Multipollutant air-quality improvement and ozone divergence across Chinese cities

Abstract. Over the past decade, China has achieved substantial reductions in particulate matter and primary gaseous pollutants, while ground-level ozone (O₃) has shown a contrasting response. We analysed air-quality changes across 356 Chinese cities during 2015–2025, with trend and city-level divergence analyses for 331 cities. Annual mean PM_2.5, PM₁₀, SO₂, NO₂ and CO concentrations decreased by 40.8%, 35.3%, 70.3%, 36.2% and 45.0%, respectively, whereas O₃ increased by 10.3% nationally. Positive O₃ Sen slopes occurred in 76.1% of cities, although only 19.9% were statistically significant. Meteorological standardization across 318 cities reduced the mean early-to-late O3 increase from 7.62 to 3.76–4.02 µg m₋₃, indicating a substantial influence of meteorological variability but leaving a persistent positive increase. Satellite observations further showed that warm-season HCHO changes between 2019–2021 and 2023–2025 were positively associated with concurrent O₃ changes (Spearman ρ = 0.36, p < 0.001), while the association with tropospheric NO₂ was weaker (ρ = 0.21, p < 0.001). The Fenwei Plain showed the largest regional O₃ increase (+14.3%), and seasonal relationships between O₃ and other pollutants reversed from positive in spring and summer to negative in winter. A city-level divergence framework further showed that similar reductions in conventional pollutants can be accompanied by markedly different O₃ responses. These findings demonstrate that reducing particulate and primary gaseous pollution alone does not guarantee lower photochemical pollution and highlight the need for coordinated, city-specific multipollutant control.