The Environmental Protection Agency (EPA) is taking final action to approve a State Implementation Plan (SIP) revision submitted by the Missouri Department of Natural Resources (MoDNR) on September 8, 2021, and supplemented on April 8, 2022, as meeting the Marginal nonattainment area requirements for the 2015 8-hour ozone National Ambient Air Quality Standard (NAAQS or standard) for the Missouri portion of the St. Louis, MO-IL nonattainment area ("St. Louis area" or "area"). The EPA is taking this action pursuant to the Clean Air Act (CAA or Act).