SUPPLEMENTARY INFORMATION:

Throughout this document whenever “we,” “us,” or “our” is used, we mean the EPA.

I. Background

A. 2015 Ozone NAAQS

On October 26, 2015 (80 FR 65452), the EPA issued its final action to revise the NAAQS for ozone to establish a new 8-hour standard. In that action, the EPA promulgated identical tighter primary and secondary ozone standards designed to protect public health and welfare that specified an 8-hour ozone level of 0.070 parts per million (ppm). Specifically, the standards require that the 3-year average of the annual fourth highest daily maximum 8-hour average ozone concentration may not exceed 0.070 ppm.

Effective August 3, 2018, the EPA designated 52 areas throughout the country as nonattainment for the 2015 ozone NAAQS ( see83 FR 25776, June 4, 2018). In a separate action, the EPA assigned classification thresholds and attainment dates based on the severity of an area's ozone problem, determined by the area's design value (DV) (see 83 FR 10376, May 8, 2018). The EPA established the attainment date for Marginal, Moderate, and Serious nonattainment areas as 3 years, 6 years, and 9 years, respectively, from the effective date of the final designations. Thus, the attainment date for Marginal nonattainment areas for the 2015 ozone NAAQS was August 3, 2021, the attainment date for Moderate areas was August 3, 2024, and the attainment date for Serious areas is August 3, 2027.

B. Determination of Attainment

Section 181(b)(2)(A) of the CAA requires that within 6 months following the applicable attainment date, the EPA shall determine whether an ozone nonattainment area attained the ozone standard based on the area's DV as of that date. Under CAA section 181(a)(5) as interpreted by the EPA in 40 CFR 51.1307, upon application by any State, the EPA may grant a 1-year extension to the attainment date when certain criteria are met.

In the event an area fails to attain the ozone NAAQS by the applicable attainment date and is not granted a 1-year attainment date extension, CAA section 181(b)(2)(A) requires the EPA to make the determination that an ozone nonattainment area failed to attain the ozone standard by the applicable attainment date, and requires the area to be reclassified by operation of law to the higher of: (1) The next higher classification for the area, or (2) the classification applicable to the area's DV as of the determination of failure to attain.

The level of the 2015 ozone NAAQS is 0.070 ppm.^[1] Under the EPA regulations at 40 CFR part 50, appendix U, the 2015 ozone NAAQS is attained at a site when the 3-year average of the annual fourth highest daily maximum 8-hour average ambient ozone concentration ( i.e., DV) does not exceed 0.070 ppm. When the DV does not exceed 0.070 ppm at each ambient air quality monitoring site within the area, the area is deemed to be attaining the ozone NAAQS. Each area's DV is determined by the highest DV among monitors with valid DVs.^[2] The data handling convention in appendix P dictates that concentrations shall be reported in “ppm” to the third decimal place, with additional digits to the right being truncated. Thus, a computed 3-year average ozone concentration of 0.071 ppm is greater than 0.070 ppm and would exceed the standard, but a computed 3-year average ozone concentration of 0.0709 ppm is truncated to 0.070 ppm and attains the 2015 ozone NAAQS.

C. The Detroit Area

The EPA designated the Detroit area as Marginal nonattainment for the 2015 ozone NAAQS on June 4, 2018 (83 FR 25776) (effective August 3, 2018). On February 1, 2023 (88 FR 6633), the EPA determined that the Detroit area failed ( printed page 31994) to attain the 2015 ozone NAAQS by its Marginal attainment date of August 3, 2021, based on the area's DV as of the attainment date ( i.e., monitoring data from 2018-2020). As a result of that determination, the area was reclassified by operation of law to Moderate nonattainment. On January 3, 2022, EGLE submitted a request to redesignate the Detroit area to attainment of the 2015 ozone NAAQS based on monitored attainment of the NAAQS for the 2019-2021 time period. On May 19, 2023 (88 FR 32584 and 32594), the EPA issued a determination that the Detroit area was attaining the 2015 ozone NAAQS based on air quality monitoring data from 2020-2022 (clean data determination) and finalized redesignation of the Detroit area to attainment of the 2015 ozone NAAQS.

On December 5, 2025, the United States Court of Appeals for the Sixth Circuit upheld the EPA's clean data determination but vacated the EPA's redesignation of the Detroit area. The Court issued its mandate in the case on February 24, 2026. See Sierra Club v. EPA, 161 F.4th 934 (6th Cir. 2025). Therefore, the Detroit area is currently designated as nonattainment for the 2015 ozone NAAQS, with a classification of Moderate.

On March 10, 2026, EGLE submitted a request that EPA initiate parallel processing of a supplement to its January 3, 2022, redesignation request. EGLE held a public comment period on the supplemental submittal, which closed on April 22, 2026. In its request for parallel processing EGLE stated that it would address all public comments received and, if appropriate, submit the supplement to EPA in May 2026. EPA will take action on this request in a separate rulemaking.

