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The air quality of Northeast Asia cannot be improved by the individual national efforts because of the proximity of countries. To foster understanding of this problem, the Fine Particle Research Initiative in East Asia considering National Differences (FRIEND) Project was launched in 2020. During the first FRIEND campaign conducted from December 15, 2020 to January 15, 2021, gaseous and aerosol components were monitored simultaneously with high temporal resolution at the following key sites in Northeast Asia: Ulaanbaatar, Mongolia; Beijing, China; Seosan and Seoul, Republic of Korea; and Noto, Japan. The chemical components of PM 1.0 were monitored with Aerosol Chemical Speciation Monitors (ACSMs) at the Beijing, Seoul, and Noto sites. Over the upwind region of Northeast Asia (Beijing and Seoul), sulfate aerosol (SO 4 2−) was a high as a fraction of PM 1.0 and nitrate aerosol (NO 3 −) was high, whereas over the downwind region (Noto), SO 4 2− was high and NO 3 − was low. We used regional numerical modeling to clarify the reason for these distinctive PM 1.0 differences over the upwind and downwind regions. The model also reproduced higher NO 3 − in the upwind region and higher SO 4 2− in the downwind region, and captured the ACSMs measurements based on the statistical analyses. The conversion ratio from SO 2 to SO 4 2− (F S) indicated lower oxidation over the Asian continent and increased oxidation toward the downwind region. The OH radical and cloud fraction suggested that the gas- and aqueous-phase SO 2 oxidation processes were inactive over the Asian continent. In contrast to F S , the conversion ratio from HNO 3 to NO 3 − (F N) indicated a higher production ratio over the Asian continent and a strong decline toward the downwind region. HNO 3 concentration in northeastern China was close to zero, whereas NH 3 concentration was high, suggesting NH 3 -rich conditions where HNO 3 was fully neutralized to form NO 3 −, whereas the conditions were NH 3 -poor toward the downwind region. Therefore, these factors govern the transformation process of PM 1.0 during long-range transport from the upwind to downwind region in Northeast Asia and determine the distinctive features of inorganic PM 1.0. [Display omitted] • FRIEND project identified the distinctive difference in inorganic PM 1.0 components. • Sulfate (SO 4 2−) was low at Beijing and Seoul whereas high at Noto (Japan). • An inactive SO 2 oxidation process was found over the Asian continent. • Nitrate (NO 3 −) was high over the Asian continent and declined toward downwind. • NH 3 -rich status can maintain a formation of ammonium-nitrate (NH 4 NO 3). [ABSTRACT FROM AUTHOR]

The Center for Fine Particle Research Initiative in East Asia considering National Differences (FRIEND) project was launched in 2020 to characterize air pollution in Northeast Asia with effective collaboration. One of the goals of this study is to elucidate the spatiotemporal characteristics of atmospheric aerosols (e.g., PM 2.5) in Northeast Asia. To achieve this objective, (1) the chemical speciation of fine particles was measured in high resolution, (2) the physical characteristics of fine particles were also elucidated, and (3) the gaseous and particulate components of the atmospheric aerosols were comprehensively monitored. The first field campaign was conducted from December 15, 2020, to January 15, 2021, and simultaneously monitored five key sites in Northeast Asia: Ulaanbaatar, Mongolia; Beijing, China; Seosan and Seoul, Republic of Korea (Korea); and Noto, Japan. The synoptic weather conditions mainly determined the periods of high PM 2.5 mass concentration. During the first high-pollution event, the air over the Northeast Asian region was generally stagnant, and the inland wind speed over China, Korea, and Japan was very low. The PM 2.5 mass concentration in Beijing, Seosan, and Seoul had increased simultaneously. Then, the prevailing westerlies in Northeast Asia caused the polluted air mass observed in Seoul to move to the background site in Noto. During the second high-pollution event, the Asian dust phenomenon occurred in the Gobi Desert and moved to all sites. Though the synoptic weather conditions largely determined the trend of the PM 2.5 mass concentrations at the five sites, the levels of chemical components in both gaseous and particulate phases were different at each site, reflecting local emission and formation characteristics. Overall, the concentrations of air pollutants were highest in Ulaanbaatar; comparable in Beijing, Seoul, and Seosan and very low in Noto. A common characteristic of the five sites was the high organic aerosol (OA) proportion in fine particles. The chemical components that increased with the PM 2.5 mass concentration were OA at Ulaanbaatar, OA and nitrate at Beijing and Seosan, nitrate at Seoul, and sulfate and OA at Noto. Further study directions for each site are discussed. • FRIEND project was launched to determine the characteristics of air pollution in Northeast Asia by simultaneous measurement. • The trend of PM 2.5 at the five sites was largely determined by synoptic weather conditions in the Northeast Asia. • The difference of major chemical components at each site reflect the significance of local emission and formation. [ABSTRACT FROM AUTHOR]