Integrating Iron Isotope and Oxidation State Measurements with In Situ Vibrational Spectroscopy to Understand Iron Dissolution during Atmospheric Processing of Iron Oxides in Wildfire Smoke

Atmospheric processing of Biomass Burning Aerosols (BBAs) changes the chemical composition of these aerosols, potentially affecting iron (Fe) solubility. To probe these changes, we conducted laboratory experiments using maghemite nanoparticles as a representative iron oxide phase. Dissolved Fe(II) concentrations were approximately 3 times higher, and total dissolved Fe was 1.4 times higher, when both catechol and oxalate were present compared to experiments with one organic compound (48 h, pH 2, HCl). In situ attenuated total reflectance Fourier transform infrared spectroscopy of the maghemite-aqueous interface shows that oxalate initially outcompetes catechol for surface binding, but as the reaction progresses, catechol forms inner-sphere complexes, promoting reductive dissolution. Iron isotope measurements indicate that oxalate-assisted dissolution accounts for most of the iron released during the first 6 h of the reaction, while catechol-assisted dissolution contributes substantially in later stages. The observed synergy in Fe(II) and total dissolved Fe arises from a combination of solution phase and surface reactions that occur when both compounds are present. These results suggest that the dark atmospheric processing of BBA can increase dissolved Fe(II) and total dissolved Fe, potentially contributing to the adverse health effects of smoke. These findings also highlight that mixtures produce outcomes not predictable from single compounds alone.