Roadside air pollution and secondary organic aerosol seasonal trends from an oxidation flow reactor in Seoul

Abstract

The aim of this study was to determine the seasonal trends in secondary aerosols (SAs) and urban pollutants formed in an oxidation flow reactor (OFR) at a key roadside location in Seoul, South Korea. Spring 2019 saw high primary concentrations overall, and all seasons saw high levels of NOx, SO2, and equivalent black carbon (eBC) trends in the early morning (6:00–7:00). This exhibited similarities to the trend in diesel vehicle traffic volume. During the campaign, the modified hydroxyl radical (OH) exposure in the OFR was ∼1.1 × 1012 molecules cm−3 s (∼8.5 days), and the potential organic aerosol enhancement (OAPE) was 0.4–57.7 μg m−3 (winter 0.4−1.6, spring 28.4−52.3, summer 40.5−57.7, fall 18.1−31.1). The highest OAPE/ΔCO value was 89.5–414.7 μg m−3 for 0.1−4 OH·day in summer, and the lowest level was 1.1–8 μg m−3 for 0.5−8.5 in OH·day winter (including holidays). OAPE in seasons without winter under ∼4.5 OH· day had a ∼5.5 times higher value compared to that of organic on the roadside, accounting for 50.9−70.2% of the total sum (organic + eBC + OAPE). This had a high contribution to secondary organic aerosol (SOA) formation and, for winter, a value of only ∼5.5%. The predicted SOA formation from intermediate volatile organic compounds (IVOCs) calculated based on literature was 11–47% during mornings (9:30–11:30), excluding the winter season. This value was higher than the SOA formation from anthropogenic VOCs (benzene, toluene, ethylbenzene, and m,p,o-xylene) and biogenic VOCs (isoprene, monoterpenes, and sesquiterpenes), indicating the need for additional research into the domestic status of IVOCs. The marked seasonal change in South Korea's climate made applying research findings from other countries difficult. Therefore, further studies must be conducted based on realistic applicability, considering various environmental conditions, chemical speciation, and the classification of emission inventory components.