Air Plan Approval; Missouri; Clean Data Determination for the 2015 8-Hour Ozone Standard for the Missouri Portion of the St. Louis Nonattainment Area

The Environmental Protection Agency (EPA) is proposing to determine under the Clean Air Act (CAA) that the Missouri portion of the St. Louis, MO-IL nonattainment area (hereafter also referred to, respectively, as the "St. Louis area" or "area") has achieved clean data for the 2015 ozone National Ambient Air Quality Standards (NAAQS or standard). This determination of clean data is based upon complete, quality-assured, and certified ambient air monitoring data for the 2023-2025 design value period showing that the Missouri portion of the area achieved attainment of the 2015 ozone NAAQS. The 2023-2025 design value relies upon EPA concurrence on a portion of the exceptional events request as submitted by the Missouri Department of Natural Resources (MoDNR) on November 3, 2025, and concurred on by the EPA on January 27, 2026. Therefore, the EPA is proposing to approve Missouri's November 3, 2025, Clean Data Determination (CDD) request. If finalized, this proposed CDD would suspend the obligations of the State of Missouri to submit certain nonattainment area planning requirements for as long as the Missouri portion of the St. Louis area continues to attain the 2015 ozone NAAQS. In a separate action, the EPA is proposing a similar determination for the Illinois portion of the St. Louis area.