Sulfur Isotope Ratios of Non-Sea Salt Sulfate in Wet Deposits in Japan

Abstract

In Japan, wet deposition is mainly from sulfuric acid rather than nitric acid, and acid rain falls even in non-industrial regions along the Japan Sea coast as well as in industrial regions facing the Pacific Ocean. In order to clarify the sources of non-sea salt sulfate in Japan, sulfur isotope ratios of non-sea salt sulfate in wet deposits were evaluated and sulfur isotope ratios of non-sea salt sulfate in wet deposits were compiled from the literature. At most sites, the isotopic ratios of non-sea salt sulfate observed in the summer were0‰. The sulfur isotope ratios of non-sea salt sulfate showed seasonal variation with an increase in winter. This seasonal variation suggests that non-sea salt sulfate in wet deposits was derived from the source, having a higher sulfur isotope ratio in winter. The sulfur isotope ratios of non-sea salt sulfate collected in Japan in winter were in agreement with the sulfur values for coals in East Asia.
The northwest Siberia monsoon dominates Japan in the winter and the deposition rate of non-sea salt sulfate increases at that time. Sulfur oxides produced by coal combustion in East Asia make a high contribution in winter. Maximum values of sulfur isotope ratios of non-sea salt sulfate in northern Japan are higher than those in southern Japan in winter. This pattern suggests that non-sea salt sulfate in northern Japan was derived from a source having a higher sulfur isotope ratio in winter. The back-trajectory at the 850 hPa level starting at Okushiri Island passed over northeast China, while those at Shimonoseki passed by the Korean Peninsula one or two days before sampling. Sulfur isotope ratios of coals from northern East Asia are higher than those in southern East Asia. It is suggested that in winter, wet deposits in northern Japan contains sulfur released by the combustion of coals in northern East Asia, while that in southern Japan contains sulfur released by the combustion of coals in southern East Asia.