Air Quality State Implementation Plans; Approvals and Promulgations: Illinois; Clean Data Determination for the Illinois Portion of the St. Louis Area for the 2015 Ozone Standard

Air Plan Approval; Illinois; Clean Data Determination for the Illinois Portion of the St. Louis Area for the 2015 Ozone Standard

SUPPLEMENTARY INFORMATION:

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

I. Background

The EPA has determined that ground-level ozone is detrimental to human health. On October 1, 2015, the EPA promulgated a revised 8-hour ozone NAAQS of 0.070 parts per million (ppm). See 80 FR 65292 (October 26, 2015). Under the EPA's regulations at 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. See 40 CFR 50.19 and appendix U to 40 CFR part 50.

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. On June 4, 2018 (83 FR 25776), the EPA designated the St. Louis, MO-IL area as Marginal nonattainment for the 2015 ozone NAAQS. On June 14, 2021 (86 FR 31438), the EPA revised the designation by expanding the boundary of the area. The current nonattainment area includes Boles Township of Franklin County, Jefferson County, St. Charles County, St. Louis County, and St. Louis City in Missouri, and Madison County, Monroe County, and St. Clair County in Illinois.

On October 7, 2022 (87 FR 60897), the EPA determined that the St. Louis area did not attain the standards by the Marginal attainment date, and the area was reclassified as Moderate by operation of law. More recently, on November 25, 2024 (89 FR 92816), and December 17, 2024 (89 FR 101901), the EPA determined the Missouri portion of the area and the Illinois portion of the area, respectively, did not attain the standards by the Moderate attainment date, and both portions of the area were reclassified as Serious by operation of law. ⁽¹⁾

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. ⁽²⁾

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. ⁽³⁾

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 December 18, 2025, the Illinois EPA submitted an exceptional events demonstration to show that ozone concentrations recorded at the Alton monitor in Madison County with Site ID 17-119-0120 and the Wood River monitor in Madison County with Site ID 17-119-3007 on June 6, 2023, were influenced by wildfires. The EPA concurred on this request on January 12, 2026.

The EPA found that Illinois' 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 demonstrate the area is attaining the standard in order to make a clean data determination for the 2015 ozone NAAQS. 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. Further details on Illinois' analyses and the EPA's concurrence can be found in the docket for this regulatory action.

While the EPA has concurred with Illinois' 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 Illinois' request. As such, the EPA proposes to take final regulatory action on the request from Illinois to remove the claimed exceptional events from the data set used for regulatory purposes.

III. Clean Data Determination

Following enactment of the CAA Amendments of 1990, the EPA discussed its interpretation of the requirements for implementing the NAAQS in the General Preamble for the Implementation of title I of the CAA Amendments of 1990 (General Preamble), 57 FR 13498, 13564 (April 16, 1992). On November 29, 2005 (70 FR 71612), the EPA set forth what has become known as its "Clean Data Policy" for the 1-hour ozone NAAQS. Under the Clean Data Policy, for a nonattainment area that can demonstrate attainment of the standard before implementing CAA nonattainment measures, the EPA interprets the requirements of the CAA that are specifically designed to help an area achieve attainment, such as the requirements for such area to submit attainment demonstrations and associated RACM, RFP plans, contingency measures for failure to attain or make reasonable progress, and other planning SIPs related to attainment of the ozone NAAQS, to be suspended for as long as air quality continues to meet the standard. Such a determination of attainment under the Clean Data Policy is known informally as a clean data determination. On December 6, 2018 (83 FR 62998), in the final rule updating implementing regulations for the 2015 ozone NAAQS, the EPA codified this policy at 40 CFR 51.1318.

An area is attaining the 2015 ozone NAAQS if it meets the 2015 ozone NAAQS based on three complete, consecutive calendar years of quality-assured air quality data for all monitoring sites in the area. To attain the 2015 ozone NAAQS, the 3-year average of the annual fourth-highest daily maximum 8-hour average ozone concentrations (ozone design values) at each monitor must not exceed 0.070 ppm. The air quality data must be collected and quality-assured in accordance with 40 CFR part 58 and recorded in AQS. Ambient air quality monitoring data for the 3-year period must also meet data completeness requirements. An ozone design value is valid if daily maximum 8-hour average concentrations are available for at least 90 percent of the days within the ozone monitoring seasons, ⁽⁴⁾ on average, for the 3-year period, with a minimum data completeness of 75 percent during the ozone monitoring season of any year during the 3-year period. See section 4 of appendix U to 40 CFR part 50.

On December 18, 2025, the Illinois EPA sent a clean data determination request to the EPA. The EPA has reviewed the available ozone monitoring data from the Illinois EPA's monitoring sites in the Illinois portion of the St. Louis area for the 2023-2025 period. 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 Illinois portion of the St. Louis area is attaining the 2015 ozone NAAQS. The annual fourth-highest 8-hour ozone concentrations and the 3-year average of these concentrations (monitoring site ozone design values) for all monitoring sites are summarized in Table 1.

