A Comparative Study to Reveal the Influence of Typhoons on the Transport, Production and Accumulation of O₃ in the Pearl River Delta, China

Abstract. The Pearl River Delta (PRD) region in South China is faced with severe ambient O₃ pollution in autumn and summer, which mostly coincides with the occurrence of typhoons above the Northwest Pacific. With increasingly severe O₃ pollution in the PRD under the influence of typhoons, it is necessary to gain a comprehensive understanding of the impact of typhoons on O₃ transport, production and accumulation for efficient O₃ reduction. In this study, we analysed the general influence of typhoons on O₃ pollution in the PRD via systematic comparisons of meteorological conditions, O₃ processes and sources on O₃ pollution days with and without typhoon occurrence (denoted as the typhoon-induced and no-typhoon scenarios, respectively), and also examined the differences in these influences in autumn and summer. The results show that the approach of typhoons was accompanied by higher wind speeds and strengthened downdrafts in autumn as well as the inflows of more polluted air masses in summer, suggesting favourable O₃ transport conditions in the typhoon-induced scenario in both seasons. However, the effect of typhoons on the production and accumulation of O₃ were distinct. Typhoons led to reduced cloud cover, and thus stronger solar radiation in autumn, which accelerated O₃ production, but the shorter residence time of local air masses was unfavourable for the accumulation of O₃ within the PRD. In contrast, in summer, typhoons increased cloud cover, and weakened solar radiation, thus restraining O₃ formation, but the growing residence time of local air masses favoured O₃ accumulation. The modelling results using the Community Multiscale Air Quality (CMAQ) model for the typical O₃ pollution days suggest increasing contributions from the transport processes as well as sources outside the PRD for O₃ pollution, confirming enhanced O₃ transport under typhoon influence in both seasons. The results of the process analysis in CMAQ suggest that the chemical process contributed more in autumn but less in summer in the PRD. Since O₃ production and accumulation cannot be enhanced at the same time, the proportion of O₃ contributed by emissions within the PRD was likely to decrease in both seasons. The difference in the typhoon influence on O₃ processes in autumn and summer can be attributed to the seasonal variation of the East Asian monsoon. From the meteorology-process-source perspective, this study revealed the complex influence of typhoons on O₃ pollution in the PRD and their seasonal differences. To alleviate O₃ pollution under typhoon influence, emission control is needed on a larger scale, rather than only within the PRD.