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

The Environmental Protection Agency (EPA) is proposing to determine under the Clean Air Act (CAA) that the Illinois portion of the St. Louis, MO-IL nonattainment area (hereafter also referred to, respectively, as the "St. Louis area" or "area") has attained the 2015 ozone National Ambient Air Quality Standards (NAAQS or standard). This determination is based upon complete, quality-assured, and certified ambient air monitoring data for the 2023-2025 design value period showing that the Illinois portion of the area achieved attainment of the 2015 ozone NAAQS. This determination also relies on the EPA concurrence of an exceptional events request submitted by the Illinois Environmental Protection Agency (Illinois EPA) on December 18, 2025, and concurred on by the EPA on January 12, 2026. Therefore, the EPA is proposing to take final agency action on Illinois' exceptional events request. 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 Reasonably Available Control Measures (RACM), Reasonable Further Progress (RFP) plans, contingency measures for failure to attain or make reasonable progress, and other planning State Implementation Plans (SIPs) related to attainment of the 2015 ozone NAAQS, for as long as the area continues to attain the 2015 ozone NAAQS.