SUPPLEMENTARY INFORMATION:

Throughout this document whenever “we,” “us,” or “our” is used, we mean the EPA. We use multiple acronyms and terms in this preamble. While this list may not be exhaustive, to ease the reading of this preamble and for reference purposes, the EPA defines the following terms and acronyms here:

AQS Air Quality System

CAA Clean Air Act

CBI Confidential Business Information

CERR Consolidated Emissions Reporting Rule

CTG Control Technology Guideline

DVs Design Values

EGLE Michigan Department of Environment, Great Lakes, and Energy

EPA Environmental Protection Agency

GHG Greenhouse Gas

I/M Inspection and Maintenance

NAAQS National Ambient Air Quality Standards

NO_X  Oxides of Nitrogen

NSR New Source Review

PBI Proprietary Business Information

PM_2.5  Fine Particulate Matter

ppm Parts per Million

PSD Prevention of Significant Deterioration

RACM Reasonably Available Control Measures

RACT Reasonably Available Control Technology

RFP Reasonable Further Progress

SEMCOG Southeast Michigan Council of Governments

SIP State Implementation Plan

VMT Vehicle Miles of Travel

VOC Volatile Organic Compound

Organization of this document. The information presented in this preamble is organized as follows:

I. What is the EPA proposing?

II. What is the background for these actions?

III. What are the criteria for redesignation?

IV. What is the EPA's analysis of Michigan's redesignation request?

A. Has the Detroit area attained the 2015 ozone NAAQS?

1. Exceptional Events Demonstration

2. Monitoring Data

B. Has Michigan met all applicable requirements of section 110 and part D of the CAA for the Detroit area, and does Michigan have a fully approved SIP for the area under section 110(k) of the CAA?

1. Michigan Has Met All Applicable Requirements of Section 110 and Part D of the CAA Applicable to the Detroit Area for Purposes of Redesignation

a. Section 110 General Requirements for Implementation Plans

b. Part D Requirements

i. Section 172 Requirements

ii. Section 176 Conformity Requirements

iii. CAA Section 182(a), Section 182(b), and Section 182(f) Requirements

2. The Detroit Area Has a Fully Approved SIP for Purposes of Redesignation Under Section 110(k) of the CAA

C. Are the air quality improvements in the Detroit area due to permanent and enforceable emission reductions?

D. Does Michigan have a fully approvable ozone maintenance plan for the Detroit area?

1. Attainment Inventory

2. Has the State demonstrated maintenance of the ozone standard in the Detroit area?

3. Continued Air Quality Monitoring, Verification of Continued Attainment, and Contingency Plan

V. Has the State adopted approvable motor vehicle emission budgets?

A. Motor Vehicle Emission Budgets

B. What is a safety margin?

VI. Adjustment of Certain SIP Submittal and Implementation Deadlines

A. Sixth Circuit Court of Appeals Decision in Sierra Club v. EPA

B. Proposed Adjustment of Certain SIP Submittal and Implementation Deadlines Associated With the Moderate Classification

VII. Adjustment of Basic I/M Implementation Deadline

VIII. What action is the EPA taking?

IX. Statutory and Executive Order Reviews

( printed page 47163)

I. What is the EPA proposing?

The EPA is proposing to take several related actions. The EPA proposes to determine that the Detroit area has met the requirements for redesignation under CAA section 107(d)(3)(E), and the EPA is thus proposing to change the legal designation of the Detroit area from nonattainment to attainment for the 2015 ozone NAAQS. The EPA is also proposing to approve, as a revision to the Michigan SIP, the State's updated maintenance plan for the area, which is intended to ensure attainment of the 2015 ozone NAAQS through 2040 in the Detroit area. As part of the maintenance plan, the EPA is proposing to approve the newly established 2035 and 2040 motor vehicle emissions budgets for the Detroit area. The EPA is also initiating the adequacy process for these budgets. Additionally, the EPA is proposing to adjust the deadline for Michigan to submit SIP revisions for the Detroit area for requirements associated with the Moderate classification, as well as the deadline to implement these measures except for Basic vehicle inspection and maintenance (I/M), to no later than December 5, 2026. The EPA is proposing to adjust the deadline for Michigan to implement Basic I/M in the Detroit area to no later than December 5, 2030. The EPA also proposes to take final agency action on an exceptional events request submitted by EGLE on April 15, 2026, and concurred on by the EPA on June 4, 2026.

II. What is the background for these actions?

Under CAA section 109, the EPA has established NAAQS for certain air pollutants and conducts periodic reviews of the NAAQS to determine whether they should be revised or whether new NAAQS should be established. One such criteria pollutant is ground-level ozone.^[1] On October 1, 2015, the EPA promulgated a revised 8-hour ozone NAAQS of 0.070 parts per million (ppm).^[2] In accordance with appendix U of 40 CFR part 50, the 2015 ozone NAAQS is attained in an area when the 3-year average of the annual fourth highest daily maximum 8-hour average concentration is equal to or less than 0.070 ppm, when truncated after the thousandth decimal place, at all of the ozone monitoring sites in the area.^[3]

Upon promulgation of a new or revised NAAQS, section 107(d)(1)(B) of the CAA requires the EPA to designate as nonattainment any areas that are violating the NAAQS, based on the most recent three years of quality assured ozone monitoring data. The Detroit area was initially designated as a Marginal nonattainment area for the 2015 ozone NAAQS on June 4, 2018, with an effective date of August 3, 2018.^[4] The Detroit area includes Livingston, Macomb, Monroe, Oakland, St. Clair, Washtenaw, and Wayne Counties.

Since the 2018 initial designation, the designation and classification of Detroit have been addressed by several submittals by EGLE and actions by the EPA:

• 2022 submittal: “Request for Redesignation to Attainment and Submittal to the State Implementation Plan (SIP) for the Clean Air Act (CAA) Section 175A Maintenance Plan for the Southeast Michigan 2015 Ozone Nonattainment Area” (January 3, 2022)
• 2022 proposed redesignation: “Redesignation of the Detroit, MI Area to Attainment of the 2015 Ozone Standards” (87 FR 14210, March 14, 2022)
• 2022 proposed reclassification: “Determinations of Attainment by the Attainment Date, Extensions of the Attainment Date, and Reclassification of Areas Classified as Marginal for the 2015 Ozone National Ambient Air Quality Standards” (87 FR 21842, April 13, 2022)
• 2023 final reclassification: “Finding of Failure To Attain and Reclassification of the Detroit Area as Moderate for the 2015 Ozone National Ambient Air Quality Standards” (88 FR 6633, February 1, 2023)
• 2023 proposed clean data determination: “Clean Data Determination for the Detroit Area for the 2015 Ozone Standard” (88 FR 7382, February 3, 2023)
• 2023 final clean data determination: “Clean Data Determination for the Detroit Area for the 2015 Ozone Standard” (88 FR 32584, May 19, 2023)
• 2023 final redesignation: “Redesignation of the Detroit, MI Area to Attainment of the 2015 Ozone Standards” (88 FR 32594, May 19, 2023)
• 2026 supplemental submittal: “Addendum to the Request for Redesignation to Attainment for the 2015 Ozone National Ambient Air Quality Standard and Revision to Michigan's State Implementation Plan and Ozone Maintenance Plan for Southeast Michigan Ozone Nonattainment Area” (May 18, 2026)
• 2026 technical amendment: “Technical Amendment” (91 FR 31953, May 29, 2026)
• 2026 proposed determination of attainment by the attainment date: “Determination of Attainment by the Attainment Date for the 2015 Ozone Standards” (91 FR 31993, May 29, 2026)

In its 2022 submittal, EGLE requested that the EPA redesignate the Detroit area to attainment of the 2015 ozone NAAQS based on attaining air quality monitoring data for the 2019-2021 period. In the 2023 final reclassification, the EPA determined that the Detroit area did not attain the standard by the Marginal attainment date based on monitoring data from the 2018-2020 period, and the area was reclassified to Moderate by operation of law. In the 2023 clean data determination, the EPA found that the Detroit area was attaining the 2015 ozone NAAQS based on monitoring data from the 2020-2022 period. In the 2023 redesignation, the EPA approved the request from EGLE to redesignate the area to attainment for the 2015 ozone NAAQS.

