We examined particulate sulfate ion concentrations across the United States from the early 1990s through 2010 using remote/rural data from the Interagency Monitoring of Protected Visual Environments (IMPROVE) network and from early 2000 through 2010 using data from the Environmental Protection Agency's (EPA) urban Chemical Speciation Network (CSN). We also examined measured sulfur dioxide (SO2) emissions from power plants from 1995 through 2010 from the EPA's Acid Rain Program. The 1992-2010 annual mean sulfate concentrations at long-term rural sites in the United States have decreased significantly and fairly consistently across the United States at a rate of -2.7 % yr(-1) (p < 0.01). The short-term (2001-2010) annual mean trend at rural sites was -4.6 % yr(-1) (p < 0.01) and at urban sites (2002-2010) was -6.2 % yr(-1) (p < 0.01). Annual total SO2 emissions from power plants across the United States have decreased at a similar rate as sulfate concentrations from 2001 to 2010 (-6.2 % yr(-1), p < 0.01), suggesting a linear relationship between SO2 emissions and average sulfate concentrations. This linearity was strongest in the eastern United States and weakest in the West where power plant SO2 emissions were lowest and sulfate concentrations were more influenced by non-power-plant and perhaps international SO2 emissions. In addition, annual mean, short-term sulfate concentrations decreased more rapidly in the East relative to the West due to differences in seasonal trends at certain regions in the West. Specifically, increased wintertime concentrations in the central and northern Great Plains and increased springtime concentrations in the western United States were observed. These seasonal and regional positive trends could not be explained by changes in known local and regional SO2 emissions, suggesting other contributing influences. This work implies that on an annual mean basis across the United States, air quality mitigation strategies have been successful in reducing the particulate loading of sulfate in the atmosphere; however, for certain seasons and regions, especially in the West, current mitigation strategies appear insufficient.
