Graphical AbstractVerifiedCharacteristics and Meteorological Causes of PM2.5-O3 Compound Pollution in Typical Cities of the Yangtze River Delta Region
In the context of accelerated urbanization, regional air composite pollution in medium and large urban agglomerations is primarily characterized by PM2.5-O3 compound pollution. To elucidate the meteorological causes of PM2.5-O3 compound pollution in the Yangtze River Delta (YRD) region over the recent seven years (2017–2023), this study analyzed monitoring data from typical cities (Nanjing, Shanghai, Hangzhou, and Hefei) using Pearson and partial correlation coefficients. Results indicate: (1) PM2.5 pollution exhibited a significant downward trend across all four cities, with notable improvement during the COVID-19 pandemic in 2020, underscoring the effectiveness of air pollution control measures. Conversely, O3 pollution remained elevated or increased in some cities, indicating persistent challenges in O3 control. (2) During O3 pollution episodes, PM2.5 and O3 concentrations were positively correlated, whereas during PM2.5 pollution episodes, they were negatively correlated. (3) Compound pollution days were predominantly observed from February to October, with the highest frequency (20 days) occurring from April to June. (4) The significant reduction in PM2.5 weakened the aerosol 'umbrella effect', enhancing surface radiation and promoting near-surface O3 formation. Concurrently, changes in the NOx/VOCs ratio weakened O3 titration, and climate warming accelerated O3 precursor generation and potentially altered boundary layer structure, collectively contributing to O3 accumulation in the cold season and an increasing frequency of compound pollution during that period. (5) The formation mechanisms of PM2.5 and O3 are driven by distinct meteorological conditions, with low overall concentration correlation; however, under compound meteorological conditions such as high temperature, stagnant air, and weak diffusion, both pollutants tend to rise synchronously, indicating that compound pollution events are typically driven by multiple adverse meteorological factors. This study demonstrates that from 2017 to 2023, PM2.5 pollution significantly decreased while O3 pollution showed an increasing trend. Compound pollution was concentrated in April–June and influenced by high temperature, stagnant air, and weak diffusion. With effective PM2.5 control, enhanced surface radiation and changes in O3 precursors led to O3 accumulation in the cold season, increasing compound pollution frequency. Overall, compound pollution is driven by multiple meteorological factors, posing complex challenges for control.
