Reconsideration of the Area Designation for the 2010 1-Hour Sulfur Dioxide (SO2) Primary National Ambient Air Quality Standard for Williamson County, Illinois

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The Environmental Protection Agency (EPA) has completed its reconsideration of the nonattainment designation under the Clean Air Act (CAA) for the Williamson County, Illinois area for the 2010 1-hour sulfur dioxide (SO 2 ) primary national ambient air quality standard (NAAQS). On June 30, 2016, the EPA Administrator signed a final action that designated the Williamson County, Illinois area as nonattainment based on a review of available information. On September 12, 2016, Southern Illinois Power Cooperative (SIPC), the owner of the largest source of SO 2 emissions in the area (the Marion Power Station), submitted to the EPA an updated modeling analysis that characterized SO 2 air quality in the area at the time of the final designation action. The EPA has reviewed that modeling and concludes the available information demonstrates that, as of the date of the Administrator's signature on the final action, the Williamson County, Illinois area was not violating the 2010 1-hour SO 2 NAAQS and was not contributing to a NAAQS violation in a nearby area. Therefore, the EPA is changing the initial designation of Williamson County, Illinois, from nonattainment to attainment/unclassifiable.