Oxalate-Promoted SO<sub>2</sub> Uptake and Oxidation on Iron Minerals: Implications for Secondary Sulfate Aerosol Formation
Ziyue Chen, Hao Li, Huan Shang, Xupeng Liu, Furong Guo, Xiufan Liu, Linghao Yu, Biao Zhou, Xiao Liu, Yanbiao Shi, Lizhi Zhang, Zhihui Ai
Abstract
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.