ABSTRACT

Monthly deposition rates of non-sea salt (nss-)sulfate from the atmosphere were observed at Toyama (1981–1991) and Wajima (1982–1984), both facing the Sea of Japan. The deposition rates at Toyama and Wajima showed similar seasonal variations with a large increase in winter, showing a good correlation to sodium deposition rate which is one of indicators of transport from the Sea of Japan. Thus, the increase in deposition rate in winter is attributed to the long-distance transport of sulfur dioxide from the Asian continent. The deposition rate and nss-SO₄/Na ratio in each winter season during 1981–1991, however, did not show an increase with time, though the annual consumption rate of coal in East Asia increased by more than 50% in the 1980s. When the nss-sulfate deposition rate is plotted against the nss-calcium deposition rate, the deposition rates in the summer season shows a correlation of 1:1 in mole. This suggests that most of the nss-sulfate in summer is due to calcium sulfate emitted from flue gas desulfurization plants. On the contrary, in winter, the large increase in nss-sulfate was coupled with a slight increase in nss-calcium, but the ratio of nss-Ca/nss-SO₄ was lower than unity. This suggests that nss-sulfate in winter can be a mixture with variable ratios of calcium sulfate in Japan to sulfuric acid converted from sulfur dioxide from the Asian continent.