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

Over the past decade, China has achieved remarkable progress in mitigating 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 the urban O₃ formation regime, shifting from being primarily volatile organic compound (VOC) limited to VOC-nitrogen oxides (NO_x) co-limited across the NCP between the 2010s and the 2020s. Notably, the primary VOC species, their respective sources, and the optimal VOCs/NO_x reduction ratios exhibit remarkable regional consistency. Modeling analyses further indicate that a 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.