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

Surface ozone (O3) is mainly photochemically formed by nitrogen oxides (NOX) 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.