II. 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 Air Quality System (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.^[3]

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.^[4]

The CAA provides for the exclusion of air quality monitoring data from design value calculations when there are NAAQS exceedances caused by events, such as wildfires, that meet the criteria for an exceptional event identified in the EPA's Exceptional Events Rule at 40 CFR 50.1, 50.14 and 51.930. For the purposes of this proposed action, on February 19, 2026, EGLE submitted an exceptional events demonstration to show that ozone concentrations recorded at the Allen Park monitor in Wayne County with Site ID 26-163-0001 on June 29 and June 30, 2023, and the East 7 Mile monitor in Wayne County with Site ID 26-163-0019 on June 19, June 29, June 30, and July 25, 2023, were influenced by wildfires. The EPA concurred on this request on March 24, 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 design value to determine if the area attained the standard by the Moderate attainment date. For this proposed action, the EPA will rely on the calculated values that exclude the event-influenced data for the purpose of demonstrating attainment of the 2015 ozone NAAQS by the Moderate attainment date. Further details on EGLE'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, the EPA is providing an opportunity for public comment on the claimed exceptional events and all supporting data prior to the EPA taking final agency action that relies on the revised data set. 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. As such, the EPA proposes to take final regulatory action on the request from Michigan to remove the claimed exceptional events from the data set used for regulatory purposes.

III. Determination of Attainment by the Attainment Date

For areas classified as Moderate nonattainment for the 2015 ozone NAAQS, the attainment date was August 3, 2024. Because the DV is based on the three most recent, complete calendar years of data, attainment must occur no later than December 31 of the year prior to the attainment date, i.e., December 31, 2023, in the case of Moderate nonattainment areas for the 2015 ozone NAAQS. Therefore, the EPA's proposed determination of attainment for the Detroit area is based upon hourly ozone concentration data for calendar years 2021, 2022 and 2023 that have been collected and quality-assured in accordance with 40 CFR part 58 and reported to the EPA's AQS database.^[5] These data demonstrate that the Detroit area attained the 2015 ozone NAAQS by the Moderate attainment deadline. As discussed in section II above, in making this proposed determination, the EPA is relying on ( printed page 31995) calculated values that exclude the event-influenced data. The annual fourth-highest 8-hour ozone concentrations and the 3-year average of these concentrations (monitoring site ozone DVs) for all monitoring sites are summarized in Table 1.

IV. Proposed Action

The EPA is proposing to determine that the Detroit area attained the 2015 ozone NAAQS by the Moderate attainment date. This proposed determination is based on complete, quality-assured, and certified ambient air quality monitoring data for the 2021-2023 design value period showing that the area achieved attainment of the 2015 ozone NAAQS. The EPA is also proposing to take final agency action on an exceptional events request submitted by EGLE on February 19, 2026, and concurred on by the EPA on March 24, 2026. This action, if finalized, will fulfill the EPA's statutory obligation to determine whether the Detroit area attained the 2015 ozone NAAQS by the Moderate attainment date.

V. Statutory and Executive Order Reviews.

A. Executive Order 12866: Regulatory Planning and Review and Executive Order 13563: Improving Regulation and Regulatory Review

This action is not a significant regulatory action and was therefore not submitted to the Office of Management and Budget (OMB) for review. This action proposed to issue a determination that the Detroit, Michigan area attained the 2015 ozone NAAQS by the Moderate attainment date.

B. Executive Order 14192: Unleashing Prosperity Through Deregulation

This action is not an Executive Order 14192 regulatory action because this action is not significant under Executive Order 12866.

C. Paperwork Reduction Act (PRA)

This rule does not impose an information collection burden under the provisions of the Paperwork Reduction Act of 1995 (44 U.S.C. 3501 et seq.).

D. Regulatory Flexibility Act (RFA)

I certify that this action will not have a significant economic impact on a substantial number of small entities under the RFA (5 U.S.C. 601 et seq.). This action will not impose any requirements on small entities beyond those imposed by State law. The proposed attainment determination does not create any new requirements and does not directly regulate any entities.

E. Unfunded Mandates Reform Act (UMRA)

This action does not contain any unfunded mandate as described in UMRA, 2 U.S.C. 1531-1538, and does not significantly or uniquely affect small governments. The action imposes no enforceable duty on any State, local, or Tribal governments or the private sector.

F. Executive Order 13132: Federalism

This action does not have federalism implications. It will not have substantial direct effects on the States, on the relationship between the national government and the States, or on the distribution of power and responsibilities among the various levels of government. This action proposed to determine that the Detroit area attained the 2015 ozone NAAQS by the Moderate attainment date, as required by the CAA.

G. Executive Order 13175: Consultation and Coordination With Indian Tribal Governments

This rule does not have Tribal implications, as specified in Executive Order 13175. It will not have substantial direct effects on Tribal governments. Thus, Executive Order 13175 does not apply to this rule.

H. Executive Order 13045: Protection of Children From Environmental Health and Safety Risks

This action is not subject to Executive Order 13045 because it is not 3(f)(1) significant as defined in Executive Order 12866, and because the EPA does not believe the environmental health or safety risks addressed by this action present a disproportionate risk to children because it merely proposed to determine that the Detroit area attained the 2015 ozone NAAQS by the Moderate attainment date.

I. Executive Order 13211: Actions That Significantly Affect Energy Supply, Distribution, or Use

This action is not subject to Executive Order 13211 because it is not a significant regulatory action under Executive Order 12866.

J. National Technology Transfer Advancement Act

This rulemaking does not involve technical standards.

91 FR 31993