Photochemical Degradation of Iron Citrate in Anoxic Viscous Films Enhanced by Redox Cascades

Iron contained in atmospheric aerosol particles can form complexes with organic ligands and initiate photochemical reactions that alter the composition and physicochemical properties of the particles. Depending on the temperature and humidity, organic particles exist in different phase states, which affects reactant diffusivity and chemical reaction rates. We performed coated-wall flow-tube experiments using citric acid films doped with iron as proxies for secondary organic aerosols. We quantified the CO₂ production under UV irradiation as a function of time and relative humidity (RH) and observed a pronounced decrease of CO₂ production with decreasing RH. The kinetic multilayer model of aerosol surface and bulk chemistry (KM-SUB) and a Monte Carlo-based global optimization method were applied to all measured data to determine the underlying effects of mass transport and chemical reactions. The model analysis revealed that after an initial rapid reaction, photooxidation becomes limited by the reoxidation of Fe^II. Under dry conditions (RH < 65%), the reoxidation of Fe^II is kinetically limited by the supply of O₂, as slow diffusion in the viscous organic matrix leads to anoxia in the interior of the film. At high humidity (RH > 85%), mass transport limitations cease, resulting in full O₂ saturation, and photooxidation becomes limited by the chemical reaction of Fe^II with oxidants. Reactive oxygen species play a key role in Fe^II reoxidation and thus in perpetuating photooxidation chemistry. A single O₂ molecule triggers a redox cascade from O₂ to HO₂, H₂O₂, and OH, leading to ≈3 cycles of the Fe^II/Fe^III redox pair. Our model and kinetic parameters provide new insights and constraints in the interplay of microphysical properties and photochemical aging of mixed organic-inorganic aerosol particles, which may influence their effects on air quality, climate, and public health.