The 36-Year Historical Variation of Precipitation Chemistry during 1976-2011 at Ryori WMO-GAW Station in Japan

Abstract

Precipitation chemistry was observed at the remote background site at Ryori, Ofunato in northeastern Japan, from 1976 to 2011 as the part of World Meteorological Organization-Global Atmospheric Watch (WMO-GAW). During this period, anthropogenic emissions in East Asia varied widely, which affected the precipitation chemistry. The precipitation amount also affects the wet deposition amount. Thus, to eliminate the effect of the precipitation amount, we used the ratio of nitrate (NO₃⁻) to non-sea-salt sulfate (nss-SO₄²⁻) concentration in precipitation on an equivalent basis (Ratio). The historical trend of NO₃⁻ and nss-SO₄²⁻ concentrations in precipitation was not clear; however, Ratio showed a strong increase in the 1990s, a sudden drop in 2001, and an increase again after 2007. Based on the analysis of the historical emission dataset, the increases in Ratio during the 1990s and after 2007 were attributed to anthropogenic emission changes in Japan and China. The drop in Ratio in 2001 was explained by the massive SO₂ release from Miyakejima volcano from mid-August 2000, rather than by anthropogenic emissions. The 36-year historical record analysis of precipitation chemistry at Ryori detected both anthropogenic and natural emission changes and indicated the transition from local to transboundary air pollution in Japan.