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2011. United States. Environmental Protection Agency. Region IX. This polygon shapefile represents nonattainment areas for carbon monoxide (CO) in EPA Region 9, including California, Nevada and Arizona, for 2011.... United States. Environmental Protection Agency. Region IX.
2010, 2011, 2012, and 2013. U.S. Census Bureau. Geography Division and Environmental Systems Research Institute (Redlands, Calif.). This polygon shapefile represents the 2010 Census urban areas in the United States. An urban area comprises a densely settled core of census tracts... Environmental Systems Research Institute (Redlands, Calif.).