The Deposition, Composition, and Potential Sources of Major Ionic Solutes in Rain of the Central Amazon Basin

Abstract

Major solute concentrations were measured for 123 individual rainstorms greater than 1 mm that occurred over a continuous 18‐month period (September 1983 through February 1985) in the central Amazon Basin. The sum of concentrations was low (volume‐weighted mean, 39 μeq/L), mean pH was 4.9, and the dominant source of acidity was organic acids. Analyses of interactions among solute concentrations in storms of differing size, frequency, and season of occurrence permitted evaluation of how rainfall variation affects depositional rates of solutes. The 20% of least frequent, large‐volume storms accounted for about 75% of the water and almost 60% of the solutes deposited annually. Depositional rates of total N and P, Na+, and SO42− did not differ substantially as rainfall varied, while Ca2+, Mg2+, Cl−, NO3− and PO43− did. Wet deposition of N and P varied less than 20%, despite a threefold intraannual variation of rainfall. Comparison between event and bulk precipitation samples indicated that dry deposition was minor except for P. Ionic ratios relative to Na+, R mode and Q mode factor analyses of rainwater composition, and differences in deposition among seasons provide evidence for the seasonal‐scale importance of biogenic emissions of aerosols and reduced S and N gases from the Amazon rain forest to the atmosphere.