Annual and seasonal trends in chemical composition of precipitation in Japan during 1989–1998

Annual and seasonal trends were discussed for precipitation chemistry in Japan on the basis of a nationwide monitoring network by Japan Environment Agency. For this analysis, 17 sites, selected from the 23 network sites after evaluation of data completeness for the present purpose, were grouped into four areas in terms of the sea which could characterize each area: the Pacific Ocean, the Japan Sea, the Seto Inland Sea and the East China Sea areas. Major ion concentrations at each site were subjected to least-squares multiple regression analysis, and discussion was principally focused on the area-groups. Each area showed significant (p<0.05) decreasing trends in non-sea-salt sulfate (nss-SO42−) concentrations with the annual mean change rate of −3.0% yr−1, whereas no significant trends were detected for NO3− at 59% of the sites. Ammonium (NH4+) showed significant increasing trends for 35%, and decreasing trends for 18% of the sites; and, the maximum change rate of 3.8% yr−1 was recorded in the Pacific Ocean area. Non-sea-salt calcium (nss-Ca2+) concentrations significantly decreased in northern and some industrialized areas. Both the annual cycle-amplitudes and effects of precipitation amounts were significant for the four ions at almost all sites. The maximum concentrations of nss-SO42− and -Ca2+ occurred from winter to spring at most sites. The seasons when the maximum concentrations of NO3− and NH4+ occurred, were found to differ from area to area. Furthermore, the trends in precipitation nss-SO42−, and NO3− were qualitatively consistent with those of anthropogenic emissions of SO2, and NOx in Japan, respectively.