Over the past decade, China has achieved substantial reductions in particulate matter and primary gaseous pollutants, while ground-level ozone (O<sub>3</sub>) 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<sub>2.5</sub>, PM<sub>10</sub>, SO<sub>2</sub>, NO<sub>2</sub> and CO concentrations decreased by 40.8%, 35.3%, 70.3%, 36.2% and 45.0%, respectively, whereas O<sub>3</sub> increased by 10.3% nationally. Positive O<sub>3</sub> 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<sub>−3</sub>, 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 O<sub>3</sub> changes (Spearman ρ = 0.36, <em>p</em> < 0.001), while the association with tropospheric NO<sub>2</sub> was weaker (ρ = 0.21, <em>p</em> < 0.001). The Fenwei Plain showed the largest regional O<sub>3</sub> increase (+14.3%), and seasonal relationships between O<sub>3</sub> 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 O<sub>3</sub> 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.