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.