Seasonal aerosol sulfate trends for selected regions of the United States

Site and regional trends in seasonally averaged particle SO42- concentrations were examined for a large portion of the United States using data collected by the CASTNet air monitoring network. Trends were analyzed for overlapping periods of 1988-1999 and 1992-1999. The largest absolute SO42- decreases-approximately -0.4 mug/m(3)/ yr-between 1988 and 1999 occurred in summer for sites in the Ohio River Valley and areas to the east. Generally, the largest SO42- reductions were found for summer, but larger relative reductions often occurred for spring and autumn. Sulfate changes during 1992-1999 were quite different from those found for 1988-1999 and were not entirely consistent with changes in SO2 emissions. In some locations, the 1992-1999 period saw smaller declines in SO42-, while in other places seasonal SO4 actually increased. Increases were mostly confined to summer and autumn across the southern and southwestern states. Multivariate analysis of ambient sulfur levels, by region, versus SO2 emissions reveals that annual emissions are associated with more than 80% of the variance in seasonal sulfur (SO2 and SO42-) in more than three-quarters of the cases examined. The weakest associations were found for the southeastern United States.