Evidence linking large reductions in acid rain to sulfur emissions cuts in the eastern United States

Concentrations of sulfate (SO₄^2-) and free acidity (H⁺) in precipitation decreased by 10 to 25 percent over large areas of the eastern United States in 1995. These decreases were extraordinary in magnitude and spatial extent, compared to the 1983-1994 record of observations from the National Atmospheric Deposition Program/National Trends Network (NADP/NTN). In contrast, nitrate, ammonium, and calcium concentrations generally increased in 1995. What's more, the H⁺ and SO₄^2- declines were highly correlated (R² = 0.72), indicating a reduction of acid rain. The largest concentration decreases in both ions occurred in and downwind of the Ohio River Valley. This is the same area where the 1990 Clean Air Act Amendments (CAAA) set limits on SO₂ emissions from 110 affected sources, 63 in states bordering the Ohio River Valley. Phase I of the CAAA required these limits be met by January 1, 1995. Indeed, sulfur dioxide emissions from Phase I sources dropped 40% in 1995 compared to 1994. This was a nearly 19% reduction in overall emissions in the 21 states with Phase I sources. Based on our analysis of emissions and NADP/NTN precipitation chemistry data, we infer that the substantial declines in acid rain in the eastern United States in 1995 occurred because of large reductions in SO₂ emissions in the same region.