Critical transition of urban ozone formation regime in the North China Plain

Abstract Over the past decade, China has achieved remarkable progress in mitigating the aerosol pollution. However, ozone (O3) pollution still shows a worsening trend, implying that China’s control measures may have been less effective in tackling O3 pollution. Herein, we conduct synchronized observations of O3 and its precursors across 37 cities in the heavily polluted North China Plain (NCP) during the summer of 2021 and apply a unified observation-based model to diagnose O3 formation mechanisms. Our results reveal a significant transition in urban O3 formation regime, shifting from primarily VOC-limited to VOC–NOx co-limited across the NCP between 2010s and 2020s. Notably, the primary VOC species, their respective sources, and the optimal VOCs/NOx reduction ratios exhibit remarkable regional consistency. Modeling analyses further indicate that long-term national “Carbon Neutrality” strategy could effectively alleviate O3 pollution, with targeted VOC emission reductions from major anthropogenic sources offering the greatest mitigation potential. These findings underscore the efficacy of China’s endeavors in mitigating O3 pollution, although the effects are not immediately evident from ambient O3 concentrations. O3 pollution control in Chinese cities has reached a critical inflection point, offering considerable flexibility and feasibility in formulating future control policies.