Particulate sulfate ion concentration and SO2 emission trends in the United States from the early 1990s through 2010
1Cooperative Institute for Research in the Atmosphere, Colorado State University, Fort Collins, Colorado, USA
2National Park Service, Cooperative Institute for Research in the Atmosphere, Colorado State University, Fort Collins, Colorado, USA
3Desert Research Institute, Reno, Nevada, USA
Abstract. 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 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). Short-term annual mean trends at rural and urban sites were −4.6% yr−1 (p < 0.01) from 2001 to 2010 and −6.2% yr−1 (p < 0.01) from 2002 to 2010, respectively. Annual total SO2 emissions from power plants across the United States have decreased at a similar rate as sulfate concentrations from 2000 to 2010 (−4.9% 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 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 behavior 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 increased concentrations could not be explained by changes in 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.