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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 FeII. Under dry conditions (RH < 65%), the reoxidation of FeII 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 FeII with oxidants. Reactive oxygen species play a key role in FeII 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 FeII/FeIII 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.
