Revisiting wintertime nitrate formation and recirculation in Seoul using box model simulations

Wintertime particulate matter (PM) episodes in Seoul are marked by elevated nitrate, yet nighttime formation remains poorly understood. We reexamined high-PM periods during the Fine Particle Research Initiative in East Asia considering National Differences (FRIEND) campaign (December 2020–January 2021) using a box model Korea Air Quality Observation-Based Box model (KAB) enhanced with a double-layer structure and expanded sensitivity tests. Contrary to previous work, residual-layer chemistry alone did not explain nighttime nitrate underestimation. Instead, model results indicate that recirculation of secondary pollutants—exported under westerlies and reintroduced under northeasterlies—was the primary driver of elevated nighttime nitrate. Prescribing a background nitrate concentration during PM episodes greatly improved model–observation agreement, highlighting the importance of regional transport. Empirical Kinetic Modeling Approach (EKMA) simulations with the tuned model show that ozone, nitrate, and secondary organic aerosol (SOA) formation during PM episodes occurred in a volatile organic compound (VOC)-limited and nitrogen oxides ( \(\:{\text{N}\text{O}}_{\text{x}}\) )-saturated regime. Under these high- \(\:{\text{N}\text{O}}_{\text{x}}\) conditions, precursor reductions produced only modest decreases in secondary pollutants although it is possible that the sensitivity was substantially underestimated because the KAB model represents recirculation using a constant boundary condition. Sensitivity analyses further show that aromatic VOCs—especially xylene species—play a disproportionately large role in secondary pollutant formation relative to their mass contribution. These results underscore the influence of regional recirculation and aromatic hydrocarbon chemistry on wintertime secondary pollution, and emphasize the need for improved treatment of regional transport, SOA formation, and VOC speciation to support effective winter air-quality management in the Seoul metropolitan area.