The Illinois portion of the St. Louis area's 3-year ozone design value for 2023-2025 is 0.070 ppm, ⁽⁵⁾ which meets the 2015 ozone NAAQS. Therefore, in this action, the EPA proposes to find that the Illinois portion of the St. Louis area is attaining the 2015 ozone NAAQS.

The EPA will not take final action to determine that the St. Louis area is attaining the NAAQS if the design value of a monitoring site in the area violates the NAAQS prior to final approval of the clean data determination.

Additionally, the EPA will not take final action to determine that the Illinois portion of the St. Louis area is attaining the NAAQS, absent a final action to determine that the Missouri portion of the St. Louis area is also attaining the NAAQS. Under the EPA's Clean Data Policy, a clean data determination only has meaning and effect when an entire area is attaining the NAAQS. In the instance of a multi-state area like the St. Louis area, this means that all monitors in all states must have attaining data.

Should this action be finalized, the requirements for the Illinois EPA to submit attainment demonstrations and associated RACM, RFP plans, contingency measures for failure to attain or make reasonable progress, and other planning SIPs related to attainment of the 2015 ozone NAAQS for the St. Louis area, would be suspended for as long as the area continues to attain the 2015 ozone NAAQS. 40 CFR 51.1318.

This action does not constitute a determination of attainment by the attainment date under CAA section 181(b)(2). In this action, the EPA is considering the area's design value for the 2023-2025 period, which is not the area's design value as of the applicable attainment date.

This action does not constitute a redesignation of any portion of the area to attainment of the 2015 ozone NAAQS under section 107(d)(3)(E) of the CAA, nor does it constitute approval of a maintenance plan for any portion of the area as required under section 175A of the CAA, nor does it find that any portion of the area has met all other requirements for redesignation. On December 30, 2025, Illinois submitted a request to redesignate the Illinois portion of the area to attainment of the 2015 ozone NAAQS under CAA section 107(d)(3)(E), and EPA will take action on Illinois' request in a separate rulemaking.

The Illinois portion of the St. Louis area will remain designated nonattainment for the 2015 ozone NAAQS until such time as the EPA determines that the Illinois portion of the area meets CAA requirements for redesignation to attainment and takes a separate action to redesignate the Illinois portion of the area.

IV. What action is the EPA taking?

The EPA is proposing to approve a determination under the CAA that the Illinois portion of the St. Louis area has attained the 2015 ozone NAAQS. This determination is based upon complete, quality-assured, and certified ambient air monitoring data for the 2023-2025 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 the Illinois EPA on December 18, 2025, and concurred on by the EPA on January 12, 2026. In a separate action, the EPA is proposing a similar determination for the Missouri portion of the St. Louis area. If finalized, this determination would suspend the requirements for the area to submit attainment demonstrations and associated RACM, RFP plans, contingency measures for failure to attain or make reasonable progress, and other planning SIPs related to attainment of the 2015 ozone NAAQS, for as long as the area continues to attain the 2015 ozone NAAQS.

V. Statutory and Executive Order Reviews

Additional information about these statutes and Executive Orders can be found at https://www.epa.gov/lawsregulations/laws-and-executive-orders.

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 proposes to issue a clean data determination for the Illinois portion of the St. Louis area for the 2015 ozone NAAQS.

B. Executive Order 14192: Unleashing Prosperity Through Deregulation

Executive Order 14192 does not apply because it is not a significant regulatory action and is therefore exempted from review 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. This action will not impose any requirements on small entities beyond those imposed by state law. The proposed clean data 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. This 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 proposes a clean data determination for the Illinois portion of the St. Louis area under the CAA.

G. Executive Order 13175: 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 Risks and Safety Risks

The EPA interprets Executive Order 13045 as applying only to those regulatory actions that concern environmental health or safety risks that the EPA has reason to believe may disproportionately affect children, per the definition of "covered regulatory action" in section 2-202 of the Executive Order. Therefore, this action is not subject to Executive Order 13045 because it merely proposes a clean data determination.

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.

⁽¹⁾  The EPA's rules dated November 24, 2024, and December 17, 2024, are the subject of ongoing litigation in the U.S. Court of Appeals for the Eighth Circuit and the U.S. Court of Appeals for the Seventh Circuit, respectively.

⁽²⁾  Under CAA section 319(b), an exceptional event means an event that (i) affects air quality; (ii) is not reasonably controllable or preventable; (iii) is an event caused by human activity that is unlikely to recur at a particular location or a natural event; and (iv) is determined by the EPA under the process established in regulations promulgated by the EPA in accordance with section 319(b)(2) to be an exceptional event. For the purposes of section 319(b), an exceptional event does not include (i) stagnation of air masses or meteorological inversions; (ii) a meteorological event involving high temperatures or lack of precipitation; or (iii) air pollution relating to source noncompliance.

⁽³⁾  40 CFR 50.14(b)(4).

⁽⁴⁾  The ozone season is defined by state in 40 CFR 58, appendix D. The ozone season for both Missouri and Illinois is March-October. See 80 FR 65292, 65466-67 (October 26, 2015).

⁽⁵⁾  The monitor ozone design value for the monitor with the highest 3-year averaged concentration.