Influence of combustion-derived ash composition on heterogeneous SO₂-NO₂ conversion under simulated atmospheric...

Secondary inorganic aerosols, particularly sulfate and nitrate, are major components of fine particulate matter (PM_2.5) and play a key role in air quality degradation. Although heterogeneous SO₂-NO₂ reactions have been extensively studied on single mineral oxides, the behavior of combustion-derived ash particles remains insufficiently understood because of their complex multi-component structures. In this study, herbaceous biomass (HB), woody biomass (WB), sewage sludge (SS-GC), and coal-derived ashes (Coal-WH, Coal-MHU, Coal-CEN) were characterized to evaluate compositional variability among combustion-derived ash particles. Heterogeneous sulfate and nitrate formation was then investigated using coal-derived ash samples under controlled laboratory conditions simulating atmospheric exposure. To support the interpretation of multi-component ash behavior, single-oxide experiments were conducted and combined with compositional analysis, including bulk oxide composition and alkalinity indices. The results show that sulfate formation in coal-derived ashes follows the order Coal-WH > Coal-MHU > Coal-CEN and does not correlate directly with Fe content or surface area alone. Instead, adsorption-related properties, redox-active components, and their accessibility within the ash matrix jointly control heterogeneous reactivity. Additional experiments demonstrate that sulfate formation is more strongly enhanced than nitrate under prolonged reaction time, SO₂-enriched conditions, high humidity, and saline environments, indicating that environmental factors can amplify heterogeneous conversion over the representative Coal-WH ash sample. These findings demonstrate that heterogeneous SO₂-NO₂ conversion on combustion-derived ash particles is controlled by compositional and structural interactions rather than single-component effects. The results provide mechanistic insight into atmospheric sulfate formation on complex particles and highlight the importance of considering realistic multi-component ash systems in atmospheric chemistry.