On July 17, 2023, Sierra Club petitioned the United States Court of Appeals for the Sixth Circuit (Court) for review of the 2023 final clean data determination and 2023 final redesignation. On December 5, 2025, the Court upheld the EPA's clean data determination and, regarding the redesignation, concluded that the EPA erred only with respect to its legal interpretation of CAA section 107(d)(3)(E)(v), which requires that a State “has met” all requirements applicable to the area as a prerequisite to redesignation. The Court vacated the EPA's redesignation of the Detroit area and issued its mandate in the case on February 24, 2026.^[5] The other EPA actions taken in the 2023 final clean data determination and 2023 final redesignation were not affected by the 2025 Sixth Circuit decision.

III. What are the criteria for redesignation?

Section 107(d)(3)(E) of the CAA allows redesignation of an area to attainment of the NAAQS provided that: (1) the Administrator (EPA) determines that the area has attained the NAAQS; (2) the Administrator has fully approved the applicable implementation plan for the area under section 110(k) of the CAA; (3) the Administrator determines ( printed page 47164) that the improvement in air quality is due to permanent and enforceable reductions in emissions resulting from implementation of the applicable SIP, applicable Federal air pollutant control regulations, and other permanent and enforceable emission reductions; (4) the Administrator has fully approved a maintenance plan for the area as meeting the requirements of section 175A of the CAA; and (5) the State containing the area has met all requirements applicable to the area for the purposes of redesignation under section 110 and part D of the CAA.

On April 16, 1992 (57 FR 13498), the EPA provided guidance on redesignations in the General Preamble for the Implementation of title I of the CAA Amendments of 1990 and supplemented this guidance on April 28, 1992 (57 FR 18070). The EPA has provided further guidance on processing redesignation requests in policy memoranda, including the Calcagni Memorandum.^[6]

IV. What is the EPA's analysis of Michigan's redesignation request?

A. Has the Detroit area attained the 2015 ozone NAAQS?

For redesignation of a nonattainment area to attainment, CAA section 107(d)(3)(E)(i) requires the EPA to determine that the area has attained the applicable NAAQS. An area is attaining the 2015 ozone NAAQS if the most recent design values (DVs) at all ambient air quality monitoring sites in the area are less than or equal to 0.070 ppm. The DV at an ambient air quality monitoring site is the 3-year average of the annual 4th highest daily maximum 8-hour average ozone concentrations, as specified in appendix U to 40 CFR part 50. Ozone DVs are based on the three most recent, complete,^[7] consecutive years of quality-assured and certified air quality monitoring data meeting all applicable requirements in 40 CFR part 58 and reported to the EPA's Air Quality System (AQS) database.

1. Exceptional Events Demonstration

Congress has recognized that it may not be appropriate for the EPA to use certain monitoring data collected by the ambient air quality monitoring network and maintained in the EPA's AQS database in certain regulatory determinations. Thus, in 2005, Congress provided the statutory authority for the exclusion of data influenced by “exceptional events” meeting specific criteria by adding section 319(b) to the CAA.^[8]

To implement this 2005 CAA amendment, on March 22, 2007 (72 FR 13560), the EPA promulgated the 2007 Exceptional Events Rule. The 2007 Exceptional Events Rule created a regulatory process codified at 40 CFR parts 50 and 51 (sections 50.1, 50.14 and 51.930). These regulatory sections, which superseded the EPA's previous guidance on handling data influenced by events, contain definitions, procedural requirements, requirements for air agency demonstrations, criteria for the EPA's approval of the exclusion of event-affected air quality data from the data set used for regulatory decisions, and requirements for air agencies to take appropriate and reasonable actions to protect public health from exceedances or violations of the NAAQS. On October 3, 2016 (81 FR 68216), the EPA promulgated a comprehensive revision to the 2007 Exceptional Events Rule. The 2016 Exceptional Events Rule revision included the requirement that, if a State demonstrates that emissions from a wildfire smoke event caused a specific air pollution concentration in excess of the NAAQS at a particular air quality monitoring location and otherwise satisfies the requirements of 40 CFR 50.14, the EPA must exclude that data from use in determinations of exceedances and violations.^[9]

For the purposes of this proposed action, on April 15, 2026, EGLE submitted an exceptional events demonstration to show that ozone concentrations recorded at the Oakland County monitor at Oak Park with Site ID 26-125-0001 and the St. Clair County monitor at Port Huron with Site ID 26-147-0005 on June 29 and June 30, 2023, and July 15, 2025, were influenced by wildfires. The EPA concurred on this request on June 4, 2026.

The EPA found that Michigan's demonstration met the Exceptional Events Rule criteria and determined that these wildfire events had regulatory significance for purposes of calculating the area's most recent DV to demonstrate the area is attaining the standard in order to redesignate the area to attainment for the 2015 ozone NAAQS. As such, the EPA now proposes to take final regulatory action on the concurred dates, as detailed in the docket, as exceptional events to be removed from the data set used for regulatory purposes. For this proposed action, the EPA will rely on the calculated DVs that exclude the event-influenced data for the purpose of demonstrating attainment of the 2015 ozone NAAQS. Further details on Michigan's analyses and the EPA's concurrence can be found in the docket for this regulatory action.

While the EPA has concurred with Michigan's request to exclude event-influenced air quality monitoring data from regulatory decisions, these regulatory actions require the EPA to provide an opportunity for public comment on the claimed exceptional events and all supporting data prior to the EPA taking final agency action. This proposed action provides the public with an opportunity to comment on the claimed exceptional events, all supporting documents and the EPA's concurrence with Michigan's request.

2. Monitoring Data

In its 2022 submittal, EGLE requested that the EPA redesignate the Detroit area to attainment of the 2015 ozone NAAQS based on attaining air quality monitoring data for the 2019 through 2021 period. The EPA is proposing to determine that the Detroit area has attained the 2015 ozone NAAQS based on monitoring data for the 2019 through 2021 period.

The EPA interprets CAA section 107(d)(3)(E)(i) to require continued attainment until the EPA's final action redesignating a nonattainment area. The EPA will not take final action to determine that the Detroit area is attaining the NAAQS nor to approve the redesignation of this area if the DV of any ozone monitoring site in the area violates the NAAQS prior to final approval of the redesignation. As discussed in section IV.D.3. of this preamble, EGLE has committed to continue monitoring ozone in this area ( printed page 47165) to verify maintenance of the 2015 ozone NAAQS.

In the EPA's 2023 final clean data determination and 2023 final redesignation, the EPA found that the Detroit area attained for the 2019 through 2021 and 2020 through 2022 periods. In the 2025 Sixth Circuit decision, the Court upheld the EPA's finding that the area attained for the 2019 through 2021 and 2020 through 2022 periods. In the 2026 proposed determination of attainment by the attainment date, the EPA proposed to determine that the area attained for the 2021 through 2023 period, and the EPA will not finalize this redesignation until it has finalized the separate determination for the 2021 through 2023 period.

