Impacts of Meteorological Conditions on Autumn Surface Ozone During 2014–2020 in the Pearl River Delta, China.

As one of the largest megacity clusters in China, the Pearl River Delta (PRD) exhibited an obvious increase in the autumn mean maximum daily 8 hr average (MDA8) ozone (O3) concentration from 2014 to 2020, although O3 precursors were effectively controlled. To understand the cause of these elevated surface MDA8 O3 concentrations in the PRD, we explored potential meteorological causes. Based on the relationships between the MDA8 O3 concentrations and the meteorological conditions, the ozone weather index (OWI) was constructed from surface downward shortwave radiation flux (DSWRF), relative humidity (RH), and wind speed (WS) data to represent the variations in the autumn MDA8 O3 concentration in the PRD. Stronger DSWRF, lower RH, and lower WS were linked to larger OWI values and stronger O3 pollution. Discrepancies in DSWRF, RH, and WS were observed on days with OWI > 1 and OWI < −1. Atmospheric stagnation (AS) was also associated with O3 pollution. The average MDA8 O3 concentrations on AS days were 53% (51.0 μg m−3) higher than those on non‐AS days. The O3 exceedance rates on AS days were 36.5%, which is 4.8 times higher than those on non‐AS days (7.6%) in the PRD. In addition to lower WS and less precipitation, AS days were also accompanied by lower total cloud cover and stronger downdraft airflow, which accelerated O3 production. Since the occurrence of AS is projected to increase, efforts to mitigate AS would contribute to decreasing the risk of severe autumn O3 pollution events in the PRD. Key Points: The ozone weather index established for the Pearl River Delta (PRD) can represent the variations in O3 concentrations in autumnAtmospheric stagnation was associated with ozone pollution in the PRD, which was accompanied by lower cloud cover and stronger downdrafts [ABSTRACT FROM AUTHOR]