Photochemical degradation of iron(III) citrate/citric acid aerosol quantified with the combination of three complementary experimental techniques and a kinetic process model

Abstract. Iron(III) carboxylate photochemistry plays an important role in aerosol aging, especially the lower troposphere. These complexes can absorb light over a broad wavelength range, inducing reduction of iron(III) and oxidation ligands. In presence O2, ensuing radical chemistry leads to further decarboxylation, production .OH, HO2., peroxides, oxygenated volatile organic compounds, contributing particle mass loss. The peroxides turn reoxidize iron(II) back iron(III), closing photocatalytic cycle. This cycle is repeated, resulting continual loss due release CO2 other compounds. cold and/or dry atmosphere, particles tend attain highly viscous states. While impact reduced mobility constituents on dark chemical reactions has received substantial attention, studies effect high viscosity photochemical processes are scarce. Here, we choose citrate (FeIII(Cit)) as model light-absorbing iron complex that induces citric acid (CA) degradation investigate how transport limitations influence processes. Three complementary experimental approaches were used kinetic limitations. single, levitated was measured with electrodynamic balance, state deposited X-ray spectromicroscopy, HO2. into gas phase observed coated-wall flow-tube experiments. We significant up 80 % within 24 h exposure. Interestingly, also always accelerated during irradiation, increase rate by about factor 10. When increased relative humidity (RH), increased. consistent strong for particles. To quantitatively compare these experiments determine physical parameters, numerical multilayered reaction diffusion (PRAD) developed treats various species. PRAD tuned simultaneously reproduce all results closely possible captured essential irradiation. particular, photolysis FeIII, reoxidation FeII, production, diffusivity O2 aqueous FeIII(Cit) ∕ CA system function RH molar ratio could be constrained. led satisfactory agreement uncertainty most but not performed. Photochemical under atmospheric conditions predicted shows repartitioning compounds may very when attempting accurately predict aging