The EPA has additionally reviewed the available ozone monitoring data from EGLE's monitoring sites in the Detroit area for the 2022 through 2024 and 2023 through 2025 periods. These data have been quality assured, are recorded in the AQS, and were certified in advance of the EPA's publication of this proposal. These data demonstrate that the Detroit area is attaining the 2015 ozone NAAQS. The annual fourth-highest daily maximum 8-hour average ozone concentrations and the 3-year averages of these values (ozone DVs) for all monitoring sites are summarized in Table 1.

The Detroit area's 3-year ozone DVs for 2022 through 2024 and 2023 through 2025 are 0.070 ppm,¹⁰ which meet the 2015 ozone NAAQS. Therefore, in this action, the EPA proposes to determine that the Detroit area is attaining the 2015 ozone NAAQS.

B. Has Michigan met all applicable requirements of section 110 and part D of the CAA for the Detroit area, and does Michigan have a fully approved SIP for the area under section 110(k) of the CAA?

For redesignation of an area from nonattainment to attainment of a NAAQS, CAA section 107(d)(3)(E)(v) requires the EPA to determine that the State has met all applicable requirements under section 110 and part D of title I of the CAA, and CAA section 107(d)(3)(E)(ii) requires the EPA to determine that the State has a fully approved SIP under section 110(k) of the CAA. The EPA proposes to find that Michigan has met all applicable SIP requirements for purposes of redesignation under section 110 and part D of title I of the CAA (requirements specific to nonattainment areas for the 2015 ozone NAAQS). Additionally, the EPA proposes to find that Michigan has a fully approved SIP under section 110(k) of the CAA. In making these proposed determinations, the EPA ascertained which requirements are applicable for purposes of redesignation, and whether the required Michigan SIP elements are fully approved under section 110(k) and part D of the CAA. As discussed more fully below, SIPs must be fully approved only with respect to these applicable requirements of the CAA.

In the 2023 final clean data determination, the EPA determined that the requirement for Michigan to submit certain planning SIPs related to attainment of the area, including the reasonably available control measures (RACM) requirement of section 172(c)(1) of the CAA, the reasonable further progress (RFP) and attainment demonstration requirements of sections 172(c)(2) and (6) and 182(b)(1) of the CAA, and the requirement for contingency measures of section 172(c)(9) of the CAA, would not be applicable to the area as long as it continues to attain the NAAQS and would cease to apply upon redesignation. In addition, in the context of redesignations, the EPA has interpreted requirements related to attainment as not applicable for purposes of redesignation. For example, in the General Preamble, the EPA stated that the section 172(c)(9) requirements are directed at ensuring RFP and attainment by the applicable date. These requirements no longer apply when an area has attained the standard and is eligible for redesignation. Furthermore, section 175A for maintenance plans provides specific requirements for contingency measures that effectively supersede the requirements of section 172(c)(9) for these areas. (General Preamble, 57 FR 13498, 13564, April 16, 1992).^[11]

1. Michigan Has Met All Applicable Requirements of Section 110 and Part D of the CAA Applicable to the Detroit Area for Purposes of Redesignation

a. Section 110 General Requirements for Implementation Plans

Section 110(a)(2) of the CAA delineates the general requirements for a SIP. Section 110(a)(2) provides that the SIP must have been adopted by the State after reasonable public notice and hearing, and that, among other things, it must: (1) include enforceable emission limitations and other control measures, means, or techniques necessary to meet the requirements of the CAA; (2) provide for establishment and operation of appropriate devices, methods, systems, and procedures necessary to monitor ambient air quality; (3) provide for implementation of a source permit program to regulate the modification and construction of stationary sources within the areas covered by the plan; (4) include provisions for the implementation of part C prevention of significant deterioration (PSD) and part ( printed page 47166) D new source review (NSR) permit programs; (5) include provisions for stationary source emission control measures, monitoring, and reporting; (6) include provisions for air quality modeling; and, (7) provide for public and local agency participation in planning and emission control rule development.

Section 110(a)(2)(D) of the CAA requires SIPs to contain measures to prevent sources in a State from significantly contributing to air quality problems in another State. To implement this provision, the EPA has required certain States to establish programs to address transport of certain air pollutants, for example, the NO_X SIP Call and the Cross State Air Pollution Rule. However, like many of the section 110(a)(2) requirements, the section 110(a)(2)(D) SIP requirements are not linked with a particular area's ozone designation and classification. The EPA concludes that the SIP requirements linked with the area's ozone designation and classification are the relevant measures to evaluate when reviewing a redesignation request for the area. The section 110(a)(2)(D) requirements, where applicable, continue to apply to a State regardless of the designation of any one particular area within the State. Thus, we believe these requirements are not applicable requirements for purposes of redesignation.^[12]

In addition, the EPA believes that other section 110 elements that are neither connected with nonattainment plan submissions nor linked with an area's ozone attainment status are not applicable requirements for purposes of redesignation. The area will still be subject to these requirements after the area is redesignated to attainment of the 2015 ozone NAAQS. The section 110 and part D requirements which are linked with a particular area's designation and classification are the relevant measures to evaluate in reviewing a redesignation request. This approach is consistent with the EPA's existing policy on applicability ( i.e., for redesignations) of conformity requirements, as well as with section 184 ozone transport requirements.^[13]

We have reviewed Michigan's SIP and propose to find that it meets the general SIP requirements under section 110 of the CAA, to the extent those requirements are applicable for purposes of redesignation. In any case, on September 28, 2021 (86 FR 53550), the EPA approved elements of the SIP submitted by Michigan to meet the requirements of section 110 for the 2015 ozone standard.

b. Part D Requirements

Section 172(c) of the CAA sets forth the basic requirements of air quality plans for States with nonattainment areas that are required to submit them pursuant to 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 Detroit area is classified as Moderate under subpart 2 for the 2015 ozone NAAQS. As such, the area is subject to the subpart 1 requirements contained in section 172(c) and section 176. Similarly, the area is subject to the subpart 2 requirements contained in section 182(a) (Marginal nonattainment area requirements) and section 182(b) (Moderate nonattainment area requirements). A thorough discussion of the requirements contained in section 172(c) and 182 can be found in the General Preamble for Implementation of title I, 57 FR 13498 (April 16, 1992). However, as discussed in section VI of this preamble, the EPA is proposing to adjust the deadline for Michigan to submit Moderate SIP revisions for the Detroit area to no later than December 5, 2026. Providing the EPA finalizes this SIP submittal deadline adjustment and the redesignation of the Detroit area prior to December 5, 2026, Moderate SIP requirements would not be applicable requirements for purposes of redesignation, because they will not have become due.

i. Section 172 Requirements

As provided in subpart 2, for ozone nonattainment areas such as the Detroit area, the attainment planning requirements that would otherwise apply under section 172(c), including the requirements under section 172(c)(1) and section 172(c)(2) for an attainment demonstration, RACM, and RFP, are addressed by the specific requirements of section 182.^[14]

Section 172(c)(3) requires submission and approval of a comprehensive, accurate, and current inventory of actual emissions. This requirement is superseded by the inventory requirement in section 182(a)(1) discussed below.

Section 172(c)(4) requires the identification and quantification of allowable emissions for major new and modified stationary sources in an area, and section 172(c)(5) requires source permits for the construction and operation of new and modified major stationary sources anywhere in the nonattainment area. The EPA approved Michigan's NSR program on December 16, 2013 (78 FR 76064), and approved revisions on May 12, 2021 (86 FR 25954). Most recently, the EPA approved Michigan's certification that its SIP satisfies the nonattainment NSR requirements of the CAA for the 2015 ozone NAAQS on June 9, 2023 (88 FR 37766). Nonetheless, the EPA has determined that, since PSD requirements will apply after redesignation, areas being redesignated need not comply with the requirement that a NSR program be approved prior to redesignation, provided that the area demonstrates maintenance of the NAAQS without part D NSR.^[15] A more detailed rationale for this view is described in the Nichols Memorandum.^[16] Michigan's PSD program will become effective in the Detroit area upon redesignation to attainment. The EPA conditionally approved Michigan's PSD program on September 16, 2008 (73 FR 53366), fully approved Michigan's PSD program on March 25, 2010 (75 FR 14352), and most recently approved revisions to Michigan's PSD program on May 12, 2021 (86 FR 25954).

