Exploring fine-aerosol episodes in urban Seoul during the cold season of the 2021 SIJAQ campaign: Measurement evidences of heterogeneous reactions on black carbon particles

Significant PM_2.5 pollution has been prevalent on a regional scale in East Asia including a megacity Seoul in South Korea. Here, we explore fine-aerosol episodes occurred in Seoul during the Satellite Integrated Joint monitoring of Air Quality (SIJAQ) campaign from October to November of the 2021, focusing on experimental evidences of heterogeneous reactions to form secondary aerosol under a highly oxidized atmospheric condition. At this urban site in Seoul, vehicle exhaust was the clear source of fresh emissions, leading to a high level of NO and small refractory black carbon (rBC) particles (mass median diameter of 162 nm ± 16 nm) in the morning time. The hourly mass concentration of PM_2.5 ranged from 5.3 μg m⁻³ to 146.1 μg m⁻³, averaging at 24.5 ± 22.2 μg m⁻³. During the campaign, the most intense episode, EP3 (November 18–21), recorded an average PM_2.5 concentration of 72.5 ± 38.2 μg m⁻³, peaking at 146.1 μg m⁻³, was characterized by relatively higher temperature (∼12 °C) and relative humidity (67 %) on average thoroughly governed by continental migratory high and westerly winds. While the average NO₃⁻ concentration was 27.7 μg m⁻³, four times the whole campaign's average, EP3 was highlighted by a high morning NO₂/NO_x ratio and significantly elevated daytime and nighttime O_x (O₃+NO₂) concentrations compared to non-episode days. Throughout the entire campaign, NO_z surrogate (NO_2(CL)-NO_2(T)), O_x, and F_{moderate + thick} (the combined number fractions of moderately and thickly coated-rBC particles) tended to increase with the PM_2.5 concentration. During the daytime, as PM_2.5 increased, F_{moderate + thick} showed a monotonic increase, accompanied by RH rising from 54 ± 16% to 63 ± 11%. In contrast, at nighttime of humid condition with RH often exceeding 70% the enhancement of F_{moderate + thick} was more sensitive to condensable gas levels than RH. Given that high levels of PM_2.5 (>60 μg m⁻³) were observed only during EP3, enhanced levels of NO_z surrogate, O_x, F_{moderate + thick} and RH were evident characteristics of EP3. Such chemical and meteorological conditions suggest that the chemically enhanced oxidation state was evident during EP3, which promoted the formation of secondary aerosols on primary particles including rBC, especially under conditions of elevated RH. Considering the recent trend of increasing number of vehicles and rising atmospheric O₃ concentrations in East Asia, future studies should be well designed to investigate the detailed mechanisms involved in heterogeneous reactions that lead to the formation of secondary aerosols.