• • PM2.5 concentrations in Nanjing, Shanghai, Hangzhou, and Hefei decreased significantly from 2017 to 2023, with the most pronounced improvement in 2020 during COVID-19 lockdowns, demonstrating the efficacy of stringent emission controls; O3 levels remained high or increased, indicating a control bottleneck.
• • Correlation analysis revealed that PM2.5 and O3 are positively correlated during O3 pollution episodes but negatively correlated during PM2.5 episodes, implying that simultaneous control of both pollutants requires season-specific strategies.
• • Compound pollution days occurred predominantly from February to October, with peak frequency (20 days) in April–June, driven by high temperature, stagnant air, and weak diffusion conditions that favor both PM2.5 accumulation and photochemical O3 production.
• • The reduction in PM2.5 weakened the aerosol 'umbrella effect', enhancing surface radiation and promoting O3 formation; combined with altered NOx/VOCs ratios and climate warming, this led to increased O3 accumulation in the cold season, raising compound pollution frequency during colder months.