Section 172(c)(6) requires the SIP to contain control measures necessary to provide for attainment of the NAAQS. Because attainment has been reached, no additional measures are needed to provide for attainment.

Section 172(c)(7) requires the SIP to meet the applicable provisions of section 110(a)(2). As noted above, we believe the Michigan SIP meets the requirements of section 110(a)(2) for purposes of redesignation.

Section 172(c)(8) allows for equivalent modeling, emission inventory, and planning procedures in certain circumstances upon application ( printed page 47167) by the State, which is not applicable to this action.

CAA section 172(c)(9) requires the submission of contingency measures. Because the area has attained, CAA section 172(c)(9) requirements are no longer applicable as long as the area continues to attain the standard.

ii. Section 176 Conformity Requirements

Section 176(c) of the CAA requires that federally supported or funded projects conform to the applicable SIP. The requirement to determine conformity applies to transportation plans, programs, and projects that are developed, funded, or approved under title 23 of the United States Code (U.S.C.) and the Federal Transit Act (transportation conformity) as well as to all other federally supported or funded projects (general conformity). State transportation conformity SIP revisions must be consistent with Federal conformity regulations relating to consultation, enforcement, and enforceability that the EPA promulgated pursuant to its authority under the CAA.

The EPA interprets the conformity SIP requirements ^[17] as not applying for purposes of evaluating a redesignation request under section 107(d) because State conformity rules are still required after redesignation and Federal conformity rules apply where State conformity rules have not been approved.^[18] Nonetheless, Michigan has an approved conformity SIP for the Detroit area.^[19]

iii. CAA Section 182(a), Section 182(b), and Section 182(f) Requirements

CAA section 182(a)(1) requires States to submit a comprehensive, accurate, and current inventory of actual emissions from sources of NO_X and VOC emitted within the boundaries of the ozone nonattainment area within two years of designation. The EPA approved Michigan's base year emissions inventory for the Detroit area on July 6, 2022 (87 FR 40097).

Under CAA section 182(a)(2)(A), States with ozone nonattainment areas that were designated prior to the enactment of the 1990 CAA amendments were required to submit, within six months of classification, all rules and corrections to existing VOC reasonably available control technology (RACT) rules that were required under CAA section 172(b)(3) prior to the 1990 CAA amendments. The Detroit area is not subject to the CAA section 182(a)(2) RACT “fix up” requirement for the 2015 ozone NAAQS because it was designated as nonattainment for this standard after the enactment of the 1990 CAA amendments and, in any case, Michigan complied with this requirement for the Detroit area under the prior 1-hour ozone NAAQS.^[20]

CAA section 182(a)(2)(B) requires each State with a Marginal ozone nonattainment area that implemented or was required to implement a vehicle I/M program prior to the 1990 CAA amendments to submit a SIP revision for an I/M program no less stringent than that required prior to the 1990 CAA amendments or already in the SIP at the time of the CAA amendments, whichever is more stringent. For the purposes of the 2015 ozone NAAQS and the consideration of Michigan's redesignation request for this standard, the Detroit area is not subject to the CAA section 182(a)(2)(B) requirement because the Detroit area was designated as nonattainment for the 2015 ozone NAAQS after the enactment of the 1990 CAA amendments and because Michigan complied with this requirement for the Detroit area under the prior 1-hour ozone NAAQS.

Regarding the source permitting and offset requirements of CAA sections 182(a)(2)(C), 182(a)(4), and 182(b)(5), Michigan currently has a fully approved part D NSR program in place. The EPA approved Michigan's NSR program on December 16, 2013 (78 FR 76064), and most recently approved revisions to Michigan's NSR program on May 12, 2021 (86 FR 25954). In addition, the EPA conditionally approved Michigan's PSD program on September 16, 2008 (73 FR 53366), fully approved Michigan's PSD program on March 25, 2010 (75 FR 14352), and most recently approved revisions to Michigan's PSD program on May 12, 2021 (86 FR 25954). The State's PSD program will become effective in the Detroit area upon redesignation to attainment.

CAA section 182(a)(3) requires States to submit periodic emission inventories and a revision to the SIP to require the owners or operators of stationary sources to annually submit emission statements documenting actual VOC and NO_X emissions. Michigan will continue to update its emissions inventory at least once every three years, consistent with the requirements of 40 CFR part 51, subpart A, and in 40 CFR 51.122. The Consolidated Emissions Reporting Rule (CERR) was promulgated by the EPA on June 10, 2002 (67 FR 39602). The CERR was replaced by the Air Emissions Reporting Requirements on December 17, 2008 (73 FR 76539). The most recent triennial inventory for Michigan was compiled for 2020, and 2023 is in progress. The EPA approved Michigan's emission statement SIP for the Detroit area for the 2015 ozone NAAQS on July 6, 2022 (87 FR 40097).

CAA section 182(b) contains the requirements for areas classified as Moderate. As discussed in section VI of this preamble, in this action the EPA is proposing to adjust the deadline for Michigan to submit Moderate SIP revisions for the Detroit area to no later than December 5, 2026. Provided the EPA finalizes this SIP submittal deadline adjustment and the redesignation of the Detroit area prior to December 5, 2026, Moderate SIP requirements would not be considered applicable requirements for purposes of redesignation because they will not have become due and thus are not a prerequisite to redesignation.

CAA section 182(b)(1) requires the submission of an attainment demonstration and RFP plan. Because the area has attained, CAA section 182(b)(1) requirements are no longer applicable as long as the area continues to attain the standard.

CAA section 182(b)(2) requires States with Moderate nonattainment areas to implement VOC RACT with respect to each of the following: (1) all sources covered by a Control Technology Guideline (CTG) document issued between November 15, 1990, and the date of attainment; (2) all sources covered by a CTG issued prior to November 15, 1990; and (3) all other major non-CTG stationary sources. However, as described in section VI.B. of this preamble, the EPA is proposing to extend the deadlines for SIP submissions associated with the Moderate classification. Therefore, if finalized, CAA section 182(b)(2) requirements would not be applicable for purposes of redesignation.

CAA section 182(b)(3) requires States to adopt Stage II gasoline vapor recovery regulations. On May 16, 2012 (77 FR 28772), the EPA determined that the use of onboard vapor recovery technology for capturing gasoline vapor when gasoline-powered vehicles are refueled is in widespread use throughout the highway motor vehicle fleet and waived ( printed page 47168) the requirement that current and former ozone nonattainment areas implement Stage II vapor recovery systems on gasoline pumps.

CAA section 182(b)(4) requires a Basic vehicle I/M program in each State with a Moderate ozone nonattainment area. However, as described in section VI.B. of this preamble, the EPA is proposing to extend the deadlines for SIP submissions associated with the Moderate classification. Therefore, if finalized, section 182(b)(4) requirements would not be applicable for purposes of redesignation.

CAA section 182(f) of the CAA establishes NO_X requirements for ozone nonattainment areas. CAA section 182(f)(1) generally requires major sources of NO_X to be covered by the same levels of emission controls as required for major sources of VOC. Since section 182(b)(2)(C) of the CAA requires areas classified as Moderate (or above) to implement RACT for major VOC sources, these ozone nonattainment areas are also required to implement NO_X RACT for major sources of NO_X. However, as described in section VI of this preamble, the EPA is proposing to extend the deadlines for those submittals. Therefore, if finalized, CAA section 182(f) requirements would not be applicable for purposes of redesignation.

