The role of source emissions in sulfate formation pathways based on chemical thermodynamics and kinetics model

Sulfate is a major PM_2.5 constituent and poses a significant threat to ecosystems and human health, which has attracted lots of attention to the sulfate formation mechanism. In recent years, there has been great scientific interest in the multiphase oxidation of SO₂ in aqueous aerosol particles. Many factors are involved in the reaction process, including precursor SO₂, oxidants/catalysts, and aerosol acidity, which are three channels closely related to the source emission. The conjoint analysis of source emissions and sulfate aqueous formation can provide a scientific basis for designing effective strategies, though the related research is extremely limited. Here, we applied an improved solute strength-dependent chemical Thermodynamics & Kinetics model (for aqueous pathway contribution) and the Partial Target Transformation-Positive matrix factor model (for source apportionment) to explore the role of source emission in sulfate aqueous formation. The results indicated H₂O₂ aqueous oxidation was the dominant pathway (65.9 %), and secondary nitrate source may grow together with sulfate formation from H₂O₂ pathway. H₂O₂ and TMI pathways were related to higher SOR (sulfur oxidation rate). TMI pathway was significant in summer (54.6 %) and increased with secondary sources and vehicle exhaust. NO₂ pathway was more significant at low secondary source and high coal combustion (higher contribution of NO₂ pathway appeared in winter, 24.7 %). While high formation rate of the O₃ pathway always occurred at low source levels. Coal combustion and vehicle exhaust showed obvious effects on sulfate aqueous formation. Notably, aerosol acidity is a significant factor related to sources and plays a key role in sulfate formation. The result also suggested aerosol pH may be more important than the amounts of substances involved in the oxidation reaction. The findings in this work can provide useful information for better understanding sulfate aqueous formation and offer a scientific basis for designing strategies for air pollution control and sulfate mitigation.