Aggravating O ₃ pollution due to NO _x emission control in eastern China

During the past five years, China has witnessed a rapid drop of nitrogen oxides (NO _x )owing to the wildly-applied rigorous emission control strategies across the country. However, ozone (O ₃ )pollution was found to steadily deteriorate in most part of eastern China, especially in developed regions such as Jing-Jin-Ji (JJJ), Yangtze River Delta region (YRD)and Pearl River Delta region (PRD). To shed more light on current O ₃ pollution and its responses to precursor emissions, we integrate satellite retrievals, ground-based measurements together with regional numerical simulation in this study. It is indicated by multiple sets of observational data that NO _x in eastern China has declined more than 25% from 2012 to 2016. Based on chemical transport modeling, we find that O ₃ formation in eastern China has changed from volatile organic compounds (VOCs)sensitive regime to the mixed sensitive regime due to NO _x reductions, substantially contributing to the recent increasing trend in urban O ₃ . In addition, such transitions tend to bring about an ~1–1.5 h earlier peak of net O ₃ formation rate. We further studied the O ₃ precursors relationships by conducting tens of sensitivity simulations to explore potential ways for effective O ₃ mitigation. It is suggested that the past control measures that only focused on NO _x may not work or even aggravate O ₃ pollution in the city clusters. In practice, O ₃ pollution in the three regions is expected to be effectively mitigated only when the reduction ratio of VOCs/NO _x is greater than 2:1, indicating VOCs-targeted control is a more practical and feasible way.