Thus, as discussed above, the EPA finds that the Detroit area satisfies all applicable requirements for purposes of redesignation under section 110 and part D of the CAA.

2. The Detroit Area Has a Fully Approved SIP for Purposes of Redesignation Under Section 110(k) of the CAA

At various times, Michigan has adopted and submitted, and the EPA has approved, provisions addressing the various SIP elements applicable for the ozone NAAQS. Providing the EPA finalizes the SIP submittal deadline adjustment and the redesignation of the Detroit area prior to December 5, 2026, the EPA will have fully approved the Michigan SIP for the Detroit area under CAA section 110(k) for all requirements applicable for purposes of redesignation under the 2015 ozone NAAQS. The EPA may rely on prior SIP approvals in approving a redesignation request.^[21] Additional measures may also be approved in conjunction with a redesignation action.^[22]

C. Are the air quality improvements in the Detroit area due to permanent and enforceable emission reductions?

To redesignate an area from nonattainment to attainment, CAA section 107(d)(3)(E)(iii) requires the EPA to determine that the air quality improvement in the area is due to permanent and enforceable reductions in emissions resulting from the implementation of the SIP and applicable Federal air pollution control regulations and other permanent and enforceable emission reductions.

The EPA determined in the 2023 final redesignation that the observed ozone air quality improvement in the Detroit area was due to permanent and enforceable reductions in VOC and NO_X emissions resulting from State measures adopted into the SIP and Federal measures. In making this determination, the EPA reviewed analyses from EGLE's 2022 submittal. The State calculated the change in emissions between 2014 and 2019. The reduction in emissions and the corresponding improvement in air quality over this time period was attributed to several regulatory control measures that the Detroit area and upwind areas had implemented. In addition, Michigan provided an analysis to demonstrate the improvement in air quality was not due to unusually favorable meteorology. Michigan's analysis and the EPA's discussion of the analysis are available in the docket for this regulatory action (for example, in the 2022 submittal, the 2023 proposed redesignation, and the 2023 final redesignation).

The determination in the 2023 final redesignation is supported by more recent air quality data, which show that the area has continued to attain the standard, after accounting for wildfire impacts, for the 2021 through 2023, 2022 through 2024, and 2023 through 2025 periods. This air quality data is additional evidence that the permanent and enforceable emissions reductions documented in the 2022 submittal have been sufficient to keep the Detroit area in attainment of the 2015 ozone NAAQS.

The EPA's determination under CAA section 107(d)(3)(E)(iii) was challenged by petitioners but upheld by the Court in the 2025 Sixth Circuit decision. The Court wrote that “the EPA's determination that the air-quality improvements in the Detroit area were due to permanent and enforceable emission reductions was not arbitrary or capricious.” For purposes of finalizing a future redesignation of the Detroit area, the EPA proposes to continue to rely on the EPA's earlier determination.

D. Does Michigan have a fully approvable ozone maintenance plan for the Detroit area?

To redesignate an area from nonattainment to attainment, CAA section 107(d)(3)(E)(iv) requires the EPA to determine that the area has a fully approved maintenance plan pursuant to section 175A of the CAA. Section 175A of the CAA sets forth the elements of a maintenance plan for areas seeking redesignation from nonattainment to attainment. Under CAA section 175A, the maintenance plan must demonstrate continued attainment of the NAAQS for at least 10 years after the Administrator approves a redesignation to attainment. Eight years after the redesignation, the State must submit a revised maintenance plan which demonstrates that attainment of the NAAQS will continue for an additional 10 years beyond the initial 10-year maintenance period. To address the possibility of future NAAQS violations, the maintenance plan must contain contingency provisions, as the EPA deems necessary, to assure prompt correction of the future NAAQS violation.

The Calcagni Memorandum provides further guidance on the content of a maintenance plan, explaining that a maintenance plan should address five elements: (1) an attainment emission inventory; (2) a maintenance demonstration; (3) a commitment for continued air quality monitoring; (4) a process for verification of continued attainment; and (5) a contingency plan.

In conjunction with its request to redesignate the Detroit area to attainment for the 2015 ozone NAAQS, EGLE's 2022 submittal included a SIP revision to provide for maintenance of the 2015 ozone NAAQS through 2035. In the 2023 final redesignation, the EPA approved this maintenance plan into the Michigan SIP, and determined that this maintenance plan satisfied the requirement for redesignation to attainment under CAA section 107(d)(3)(E)(iv) because it met the requirements of CAA section 175A. Neither the EPA's approval of the maintenance plan into the SIP nor the EPA's reliance on that maintenance plan for purposes of redesignation were challenged by petitioners, and EPA's actions related to the maintenance plan were not vacated by the 2025 Sixth Circuit decision.

In its 2022 submittal, EGLE used 2019 to represent the attainment year, and 2035 to represent the maintenance year. In its 2026 supplemental submittal, EGLE submitted revised versions of these inventories, and EGLE is now ( printed page 47169) using the 2035 inventory to represent an interim year. In the 2026 supplemental submittal, EGLE submitted a new maintenance year inventory for 2040, which is more than 10 years after the expected effective date of a future redesignation to attainment.

As discussed below, the EPA proposes to find that Michigan's SIP-approved maintenance plan, as updated by the 2026 supplemental submittal, includes the necessary components under CAA section 175A and CAA section 107(d)(3)(E)(iv). The EPA proposes to approve the updates to the maintenance plan as a revision of the Michigan SIP.

1. Attainment Inventory

The EPA is proposing to determine that the Detroit area has attained the 2015 ozone NAAQS based on monitoring data for the 2019-2021 period. Michigan selected 2019 as the attainment emissions inventory year, which is appropriate because it is one of the years in the 2019-2021 period. The attainment emissions inventory identifies the levels of VOC and NO_X emissions in the Detroit area that are sufficient to attain the 2015 ozone NAAQS. The emissions for the 2019 attainment year, by source category, are summarized in Table 2 and Table 3 below.

Michigan provided inventories for point, nonpoint, onroad, and nonroad sources. The point source category includes facilities that report their emissions directly to EGLE, as well as sources such as airports and rail yards. Nonpoint sources, sometimes called area sources, include emissions from sources that are more ubiquitous, such as consumer products or architectural coatings. Onroad sources are vehicles that are primarily used on public roadways, such as cars, trucks, and motorcycles. Nonroad sources include engine-based emissions that do not occur on roads, such as trains or boats.

For the point, nonpoint, and nonroad source categories, in its 2026 supplemental submittal, EGLE made no changes to the emission levels submitted in its 2022 submittal. EGLE's primary data source was the EPA's 2016v2 modeling platform, which includes emissions data for the years 2016, 2023, 2026 and 2032. To derive point, nonpoint, and nonroad inventories for 2019, EGLE interpolated between 2016 and 2023 data from the 2016v2 modeling platform. The 2016v2 modeling platform and 2014 NEI have been quality-assured, and documentation regarding these datasets and their methods is available on the EPA's website.^[23] Additional information regarding point, nonpoint, and nonroad inventories is available in the docket for this regulatory action (for example, in the 2022 submittal, the 2023 proposed redesignation, and the 2023 final redesignation).

For its onroad emissions inventory, Michigan submitted an analysis by the Southeast Michigan Council of Governments (SEMCOG). In the 2022 submittal, this analysis used the EPA's MOVES3 model to generate July weekday onroad emissions. In its 2026 supplemental submittal, EGLE submitted updated inventories generated using the EPA's MOVES5 model, the latest version of the EPA's MOVES model. SEMCOG's analysis relied on local travel inputs including demographic data, travel demand forecasting, road types, Vehicle Miles of Travel (VMT), Vehicle Hours of Travel, vehicle population, and vehicle age, as well as meteorological data. In attachment A of its 2026 supplemental submittal, Michigan has included a detailed narrative of SEMCOG's methods.

