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, 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 PM_2.5, 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 PM_2.5, 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, NO_x–VOC precursor management and seasonal O₃ formation conditions.