Trends of sulfur and nitrogen deposition from 2003 to 2017 in Japanese remote areas

Emissions of sulfur (S) and nitrogen (N) compounds in East Asia has drastically changed over the last two decades. To assess the influence of the drastic changes in air pollution on ecosystems in Japan, we investigated the trends of S and N deposition during 2003–2017 at remote sites of Acid Deposition Monitoring Network in East Asia (EANET). We measured wet deposition and inferentially estimated dry deposition of S and N using monitoring data from 2003 to 2017 at eight sites. We estimated dry deposition using the inferential method with an updated parameterization for gaseous surface resistance. The linear regression method and nonparametric Mann–Kendall test was used to analyze the temporal trends based on the monthly data sets. High S and N deposition amounts over 10 kg ha−1 year−1 were frequently found at most sites. There were significant increase trends in N deposition to S deposition (N/S) ratio at all sites throughout the 15-year period. Some trends were significantly found when the 15-year period was divided into three: 2003–2007, 2008–2012, and 2013–2017. S deposition had significantly decreased over a wide area in Japan, especially at Sado-seki, Happo, Oki, Hedo, and Ogasawara, in 2013–2017. Significant decreases in oxidized N deposition at Sado-seki and Oki were also found in 2013–2017. Because of almost flat N deposition mainly contributed by reduced N deposition, the N/S ratio clearly increased. These trends were associated with the recent reductions in SO2 and NOx emissions in China. The NOx emission reduction of China has not caught up with that of SO2, and NH3 emissions have not been reduced. This caused the significant increases in the N/S ratio not only in 2013–2017 but also in 2003–2017. [Display omitted] •Long-term trends of sulfur (S) and nitrogen (N) deposition were investigated.•S deposition had significantly decreased over a wide area in Japan in 2013–2017.•N/S deposition ratio had significantly increased in many sites in 2013–2017.•These trends were associated with recent emission reductions in China. The ratio of nitrogen deposition to sulfur deposition had significantly increased in Japanese remote areas in 2013–2017, associated with emission reductions in China.