Trends in precipitation chemistry in the United States: A national perspective, 1980-1992

Thirteen years (1980-1992) of precipitation chemistry data from 58 National Atmospheric Deposition Program/National Trends Network (NADP/NTN) sites are examined for trends using a general linear least squares model. SO₄^2- concentrations are decreasing throughout the United States; significant (p < 0.05) trends are evident at 42 of the 58 sites included in this analysis and the average decrease is 12.026 μeq ℓ^-1. The largest concentration of sites with significant decreasing SO₄^2- trends is located in the north central and western regions of the country. Eleven sites exhibit significant NO₃^- trends, nine of which are decreasing. Ca²⁺ and Mg²⁺ concentrations show the most widespread decline (44 and 52 sites, respectively) of all the major cations and anions in precipitation. Ca²⁺ concentrations have decreased 4.587 μeq ℓ^-1 since 1980; Mg²⁺ concentrations have decreased 2.034 μeq ℓ^-1. The most consistent and statistically most significant (p < 0.001) Ca²⁺ and Mg²⁺ trends occur in the northeast. Decreasing trends are also evident at 28 sites for Na⁺ and at 35 sites for K⁺. NH₄⁺ concentrations exhibit very little change over this 13-year period. The widespread decline in base cations, particularly Ca²⁺ and Mg²⁺, appears to have offset the effects that decreasing SO₄^2- concentrations should have on free acidity in precipitation. Only 17 sites exhibit both significant decreasing H⁺ and SO₄^2- concentration trends, most of which are located in the north central portion of the United States. The average decrease in H⁺ at the 17 sites is 9.991 μeq ℓ^-1. Hawaii is the only site in the NADP/NTN network to exhibit a significant increasing H⁺ trend.