SUPPLEMENTARY INFORMATION:

Throughout this proposed rule, the use of “we,” “us,” or “our” is intended to refer to the EPA. We use multiple abbreviations and terms in this proposed rule. While this list may not be exhaustive, for ease of reading and for reference purposes, the EPA defines the following terms and acronyms here:

ADEQ Arizona Department of Environmental Quality

BACM Best Available Control Measures

BACT Best Available Control Technology

CAA Clean Air Act

CBI Confidential Business Information

EI Emissions Inventory

EPA U.S. Environmental Protection Agency

FHWA Federal Highway Administration

FTA Federal Transit Administration

MAG Maricopa Association of Governments

µg/m³  Micrograms per Cubic Meter

MPO Metropolitan Planning Organization

NAAQS National Ambient Air Quality Standards

NEI National Emission Inventory

NH₃  Ammonia

NO_X  Oxides of Nitrogen

PCAQCD Pinal County Air Quality Control District

PM Particulate Matter

PM₁₀  Particulate Matter with a Diameter of Ten Microns or Less

RACM Reasonably Available Control Measures

RFP Reasonable Further Progress

SIP State Implementation Plan

SO₂  Sulfur Dioxide

TSD Technical Support Document

VOC Volatile Organic Compounds

VMT Vehicle Miles Traveled

West Pinal County Pinal County (Part); West Pinal

Table of Contents

I. Executive Summary

A. What proposed action is the EPA taking?

B. What is the legal authority and what are the requirements?

II. PM₁₀ Air Quality Planning in the West Pinal County PM₁₀ Nonattainment Area

III. Overview of Applicable CAA Requirements

A. Emissions Inventories

B. PM₁₀ Precursors

C. Best Available Control Measures and Reasonably Available Control Measures Demonstration

D. CAA Section 189(d) Attainment Demonstration and Five Percent Requirement

E. Reasonable Further Progress and Quantitative Milestones

F. Contingency Measures

G. Transportation Conformity and Motor Vehicle Emissions Budgets

H. Adequate Authority

IV. Evaluation of the 2023 Five Percent Plan's Compliance With CAA Requirements

A. Emissions Inventories

B. PM₁₀ Precursors

C. Best Available Control Measures and Reasonably Available Control Measures Demonstration

D. CAA Section 189(d) Attainment Demonstration and Five Percent Requirement

E. Reasonable Further Progress and Quantitative Milestones

F. Contingency Measures

G. Transportation Conformity and Motor Vehicle Emissions Budgets

H. Adequate Legal Authority

V. Summary of Proposed Actions

VI. Statutory and Executive Order Reviews

I. Executive Summary

A. What proposed action is the EPA taking?

The EPA is proposing to approve a SIP revision submitted by the State of Arizona to meet all Moderate area, Serious area, and CAA section 189(d) requirements applicable to the West Pinal County PM₁₀ Nonattainment Area for the PM₁₀ NAAQS. The EPA is also proposing to approve the PM₁₀ motor vehicle emissions budgets included with the SIP revision for transportation conformity purposes and is starting the adequacy process for these budgets.

B. What is the legal authority and what are the requirements?

CAA sections 188 and 189 contain specific planning and control requirements relevant to the PM₁₀ NAAQS while CAA sections 110 and 172 contain general planning and control requirements for all nonattainment plans.

The EPA is proposing approval of the baseline and attainment year emission inventory requirements of CAA section 172(c)(3), the BACM demonstration requirements of CAA section 189(b)(1)(B), the RACM demonstration requirements of CAA sections 189(a)(1)(C) and 172(c)(1), the attainment demonstration requirements of CAA sections 189(d) and 179(d)(3), the five percent demonstration requirements of CAA section 189(d), the reasonable further progress and quantitative milestone demonstration requirements of CAA sections 172(c)(2) and 189(c), the contingency measures requirements of CAA section 172(c)(9), and the motor vehicle emissions budgets as meeting the criteria for budgets in CAA section 176(c) and 40 CFR 93.118(e). The EPA is also starting the adequacy process for these budgets under 40 CFR 93.188(f).

II. PM 10 Air Quality Planning in the West Pinal County PM 10 Nonattainment Area

The NAAQS are standards for certain ambient air pollutants set by the EPA to protect public health and welfare. Particulate matter with a diameter of ten microns or less (PM₁₀) is among the ambient air pollutants for which the EPA has established health-based standards. PM₁₀ causes adverse health effects by penetrating deep in the lungs, aggravating the cardiopulmonary system. Children, the elderly, and people with asthma and heart conditions are the most vulnerable.

On July 1, 1987, the EPA revised the health-based NAAQS for particulate matter, establishing two PM₁₀ standards, an annual standard and a 24-hour standard.^[1] Effective December 18, 2006, the EPA revoked the annual PM₁₀ standard but retained the 24-hour PM₁₀ standard.^[2] The 24-hour PM₁₀ standard of 150 micrograms per cubic meter (µg/m^[3] ) is attained when the expected number of days with a 24-hour average concentration above 150 µg/m^[3] per calendar year averaged over a three-year period, as determined in accordance with appendix K to 40 CFR part 50, is equal to or less than one.^[3]

On May 31, 2012, the EPA designated a portion of State lands in Pinal County, Arizona (“West Pinal County”) as nonattainment for the 1987 PM₁₀ NAAQS based on 2006-2008 data.^[4] West Pinal County 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 ( printed page 66050) different characteristics relevant to pollution formation. The West Pinal County PM₁₀ Nonattainment Area is located within the western region, characterized by low desert valleys and an arid climate. The eastern region is mountainous, with elevations up to 6,441 feet. The West Pinal County PM₁₀ Nonattainment Area encompasses all land geographically located within Pinal County west of the north-south line defined by the boundary between Townships 10E and 11E, but excluding the main reservation of the Tohono O'odham Nation and excluding the Apache Junction portion of the existing Phoenix PM₁₀ nonattainment area.

