Acid rain: ionic correlations in the eastern United States, 1980-1981.

Hydrogen ions in precipitation are correlated much more closely with sulfate than with nitrate, whereas ammonium ions are correlated more closely with nitrate than with sulfate. Target loadings of 14 to 16 kilograms of wet sulfate deposition per hectare per year, instead of 20 as suggested hitherto, are probably necessary to produce average pH values of 4.6 to 4.7, the approximate boundary levels for damage to aquatic ecosystems. Cluster analysis reveals that there are three linked groups of ions related to air pollution, agriculture, and sea spray.