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Abstract: Formation of organic aerosol by oxidation of gas phase compounds has been intensely studied, and is much better understood than the aerosol aging transformations during the lifetime of organic aerosol. Aerosol ageing influences how these particles affect climate and human health and is still not well constrained in current models. Photochemistry in the condensed phase is an important mechanism responsible for aging. In the lower troposphere, where UV light intensity with sufficiently low wavelength to directly photolyze aerosol components is low, indirect photochemistry (catalyzing redox processes of non-absorbing molecules) is especially relevant. Here, I will concentrate on the feebacks between microsphysics and photochemical aging in a surrogate system, aqueous citric acid aerosol containing iron(III) citrate.Fe(III) carboxylate complexes absorb light up to about 500 nm and absorption induces a reduction of Fe(III) and oxidation of carboxylate ligands. In the presence of O2, ensuing radical chemistry leads to more decarboxylation and production of peroxides (e.g., OH•, HO2• and H2O2) and oxygenated volatile organic compounds (OVOC). The peroxides in turn allow the re-oxidation of Fe(II) to Fe(III), closing this photocatalytic cycle.When atmospheric relative humidity (RH) is low, organic aerosol particles turn from a liquid to a semi-solid, high viscosity state. Here, we use three experimental setups to investigate the influence of kinetic transport limitations on photochemical aging: single particles levitated in an electrodynamic balance (EDB), coated wall flow tube experiments and scanning transmission X-ray microscopy to study the internal composition of particles. For a coherent description of all three experimental data sets, we developed a detailed numerical model, including main equilibria, chemical reactions, and diffusivities of major species.