As a result of the nonattainment designation, the West Pinal County PM₁₀ Nonattainment Area was classified as a Moderate PM₁₀ nonattainment area with an attainment date of as expeditiously as practicable, but no later than December 31, 2018. The Arizona Department of Environmental Quality (ADEQ) submitted a SIP revision intended to meet the Moderate PM₁₀ nonattainment requirements on December 21, 2015 (“2015 West Pinal Plan”).

On June 24, 2020, the EPA determined that West Pinal County PM₁₀ Nonattainment Area did not attain the 1987 PM₁₀ NAAQS by December 31, 2018.^[5] Pursuant to CAA section 188(b)(2), the area was reclassified to “Serious” nonattainment with an attainment date of as expeditiously as practicable, but no later than December 31, 2022.^[6]

On January 8, 2021, the EPA proposed to disapprove the 2015 West Pinal Plan.^[7] On May 17, 2021, ADEQ withdrew the 2015 West Pinal Plan, with the exception of several rules and statutes that had been approved into the SIP in 2017.^[8] Consequently, the EPA did not finalize the actions proposed on January 8, 2021, with respect to the Moderate area plan requirements. Instead, on July 23, 2021, the EPA found that Arizona had failed to submit a plan to address these requirements.^[9]

On May 31, 2022, ADEQ adopted and submitted the “2022 Serious Area Particulate Plan for PM₁₀ for the West Pinal County Nonattainment Area” (“2022 West Pinal Plan”),^[10] as a revision to the Arizona SIP.^[11] In addition to addressing the Moderate and Serious area plan requirements, the 2022 West Pinal Plan included a request for an extension of the December 31, 2022 attainment date pursuant to CAA section 188(e). On November 30, 2022, the EPA issued a letter finding the 2022 West Pinal Plan complete under CAA section 110(k).^[12] However, the EPA did not approve the Plan or the attainment date extension request. On May 19, 2025, ADEQ withdrew the 2022 West Pinal Plan.^[13]

On July 21, 2023, the EPA determined that the West Pinal County PM₁₀ Nonattainment Area did not attain the 1987 PM₁₀ NAAQS by the December 31, 2022 Serious area attainment date.^[14] Pursuant to CAA section 189(d), Arizona was required to submit a revision to the SIP for the West Pinal County PM₁₀ Nonattainment Area by December 31, 2023.

On November 29, 2023, the Maricopa Association of Governments (MAG) adopted the “2023 Five Percent Particulate Plan for PM-10 for the West Pinal County Nonattainment Area” (“2023 Five Percent Plan” or “Plan”). On December 14, 2023, ADEQ submitted the 2023 Five Percent Plan on behalf of MAG.^[15] The EPA reviewed the submission for completeness under our completeness criteria in 40 CFR part 51, appendix V and determined that the submittal meets these minimum criteria on June 14, 2024.^[16] The EPA is proposing approval of the submission as meeting the requirements of CAA section 189(d) as well as Moderate and Serious area requirements.

III. Overview of Applicable CAA Requirements

As a Serious PM₁₀ nonattainment area that failed to attain the NAAQS by its applicable attainment date of December 31, 2022, the West Pinal County PM₁₀ Nonattainment Area is subject to the requirements of CAA section 189(d). In addition, because the EPA has not previously approved a Moderate or a Serious area plan for the West Pinal County PM₁₀ Nonattainment Area, the area is also required to demonstrate that it has met the requirements associated with these classifications.

CAA section 189(d) provides that the State shall “submit within 12 months after the applicable attainment date, plan revisions that provide for attainment of the PM₁₀ air quality standard and, from the date of such submission until attainment, for an annual reduction of PM₁₀ or PM₁₀ precursor emissions within the area of not less than 5 percent of the amount of such emissions as reported in the most recent inventory prepared for the area.”

The general planning and control requirements for all nonattainment plans are found in CAA sections 110 and 172. More specific planning and control requirements relevant to the PM₁₀ NAAQS are found in part D, subpart 4, in CAA sections 188 and 189. The EPA has issued a General Preamble ^[17] and Addendum to the General Preamble ^[18] to provide guidance to States for meeting the CAA's requirements for the PM₁₀ NAAQS. The General Preamble mainly addresses the requirements for Moderate nonattainment areas and the Addendum addresses the requirements for Serious nonattainment areas. The EPA has also issued other guidance documents ( printed page 66051) related to PM₁₀ plans that are discussed and cited below. The specific PM₁₀ attainment planning requirements addressed by this SIP submission are summarized below.

A. Emissions Inventories

CAA section 172(c)(3) requires a “base year inventory” that is a comprehensive, accurate, current inventory of actual emissions from all sources of the relevant pollutant or pollutants in the area. The plan should also include a second projected attainment year inventory for the year that the area is projected to attain the PM₁₀ standards.

