Iron(III) carboxylate photochemistry plays an important role in aerosol aging, especially in the lower troposphere. These complexes can absorb light over a broad wavelength range, inducing the reduction of iron(III) and the oxidation of carboxylate ligands [1]. In the presence of O₂, ensuing radical chemistry leads to further decarboxylation, and the production of OH, HO₂, peroxides, and oxygenated volatile organic compounds, contributing to particle mass loss. The oxidants, in turn, re-oxidize iron(II) back to iron(III), closing a photocatalytic cycle [2]. In a cold and/or dry atmosphere, organic aerosol particles tend to attain highly viscous states. While the impact of reduced mobility of aerosol constituents on dark chemical reactions has received substantial attention, studies on the effect of high viscosity on photochemical cycles and processes are scarce. Here, we choose iron(III)-citrate (Fe^III(Cit)) as a model light absorbing iron carboxylate complex that induces citric acid (CA) degradation to investigate how transport limitations influence photochemical processes. Three complementary experimental approaches were used to investigate kinetic transport limitations. The mass loss of single, levitated particles was measured with an electrodynamic balance, the oxidation state of deposited particles was measured with X-ray spectromicroscopy, and HO₂ radical production and release into the gas phase was observed in coated wall flow tube experiments. We developed a numerical multi-layered photochemical reaction and diffusion (PRAD) model that treats chemical reactions and transport of various species to quantitatively compare with our experiments and determine important physical and chemical parameters for our system. We observed significant photochemical degradation, with up to 80 % mass loss within 24 hours of light exposure. When we decreased relative humidity our particles were exposed to, the observed mass loss rate also decreased. This is consistent with strong kinetic transport limitations for highly viscous particles. The PRAD model reproduced all experimental results and captured the essential chemistry and molecular transport during irradiation. In particular, the photolysis rate of Fe^III, the re-oxidation rate of Fe^II, HO₂ production, and the diffusion coefficients of species in aqueous Fe^III(Cit)/CA system as function of relative humidity and Fe^III(Cit)/CA molar ratio could be constrained. Photochemical degradation under atmospheric conditions predicted by the PRAD model shows that release of CO₂ and re-partitioning of organic compounds to the gas phase may be very significant to accurately predict organic aerosol aging processes.

[1] Jing Dou, Beiping Luo, Thomas Peter, Peter A. Alpert, Pablo Corral Arroyo, Markus Ammann, Ulrich K. Krieger, The Journal of Physical Chemistry Letters, 2019, 10, 4484–4489.
[2] Christian Weller, Andreas Tilgner, Peter Bräuer, Hartmut Herrmann, Environmental Science & Technology, 2014, 48, 5652–5659.