Statistical Analysis of Precipitation Chemistry Measurements over the Eastern United States. Part III: The Ionic Balance among Chemical Constituents

Abstract
The relationships between acidity and the principal chemical constituents of precipitation (including sulfate, nitrate, ammonium, calcium, magnesium, sodium, and chloride) were investigated using a new archive of daily precipitation chemistry measurements for the eastern United States. This archive consist of measurements from three networks during the period 1979 through 1983. The chemical relationships between acidity (hydrogen ion concentrations) and the other chemical constituents were studied using six sites that span the eastern United States and represent geographic variations. At these sites, the two predominant cations were hydrogen and ammonium, and the predominant anions were sulfate and nitrate; these four largely controlled the ionic balance, particularly at locations where the precipitation was most acidic. When other less predominant analytes (sodium, magnesium, potassium, chloride, and phosphate) were also considered, the average ionic balance indicated an apparent excess of cations relative to anions. This imbalance arose either from a bias in the measurement of cations, or from anions (such as organic acids) that were not measured. At all sites, there was wide variability from day to day in the concentrations of all analytes. For example, the relative amounts of the four predominant analytes changed substantially from storm to storm. This variability indicates that air mass trajectories over emission sources, and variations in chemical processes and meteorological conditions, interacted in complex ways to produce the observed constituent concentrations.

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