Characteristics of local recirculation affecting summer ozone in coastal areas of the Korean Peninsula

Abstract

This study investigates the influence of local circulations, specifically stagnation and recirculation, on high ozone concentration events in Seoul during the summer (May–August) from 2016 to 2021. Local circulations were classified into Stagnation, Recirculation, and Ventilation types using the recirculation factor (RF), calculated as the ratio of net displacement distance (L) to actual displacement distance (S) based on wind observation data. Stagnation was the most frequently observed type, characterized by the highest daily maximum ozone concentrations and meteorological conditions unfavorable for dispersion, such as weak winds. During stagnation periods, limited air flow resulted in the accumulation of ozone within the urban area, with horizontal transport (HTRANS process) identified as a significant contributor to increased ozone concentrations near the surface. Additionally, nighttime ozone concentrations decreased due to active NO titration, accompanied by an increase in NO₂. In contrast, recirculation periods were marked by the presence of elevated ozone concentrations (∼0.07 ppm) at altitudes above 500 m during the early morning. This residual ozone, transported offshore by nighttime land breezes, was brought back inland by daytime sea breezes and subsequently transported downward to the surface through vertical transport (VTRANS process), contributing significantly to surface ozone levels. The differences in ozone formation mechanisms between stagnation and recirculation highlight the role of limited dispersion in stagnation and the combined effects of sea breeze and vertical transport in recirculation. These findings emphasize the importance of understanding local circulation patterns, particularly the interaction between land and sea breezes, in predicting and managing high ozone events during the summer. The study underscores the need for long-term climatological research on local circulations and their impact on urban air quality to inform effective air quality management strategies.