B. PM₁₀ Precursors

CAA section 189(e) requires that the control requirements for major stationary sources of direct PM₁₀ also apply to major stationary sources of PM₁₀ precursors, except where the Administrator determines that such sources do not contribute significantly to PM₁₀ levels that exceed the standard in the area. While CAA section 189(e) expressly requires control of precursors from major stationary sources, subpart 4 and other CAA provisions collectively require the control of direct PM₁₀ and PM₁₀ precursors from all types of sources ( i.e., stationary sources, area sources and mobile sources) as may be needed for the purposes of demonstrating attainment as expeditiously as practicable in a given nonattainment area.^[19]

C. Best Available Control Measures and Reasonably Available Control Measures Demonstration

CAA section 189(b)(1)(B) requires that Serious area PM₁₀ attainment plans provide for the implementation of best available control measures (BACM) within four years of reclassification to Serious. Because the Serious area attainment date of December 31, 2022, has passed, and the EPA found that the area failed to attain by the Serious area attainment date,^[20] we are evaluating the submission for compliance with the BACM requirements now, in conjunction with the State's SIP submission intended to meet the Moderate area, Serious area, and CAA section 189(d) plan requirements. Furthermore, because the EPA has not previously taken action to approve the SIP as meeting the subpart 4 reasonably available control measures (RACM) Moderate area planning requirements under CAA section 189 for the West Pinal County PM₁₀ Nonattainment Area, the EPA is reviewing the 2023 Five Percent Plan for compliance with those requirements as well.^[21]

D. CAA Section 189(d) Attainment Demonstration and Five Percent Requirement

For Serious PM₁₀ nonattainment areas that do not attain the PM₁₀ NAAQS by the applicable attainment date, CAA section 189(d) requires the State to submit plan revisions that provide for attainment of the NAAQS ( i.e., an attainment demonstration) as expeditiously as practicable, but no later than five years from the publication date of the notice of a nonattainment finding. The plan must also provide for an annual five percent reduction in PM₁₀ or PM₁₀ precursor emissions for each year from the date of submission until attainment. CAA section 189(d) specifies that the State must submit these plan revisions within 12 months of the applicable attainment date that the area failed to meet.

E. Reasonable Further Progress and Quantitative Milestones

CAA section 172(c)(2) requires that implementation plans demonstrate reasonable further progress (RFP) as defined in CAA section 171(1). CAA section 171(1) defines RFP as “such annual incremental reductions in emissions of the relevant air pollutant as are required by this part [part D of title I] or may reasonably be required by the Administrator for the purpose of ensuring attainment of the applicable national ambient air quality standard by the applicable date.” The general RFP requirement of CAA section 172(c)(2) applies to SIP submissions necessary to meet CAA section 189(d) for the PM₁₀ NAAQS.

In addition, CAA section 189(c)(1), specifically applicable to the PM₁₀ NAAQS, requires that an implementation plan contain quantitative milestones that will be achieved every three years and that will demonstrate that RFP is being met.

F. Contingency Measures

CAA section 172(c)(9) requires that implementation plans provide for “the implementation of specific measures to be undertaken if the area fails to make reasonable further progress, or to attain the NAAQS by the attainment date applicable under this part [part D of title I]. Such measures are to take effect in any such case without further action by the State or the Administrator.” The contingency measure requirement of CAA section 172(c)(9) applies to the SIP submissions necessary to meet CAA section 189(d) for the PM₁₀ NAAQS.

G. Transportation Conformity and Motor Vehicle Emissions Budgets

Transportation conformity is required by CAA section 176(c). The conformity regulations found at 40 CFR part 93, subpart A require that transportation plans, programs, and projects conform to the purpose of the SIP and establish the criteria and procedures for determining whether or not they do so. Conformity to the purpose of the SIP means that transportation activities will not produce new air quality violations, worsen existing violations, or delay timely attainment of the NAAQS or any interim milestones. Once a SIP that contains motor vehicle emissions budgets (“budgets”) has been submitted to the EPA, and the Agency has found them adequate, these budgets are used for determining conformity: emissions from planned transportation activities must be less than or equal to the budgets.^[22]

H. Adequate Authority

CAA section 110(a)(2)(E)(i) requires that implementation plans provide necessary assurances that the State (or the general purpose local government or regional agency designated by the State for this purpose) will have adequate personnel, funding and authority under State law to carry out the requirements of such plan. Requirements for legal authority are further defined in 40 CFR part 51, subpart L (51.230-51.232) and for resources in 40 CFR 51.280. States and responsible local agencies must also demonstrate that they have the legal authority to adopt and enforce provisions of the SIP and to obtain information necessary to determine compliance. ( printed page 66052)

IV. Evaluation of the 2023 Five Percent Plan's Compliance With CAA Requirements

A. Emissions Inventories

CAA section 172(c)(3) requires all nonattainment area plans to include a base year emissions inventory that is a comprehensive, accurate, and current inventory of actual emissions from all sources of the relevant pollutant or pollutants in the area at issue. Also, the State should submit a projected attainment year emissions inventory for the year in which the State projects that the area will attain the PM₁₀ standards. The EPA's emission inventory guidance provides recommendations on how to develop and document emission inventories.^[23]

The 2023 Five Percent Plan uses a 2017 base year emissions inventory for Pinal County (“2017 PM₁₀ Inventory”) as a starting point in the analysis. The 2017 PM₁₀ Inventory was developed by ADEQ, Pinal County Air Quality Control District (PCAQCD), and MAG. The 2017 PM₁₀ Inventory included emissions from point sources (permitted sources), nonpoint sources (area sources), nonroad mobile sources, and on-road mobile sources. This emission inventory is sufficiently comprehensive, covering all sources of PM₁₀ that have been found to be important sources of relevant emissions in this and other PM₁₀ nonattainment areas.

