Ozone production characteristics during the Korea-United States Air Quality campaign over the Seoul Metropolitan Area deciphered by observationally constrained hydroxyl radical reactivity

Despite notable reductions in precursor emissions, ambient ozone levels continue to increase in many urban areas, including Seoul, South Korea. This study examines the photochemical ozone production process in the Seoul Metropolitan Area using observational data from the Korea-United States Air Quality (KORUS-AQ) airborne campaign. By analyzing hydroxyl radical reactivity linked to nitrogen oxides (NOX) and volatile organic compounds (VOCs) (RNOx/RVOCs), we identify a shift from a NOX-saturated regime during the low-ozone period toward a transitional regime during the high-ozone period. Observed increases in VOC reactivity relative to NOX reactivity correlate with higher ozone production. We use a 0-D box model with observational constraints to generate ozone isopleths under different photochemical conditions. The results indicate that the isopleth varies significantly depending on atmospheric trace gas composition even in the same location with differences in trace gas mixtures, mostly driven by VOCs. RNOx/RVOCs generally correlate well with traditional metrics such as CH2O/NO2 and ozone production efficiency. However, the correlation becomes less reliable as the ozone production regime approaches the transitional regime. Our findings highlight the limitations of static regime classifications and emphasize the importance of dynamic, observation-based indicators for guiding ozone pollution control strategies.