A 15-year record (2001–2015) of the ratio of nitrate to non-sea-salt sulfate in precipitation over East Asia

Abstract. Acidifying species in precipitation can have severe impacts on ecosystems. The chemical composition of precipitation is directly related to the amount of precipitation; accordingly, it is difficult to identify long-term variation in chemical concentrations. The ratio of the nitrate (NO₃⁻) to non-sea-salt sulfate (nss-SO₄²⁻) concentration in precipitation on an equivalent basis (hereinafter, Ratio) is a useful index to investigate the relative contributions of these acidifying species. To identify the long-term record of acidifying species in precipitation over East Asia, the region with the highest emissions worldwide, we compiled ground-based observations of the chemical composition of precipitation over China, Korea, and Japan from 2001 to 2015 based on the Acid Deposition Monitoring Network in East Asia (EANET). The spatial coverage was limited, but additional monitoring data for Japan, southern China, and northern China around Beijing were utilized. The period of analysis was divided into three phases: Phase I (2001–2005), Phase II (2006–2010), and Phase III (2011–2015). The behaviors of NO₃⁻ and nss-SO₄²⁻ concentrations and hence the Ratio in precipitation were related to these precursors. The anthropogenic NO_x and SO₂ emissions and the NO_x ∕ SO₂ emission ratio were analyzed. Further, satellite observations of the NO₂ and SO₂ column density to capture the variation in emissions were applied. We found that the long-term trend in the NO₃⁻ concentration in precipitation was not related to the variation in NO_x emission and the NO₂ column. In comparison, the nss-SO₄²⁻ concentration in precipitation over China, Korea, and Japan was partially connected to the changes in SO₂ emissions from China, but the trends were not significant. The long-term trends of Ratio over China, Korea, and Japan were nearly flat during Phase I, increased significantly during Phase II, and were essentially flat again during Phase III. This variation in Ratio in East Asia clearly corresponded to the NO_x ∕ SO₂ emission ratio and the NO₂ ∕ SO₂ column ratio in China. The initial flat trend during Phase I was due to increases in both NO_x and SO₂ emissions in China, the significantly increasing trend during Phase II was triggered by the increase in NO_x emissions and decrease in SO₂ emissions in China, and the return to a flat trend during Phase III was caused by declines in both NO_x and SO₂ emissions in China. These results suggest that emissions in China had a significant impact not only on China but also on downwind precipitation chemistry during the 15-year period of 2001–2015. In terms of wet deposition, the NO₃⁻ wet deposition over China, Korea, and Japan did not change dramatically, but the nss-SO₄²⁻ wet deposition declined over China, Korea, and Japan from Phase II to III. These declines were caused by a strong decrease in the nss-SO₄²⁻ concentration in precipitation accompanied by a reduction in SO₂ emission from China, which counteracted the increase in precipitation. These findings indicated that the acidity of precipitation shifted from sulfur to nitrogen.