Meteorological and chemical factors controlling ozone formation in Seoul during MAPS-Seoul 2015

To understand the chemical mechanisms of controlling factors in ozone (O₃) formation in early summer in Seoul, a comprehensive study encompassing measurement and modeling was conducted under the Megacity Air Pollution Study-Seoul (MAPS-Seoul) campaign. From May 18 to June 12, 2015, O₃ and peroxyacetyl nitrate (PAN) were measured, along with their precursors, including NO_x and volatile organic compounds (VOCs), at the Korea Institute of Science and Technology, located in northeast Seoul. VOCs were sampled in a canister twice a day (at 09:30 and 15:00) and analyzed via gas chromatography. The meteorological conditions and chemical regimes of the air masses were clearly distinguished during the study period. In May, NO_x concentrations were higher with more pronounced diurnal cycles of precursors and O₃ under constant westerly winds. By contrast, stagnant conditions developed in June, which reduced the inflow of primary emissions from the downtown area but increased the influence from the neighboring forest under high temperatures. As a result, the ratio of O₃ to odd oxygen was higher in June, indicating a less efficient removal of O₃ by NO_x. In the same context, the air mass was chemically more aged with a higher NO₂/NO_x ratio and enhanced OH reactivity of oxygenated and biogenic VOCs in June. The overall measurement results suggest that O₃ formation is slightly more sensitive to VOCs than to NO_x in Seoul during this season, when O₃ concentrations are the highest of the year.