Determination of seasonal and regional variation in the provenance of dissolved cations in rain in Japan based on Sr and Pb isotopes

We determined the elemental and Pb and Sr isotopic compositions of monthly wet precipitation at five sites in Japan (Sapporo, Morioka, Toyama, Tsukuba, and Kumamoto) to elucidate the seasonal and regional variation in the provenance of water-soluble materials. The concentrations of As, Cd, and Pb in the precipitation became high in winter to spring when the prevailing winds are westerly, but those of some metals (Mn, Cu, Ni) did not show distinct seasonal variations. Comparison of Pb isotopic ratios of the precipitation with those of aerosols from the Asian region showed (1) a geographical trend such that precipitation at Sapporo and Morioka in northeastern Japan contained Pb from aerosols from Russia and Mongolia, whereas that at Kumamoto in southwestern Japan contained Pb from aerosols from southern China, and (2) a seasonal variation in that precipitation in winter and spring became enriched in the airborne Pb from northern China, but the degree of the enrichment differed geographically. The sea-salt component (Cl, Na, and Mg) was high in winter and low in summer, particularly at Sapporo and Toyama, which face the Sea of Japan, whereas the non-sea-salt content of Ca and Sr and the 87Sr/86Sr tended to become high in spring and low in summer to autumn. The pH was low in winter and became high in spring and summer, except at Tsukuba, which faces the Pacific Ocean. These results show that spring precipitation was partly neutralized by the dissolution of calcium carbonate in the Asian dust with high 87Sr/86Sr and Ca/Sr. Summer and autumn precipitation, which was less affected by the Asian aerosols, had distinct Pb and Sr isotopic ratios depending on the site, suggesting that there are several Japanese sources of the base cations and heavy metals in the atmosphere over Japan.