The 2017 PM₁₀ Inventory shows that the most significant sources of annual emissions in the West Pinal County PM₁₀ Nonattainment Area are unpaved road dust: private roads (31.5 percent), unpaved road dust: agricultural roads (24.7 percent), unpaved road dust: public roads (16.2 percent), windblown dust (9.0 percent), tilling and harvesting (5.0 percent), feedlots (3.3 percent), and construction (2.7 percent). The annual and daily PM₁₀ emissions inventories and the emission estimation methodologies used to develop the 2017 PM₁₀ Inventory are described in detail in Appendix A, Exhibit 1 of the 2023 Five Percent Plan.

In general, the base year emissions inventory represents the emissions contributing to the air quality problem addressed by the SIP revision. Because the EPA's final rule determining that the West Pinal County PM₁₀ Nonattainment Area failed to attain the PM₁₀ standard by its attainment date used 2020-2022 monitoring data to make its determination, a year that is included within that time period and the triennial schedule of the National Emission Inventory (NEI) would generally be an appropriate base year. While the 2020 NEI year would have been the most recent such year to consider at the time of this plan's development, due to COVID-related emissions anomalies, the 2023 Five Percent Plan concludes that 2020 was not an appropriate inventory year. The base year, 2017, was chosen as it is a reasonably current NEI year, and the most recent NEI year available that was not 2020 at the time of plan development.

The 2017 PM₁₀ Inventory includes only primary PM₁₀ emissions and does not include emissions for PM₁₀ precursors. As described in section IV.B of this document, MAG determined that precursors do not play a significant role in the PM₁₀ exceedances in the West Pinal County PM₁₀ Nonattainment Area and the EPA's analysis supports this conclusion.

The 2023 Five Percent Plan includes a 2026 projected attainment year emissions inventory (“attainment year inventory”) that was developed using annual PM₁₀ emissions under implementation of Moderate area (existing) controls from the 2017 base year inventory through 2021 and Serious area controls from 2022 through 2026. Emission growth factors were developed for each source category relative to 2017 (growth or decline) and applied based on factors including, but not limited to, forecasted total population growth and vehicle miles traveled (VMT). Control reductions were then applied as appropriate, based on category-specific control effectiveness estimates for each individual measure. The resulting 2026 attainment year inventory was then provided for each source category and in total.^[24]

In developing the emissions inventories, MAG followed the EPA's 2017 EI Guidance and recommendations regarding the use of emission factors, activity estimates, and control factors, and the other source-specific inventory development methodologies. The relative accuracy of each estimate underwent the prescribed quality assurance procedures, documented in the State's emissions inventory preparation plan.^[25]

The EPA's analysis indicates the inventories are sufficiently accurate for the purposes of the 2023 Five Percent Plan. Because we find that the inventory is current, comprehensive, and accurate, we propose to approve the 2017 PM₁₀ base year inventory and 2026 attainment year inventory under CAA section 172(c)(3).

B. PM₁₀ Precursors

The provisions of subpart 4 of part D, title I of the CAA do not define the term “precursor” for purposes of PM₁₀, nor do they explicitly require the control of any specific particulate matter (PM) precursor. The statutory definition of “air pollutant” in CAA section 302(g), however, provides that the term “includes any precursors to the formation of any air pollutant, to the extent the Administrator has identified such precursor or precursors for the particular purpose for which the term `air pollutant' is used.” The EPA has identified sulfur dioxide (SO₂), oxides of nitrogen (NO_X), volatile organic compounds (VOC), and ammonia (NH₃ ) as precursors to the formation of PM.^[26] Accordingly, a State must include emissions of direct PM and these four precursors in emissions inventories and must control emissions from sources of all of these pollutants, unless the State demonstrates to the EPA's satisfaction that control of one or more of these pollutants is not needed for expeditious attainment of the NAAQS in the nonattainment area at issue.

On December 11, 2025, the EPA published a final notice approving the portion of the Arizona SIP that addresses the CAA section 189(e) requirement regarding precursor emissions from major stationary sources of PM₁₀ for the purpose of nonattainment new source review in the West Pinal County PM₁₀ Nonattainment Area.^[27]

Regarding the remaining plan requirements, the 2023 Five Percent Plan contains a demonstration that precursors do not significantly contribute to PM₁₀ exceedances in the West Pinal County PM₁₀ Nonattainment Area.^[28] The EPA has evaluated this demonstration and included a detailed analysis in the West Pinal PM₁₀ Modeling Technical Support Document (“EPA's Modeling TSD”).^[29] The EPA proposes to find that precursors do not significantly contribute to PM₁₀ exceedances in the West Pinal County PM₁₀ Nonattainment Area, and thus that the area is not required to regulate ( printed page 66053) precursors for the purposes of RACM, BACM, and attainment planning.

