requirements. We will accept comments from the public on this proposal until October 14, 2025, and will consider those comments before taking final action.

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 Act 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 Act. Accordingly, this proposed action merely proposes to approve 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 underExecutive Order 12866 (58 FR 51735, October 4, 1993);
• Is not subject toExecutive Order 14192 (90 FR 9065, February 6, 2025) because SIP actions are exempt from review under Executive Order 12866;
• 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 subject toExecutive Order 13045 (62 FR 19885, April 23, 1997) because it proposes to approve a state program;
• 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 CAA.

In addition, the SIP is not approved to apply on any Indian reservation land or in any other area where the EPA or an Indian Tribe has demonstrated that a Tribe has jurisdiction. In those areas of Indian country, the rule does not have Tribal implications and will not impose substantial direct costs on Tribal governments or preempt Tribal law as specified by Executive Order 13175 (65 FR 67249, November 9, 2000).

90 FR 43980