Impacts of aerosol acidity and liquid water content on secondary inorganic aerosol pollution in East Asian megacities: Beijing and Seoul

Despite considerable progress in air quality improvement, fine particulate matter (PM_2.5) pollution remains prevalent during the autumn and winter seasons in eastern Asian megacities: Beijing and Seoul. Here, we conducted synchronous field observations of PM_2.5 chemical composition in Beijing and Seoul as part of the FRIEND Project, spanning the period from November 10th to December 10th, 2022 and we aimed to understand sulfate, nitrate, and ammonium (SNA) formation mechanism. The observations revealed that SNA, especially nitrate, were the primary contributor to particle matter pollution, accounting for 66 % and 76 % of PM_2.5 in Beijing and Seoul, respectively, during pollution period (PM_2.5>60 μg/m³). This indicated secondary inorganic aerosol formation was a key driver to force the urban fine particulate matter pollution in eastern Asian megacities. We found that the enhanced aerosol liquid water content (ALWC) during pollution episode facilitated SNA formation, then, contributing to the PM_2.5 pollution. Such phenomenon was found in both cities. Aerosol acidity in Seoul was higher than that of in Beijing, leading to the different sulfate formation pathways. During observed pollution episode, liquid-phase sulfate was primarily formed through oxidation by transition metal ions (TMI) and hydrogen peroxide (H₂O₂) in Beijing, while in Seoul, sulfate was mainly formed through TMI-catalyzed oxidation. Nitrate primarily existed in the particulate phase during pollution period with ambient relative humidity larger than 60 % in Beijing. In contrast, around 20 % of nitrate was still in the gas phase in Seoul due to low aerosol pH. The sensitivity analysis of particulate matter to precursors in both cities highlighted the needs for differentiated management strategies to reduce NOx and NH₃ emissions under varying ALWC and pH conditions.