A 10‐year spatial and temporal trend of sulfate across the United States

Legislative and regulatory mandates have resulted in reduced sulfur dioxide (SO 2 ) emissions in both the eastern and western United States with anticipation that concurrent levels of ambient SO 2 , SO 4 2− , and rainwater acidity would decrease. This paper examines spatial and temporal trends in ambient SO 4 2− concentration from 1988 to 1999, SO 2 emissions from 1990 to 1999, and the relationship between these two variables. The SO 4 2− concentration data came from combining data from the Interagency Monitoring of Protected Visual Environments (IMPROVE) and the Clean Air Status and Trends Network (CASTNet). Over 70 sites spread across the continental United States are considered in this analysis. From a spatial perspective, the 90th percentile summer sulfate concentrations are highest along the Ohio River Valley and in central Tennessee where the emission density of SO 2 is greatest. These concentrations are a factor of 2 greater than the Northeast, northern Michigan, and coastal areas of the Southeast and about a factor of 15 greater than the central western United States. In the East, the largest SO 4 2− decreases in the 80th percentile concentrations occurred north of the Ohio River Valley, while most monitoring sites south of Kentucky and Virginia showed increasing and decreasing trends that were not statistically significant. Big Bend National Park, Texas, Cranberry, North Carolina, and Lassen Volcanic National Park, California, are the only areas that show a statistically significant increase in SO 4 2− mass concentrations. The 1990–1999 annual 80th percentile SO 4 2− time series were compared to the annual SO 2 emissions over four broad United States regions. Each region had a unique time series pattern with the SO 4 2− concentrations and SO 2 emissions closely tracking each other over the 10‐year period. Both the SO 4 2− and SO 2 emissions decreased in the Northeast (28%) and the West (15%), while there was little change in the Southeast and a 15% increase over Texas, New Mexico, and Colorado.