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

Abstract

Regional ozone (O<inf>3</inf>) 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 O<inf>3</inf> 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 O<inf>3</inf> simulations indicated that intensive noontime O<inf>3</inf> production rates in upwind cities (e.g., Foshan and Guangzhou) substantially contributed to O<inf>3</inf> episodes in downwind ones along the north/northeastern transport pathways. Additionally, O<inf>3</inf> 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 (NO<inf>x</inf>) 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 O<inf>3</inf> concentrations in downwind Zhuhai and Jiangmen, respectively, implying the necessity of spatiotemporally targeted control strategies. This study enhances our understanding of inter-city O<inf>3</inf> photochemical evolution and provides actionable insights for spatiotemporally differentiated control strategies on regional O<inf>3</inf> mitigation.