Widespread Reductions in Sulfate Across the United States Since the Early 1990s

Particulate sulfate ion concentrations were examined across the United States at remote/rural sites from the Interagency Monitoring of Protected Visual Environments (IMPROVE) and urban sites from the Environmental Protection Agency's (EPA) Chemical Speciation Network (CSN) from the early 1990s through 2010. Temporal trends in sulfate concentrations revealed widespread reductions of sulfate at both urban and rural sites on an annual basis, with higher rates of decrease in the eastern United States relative to the West. In the western United States sulfate concentrations have not decreased as strongly and in some regions and seasons actually increased. Similar temporal trends in measured power plant SO2 emissions suggested a near linear relationship between emissions and sulfate concentrations that was strongest in the eastern United States. This work suggests that regulating SO2 emissions has been extremely successful in lowering sulfate concentrations in the East, however, impacts from unregulated sources and potentially international contributions appear to be affecting sulfate concentrations in the West and may become increasingly important, especially as regulated sources of SO2 emissions continue to decrease.