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 (O-3) concentration from 2014 to 2020, although O-3 precursors were effectively controlled. To understand the cause of these elevated surface MDA8 O-3 concentrations in the PRD, we explored potential meteorological causes. Based on the relationships between the MDA8 O-3 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 O-3 concentration in the PRD. Stronger DSWRF, lower RH, and lower WS were linked to larger OWI values and stronger O-3 pollution. Discrepancies in DSWRF, RH, and WS were observed on days with OWI > 1 and OWI < -1. Atmospheric stagnation (AS) was also associated with O-3 pollution. The average MDA8 O-3 concentrations on AS days were 53% (51.0 mu g m(-3)) higher than those on non-AS days. The O-3 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 O-3 production. Since the occurrence of AS is projected to increase, efforts to mitigate AS would contribute to decreasing the risk of severe autumn O-3 pollution events in the PRD.