On December 27, 2017, the State of Missouri submitted a request for the Environmental Protection Agency (EPA) to redesignate the Jefferson County, Missouri, 2010 1-hour sulfur dioxide (SO<INF>2</INF>) National Ambient Air Quality Standard (NAAQS) nonattainment area to attainment and to approve a State Implementation Plan (SIP) revision containing a maintenance plan for the area. The State provided supplemental information on: May 15, 2018; February 7, 2019; February 25, 2019; and April 9, 2021.