C. Best Available Control Measures and Reasonably Available Control Measures Demonstration

CAA section 189(b)(1)(B) requires that Serious area PM₁₀ plans provide for the implementation of BACM within four years of reclassification to Serious.^[30] The EPA has established a process for evaluating potential BACM (including best available control technology (BACT)) in Serious areas.^[31] The first step in determining BACM is to develop a comprehensive emissions inventory of the sources of PM₁₀ that can be used with modeling to determine the effects of these sources on ambient PM₁₀ levels. Based on our review of the emission inventories provided in the 2023 Five Percent Plan, the EPA's analysis indicates that the 2023 Five Percent Plan appropriately identifies all sources of PM₁₀ that are subject to evaluation for potential control consistent with the requirements of subpart 4 of part D, title I of the Act.

The remaining steps are to identify potential control measures for each source category, determine whether available control measures or technologies are technologically and economically feasible for implementation in the area, and determine the earliest date by which those control measures or technologies found to be feasible can be implemented, in whole or in part. In accordance with these steps, the significant source categories examined for BACM evaluation in the 2023 Five Percent Plan are: Construction Sites, Cleared Areas, Dairies, Cattle Concentrated Animal Feeding Operations, Agriculture (Cropland), Unpaved Roads, Unpaved Lots, and Paved Roads.

We provide a detailed evaluation of the 2023 Five Percent Plan's control measures for sources in the nonattainment area in the EPA's “Technical Support Document, EPA Evaluation of BACM, West Pinal County 2023 Five Percent Plan for the 1987 PM-10 NAAQS,” July 2026 (“EPA's BACM TSD”).^[32] The EPA notes that its BACM analysis is based on the rules currently approved in the Pinal County SIP, as well as rules that the EPA is proposing to approve elsewhere in today's Federal Register —specifically, regulations addressing fugitive dust and agricultural best management practices. As described in further detail in EPA's BACM TSD, we find that the evaluation process followed by MAG in the 2023 Five Percent Plan to identify potential BACM is consistent with the requirements, the evaluation of potential measures is appropriate, and reasoned justifications are provided for the rejection of potential measures based on technological or economic infeasibility.

For the foregoing reasons, we propose to find that the 2023 Five Percent Plan provides for the implementation of BACM for all required sources of PM₁₀ in accordance with the requirements of CAA section 189(b)(1)(B).

The EPA does not normally conduct a separate evaluation to determine whether a Serious area plan's measures also meet the RACM requirements. However, because in this instance, we have not previously approved a Moderate area plan as meeting the RACM requirement, we must evaluate the control strategy for RACM as well. As explained in the General Preamble Addendum, we interpret the BACM requirement as generally subsuming the RACM requirement— i.e., if we determine that the measures are indeed the “best available,” we have necessarily concluded that they are “reasonably available.” ^[33] Therefore, we have not conducted a separate analysis to determine if the measures represent a RACM level of control, and propose to find that the measures satisfy the RACM requirement, because our analysis indicates that they meet the stricter BACM requirement.

D. CAA Section 189(d) Attainment Demonstration and Five Percent Requirement

CAA section 189(d) requires the submittal of plan revisions that provide for expeditious attainment of the PM₁₀ NAAQS. The attainment deadline applicable to an area that fails to attain by the Serious area attainment date is as soon as practicable, but no later than five years from the publication date of the notice of a nonattainment finding unless extended by the EPA as meeting certain specified requirements.^[34] Because the EPA published the nonattainment finding for the West Pinal County PM₁₀ Nonattainment Area on July 21, 2023, the State must demonstrate attainment no later than July 21, 2028, unless the date is extended.

1. Attainment Deadline

The 2023 Five Percent Plan models attainment of the PM₁₀ NAAQS by December 31, 2026. The 2023 Five Percent Plan contains a demonstration supporting the conclusion that December 31, 2026, is as expeditiously as the area can attain the standard. Due to the timeline for feasible development and implementation of control measures, the first full year that all control measures relied upon in the plan would be implemented was 2024. Therefore the 2026 design value, which includes 2024-2026 data, is the first design value year to include three years of fully implemented control measures. MAG also notes that maximum compliance rates will not be achieved until 2026. For these reasons, the EPA agrees that the submission shows that the area could not practicably attain the NAAQS prior to December 31, 2026.

2. Modeled Attainment Demonstration

The 2023 Five Percent Plan shows attainment of the PM₁₀ NAAQS through modeled attainment demonstrations for three sites (Hidden Valley, Pinal County Housing, and Stanfield) in the West Pinal County PM₁₀ Nonattainment Area.^[35] MAG conducted modeling for seven design days across the three monitoring sites, representing the meteorological conditions associated with exceedances in the nonattainment area.^[36]

( printed page 66054)

The methods used for modeling the design days are consistent with EPA guidance.^[37 38] The results of the modeling demonstrate that the emission reductions in the plan are expected to result in PM₁₀ levels that provide for attainment of the NAAQS by December 31, 2026. The EPA notes that preliminary ambient air quality data for the area suggest that the control strategy may not have achieved reductions at the levels predicted in the Plan. The EPA has not identified deficiencies in the modeling, or in the substantive requirements contained in the rules that constitute the control strategy that would cause this discrepancy. However, the EPA has previously noted that an earlier version of the AgBMP rules, which regulate sources that predominate in the area surrounding the design value monitor, may not have contained implementation requirements sufficient to achieve the level of compliance predicted in the Plan.^[39] The State has updated the AgBMP rules, substantially strengthening the enforceability of the rules in a manner that the EPA expects will result in reductions in line with those modeled in the Plan.^[40] Therefore, the EPA proposes to find that the 2023 Five Percent Plan's attainment demonstration adequately demonstrates attainment of the PM₁₀ standard in the West Pinal County PM₁₀ Nonattainment Area. The EPA's detailed analysis of the modeling can be found in EPA's Modeling TSD for this action.

