Photochemistry of iron in simulated crustal aerosols with dimethyl sulfide oxidation products

Iron contained in dust-derived aerosol particles deposited into remote oceans is essential for phytoplankton productivity, which controls photosynthesis rate and the uptake and release of climate forcing gases. Understanding chemical mechanisms that control iron bioavailability, that is, its speciation, is therefore crucial for global climate predictions. In the present study, the photoredox chemistry of iron in marine atmospheric aerosol particles was investigated by using ferrihydrite as a surrogate iron phase in the presence of dimethyl sulfide (DMS) derived oxidation products: dimethyl sulfoxide (DMSO), dimethyl sulfone (DMSO2), methane sulfinic acid (MSIA), and methane sulfonic acid (MSA). Reactants and products were analyzed with UV-vis absorption spectroscopy, ion chromatography, and a hydrogen peroxide sensitive electrode. Results show that MSIA enhances the photoreductive dissolution of iron in a ligand-to-metal charge transfer reaction producing Fe(II), MSA, and H2O2. The rate law for Fe(II) is close to first order (0.79) with regard to adsorbed MSIA and has an empirical rate constant of 1.4 x 10(-4) s(-1). This mechanism may represent a significant pathway through which iron becomes more bioavailable, and it contributes to models of iron and sulfur chemistries in the marine atmosphere.