The Environmental Protection Agency (EPA) is finalizing approval of revisions to the Tennessee State Implementation Plan (SIP) submitted through two letters dated June 25, 2008, and September 12, 2018. The SIP revisions were submitted by the Tennessee Department of Environment and Conservation (TDEC) on behalf of the Chattanooga/ Hamilton County Air Pollution Control Bureau and modify the Prevention of Significant Deterioration (PSD) regulations in the Chattanooga portion of the Tennessee SIP to address changes to the federal new source review (NSR) regulations in recent years for the implementation of the national ambient air quality standards (NAAQS). Additionally, the SIP revisions include updates to Chattanooga's regulations of nitrogen oxides (NO<INF>X</INF>) and other miscellaneous typographical and administrative updates. This action is being taken pursuant to the Clean Air Act (CAA or Act).