3. Five Percent Requirement

Under CAA section 189(d), the SIP revision submitted by the State must provide for an annual reduction in PM₁₀ or PM₁₀ precursor emissions of not less than five percent from the date of such submission until attainment. The 2023 Five Percent Plan's demonstration of annual five percent reductions is found in Chapter 6. MAG used the 2017 PM₁₀ Inventory as the baseline inventory and derived emissions levels for years 2017-2027 based on the 2017 PM₁₀ Inventory.^[41] The demonstration of annual five percent reductions uses 2024 as the point at which the reductions should start.^[42] Since the plan demonstrates attainment by December 31, 2026, annual five percent reductions are shown for three years: 2024, 2025, and 2026. The 2023 Five Percent Plan's demonstration is summarized in Table 2 of this document.

The “2017 Baseline Inventory (tons)” values are derived from the 2017 PM₁₀ Inventory. The “Five Percent Annual Requirement (tons)” values indicate the annual five percent reduction target as calculated by multiplying the total “2017 Baseline Inventory (tons)” (41,168 tons) by five percent, resulting in a value of 2,058 tons. The “Five Percent Cumulative Reduction (tons)” indicate the cumulative yearly ( printed page 66055) reductions, increasing by five percent of the 2017 Baseline Inventory each year.

The “2017 Base PM₁₀ Emissions Minus Five Percent Annual Reductions (tons)” values are calculated as the “2017 Baseline Inventory (tons)” minus the “Five Percent Annual Requirement (tons),” representing the maximum total emission in tons that could be emitted each year to meet the five percent requirement. The “Projected Annual PM₁₀ Emissions with BACM Implementation (tons)” values are the projected actual emissions between 2024 and 2026 reflecting the implementation of control measures. The “Excess Benefit = Total PM₁₀ Emissions Reductions Minus Five Percent Reductions (tons)” indicates the surplus of cumulative emissions reductions each year beyond the five percent requirement.

The 2023 Five Percent Plan demonstrates compliance with the five percent reduction requirement by comparing the cumulative reductions against the total five percent reduction values each year and shows that the reductions exceed the five percent per year requirement. Most of the required reductions were achieved in the early years of the plan. The EPA encourages this approach as it accelerates the environmental benefits of the reductions.^[43] Because the 2023 Five Percent Plan demonstrates that the area will achieve the requisite reductions from 2024 until the modeled attainment date in 2026, the EPA proposes to find that Plan meets the CAA section 189(d) five percent requirement.

E. Reasonable Further Progress and Quantitative Milestones

Pursuant to CAA sections 172(c)(2) and 189(c)(1), the State must demonstrate RFP in the 2023 Five Percent Plan. We have explained in guidance that for areas such as the West Pinal County PM₁₀ Nonattainment Area where “the nonattainment problem is attributed to area type sources ( e.g., fugitive dust, etc.), RFP should be met by showing annual incremental emission reductions sufficient generally to maintain linear progress towards attainment. Total PM₁₀ emissions should not remain constant or increase from 1 year to the next in such an area.” ^[44] Further, we have stated that, “in reviewing the SIP, EPA will determine whether the annual incremental emission reductions to be achieved are reasonable in light of the statutory objective to ensure timely attainment of the PM₁₀ NAAQS.” ^[45]

CAA section 189(c) further requires PM₁₀ attainment plans to contain quantitative milestones that are to be achieved every three years and that demonstrate RFP for the area. These quantitative milestones should consist of elements that allow RFP to be quantified or measured objectively. Specifically, States should identify and submit quantitative milestones that allow for evaluation of whether the plan is achieving emissions reductions adequate to attain the NAAQS by the applicable attainment date.^[46]

The 2023 Five Percent Plan includes an RFP demonstration in Chapter 9.^[47] This analysis uses the 2017 inventory totals by year as shown in Table 2 of this document. Specifically, the 2023 Five Percent Plan shows the following levels of PM₁₀, which decline between 2022 and 2027, with BACM control implementation in Table 3 below.

The analysis required for the five percent demonstration provides annual emission targets between 2024 and the attainment year of 2026. These annual totals show a steady downward trend in emissions that fulfills the milestone requirement of every three years.^[48] The trend is more sharply downward in 2024 due to the implementation schedule of control measures. The EPA proposes to find that the 2023 Five Percent Plan has demonstrated RFP and that by setting annual target emission levels, the plan has met the requirement to provide for milestones every three years.

F. Contingency Measures

CAA section 172(c)(9) requires that attainment plans provide for the implementation of specific measures to be undertaken if the area fails to meet RFP requirements or fails to attain the PM₁₀ standard as projected in the plan. That section further requires that such measures are to take effect in any such case without further action by the State or the EPA. The CAA does not specify how many contingency measures are necessary nor does it specify the level of emission reductions they must achieve.

