Spatiotemporal characteristics of PM and ozone concentrations in Chinese urban clusters.

Despite China's public commitment to emphasise air pollution investigation and control, trends in PM and ozone concentrations in Chinese urban clusters remain unclear. This study quantifies the spatiotemporal variations in PM and surface ozone at the scale of Chinese urban clusters by using a long-term integrated dataset from 2015 to 2020. Nonlinear Granger causality testing was used to explore the spatial association patterns of PM and ozone pollution in five megacity cluster regions. The results show a significant downward trend in annual mean PM concentrations from 2015 to 2020, with a decline rate of 2.8 μg m yr. By contrast, surface ozone concentrations increased at a rate of 2.1 μg m yr over the 6 years. The annual mean PM concentrations in urban clusters show significant spatial clustering characteristics, mainly in Beijing-Tianjin-Hebei (BTH), Fenwei Plain (FWP), Northern slope of Tianshan Mountains urban cluster (NSTM), Sichuan Basin urban cluster (SCB), and Yangtze River Delta (YRD). Surface ozone shows severe summertime pollution and distributional variability, with increased ozone pollution in major urban clusters. The highest increases were observed in BTH, Yangtze River midstream urban cluster (YRMR), YRD, and Pearl River Delta (PRD). Nonlinear Granger causality tests showed that PM was a nonlinear Granger cause of ozone, further supporting the literature's findings that PM reduction promoted photochemical reaction rates and stimulated ozone production. The nonlinear test statistic passed the significance test in magnitude and statistical significance. FWP was an exception, with no significant long-term nonlinear causal link between PM and ozone. This study highlights the challenges of compounded air pollution caused primarily by ozone and secondary PM. These results have implications for the design of synergistic pollution abatement policies for coupled urban clusters.