Abstract
<jats:p>Abstract. Over the past decade, China has achieved substantial reductions in particulate matter and primary gaseous pollutants, yet the response of ground-level ozone (O3) remains ambiguous. This study analyzed the multipollutant evolution of air quality in 356 Chinese cities during 2015–2025 using ground-based monitoring data; trend and city-level divergence analyses were performed for 331 cities with sufficient observational coverage. Annual mean concentrations of PM2.5, PM10, SO2, NO2 and CO decreased by 40.8 %, 35.3 %, 70.3 %, 36.2 % and 45.0 %, respectively. In contrast, O3 increased by 10.3 %, with positive trends in 76.1 % of cities. Sen’s slope analysis showed spatially coherent declines in conventional pollutants but more heterogeneous O3 dynamics. Regionally, the largest O3 increase occurred in the Fenwei Plain (+14.3 %), whereas the smallest occurred in Northeast China. Seasonal correlations revealed a reversal in O3-centred relationships: positive associations with PM2.5, PM10 and NO2 in spring and summer shifted to negative relationships in winter, reflecting seasonal changes in photochemical and meteorological conditions. A city-level divergence framework classified cities into four types based on the five-pollutant reduction index (R5) and O3 response. Type I and Type II cities showed similar conventional pollutant reductions but sharply different O3 responses, while Type II cities were more frequent in northern and inland China. These results show that controlling particulate and primary gaseous pollution does not guarantee lower secondary photochemical pollution. Further air-quality improvement requires coordinated, city-specific and regionally differentiated multipollutant strategies integrating PM reduction, NOx–VOC precursor management and seasonal O3 formation conditions.</jats:p>