The purpose of contingency measures is to continue progress in reducing emissions while a State revises its SIP to meet the missed RFP milestone or to develop a new plan demonstrating attainment of the NAAQS. Contingency measures must consist of measures that the State is not otherwise relying on to meet other attainment plan requirements in the area. Thus, these additional emission reductions that will be achieved by the contingency measures ensure continued progress towards attainment while the State is revising the SIP to correct the failure to meet RFP or to attain.

In interpreting the requirement that the contingency measures must “take effect without further action by the State or the Administrator,” the General Preamble provides the following general guidance: “[s]tates must show that their contingency measures can be implemented with minimal further action on their part and with no additional rulemaking actions such as public hearings or legislative review.” Further, “[i]n general, EPA will expect all actions needed to affect full implementation of the measures to occur within 60 days after EPA notifies the State of its failure.”

In December 2024, the EPA released the “Guidance on the Preparation of State Implementation Plan Provisions that Address the Nonattainment Area Contingency Measure Requirements for Ozone and Particulate Matter (December 3, 2024)” (“Revised Contingency Measure Guidance”).^[49] This guidance primarily relates to the EPA's recommendations concerning the specific amount of emissions reductions that implementation of contingency ( printed page 66056) measures should achieve and the timing for when the emissions reductions from the contingency measures should occur.

Prior to the release of the Revised Contingency Measure Guidance, the EPA's recommendation for the amount of emissions reductions to achieve from implementation of contingency measures was one year's worth of RFP. For PM₁₀, prior EPA guidance recommended that RFP should represent generally linear progress towards attainment ( i.e., the difference between the attainment projected inventory ^[50] and the base year emissions inventory, divided by the number of years between the base year and the attainment year).^[51]

The Revised Contingency Measure Guidance introduces “one year's worth of progress,” a metric intended to be more closely tied to the emissions reductions required for attainment of the NAAQS, for determining the amount of emissions reductions that contingency measures should achieve. One year's worth of “progress” is calculated by determining the average annual reductions between the base year emissions inventory and the projected attainment year emissions inventory, determining what percentage of the base year emissions inventory this amount represents, and then applying that percentage to the projected attainment year emissions inventory to determine the amount of reductions needed to ensure ongoing progress if contingency measures are triggered.^[52]

The 2023 Five Percent Plan identified one adopted contingency measure of a speed limit reduction on public unpaved roads. This contingency measure is PCAQCD Rule 4-1-030.6.C, which the State submitted to the EPA for approval into the SIP on February 13, 2023. The measure would require owners or operators of unpaved public roads of 26 annual average daily traffic or more to establish a speed limit of 15 miles per hour in the event that the area fails to make RFP or fails to attain the PM₁₀ NAAQS by the attainment date.^[53] The EPA is proposing to approve Rule 4-1-030.6.C in a separate notice in today's Federal Register . While that rulemaking action evaluates the approvability of the rule into the SIP, in this action we evaluate whether the contingency provision in that rule satisfies the contingency measure planning requirement for the PM₁₀ NAAQS associated with the submitted plan.

If the EPA finalizes the measure's approval into the SIP, our analysis indicates that the measure meets all relevant requirements to satisfy the contingency measure requirement for the PM₁₀ NAAQS. The measure has been fully adopted and will be triggered upon an EPA finding that the area has failed to attain the NAAQS by the applicable attainment date or has failed to meet the RFP requirement. The measure goes into effect immediately upon such a finding, with no additional action required by the State. In addition, the method used to estimate emissions reductions from this contingency measure is consistent with the EPA's recommended calculation methods for such measures, and the total reductions exceed the target of one year's worth of progress.^[54] Because it meets all applicable requirements, the EPA proposes to approve the contingency measure element described in the 2023 Five Percent Plan.

G. Transportation Conformity and Motor Vehicle Emissions Budgets

Actions involving Federal Highway Administration (FHWA) or Federal Transit Administration (FTA) funding or approval are subject to the EPA's transportation conformity rule, codified at 40 CFR part 93, subpart A. Under this rule, metropolitan planning organizations (MPOs) in nonattainment and maintenance areas coordinate with State and local air quality and transportation agencies, the EPA, the FHWA, and the FTA to determine that an area's regional transportation plans and transportation improvement programs conform to the applicable SIP. This determination typically includes a showing that estimated emissions from existing and planned highway and transit systems are less than or equal to the budgets that the EPA has found adequate or approved in the area's control strategy SIP.^[55] Budgets are generally established for specific years and specific pollutants or precursors. Attainment plans for PM₁₀ nonattainment areas should identify budgets for on-road mobile source emissions of PM₁₀, i.e., vehicle emissions and fugitive dust emissions, in the area for each RFP milestone year, as appropriate, and the attainment year, if the plan demonstrates attainment.^[56]

For budgets to be approvable, they must meet, at a minimum, the EPA's adequacy criteria at 40 CFR 93.118(e)(4). While we have not yet made an adequacy finding, the EPA is starting the adequacy process with this notice and as such, the EPA is also proposing to approve the budgets based on these criteria. To meet these requirements, the budgets must be consistent with the attainment and RFP requirements and reflect all the motor vehicle control measures contained in the attainment and RFP demonstrations.^[57]

The EPA's process for determining the 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 budget during a public comment period; and (3) making a finding of adequacy or inadequacy.^[58] During the comment period for this notice, the public can comment on the adequacy of the budgets in addition to other aspects of this proposed rule.

