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

Abstract

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 PM1.0 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 (SO42−) was a high as a fraction of PM1.0 and nitrate aerosol (NO3−) was high, whereas over the downwind region (Noto), SO42− was high and NO3− was low. We used regional numerical modeling to clarify the reason for these distinctive PM1.0 differences over the upwind and downwind regions. The model also reproduced higher NO3− in the upwind region and higher SO42− in the downwind region, and captured the ACSMs measurements based on the statistical analyses. The conversion ratio from SO2 to SO42− (FS) 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 SO2 oxidation processes were inactive over the Asian continent. In contrast to FS, the conversion ratio from HNO3 to NO3− (FN) indicated a higher production ratio over the Asian continent and a strong decline toward the downwind region. HNO3 concentration in northeastern China was close to zero, whereas NH3 concentration was high, suggesting NH3-rich conditions where HNO3 was fully neutralized to form NO3−, whereas the conditions were NH3-poor toward the downwind region. Therefore, these factors govern the transformation process of PM1.0 during long-range transport from the upwind to downwind region in Northeast Asia and determine the distinctive features of inorganic PM1.0. © 2023 The Authors