Extreme Weather exacerbates Ozone Pollution in the Pearl River Delta, China: Role of Natural Processes

Abstract. Ozone (O₃) pollution research and management in China have mainly focused on anthropogenic emissions, while the importance of natural processes is often overlooked. With the increasing frequency of extreme weather events, the role of natural processes in exacerbating O₃ pollution is gaining attention. In September 2022, the Pearl River Delta (PRD) in South China experienced an extended period (25 days) of regional O₃ exceedances and high temperatures (2^nd highest over last 2 decades) due to extreme weather conditions influenced by the Subtropical High and typhoon peripheries. Employing an integrated approach involving field measurements, machine learning, and numerical model simulations, we investigated the impact of weather-induced natural processes on O₃ pollution by considering meteorological factors, natural emissions, chemistry pathways, and atmospheric transport. It was found that the hot weather significantly promoted regional photochemical reactions, with meteorological factors contributing to an additional 10.8 ppb of O₃ levels compared to normal conditions. Temperature was identified as the dominant factor influencing O₃ pollution. The hot weather also intensified the emission of biogenic volatile organic compounds by ~10 %. Notably, isoprene and biogenic formaldehyde accounted for about half of the in-situ O₃ production. The chemical mechanism of isoprene contributing to O₃ formation was further explored, with O₃ production more attributable to the further degradation of early generation isoprene oxidation products than the direct isoprene oxidation itself. Furthermore, the typhoon nearing landfall significantly enhanced the cross-regional transport of O₃ from northern to southern China through stratosphere-to-troposphere exchange (STE). The CAM-Chem model simulations revealed that the STE-induced O₃ on the PRD surface could reach a maximum of ~ 8 ppb, highlighting the non-negligible impact of STE. This study highlights the importance of natural processes exacerbated by extreme weather events in O₃ pollution and provides valuable insights for O₃ pollution control under global warming.