Deposition of anthropogenic sulfate and Pb-210 in the western North Pacific area.

Abstract

The monthly deposition rates of sodium, calcium, chloride, sulfate and Pb-210 from the atmosphere were measured at 12 stations in various parts of the Japanese Islands for two years. The deposition rates of sea salt particles were higher in the coastal region near the Sea of Japan in winter, demonstrating the fact that the strong winter monsoon produces many sea salt particles. Amounts of chloride, sulfate and calcium in excess of the sea salt component were always observed. The rather uniform annual deposition rates of excess sulfate (3–10g/m2/y) suggest that a large part of this excess sulfate originates in global air pollution. Pb-210, much of which is of upper atmospheric origin, is also deposited in significant amounts in winter. This seasonal change may be due to the replacement of lower air masses over the continental margin area, which yields a two-layer model for the atmospheric transport of chemical constituents. The geographical variation of Pb-210 deposition can be explained using the two-layer model and by assuming a 12 day residence time for the tropospheric aerosols. The deposition rates for sulfate over the ocean area decrease more rapidly with distance from the continental coast than those for Pb-210. This result may show that the atmospheric residence time of sulfate is shorter than that of Pb-210.