Spatiotemporal differences on the real-time physicochemical characteristics of PM_2.5 particles in four Northeast Asian countries during Winter and Summer 2020–2021

Northeast Asian countries have suffered from high concentration of the particulate matter (PM_2.5). However, the lack of simultaneous real-time measurements on the PM_2.5 physicochemical characteristics in the different Northeast Asian countries has retarded to understand and mitigate high PM_2.5 air pollution in the continent. In this study, two simultaneous monitoring campaigns were conducted at five supersites in Northeast Asia by deploying same methods using real-time instruments during the winter and summer 2020–2021. Through the campaigns, several high PM_2.5 events were observed in the same periods in the winter among some countries, which were related to the weather conditions. However, the formation and growth trends of PM_2.5 were largely different among the countries, affecting on the spatiotemporal differences of PM_2.5 physicochemical characteristics with local emission. PM_2.5 mass concentration (C_PM2.5) measured in Ulaanbaatar (Mongolia) in the winter showed the highest value due to primary emission by fuel-combustion, but lowest in the summer. In contrast, C_PM2.5 in Noto (Japan) was lowest during the two campaigns among the countries as a background measurement site. Compared to Seosan and Seoul (Republic of Korea), Beijing (China) showed lower total number of PM_2.5 particles during the winter campaign. However, the PM_2.5 particles were grown larger in Beijing than the two Korean sites, resulting similar average C_PM2.5 with the Korean sites in the winter. During the C_PM2.5 increased, the real-time concentration of the nitrate increased simultaneously and became the dominant PM_2.5 constituent, which was commonly observed at Seoul, Seosan and Beijing. However, the seasonal changes of the PM_2.5 chemical constituents were different depending on the sites, causing the different seasonal changes of C_PM2.5. Otherwise, sulfate and organics significantly contributed to the C_PM2.5 (>80%) at Noto, where the C_PM2.5 was generally <35 μg/m³ with little amount of nitrate.