This final rule establishes the initial air quality designations for certain areas in the United States (U.S.) for the 2010 primary sulfur dioxide (SO<INF>2</INF>) National Ambient Air Quality Standard (NAAQS). The Environmental Protection Agency (EPA) is designating the areas as either nonattainment, attainment/ unclassifiable, or unclassifiable. The designations are based on application of the EPA's nationwide analytical approach and technical analysis, including evaluation of monitoring data and air quality modeling, to determine the appropriate designation and area boundary based on the weight of evidence for each area. The Clean Air Act (CAA or Act) directs areas designated as nonattainment to undertake certain planning and pollution control activities to attain the SO<INF>2</INF> NAAQS as expeditiously as practicable. This is the fourth and final set of actions to designate areas of the U.S. for the 2010 SO<INF>2</INF> NAAQS; there are no remaining undesignated areas in the U.S. for the 2010 SO<INF>2</INF> NAAQS.