2. Has the State demonstrated maintenance of the ozone standard in the Detroit area?

Michigan has demonstrated maintenance of the 2015 ozone NAAQS through 2040 by projecting that current and future emissions of VOC and NO_X for the Detroit area remain at or below attainment year emission levels. A maintenance demonstration need not be based on modeling.^[24]

Michigan is using emissions inventories for the years 2035 and 2040 to demonstrate maintenance. 2040 was selected because it is more than 10 years after the expected effective date of the redesignation to attainment, and 2035 was selected to demonstrate that emissions are not expected to spike in the interim between the 2019 attainment year and the 2040 final maintenance year.

To develop emissions inventories for the years 2035 and 2040, Michigan used the same data sources discussed above in section IV.D.1. of this proposed rule.

For its onroad emissions inventory, in the 2026 supplemental submittal, Michigan again relied upon the SEMCOG analysis, which used the EPA's MOVES5 model, the latest version of the EPA's MOVES model, to generate July weekday onroad emissions for the 2019 attainment year, as well as 2035 and 2040.

For the point, nonpoint, and nonroad source categories, for the 2035 year, in the 2026 supplemental submittal EGLE made no changes to the emission levels from its 2022 submittal. For the 2040 year, EGLE again used the EPA's 2016v2 modeling platform, and extrapolated forward from the modeling platform data using the 2016 and 2032 years. To convert annual emissions totals into a value of tons per ozone season day, EGLE calculated a conversion factor for each county and each source category. This conversion factor was generated from the 2016v2 modeling platform by taking the July category emissions and dividing them by the annual category emissions. EGLE selected July as the standard ozone season month after evaluating the summer months for ozone season production.

By calculating its inventories through interpolation and extrapolation, EGLE projects that changes within a source category and county are linearly constant. For point sources, actual reductions may not align with inventories derived from linear interpolation, because shutdowns and the operation of new control equipment may be staggered across several years. However, given the magnitude of the reductions in other categories of sources, any uncertainty caused by linear interpolation would be outweighed by the emissions reductions in other sectors. Similarly, inventories derived from extrapolation may not align with actual reductions for some types of sources. However, even if Michigan as a cautious measure had projected that emissions from the 2016v2 modeling platform for the year 2032 would remain constant through 2040, this level of emissions would still have been sufficient to show that the area would maintain the standard through 2040.

Emissions data for the 2019 attainment year, 2035 interim year, and 2040 maintenance year are shown in Table 3 and Table 4 below. Data are expressed in terms of tons per ozone season day. ( printed page 47170)

As shown in Table 2 and Table 3, NO_X and VOC emissions in the Detroit area are projected to decrease by 137.84 tons per ozone season day and 40.00 tons per ozone season day, respectively, between the 2019 attainment year and 2040 maintenance year. Michigan's maintenance demonstration for the Detroit area shows maintenance of the 2015 ozone NAAQS by providing emissions information to support the demonstration that future emissions of NO_X and VOC will remain at or below 2019 emission levels when considering future source growth.

On February 12, 2026, the EPA finalized the rescission of the GHG Endangerment Finding and repealed all greenhouse gas (GHG) emission standards for light-duty, medium-duty, and heavy-duty vehicles and engines (“Endangerment Finding Rescission Rule”).^[25] Due to this action, aspects of the State's planning assumptions from the EPA's Motor Vehicle Emission Simulator model versions 5 (MOVES5) are no longer entirely consistent with the latest information. The EPA recognizes that States have relied on MOVES4 and MOVES5, including the national default electric vehicle (EV) projections tied in part to the implementation of these now-repealed standards, in developing some SIP actions that have been submitted to the EPA.

The Detroit maintenance plan was developed using MOVES5 and the latest EPA guidance on the use of MOVES in SIP development that was available at the time. Per the EPA's MOVES Policy Guidance, States “should use the latest version of MOVES that is available at the time that a SIP is developed.” ^[26] In this guidance, the EPA also stated that an earlier version of MOVES can be used in SIP development if significant work has already been completed on a SIP using that earlier version.^[27] Therefore, the State developed this SIP based on an emissions model that the EPA expected States to use. In addition, the EPA indicated that for fuel type distributions for onroad vehicles that include estimates of EVs, States should use local data if available and could rely on the default information within the model as planning assumptions.^[28] Therefore, based on EPA's statements in the EPA's MOVES Policy and Technical Guidance, the State reasonably relied on the assumptions in MOVES5.

As a general principle, the SIP is an iterative document, subject to various revisions over time. This is because the SIP revision process is cyclical by nature per the different obligations under the CAA. The EPA acknowledges the significant amount of time and resources required by States to develop a SIP revision. In the evaluation of this SIP submittal, the EPA is considering the modeling and information provided by Michigan and affirming that Michigan relied on the latest EPA model and guidance information available at the time it developed the SIP. To acknowledge and accommodate reliance interests States may have had in MOVES5 based on the EPA's statements in the EPA's MOVES Policy and Technical Guidance, and to treat all States consistently that submitted SIPs prior to or shortly after the Endangerment Finding Rescission Rule where significant work had been completed, the EPA is proposing to find that Michigan appropriately estimated onroad mobile source emissions for this SIP based on MOVES5.

3. Continued Air Quality Monitoring, Verification of Continued Attainment, and Contingency Plan

With respect to the requirements for a commitment for continued air quality monitoring, a process for verification of continued attainment, and contingency plan, information regarding Michigan's maintenance plan is available in the ( printed page 47171) docket for this regulatory action (for example, in the 2022 submittal, the 2023 proposed redesignation, and the 2023 final redesignation). In its 2026 supplemental submittal, EGLE made no changes to these aspects of Michigan's SIP-approved maintenance plan. On May 19, 2023 (88 FR 32594), the EPA determined these components of the maintenance plan meet the requirements at CAA section 175A and we propose to confirm that finding.

The EPA has determined that Michigan's maintenance plan adequately addresses the five basic components of a maintenance plan: attainment inventory, maintenance demonstration, monitoring network, verification of continued attainment, and a contingency plan. In addition, as required by section 175A(b) of the CAA, Michigan has committed to submit to the EPA an updated ozone maintenance plan eight years after redesignation of the Detroit area to cover an additional ten years beyond the initial 10-year maintenance period. Thus, the EPA finds that the maintenance plan for the Detroit area, as updated by the 2026 supplemental submittal, meets the requirements of section 175A of the CAA. The EPA proposes to approve the updates to the maintenance plan as a revision to the Michigan SIP.

V. Has the State adopted approvable motor vehicle emission budgets?

A. Motor Vehicle Emission Budgets

Under section 176(c) of the CAA, new transportation plans, programs, or projects that receive Federal funding or support, such as the construction of new highways, must “conform” to ( i.e., be consistent with) the SIP. Conformity to the SIP means that transportation activities will not cause or contribute to any new air quality violations, increase the frequency or severity of any existing air quality problems, or delay timely attainment or any required interim emissions reductions or any other milestones. Regulations at 40 CFR part 93 set forth EPA policy, criteria, and procedures for demonstrating and ensuring conformity of transportation activities to a SIP. Transportation conformity is a requirement for nonattainment and maintenance areas. Maintenance areas are areas that were previously nonattainment for a particular NAAQS, but that have been redesignated to attainment with an approved CAA section 175A maintenance plan for the NAAQS.

