Chemical composition and deposition of rain in the central Amazon, Brazil

Major solute concentrations in wet deposition were measured for 115 individual events from October 1988 to June 1990 at Lake Calado, Amazonas, Brazil. A continuous record from July 1989 through June 1990 included 210 events that ranged in size from 0.2 to 85 mm (annual total, 2754 mm). The 95 events chemically analyzed during this period (45% of the total) were evenly distributed over all storm sizes. The volume-weighted mean (VWM) of the measured ionic sum was low (36 μeq −1), and H+ was the most abundant ion (VWM pH = 4.8). Organic acids made substantial contributions to both measured dissolved organic carbon and H+. Small interannual differences are apparent in a comparison with rain data collected from the same study site in 1984. The composition of rain appeared to be derived from a constant source of oceanic and rain-forest aerosols, and was not significantly influenced by local or regional biomass burning.