Photochemical formation process of ozone and spatiotemporally targeted control strategies along the transport pathways in the Pearl River Delta region.

Regional ozone (O3) pollution is currently a major challenge for urban agglomerations in China, including the Pearl River Delta (PRD) region. Understanding the cross-city photochemical evolution of O3 is crucial for developing spatiotemporally targeted control strategies. In this study, three major transport pathways across cities in the PRD region were identified through multiyear airflow trajectory cluster analysis. A photochemical trajectory model (PTM) coupled with the near-explicit Master Chemical Mechanism (MCM) was employed to elucidate the photochemical processes along these pathways. The spatiotemporal variations in O3 simulations indicated that intensive noontime O3 production rates in upwind cities (e.g., Foshan and Guangzhou) substantially contributed to O3 episodes in downwind ones along the north/northeastern transport pathways. Additionally, O3 formation regimes shifted from volatile organic compounds (VOCs) regime to a transitional regime around midday, suggesting the effectiveness of time-resolved precursor controls, specifically, VOCs reduction during morning hours followed by coordinated nitrogen oxides (NOx) and VOCs reductions post-noon in the relevant cities. Furthermore, key VOC species and their sources (i.e., industrial processes and on-road mobile emissions) from responsible cities were traced and their downstream impacts were estimated. In the northern and northeastern cases, morning anthropogenic emissions from Foshan and Guangzhou accounted for 29 % and 42 % of O3 concentrations in downwind Zhuhai and Jiangmen, respectively, implying the necessity of spatiotemporally targeted control strategies. This study enhances our understanding of inter-city O3 photochemical evolution and provides actionable insights for spatiotemporally differentiated control strategies on regional O3 mitigation.