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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. • FRIEND project conducted simultaneous real-time measurements in Northeast Asia. • Same methods used during the campaigns enabled direct spatiotemporal comparison. • Real-time particle formation and growth were largely different among the countries. • Local emission and formation also affected on PM 2.5 characteristic differences. [ABSTRACT FROM AUTHOR]