In this study, a separation and quantification method for ammonium sulfate in ambient PM_2.5 was developed. Aqueous solutions of four major ammonium salts in PM_2.5, i.e., ammonium sulfate, ammonium hydrogen sulfate, ammonium nitrate and ammonium chloride, were added to a quartz fiber filter and dried. Next, the filter was subjected to heat treatment to separate the ammonium sulfate from the other ammonium salts. By heating the filter to 110℃ for 30 min, ammonium nitrate and ammonium chloride volatilized, and only ammonium sulfate and ammonium hydrogen sulfate remained on the filter. Furthermore, by heating the filter to 150℃ for 30 min, the ammonium sulfate splits into ammonium hydrogen sulfate and ammonia, so half of the NH₄ in the ammonium sulfate volatilized. The other sulfate salts that were estimated to be present in PM_2.5, i.e., sodium sulfate, potassium sulfate and calcium sulfate, did not decrease by the heat treatments at 110℃ and 150℃. Hence, after the heat treatments, the ammonium ion, which decreased by heating at 150℃ compared to heating at 110℃, was confirmed to originate from the ammonium sulfate. Thus the concentration of ammonium sulfate in the PM_2.5 was estimated from the decreased ammonium ion concentration and the ratio of the formula weight. Using this developed method, the concentration of ammonium sulfate in ambient PM_2.5 was estimated to be 1.8 μg/m³ in January and 2.5 μg/m³ in February, 2018.

Before the environmental standard value for PM_2.5 was introduced in 2009, the observed PM_2.5 in Japan showed the gradual decreasing trend. In this study, in order to clarify the reason of this gradual decreasing trend of PM_2.5, the long-term source apportionments of PM_2.5 in Japan during 2000–2008 were evaluated based on the air quality model by applying the tagged tracer method. The source groups were divided into total of 11; six domestic anthropogenic sources, three foreign anthropogenic sources from China, Korea, and other regions, and ship and natural sources. The performances of the air quality model were validated by a statistical analysis, and confirmed that the modeling system satisfied the model performance criteria. The domestic anthropogenic sources were higher than the abroad anthropogenic source on 2000–2001; and subsequently the relationships were reversed. This increasing abroad anthropogenic sources was mainly caused by the impact from China. The source apportionment of abroad anthropogenic sources had increased; however, due to the decline of natural sources and significant decline of domestic automobile sources (p<0.001), it was clarified that PM_2.5 in Japan showed the gradual decreasing trend. Other sources of power plant, industry, domestic, and others in Japan showed unclear trends.