Phase-resolved multi-path drivers of PM_2.5 evolution in eastern China under Clean Air Actions and carbon neutrality...

Although China has achieved substantial PM_2.5 reductions over the past decade, the relative contributions of aerosol-radiation interactions (ARI; the effect of aerosols on solar radiation and photochemistry) and heterogeneous chemistry (HET; multiphase reactions occurring on aerosol surfaces) to these changes remain poorly understood, particularly during the Clean Air Action Plan (CAAP; Phase I: 2013-2017, Phase II: 2018-2020) and under future carbon neutrality scenarios. We applied a phase- and season-resolved WRF-Chem framework incorporating ARI and HET to quantify historical and projected PM_2.5 changes in the Yangtze River Delta, explicitly integrating aerosol effects with anthropogenic emissions, meteorological variability, and carbon-neutrality pathways. Emission reductions were the dominant driver of the decadal PM_2.5 decrease, with the largest decline during Phase I. HET consistently exerted stronger impacts than ARI, reducing winter PM_2.5 by - 3.94 (85.66 %) and - 0.66 (14.34 %) μg m^-3 in Phases I and II, and summer PM_2.5 by - 2.11 μg m^-3 in Phase I but increasing it slightly (+ 0.15 μg m^-3) in Phase II. Combined aerosol effects contributed net PM_2.5 reductions of - 4.29 μg m^-3 (winter) and - 1.14 μg m^-3 (summer) over the CAAP period, mainly in the 0.16-0.63 μm accumulation mode, with sulfate as the primary component. Sensitivity experiments indicated that coordinated controls of primary PM_2.5 and NH₃ produced the greatest year-round benefits when aerosol effects (AEs = ARI + HET) were considered. Under carbon peaking and neutrality scenarios with AEs (aerosol effects), PM_2.5 decreased by 27.32 % (21.24 %) and 47.64 % (40.69 %) in winter (summer), respectively. These findings underscore that a mechanistic understanding of aerosol processes governing PM_2.5 formation and evolution is essential for guiding targeted emission controls and designing synergistic air quality-climate policies along China's carbon-neutral pathway.