Under the CAA, States are required to submit, at various times, control strategy SIPs for nonattainment areas and maintenance plans for areas seeking redesignations to attainment of the ozone standard and maintenance areas.^[29] These control strategy SIPs (including RFP plans and attainment plans) and maintenance plans must include motor vehicle emissions budgets for criteria pollutants and/or their precursor pollutants to address pollution from onroad transportation sources. In ozone areas, budgets are established for the precursor pollutants VOC and NO_X, per 40 CFR 93.102(b)(2)(i). The budgets are the portion of the total allowable emissions that are allocated to highway and transit vehicle use that, together with emissions from other sources in the area, will provide for attainment or maintenance.^[30] The budget serves as a ceiling on emissions from an area's planned transportation system.

Under 40 CFR part 93, a budget for an area seeking redesignation to attainment must be established, at minimum, for the last year of the maintenance plan.^[31] A State may adopt budgets for other years as well. The preamble also describes how to establish the budget(s) in the SIP and how to revise the budget(s), if needed, after initially establishing a budget in the SIP.

As discussed earlier, Michigan's maintenance plan includes NO_X and VOC budgets for the Detroit area for 2035, which is an interim year, as well as 2040, which is the last year of the maintenance period. Establishing budgets for 2035 allows the area to use them for transportation conformity determinations instead of making the qualitative finding described in 40 CFR 93.118(b)(2)(i). The EPA has reviewed Michigan's NO_X and VOC budgets for the area and, in this action, is proposing to approve them.

We are also starting the adequacy review process for these budgets to determine if they meet the adequacy criteria in the transportation conformity regulations (40 CFR 93.118(e)(4)).^[32] Michigan's updated maintenance plan from its 2026 supplemental submittal, including the budgets for this area, is available for public comment via this proposed rulemaking. After the public comment period, the EPA can make a finding of adequacy for the budgets.

The EPA notes that Michigan's submission meets the adequacy criteria at 40 CFR 93.118(e)(4): The submission was endorsed by the Governor's designee and Michigan provided opportunity for a public hearing. The budgets were developed as part of an interagency consultation process which includes Federal, State, and local agencies. The budgets were clearly identified and precisely quantified. These budgets, when considered together with all other emissions sources, are consistent with maintenance of the 2015 ozone NAAQS. These criteria also support the EPA's proposed approval of the budgets as part of the approval of the maintenance plan.

As shown in Table 4, the 2035 and 2040 budgets are larger than the estimated 2035 and 2040 onroad sector emissions. To accommodate future variations in VMT in the area, EGLE allocated to the mobile sector a portion ( printed page 47172) of the safety margin, as described further below.^[33] Michigan has demonstrated that the Detroit area can maintain the 2015 ozone NAAQS in the 2040 maintenance year with mobile source emissions of 100.51 tons per ozone season day of NO_X and 46.50 tons per ozone season day of VOCs. Similarly, the Detroit area can maintain the 2015 ozone NAAQS in the 2035 interim year with mobile source emissions of 101.68 tons per ozone season day of NO_X and 49.14 tons per ozone season day of VOCs. Despite partial allocation of the safety margin, emissions will remain under emission levels in the 2019 attainment year.

The EPA is proposing to approve the budgets for use to determine transportation conformity in the Detroit area, because the EPA has determined that the area can maintain attainment of the 2015 ozone NAAQS for the relevant maintenance period with mobile source emissions at the levels of the budgets.

B. What is a safety margin?

A “safety margin” is the amount by which the total projected emissions from all sources of a given pollutant are less than the total emissions that would satisfy the applicable requirement for maintenance. 40 CFR 93.101. As noted in Table 2 and Table 3, the emissions in the Detroit area are projected to have safety margins of 137.84 tons per ozone season day for NO_X and 40.00 tons per ozone season day for VOC in 2040 (the difference between emissions in the 2019 attainment year, and projected emissions in the 2040 maintenance year, for all sources in the Detroit area). Similarly, there is a safety margin of 117.05 tons per ozone season day for NO_X and 32.68 tons per ozone season day for VOC in 2035. Even if the full amount of the safety margin were added to the projected onroad mobile source levels, these counties would still demonstrate maintenance since emissions levels would equal those in the attainment year.

As shown in Table 4 above, Michigan is allocating a portion of that safety margin to the mobile source sector. Specifically, in 2035, Michigan is allocating 76.08 of the 117.5 tons per ozone season day NO_X safety margin available to the NO_X budget, and 21.24 tons of the 32.68 tons per ozone season day VOC safety margin available to the VOC budget. In 2040, Michigan is allocating 82.71 of the 137.84 tons per ozone season day NO_X safety margin available to the NO_X budget, and 24.00 tons of the 40.00 ton per ozone season day VOC safety margin available to the VOC budget. Michigan is requesting that only a portion of the available safety margins be allocated to the budgets. Therefore, even though the State has submitted budgets that are larger than the projected onroad mobile source emissions for 2035 and 2040 contained in the demonstration of maintenance, the increase in onroad mobile source emissions that can be considered for transportation conformity purposes is within the safety margins of the ozone maintenance demonstration. Further, once the budgets are found adequate, these portions of the safety margins that are allocated to onroad mobile sources will not be available for use by other sources.

VI. Adjustment of Certain SIP Submittal and Implementation Deadlines

A. Sixth Circuit Court of Appeals Decision in Sierra Club v. EPA

As discussed in section III of this preamble, section 107(d)(3)(E) of the CAA contains the criteria for redesignation. CAA section 107(d)(3)(E)(v) specifically requires that “the State containing such area has met all requirements applicable to the area under section [110] of this title and part D of this subchapter.” The EPA's interpretation of that provision since passage of the 1990 CAA Amendments has been that requirements with submission deadlines occurring after the State's submission of its redesignation request are not “applicable” under CAA section 107(d)(3)(E)(v) for purposes of evaluating the approvability of the redesignation.

In the 2023 final redesignation of the Detroit area, the EPA relied on its longstanding interpretation of CAA section 107(d)(3)(E)(v) in finding that Michigan had satisfied that requirement, because the State had met all applicable requirements that were due as of the time of the State's submission requesting redesignation. The EPA's redesignation of the Detroit ozone nonattainment area was challenged, partially based on the argument that the CAA required Michigan to have met all applicable requirements due as of the time the EPA issued the final redesignation, rather than the requirements due as of the time Michigan submitted its redesignation application.

On December 5, 2025, the Sixth Circuit Court of Appeals vacated the EPA's redesignation of the Detroit area. In the 2025 Sixth Circuit decision, the Court held that Michigan was required to have met all requirements due at the time of the EPA's redesignation action, not just the requirements that had been due as of the time of submittal of the redesignation request.

B. Proposed Adjustment of Certain SIP Submittal and Implementation Deadlines Associated With the Moderate Classification

As noted, the EPA's interpretation of CAA section 107(d)(3)(E)(v) has informed State and Federal implementation of redesignations for more than three decades. Michigan devoted considerable resources towards preparing its request to redesignate the Detroit nonattainment area to attainment. And, understandably, it had done so under the longstanding pre- Sierra Club framework that had governed redesignations and the expectations of what States were required to submit in order for areas to have their redesignation requests granted. Therefore, the 2022 submittal did not address Moderate area SIP requirements, because the EPA had not yet proposed or finalized a reclassification of the Detroit area to Moderate nonattainment, nor had the EPA set any deadline for Moderate requirements.

The final determination that the area failed to attain, issued February 1, 2023 (88 FR 6633), reclassified the Detroit area to Moderate nonattainment and established a SIP due date of March 1, 2023. The area was then redesignated to attainment on May 19, 2023 (88 FR 32594), terminating any obligation for the Detroit area to submit Moderate area requirements. The Court's vacatur of the redesignation in its December 5, 2025, decision had the effect of restoring the area's designation and classification to Moderate nonattainment. However, during the intervening years while the redesignation has been effective, Michigan has understandably believed itself not to be subject to the requirement to submit the Moderate elements.

