Analysis of Synergistic Changes in PM2.5 and O3 Concentrations Based on Structural Equation Model Study

Given the increasing importance of effectively identifying synergistic changes between PM2.5 and O-3 and comprehensively analyzing their impact on air quality management in China, we employ the Sen+Mann-Kendall (Sen+M-K) trend test in this study to examine the temporal and spatial variation trends of PM2.5 and O-3 in the Yangtze River Delta (YRD), from 2003 to 2020. We identified the regions where these pollutants exhibited synergistic changes and established the pathways between the pollutants and their potential drivers, using geographically weighted random forest algorithms and structural equation modeling. The study results revealed as follows: (1) Overall, the PM2.5 concentrations show a decreasing trend, while the O-3 concentrations exhibit an increasing trend, in the YRD. Analysis of the combined trends indicates that approximately 95% of the area displays opposing trends for PM2.5 and O-3, with only about 4% in the southern region showing synergistic trends for both pollutants. (2) Drought and the average temperature are the main drivers of the changes in PM2.5 and O-3 concentrations in areas experiencing synergistic changes. Their combined effects alleviate the aggregation of PM2.5 and reduce the formation of VOCs, indirectly reducing the generation of pollutants. The negative effect of the average temperature on the O-3 concentration may indicate the existence of nonlinear effects and complex interaction effects between the drivers. NOx and VOCs play important dual roles in the generation and conversion of pollutants, although their overall impact is smaller than meteorological factors. They produce significant indirect effects through their interaction with meteorological and other human factors, further affecting the concentrations of PM2.5 and O-3. In areas without coordinated changes, the main impact of meteorological factors remains unchanged, and the relationship between the two anthropogenic emission sources and their effects on PM2.5 and O-3 are complex, with different directions and levels involved. This study provides detailed insights into the drivers of air quality changes in the YRD and offers a scientific basis for environmental management authorities to develop more comprehensive and targeted strategies for balancing the control of PM2.5 and O-3 pollution.