Dominant Mechanisms of Sulfate Formation Across Pollution Regimes in China: A GEOS-Chem Modeling Study

Sulfate, a major component of secondary inorganic aerosols, substantially impacts environment and climate. However, atmospheric models often underestimate sulfate formation because key heterogeneous pathways remain incompletely represented, leading to large simulation biases and uncertainty in dominant mechanisms across multiple pollution regimes. We therefore incorporate dust heterogenous chemistry and key reactions under haze conditions into GEOS-Chem, building a comprehensive framework to assess their impacts on sulfate. Compared to baseline, this framework markedly improve sulfate simulation for 2018 dust event, 2016 haze episode and 2018 annual-mean, reducing NMBs from -62.76%, -41.97% and -35.01% to 0.44%, 14.85%, and -16.52%, respectively, outperforming simulation only including dust heterogenous chemistry(-19.05%). Process diagnostics show during 2018 dust event, sulfate formation is dominated by SO2+OH oxidation on coarse-mode particles. Post-event, decreasing dust and OH weaken dust chemistry and elevated NH3 by buffering aerosol acidity strengthens particle-surface NO2 oxidation. During 2016 haze episode, heterogeneous chemistry dominates sulfate formation(67.41%), with Mn-catalyzed oxidation contributing 17.11%, lower than previous estimates. Sensitivity simulations indicate this reduction is driven by competition with dust chemistry through SO2 consumption and lower simulated Mn concentrations. On 2018-annual-mean, dominant sulfate formation pathways exhibit pronounced regional contrasts. In Sichuan-Basin, gas-/aqueous-phase oxidation(54.37%) and dust-phase SO2+O3 oxidation(26.19%) prevail. In Beijing-Tianjin-Hebei, dust chemistry(38.07%) dominates, followed by particle-surface NO2 oxidation(6.96%), and BC-catalyzed oxidation(9.57%). Sulfate formation in Yangtze-River-Delta exhibits similar patterns but weaker heterogeneous contributions. Overall, results show explicitly representing heterogeneous chemistry across dust and haze regimes improves sulfate simulations and provide scientific basis for improving atmospheric-chemistry parameterizations and informing region-specific sulfur mitigation strategies.