Synergistic PM_2.5 and O₃ control to address the emerging global PM_2.5-O₃ compound pollution challenges

In recent years, the issue of PM_2.5-O₃ compound pollution has become a significant global environmental concern. This study examines the spatial and temporal patterns of global PM_2.5-O₃ compound pollution and exposure risks, firstly at the global and urban scale, using spatial statistical regression, exposure risk assessment, and trend analyses based on the datasets of daily PM_2.5 and surface O₃ concentrations monitored in 120 cities around the world from 2019 to 2022. Additionally, on the basis of the common emission sources, spatial heterogeneity, interacting chemical mechanisms, and synergistic exposure risk levels between PM_2.5 and O₃ pollution, we proposed a synergistic PM_2.5-O₃ control framework for the joint control of PM_2.5 and O₃. The results indicated that: (1) Nearly 50% of cities worldwide were affected by PM_2.5-O₃ compound pollution, with China, South Korea, Japan, and India being the global hotspots for PM_2.5-O₃ compound pollution; (2) Cities with PM_2.5-O₃ compound pollution have exposure risk levels dominated by ST + ST (Stabilization) and ST + HR (High Risk). Exposure risk levels of compound pollution in developing countries are significantly higher than those in developed countries, with unequal exposure characteristics; (3) The selected cities showed significant positive spatial correlations between PM_2.5 and O₃ concentrations, which were consistent with the spatial distribution of the precursors NOx and VOCs; (4) During the study period, 52.5% of cities worldwide achieved synergistic reductions in annual average PM_2.5 and O₃ concentrations. The average PM_2.5 concentration in these cities decreased by 13.97%, while the average O₃ concentration decreased by 19.18%. This new solution offers the opportunity to construct intelligent and healthy cities in the upcoming low-carbon transition.