Estimates of sulfate deposition in the middle eastern United States: 1975,1990, and 2010

Deposition of airborne sulfates and nitrates is considered to be an importa nt factor in the decline of certain fish populations and of forest health i n the eastern United States. A solution has been sought through legislation that mandates significant reductions in anthropogenic emissions of oxides of sulfur and nitrogen, precursors of sulfates and nitrates. This paper loo ks at the changes in sulfate deposition in the middle eastern United States from 1975 to 1990 and 2010 due to SO2 emission reductions brought about by 1970, 1977, and 1990 clean-air legislation. Recently developed emission in ventories, together with a statistical, long-range transport model, were us ed to construct the estimates. The results show that sulfate deposition dec reased over the middle eastern United States by about 25% between 1975 and 1990 and will decrease another 45% between 1990 and 2010-reductions commens urate with the overall decreases in SO2 emissions. The results further indi cate that about half of the sulfur deposited in the environmentally sensiti ve southern Appalachian Mountains comes from states not a part of the south ern Appalachian region. These estimates contain a measure of uncertainty du e to uncertainty in emission estimates, the interannual variability of mete orology, and uncertainty arising from model assumptions and choices of oxid ation and deposition rates. The model was able to reproduce about 10% of th e variability in observations of wet sulfate deposition at over 100 sites o ver a 12-year period.