A 15-year record (2001–2015) of the ratio of nitrate to non-seasalt
sulfate in precipitation over East Asia
Authors: Itahashi Syuichi , Yumimoto Keiya , Uno Itsushi , Hayami Hiroshi , Fujita Shin-ichi , Pan Yuepeng , Wang Yuesi 
Published: October 2017

Abstract. Acidifying species in precipitation can cause severe impacts on ecosystem. The chemical concentration of precipitation is directly related to the precipitation amount, so it is partly difficult to identify the long-term variation from precipitation concentration. The ratio of nitrate (NO3−) to non-seasalt sulfate (nss-SO42−) concentration in precipitation on an equivalent basis (hereinafter, Ratio) will be a useful index. To identify the long-term record of acidifying species in precipitation over East Asia, where is the highest emission region in the world, we have compiled the ground-based observations of the chemical concentration of precipitation over China, Korea, and Japan from 2001 to 2015 based on the Acid Deposition Monitoring Network in East Asia (EANET). The period was partly limited but other monitoring data in Japan, southern China, and northern China around Beijing were additionally utilized. The analyzed period was categorized into three phases: Phase I (2001–2005), Phase II (2006–2010), and Phase III (2011–2015). The behavior of NO3− and nss-SO42− concentration, and hence Ratio in precipitation will be related to these precursors. The anthropogenic NOx and SO2 emission amount, and NOx/SO2 emission ratio are analyzed. Further, satellite observations of NO2 and SO2 column density to capture the variation in emission was applied. We found that the long-term trend of NO3− concentration in precipitation was not related to the variation in NOx emission and the NO2 column. In comparison, the nss-SO42− concentration in precipitation over China, Korea, and Japan was partly connected to the changes in SO2 emission from China, but the trends were not significant. The long-term trend of Ratio over China, Korea, and Japan were nearly flat during Phase I, increasing significantly during Phase II, and almost flat again during Phase III. These variations of Ratio in East Asia clearly corresponded to the NOx/SO2 emission ratio and the NO2/SO2 column ratio in China. The first flat trend during Phase I was due to both increases in NOx and SO2 emissions in China, the significant increasing trend during Phase II was triggered by the increase in NOx emission and decrease in SO2 emission in China, and the returned flat trend during Phase III was caused by both declines in NOx and SO2 emissions in China. This suggests that China’s emission has a significant impact not only on China but also on downwind precipitation chemistry during the analyzed 15–year period of 2001–2015. In terms of wet depositions, the NO3− wet deposition amount over China, Korea, and Japan has not changed dramatically, but the nss-SO42− wet deposition amount declined over China, Korea, and Japan from Phase II to III. These declines were caused by a strong decrease in nss-SO42− concentration in precipitation accompanied by a reduction in SO2 emission from China, which counteracted an increase in precipitation amount. It was indicated the decision on the acidity of precipitation would be shift from sulfur to nitrogen.