This paper discusses (1) the composition of precipitation on Amsterdam Island, in the central Indian Ocean, (2) the processes controlling the precipitation composition and (3) the acids responsible for the observed acidity.

The data set comprises 66 rain events collected over a 30 month period and analyzed for SO2−4, NO3, Cl, CH3COO, HCOO, Na+, Mg2+, K+, Ca2+, H+, strong acidity and total acidity. Supporting data used in data analysis include 222Rn and meteorological parameters.

Precipitation on Amsterdam Island has two components, seawater and acid. Using Mg2+ as the seawater tracer, there is no significant difference between the K+/Mg2+, Cl/Mg2+, Na+/Mg2+ and Ca2+/Mg2+ ratios of the rain and those of seawater. The SO2−4/Mg2+ ratio is significantly larger than the seawater ratio. The acid component is substantially smaller than the seasalt component. The volume weighted pH of the precipitation samples is 5.06 with primary proton donors being H2SO4, organic acids (HCOOH and CH3COOH) and HNO3. The maximum contributions of these acids to precipitation acidity are 30, 25 and 15%, respectively.

There is an interaction between the alkaline seawater component and the acidic component. Specifically, using Mg2+ as a tracer of seawater alkalinity, there is, on the average, a loss of 10% of the original free acidity due to neutralization. Specific samples ranged from <10 to 50% neutralization, depending on the relative magnitude of the acid and seawater components.

222Rn was used as a tracer to estimate the influence of the long range transport of marine and continental material on the composition of Amsterdam Island precipitation. A comparison of the SO2−4, NO3, H+ and CH3COO and HCOO concentrations between events with a continental source area and those with a marine source area revealed a possible continental influence on NO3 only.

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