Modeling investigation of controlling factors in the increasing ratio of nitrate to non-seasalt sulfate in precipitation over Japan

Anthropogenic emissions in East Asia have been increasing during the three decades since 1980, as the population of East Asia has grown and the economies in East Asian countries have expanded. This has been particularly true in China, where NOx emissions have been rising continuously. However, because of fuel-gas desulfurization systems introduced as part of China’s 11th Five-Year Plan (2006–2010), SO2 emissions in China reached a peak in 2005–2006 and have declined since then. These drastic changes in emission levels of acidifying species are likely to have caused substantial changes in the precipitation chemistry. The absolute concentration of compounds in precipitation is inherently linked to precipitation amount; therefore, we use the ratio of nitrate ( NO 3 − ) to non-seasalt sulfate (nss- SO 4 2 − ) concentration in precipitation as an index for evaluating acidification, which we call Ratio. In this study, we analyzed the long-term behavior of Ratio in precipitation over the Japanese archipelago during 2000–2011 and estimated the factors responsible for changes in Ratio in precipitation by using a model simulation. This analysis showed that Ratio was relatively constant at 0.5–0.6 between 2000 and 2005, and subsequently increased to 0.6–0.7 between 2006 and 2011. These changes in Ratio corresponded remarkably well to the changes of NOx/SO2 emissions ratio in China; this correspondence suggests that anthropogenic emissions from China were responsible for most of the change in precipitation chemistry over Japan. Sensitivity analysis elucidated that the increase in NOx emissions and the decrease in SO2 emissions contributed equally to the increases in Ratio. Considering both emission changes in China enables to capture the observed increasing trend of Ratio in Japan.