The 2023 Five Percent Plan specifies the maximum transportation-related PM₁₀ emissions for the RFP milestone year of 2023, and for the RFP milestone and attainment year of 2026. The State's submitted 2023 and 2026 budgets for PM₁₀ are included in Table 4 of this document. These budgets include emissions from vehicle exhaust, tire wear, brake wear, road construction, re-entrained dust from vehicles traveling on paved roads, and fugitive dust generated from vehicles traveling on public unpaved and private non-agricultural ^[59] unpaved roads.

( printed page 66057)

The methodologies MAG used to develop the budgets are provided in Chapter 9 and Appendix B, Exhibit 1 of the 2023 Five Percent Plan.^[60] MAG used MOVES3 in the development of these budgets; this emissions factor model was the latest EPA-approved version at the time the 2023 Five Percent Plan was developed.^[61] Annual emission estimates from road construction were developed from calculations provided by PCAQCD, published emission factors for construction, and local data inputs.^[62] MAG used the most recently approved EPA-provided AP-42 emissions factor equations to develop paved and unpaved road fugitive dust emissions estimates.^[63] Paved road vehicle miles traveled (VMT) estimates for estimating direct and fugitive PM₁₀ emissions were provided by MAG for 2026, and estimated for 2023 using an interpolation of the 2017 base year VMT.^[64] Unpaved public road VMT estimates for estimating fugitive PM₁₀ emissions were interpolated from the 2017 base year VMT.^[65] Unpaved private non-agricultural VMT estimates for estimating fugitive PM₁₀ were held constant from the 2017 base year VMT estimates.^[66]

We are proposing to approve the budgets for 2023 and 2026 as part of our approval of the 2023 Five Percent Plan. The EPA has determined that the 2023 and 2026 motor vehicle emissions budgets are consistent with emission control measures in the SIP and the attainment demonstration, five percent demonstration, and RFP demonstration. The details of the EPA's evaluation of the budgets for compliance with the budget adequacy criteria of 40 CFR 93.118(e) is provided in a separate document included in the docket for this rulemaking.^[67] If we find the budgets adequate or finalize approval of the budgets in the 2023 Five Percent Plan as proposed, then these budgets must be used in future transportation conformity analyses for the West Pinal County PM₁₀ Nonattainment Area.^[68]

H. Adequate Legal Authority

CAA section 110(a)(2)(E)(i) requires that implementation plans provide necessary assurances that the State (or the local government in the case of delegation) will have adequate personnel, funding and authority under State law. Requirements for legal authority are further defined in 40 CFR part 51, subpart L (sections 51.230-51.232) and for resources in 40 CFR 51.280.

States and responsible local agencies must demonstrate that they have the legal authority to adopt and enforce provisions of the SIP and to obtain information necessary to determine compliance. These requirements are addressed in the cover letters and submittal package for the 2023 Five Percent Plan.^[69]

MAG derives its authority to develop and adopt air quality plans, including the 2023 Five Percent Plan, from Arizona Revised Statute (ARS) 49-406 and from a June 22, 2016 letter from the Governor of Arizona designating MAG as responsible for those tasks.^[70] ADEQ is authorized to adopt and submit the 2023 Five Percent Plan by ARS 49-404 and ARS 49-406. PCAQCD implements air quality programs within Pinal County.

For the reasons discussed above, we propose to find that the requirements of CAA section 110(a)(2)(E) and related regulations have been met with respect to legal authority.

V. Summary of Proposed Actions

The EPA is proposing to approve the CAA section 189(d) plan for the West Pinal County PM₁₀ Nonattainment Area as meeting all relevant Moderate, Serious, and CAA section 189(d) requirements. Specifically, we propose to approve the following:

(1) The 2017 baseline emissions inventory and the attainment year emission inventories as meeting the requirements of CAA section 172(c)(3); ( printed page 66058)

(2) the BACM demonstration as meeting the requirements of CAA section 189(b)(1)(B), and the RACM demonstration as meeting the requirements of CAA sections 189(a)(1)(C) and 172(c)(1);

(3) the attainment demonstration as meeting the requirements of CAA sections 189(d) and 179(d)(3), with an attainment date of December 31, 2026;

(4) the five percent demonstration as meeting the requirements of CAA section 189(d);

(5) the reasonable further progress and quantitative milestone demonstrations as meeting the requirements of CAA sections 172(c)(2) and 189(c);

(6) the contingency measure as meeting the requirements of CAA section 172(c)(9); and

(7) the motor vehicle emissions budgets, as they meet the criteria for budgets in CAA section 176(c) and 40 CFR 93.118(e)(4).

The EPA is also starting the adequacy process for these budgets.

VI. 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 CAA. 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, Oct. 4, 1993);
• Is not anExecutive Order 14192 (90 FR 9065, Feb. 6, 2025) regulatory action because this action is not significant 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, Aug. 10, 1999);
• Is not subject toExecutive Order 13045 (62 FR 19885, Apr. 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, Nov. 9, 2000).

Authority: 42 U.S.C. 7401 et seq.