Changes in the Chemistry of Precipitation in the United States, 1981–1998

Abstract

Regulatory measures in the United States, such as Title IV of the Clean Air Act Amendments of 1990, have primarily restricted sulfur dioxide emissions as a way to control acidic deposition. These restrictions, coupled with increasing concentrations of NH₄ ⁺ in wet deposition in some regions of the U.S. and continued high emissions of nitrogen oxides have generated a significant shift in the chemistry of precipitation as measured at National Atmospheric Deposition Program/National Trends Network sites. Trends in precipitation chemistry at NADP/NTN sites were evaluated for statistical significance for the period 1981–1998 using a Seasonal Mann-Kendall Test, a robust non-parametric test for detection of monotonic trends. SO₄ ²⁻ declines were detected at 100 of the 147 sites examined while no sites exhibited increasing SO₄ ²⁻ trends. On average, SO₄ ²⁻ declined 35% over the period 1981–1998 with downward SO₄ ²⁻ trends being most pronounced in the north-eastern United States. In contrast, no consistent trends in NO₃ ⁻ concentrations were observed in precipitation in any major region of the United States. Although the majority of sites did not exhibit significant trends in NH₄ ⁺ concentration, 30 sites exhibited upward trends. For Ca²⁺ concentration in precipitation, 64 sites exhibited a significant decreasing trend and no sites exhibited an upward trend.