Understanding the nocturnal ozone increase in Nanjing, China: Insights from observations and numerical simulations

Surface ozone (O₃) is mainly photochemically formed by nitrogen oxides (NO_X) and volatile organic compounds (VOCs), and therefore O₃ usually has a distinct diurnal variation with high concentrations in the afternoon and low values at night. However, eight nocturnal O₃ increase (NOI) events were identified in Nanjing in June 2021. To understand the mechanism of NOI events, we selected two events (June 6-7, and 24-25) for observational data analysis. The Community Multiscale Air Quality (CMAQ) model was employed for the process analysis (PA) and regional transport of O₃. By analyzing the O₃ observation data and meteorological factors, we found that there were clear southeastward O₃ transport paths. The O₃ peak clearly moved from the upwind to the downwind cities in both events. Model simulations showed that when nocturnal O₃ enhancement occurred, horizontal transport resulted in a negative to positive net O₃ production rate. O₃ continued to get accumulated in Nanjing. Nocturnal O₃ in the first event was dominated by long-range transport, with the top two contributing cities being Huzhou (5.6 %) and Jiaxing (4.7 %). NOI during the second event was from the nearby upwind cities. The top three contributing cities were Shanghai (18.3 %), Wuxi (9.1 %), and Suzhou (8.8 %). We conclude that the June NOI events in Nanjing were mainly driven by the horizontal transport of southeasterly winds. This study provides scientific support for O₃ prevention and control in Nanjing in the summer.