Mississippi Commission on Environmental Quality v. Environmental Protection Agency

Congress enacted the Clean Air Act (CAA) to protect and promote air quality. The CAA required the Environmental Protection Agency (EPA) to promulgate National Ambient Air Quality Standards (NAAQS) to set the maximum level of permissible pollutant concentrations for six potentially dangerous pollutants in the atmosphere. The Environmental Protection Agency (defendant) would designate areas of states that had ambient air concentrations of pollutants that complied with the NAAQS as “attainment” and ones that did not as “nonattainment.” In March of 2008, the EPA promulgated new, lower primary and secondary NAAQS for ozone. The EPA reviewed each state’s initial designations and then made determinations on whether to alter the states’ recommended nonattainment boundaries. The EPA identified areas that indicated NAAQS violations and then made nonattainment determinations based on a multi-factor, weight-of-the-evidence test that was based on a nine-factor test established on a 2008 guidance that was nonbinding on the EPA concerning designations of nonattainment areas. The EPA condensed the nine-factor test into a five-factor test that examined air quality data, emissions data, meteorology, geography, and jurisdictional boundaries. The EPA assessed areas that exceeded NAAQS as well as nearby areas that contributed to a NAAQS violation. Each county was assessed on a case-by-case basis. In 2011 and 2012, the EPA conducted the designation process and detected a NAAQS violation in Shelby County, Tennessee, and determined that part of DeSoto County, Mississippi, contributed to the violation. This determination was based on weather and wind patterns, ozone precursor emissions data, vehicle usage, and population-growth trends in DeSoto County. Mississippi (plaintiff) challenged the EPA’s designation process and argued that the EPA acted arbitrarily by consolidating the 2008 guidance nine-factor test and its application to DeSoto County. Mississippi petitioned for review.