Oxalate-Promoted SO₂ Uptake and Oxidation on Iron Minerals: Implications for Secondary Sulfate Aerosol Formation

Mineral dust serves as a significant source of sulfate aerosols by mediating heterogeneous sulfur dioxide (SO₂) oxidation in the atmosphere. Given that a considerable proportion of small organic acids are deposited onto mineral dust via long-range transportation, understanding their impact on atmospheric SO₂ transformation and sulfate formation is of great importance. This study investigates the effect of oxalate on heterogeneous SO₂ uptake and oxidation phenomenon by in situ FTIR, theoretical calculation, and continuous stream experiments, exploiting hematite (Fe₂O₃) as an environmental indicator. The results highlight the critical role of naturally deposited oxalate in mononuclear monodentate coordinating surface Fe atoms of Fe₂O₃ that enhances the activation of O₂ for oxidizing SO₂ into sulfate. Meanwhile, oxalate increases the hygroscopicity of Fe₂O₃, facilitating H₂O dissociation into reactive hydroxyl groups and further augmenting the SO₂ uptake capacity of Fe₂O₃. More importantly, other conventional iron minerals, such as goethite and magnetite, as well as authentic iron-containing mineral dust, exhibit similar oxalate-promoted sulfate accumulation behaviors. Our findings suggest that oxalate-assisted SO₂ oxidation on iron minerals is one of the important contributors to secondary sulfate aerosols, especially during the nighttime with high relative humidity.