Origin and composition of Samoan acid precipitation

Abstract

Samoan precipitation events are both plentiful and well distributed throughout the year. The chemistry of this rain, determined from weekly samples from 1980 to 1983, shows that the annual fluctuations of free acidity are almost negligible. The 3-year pH is 5.3, with no significant yearly trend. Classification using trajectory analyses of storm events that originated from westerly sources and occurred mainly during the March-April austral fall period, have revealed consistently poorer equivalent ion balance, e.g., anion deficits of 10 μeq L-1 larger than observed from other sources. The austral spring period (October-November) is characterized by very steady, strong, easterly trade winds and the Ocean is probably the primary source of aerosol. Decreases of acidity are also observed in samples measured first in the field and then in the laboratory, suggesting the existence of another contributor of free protons in Samoan precipitation in addition to the long-range transport of sulfate aerosols. Air masses arriving from westerly sources and associated with precipitation events had almost 25% of their points of origin in the New Guinea area. It is speculated that during these flows, organic acids may be transported to Samoa.