Distinctive features of inorganic PM1.0 components during winter pollution events over the upwind and downwind regions in Northeast Asia.

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]