Aggravating O3 pollution due to NOx emission control in eastern China.

During the past five years, China has witnessed a rapid drop of nitrogen oxides (NOx) owing to the wildly-applied rigorous emission control strategies across the country. However, ozone (O3) 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 O3 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 NOx in eastern China has declined more than 25% from 2012 to 2016. Based on chemical transport modeling, we find that O3 formation in eastern China has changed from volatile organic compounds (VOCs) sensitive regime to the mixed sensitive regime due to NOx reductions, substantially contributing to the recent increasing trend in urban O3. In addition, such transitions tend to bring about an ~1-1.5 h earlier peak of net O3 formation rate. We further studied the O3 precursors relationships by conducting tens of sensitivity simulations to explore potential ways for effective O3 mitigation. It is suggested that the past control measures that only focused on NOx may not work or even aggravate O3 pollution in the city clusters. In practice, O3 pollution in the three regions is expected to be effectively mitigated only when the reduction ratio of VOCs/NOx is greater than 2:1, indicating VOCs-targeted control is a more practical and feasible way.