Decadal evolution of aerosol-mediated ozone responses in Eastern China under clean air actions and carbon neutrality policies

Abstract. Despite substantial reductions in PM2.5 and other pollutants, ozone (O3) in eastern China has increased over the past decade, yet the influence of aerosol processes – including aerosol–radiation interactions (ARI) and heterogeneous chemistry (HET) – on these trends remains insufficiently explored, particularly during Clean Air Action Plan (CAAP, Phase I: 2013–2017; Phase II: 2018–2020) and under carbon neutrality pathways. We applied a phase- and season-resolved WRF-Chem framework with explicit ARI and HET to quantify historical and projected O3 changes in the Yangtze River Delta (YRD), linking aerosol effects with CAAP and carbon-neutrality pathways. We separate O3 changes into those driven directly by anthropogenic emissions and meteorological variability, and those mediated by aerosol processes through ARI and HET. The results revealed that anthropogenic emissions and meteorological variability respectively dominated winter and summer O3 increases. Winter O3 increases were dominated by ARI: large aerosol reductions enhanced solar radiation, temperature, and photolysis, resulting in a photochemical O3 rise (+1.14 (+0.74) ppb in Phase I (II)). Summer O3 was more sensitive to HET. In Phase I, aerosol decreases weakened heterogeneous radical uptake, enhancing O3 formation (+1.62 ppb). In Phase II, however, the net HET effect reversed sign (−2.86 ppb), driven by shifts in multiple heterogeneous pathways – including changes in radical uptake, HONO and N2O5 chemistry, and aerosol liquid water – rather than radical scavenging alone. Accounting for aerosol effects (AEs = ARI + HET), reductions in PM2.5 and NOx increased O3, while VOCs reductions consistently lowered O3 in both seasons. Under carbon peaking and neutrality scenarios with AEs, winter O3 increased by 6.7 % and 10.7 %, whereas summer O3 decreased by 2.9 % and 6.7 %, highlighting seasonally contrasting responses. These results underscore the necessity of explicitly accounting for multi-path aerosol–O3 interactions in both near-term air quality management and long-term climate mitigation to prevent unintended trade-offs and maximize co-benefits.