The EPA is therefore taking these circumstances into account in proposing to adjust the deadline for certain Moderate area SIP requirements for the Detroit area. The Agency has previously made similar adjustments to implementation deadlines in response to an adverse court decision.^[34] In that instance, the EPA had for many years implemented particulate matter standards under subpart 1 of the CAA. The D.C. Circuit held that the Agency had erred in doing so, and that the CAA required the implementation of those ( printed page 47173) standards to be governed by subpart 4 instead. In response to that adverse decision, rather than find that States were immediately in default of subpart 4 obligations, the EPA established prospective, relatively expedited deadlines for States to comply with any outstanding subpart 4 requirements.^[35] The D.C. Circuit found that the Agency acted within its authority in establishing new deadlines “in the novel circumstances of this case, [where] all affected parties have been long acting on the mistaken assumption that a different framework . . . controls.” ^[36]

In establishing new deadlines in the fine particulate matter (PM_2.5) rule at issue in Wildearth Guardians, the EPA relied upon its general rulemaking authority under CAA section 301(a), which authorizes the Administrator to promulgate such regulations as are necessary to carry out his functions under the CAA. The Court held that the rule represented “a reasonable exercise of the EPA's gap-filling authority” in that it retained the CAA's attainment deadline for the areas, “even though that date was fast approaching by the time of the Rule's promulgation,” and “[m]oreover, the agency's plan submission deadline . . . was less than two years after the NRDC decision [requiring implementation of the PM_2.5 NAAQS under subpart 4] and some six months after the [final publication of] the Rule.” Id. at 541. The Court noted that the “short timeframe suggests a reasonable effort to expedite compliance with the Subpart 4 framework without imposing unfair obligations on states.” Id.

Here, the EPA is proposing to alter deadlines for reasons similar to the EPA's action establishing new deadlines for PM_2.5 SIPs. Additionally, the EPA has specific authority over reclassified ozone nonattainment areas under CAA section 182(i) to “adjust any applicable deadlines (other than attainment dates) to the extent such adjustment is necessary or appropriate to assure consistency among the required submissions.” For reclassified areas, SIP submission and control measure implementation deadlines are established by rule at 40 CFR 51.1402(b)(1)(i).^[37] As noted in that regulatory provision, the default deadline for reclassified areas applies “unless the Administrator establishes a different deadline in a separate action.”

The EPA is therefore proposing under its statutory and regulatory authority to establish a different Moderate SIP submission and control measure implementation deadline for the Detroit area. This one-time adjustment of the Moderate area SIP submission and implementation deadline for the reclassified Detroit ozone nonattainment area) is necessary and appropriate because without such adjustment, Michigan would immediately be in default of its obligation to submit and implement certain Moderate elements for the Detroit area, despite its reasonable assumption since May 2023 that it had no obligation to do so as a result of the area's redesignation to attainment. This proposed adjustment will allow the State to make any submissions it determines necessary. The EPA proposes to adjust the deadline for Michigan to submit SIP revisions for the Detroit area for requirements associated with the Moderate classification, such as RACM/RACT and Basic I/M, to no later than December 5, 2026, one year after the 2025 Sixth Circuit decision. Except for Basic I/M, the EPA proposes that the implementation deadline for these requirements should also be adjusted to no later than December 5, 2026.

VII. Adjustment of Basic I/M Implementation Deadline

In the 2022 proposed reclassification, the EPA noted that “given the unique nature of I/M programs, there are many challenges, tasks, and milestones that must be met in establishing and implementing an I/M program.” For States that did not intend to rely upon emission reductions from their Basic I/M program in attainment or RFP SIPs, the EPA proposed to allow Basic I/M programs to be fully implemented no later than 4 years after the effective date of reclassification. In the 2023 final reclassification, the EPA finalized this implementation deadline of no later than 4 years after the effective date of reclassification, should EGLE not intend to rely upon emission reductions from their I/M program in attainment or RFP SIPs.

Similarly, on January 17, 2025 (90 FR 5651), in a final rule titled State Implementation Plan Submittal Deadlines and Implementation Requirements for Reclassified Nonattainment Areas Under the Ozone National Ambient Air Quality Standards, the EPA established default SIP submittal and control measure implementation deadline requirements for reclassified ozone nonattainment areas. For I/M, that rule required implementation as expeditiously as practicable, but no later than 4 years after the effective date of the relevant reclassification, unless the I/M program is needed for attainment by the attainment date or RFP, in which case the State shall provide for full implementation of such I/M program no later than the beginning of the attainment year ozone season.^[38]

In the 2023 final clean data determination, the EPA suspended the requirement for attainment or RFP SIPs, meaning the EPA does not expect EGLE to rely upon emission reductions from their I/M program for purposes of any other requirement.

Considering the numerous challenges and milestones necessary in implementing a Basic I/M program, the EPA believes it is reasonable to set an implementation deadline that is 4 years from December 5, 2026. Therefore, the EPA is proposing under CAA section 182(i) to adjust the deadline for Michigan to implement I/M for the Detroit area to no later than December 5, 2030.

VIII. What action is the EPA taking?

The EPA is proposing to determine that the Detroit area has met the requirements for redesignation under section 107(d)(3)(E) of the CAA, and the EPA is thus proposing to change the legal designation of the Detroit area from nonattainment to attainment for the 2015 ozone NAAQS in accordance with EGLE's 2022 submittal and 2026 supplemental submittal. The EPA is also proposing to approve, as a revision to the Michigan SIP, the State's updated maintenance plan for the area, which is designed to keep the Detroit area in attainment of the 2015 ozone NAAQS through 2040. As part of the maintenance plan, the EPA is initiating the adequacy process and proposing to approve the newly established 2035 and 2040 motor vehicle emissions budgets for the Detroit area. Additionally, the EPA is proposing to adjust the deadline for Michigan to submit SIP revisions for the Detroit area for requirements associated with the Moderate classification, as well as the deadline to implement these measures except for Basic I/M, to no later than December 5, 2026. The EPA is proposing to adjust the deadline for Michigan to implement ( printed page 47174) Basic I/M in the Detroit area to no later than December 5, 2030. The EPA also proposes to take final agency action on an exceptional events request submitted by EGLE on April 15, 2026, and concurred on by the EPA on June 4, 2026.

IX. Statutory and Executive Order Reviews

Under the CAA, redesignation of an area to attainment and the accompanying approval of a maintenance plan under section 107(d)(3)(E) are actions that affect the status of a geographical area and do not impose any additional regulatory requirements on sources beyond those imposed by State law. A redesignation to attainment does not in and of itself create any new requirements, but rather results in the applicability of requirements contained in the CAA for areas that have been redesignated to attainment. Moreover, the Administrator is required to approve a SIP submission that complies with the provisions of the CAA and applicable Federal regulations. 42 U.S.C. 7410(k); 40 CFR 52.02(a). Thus, in reviewing SIP submissions, EPA's role is to approve State choices, provided that they meet the criteria of the CAA. Accordingly, the proposed actions to approve Michigan's SIP submissions merely approve State law as meeting Federal requirements and do not impose additional requirements beyond those imposed by State law. For these reasons, this 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 anExecutive Order 14192 (90 FR 9065, February 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, August 10, 1999);
• Is not subject toExecutive Order 13045 (62 FR 19885, April 23, 1997) because it approves 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 EPA or an Indian Tribe has demonstrated that a Tribe has jurisdiction. In those areas of Indian country, the rulemaking 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).

91 FR 47162