Changes in Metal Solubility in PM_2.5 in Xi’an City Under Clean Heating Policies: Effects of Emission Source and Aerosol Acidity

Clean heating policies were implemented in rural areas of Shaanxi Province in 2017 to alleviate severe air pollution. To evaluate their impacts on bioavailability of PM_2.5-bound metals, the influence of emission sources and aerosol acidity on PM_2.5-bound metal solubility was explored in Xi’an over three policy-defined periods between 2016 and 2021. Results showed that aerosol pH increased progressively from 4.81 ± 1.82 to 5.29 ± 1.79 following policy implementation, closely associated with reductions in SO₂ and NO₂ concentrations due to emission controls. Metal concentrations decreased significantly over the study period. In contrast, metal solubility exhibited clear source-dependent variations. Solubilities of metals associated with coal combustion, biomass burning, and industrial activities (As, Cd, Pb, K and Zn) decreased by 16.6–50.5% with weakening aerosol acidity. In contrast, solubilities of metals related to vehicle exhaust, oil fuel combustion and dust (Cu, V, Ni, Ti and Fe) increased by 38.3–56.8%, indicating enhanced influence of emission processes. Source apportionment demonstrated that mixed contributions of coal combustion, biomass burning and industrial activities to total and water-soluble metals decreased by 12% and 11.2%, respectively, while contribution from secondary atmospheric processes increased by 4% and 3.8%. These findings highlight that clean heating policies reshape both metal sources and atmospheric chemical environments, thereby altering metal dissolution characteristics and bioavailability.