Oxalate-Promoted SO 2 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 (SO 2 ) 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 SO 2 transformation and sulfate formation is of great importance. This study investigates the effect of oxalate on heterogeneous SO 2 uptake and oxidation phenomenon by in situ FTIR, theoretical calculation, and continuous stream experiments, exploiting hematite (Fe 2 O 3 ) as an environmental indicator. The results highlight the critical role of naturally deposited oxalate in mononuclear monodentate coordinating surface Fe atoms of Fe 2 O 3 that enhances the activation of O 2 for oxidizing SO 2 into sulfate. Meanwhile, oxalate increases the hygroscopicity of Fe 2 O 3 , facilitating H 2 O dissociation into reactive hydroxyl groups and further augmenting the SO 2 uptake capacity of Fe 2 O 3 . 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 SO 2 oxidation on iron minerals is one of the important contributors to secondary sulfate aerosols, especially during the nighttime with high relative humidity.