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. SO42- 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 SO42− 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 SO42− concentrations should have on free acidity in precipitation. Only 17 sites exhibit both significant decreasing H+ and SO42− 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.