Chinese Regulations Are Working—Why Is Surface Ozone Over Industrialized Areas Still High? Applying Lessons From Northeast US Air Quality Evolution

Observational data indicate increasing trends of surface ozone (O₃) in China, despite emission controls that have resulted in reductions of precursor emissions. Here, we explore the cause of this contradiction, through analysis of surface observations (2014-2019) in China and historical observation record in the United States (US, 1990-2019). Our observation-based analysis indicates that the reductions of nitrogen oxides (NO_x) emissions led to increase of surface O₃ in North China Plain (NCP) and Yangtze River Delta (YRD) of around 8 ppb. However, NO_x controls resulted in shift of O₃ chemical regimes over NCP and YRD, with turning points between NO_x- and volatile organic compound (VOC)-limited regimes around 2019, while model simulations suggest transitional or NO_x-limited regimes over the rest of China. The impacts of high fine particles (PM_2.5) on O₃ formation has declined because of the reduction of PM_2.5 concentrations. Stricter NO_x controls can mitigate O₃ pollutions over industrialized areas in China.