Modeling Iron-Copper Cycling in Photochemically Aged Organic Aerosol Particles

Photochemical aging of redox-active transition metals in organic aerosol (OA) particles contributesto an increase in oxidative potential and changes their atmospheric fate. We evaluated thepoorly characterized role of copper as a highly emitted transition metal in a well-established iron-containing proxy for SOA material (citric acid with iron citrate). Here, we computationallymodel photochemical aging experiments from a coated-wall flow-tube to derive an iron-copper cy-cling mechanism that explains the enhanced aging with copper found in scanning transition X-raymicroscope measurements. Aging was carried out under UV light irradiation (λ = 365 nm) at at-mospherically relevant residence times as a function of relative humidity. We measure volatilizedCO2 as the first decarboxylation product of iron citrate to quantify the rate of photochemical ironredox cycling. For kinetic modeling, we utilized the kinetic multilayer model of aerosol surface andbulk chemistry (KM-SUB) for films, in which we incorporated chemical reaction mechanismsbuilt on previous work. The model explicitly treats photo- and redox chemistry along with themass transfer of reactants and products between the condensed and gas phase, and is used todescribe CO2 production in the flow reactor. The model was applied to data from experimentsusing iron citrate alone and to mixed iron and copper citrate experiments. We tested chemicalmechanisms for iron-copper cycling found in the literature and a newly developed mecha-nism [3]. Inverse modeling and global optimization techniques were used to constrain kineticparameters and optimize the chemical reaction mechanism. In addition, some physical para-meters were quantified anew by measuring the viscosity of aged and non-aged iron-copper citricacid particles. This supports the KM-SUB modeling, including exact microphysical properties un-der different humidity and/or aging conditions. The new model uniquely includes redox reactionsbetween iron and copper complexes in a multiphase system, which may elucidate the role of photo-chemically active OA in the atmosphere. In future work, the model will also be used for similaraging processes with SOA such as α-pinene and OA particles containing nitrate and/or iodinespecies.