Impact of iron-containing dust on atmospheric oxidation processes

In the atmosphere, organic and inorganic compounds can partition into clouds, fog, raindrops, and aqueous aerosols, where they undergo rapid chemical oxidation, yielding secondary aerosols. This process is governed by the availability of radicals such as hydroxyl (OH) and nitrate (NO₃) radicals in the liquid phase. The presence of dissolved iron can boost the OH reactivity via Fenton reactions. Dust is a major source of iron in the atmosphere, occurring primarily in the crystalline lattices of aluminosilicates or as iron oxides. Following its emission, iron tends to be mostly insoluble but can be converted into soluble forms when inorganic acids decrease the pH, and organic ligands create iron complexes during atmospheric transport. In this study, we address the importance of iron in global atmospheric oxidation processes by mechanistically modelling the related chemical processes in the gas and liquid phases within clouds, fog, rain droplets, and, for the first time, aqueous aerosols. We employ the atmospheric chemistry MESSy model infrastructure, coupled to the global general circulation model ECHAM5 (EMAC). We represent three mechanisms of iron dissolution into aerosol water, driven by aerosol acidity, irradiation, and the presence of oxalate in the solution, which acts as an organic ligand. In the atmosphere, oxalate is the dominant dicarboxylic acid, mainly formed via aqueous-phase oxidation of glyoxal and other organic compounds. Our new approach is to explicitly account for oxalate-related aqueous-phase chemistry. Through a series of sensitivity simulations, with and without soluble iron, we address the global impact of iron on aqueous-phase oxidation capacity. We find that iron uptake into aerosol water enhances OH reactivity, particularly in cloud droplets, thereby increasing the aqueous oxidation of isoprene oxidation products and influencing secondary organic aerosol formation.