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Aromatic volatile organic compounds (VOCs) including benzene, toluene, ethylbenzene, xylene, and trimethylbenzene (BTEX) are known to contribute the most to the formation of ozone and secondary organic aerosols (SOA) in an urban atmosphere and their representation in chemical transport models (CTM) is still uncertain. In this study, we evaluated the model representation of BTEX concentrations and their seasonal difference at Beijing and Seoul using the GEOS-Chem CTM and ground observations from the Fine particle Research Initiative in East Asia considering National Differences (FRIEND) project in winter and summer. The model captured different levels of observed BTEX at Beijing and Seoul, where benzene (BENZ) was comparable and toluene + ethylbenzene (TOLU) and xylenes + trimethylbenzenes (XYLE) were higher at Seoul. The observed seasonal variations of BTEX were similar at both sites, with benzene being higher in winter and TOLU and XYLE being higher in summer. However, the magnitude and seasonal variation of TOLU at Seoul and BENZ at Beijing were not reproduced by the model. TOLU was underestimated in both seasons at Seoul especially in summer due to the underestimation of emission rates and evaporative emission being not included in the current emission inventory. At Beijing, BENZ was underestimated especially in winter and showed no seasonal variation which was different from the observations. Underestimated benzene emission rates or missing sources of it cause discrepancies between observed and modeled benzene concentrations at Beijing. • The observed toluene and xylenes were higher in summer while benzene was higher in winter at Beijing and Seoul. • The evaporative emission of toluene at Seoul needs to be considered in the model to reproduce the observed seasonal variation of toluene. • Benzene concentration at Beijing is underestimated likely due to a low bias in emissions from Beijing's own or upwind region. [ABSTRACT FROM AUTHOR]

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]