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 heterogeneous 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 improves sulfate simulation for a 2018 dust event, 2016 haze episode, and the 2018 annual mean, reducing NMBs (normalized mean biases) from −62.76%, −41.97% and −35.01% to 0.44%, 14.85% and −16.52%, respectively, outperforming the simulation only including dust heterogeneous chemistry (−19.05%). Process diagnostics show that during the 2018 dust event, sulfate formation is dominated by SO₂+OH oxidation on coarse-mode particles. Post-event, decreasing dust and OH weaken the dust chemistry and elevate the NH₃ by buffering aerosol acidity to strengthen particle-surface NO₂ oxidation. During the 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. In terms of the 2018 annual mean, dominant sulfate formation pathways exhibit pronounced regional contrasts. In the Sichuan Basin, gas-/aqueous-phase oxidation (54.37%) and dust-phase SO₂+O₃ oxidation (26.19%) prevail. In the Beijing–Tianjin–Hebei region, dust chemistry dominates (38.07%), followed by particle-surface NO₂ oxidation (6.96%) and black carbon–catalyzed oxidation (9.57%). Sulfate formation in the Yangtze River Delta region exhibits similar patterns but weaker heterogeneous contributions. Overall, the results show that explicitly representing the heterogeneous chemistry across dust and haze regimes improves sulfate simulation and provides a scientific basis for improving atmospheric chemistry parameterizations and informing region-specific sulfur mitigation strategies.