Modeling Iron-Copper Cycling in Photochemically Aged Organic Aerosol Particles

Photochemical aging of redox-active transition metals in organic aerosol (OA) particles contributes
to an increase in oxidative potential and changes their atmospheric fate. We evaluated the
poorly 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 computationally
model 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-ray
microscope 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 volatilized
CO₂as the first decarboxylation product of iron citrate to quantify the rate of photochemical iron
redox cycling. For kinetic modeling, we utilized the kinetic multilayer model of aerosol surface and
bulk chemistry (KM-SUB) for films, in which we incorporated chemical reaction mechanisms
built on previous work. The model explicitly treats photo- and redox chemistry along with the
mass transfer of reactants and products between the condensed and gas phase, and is used to
describe CO₂production in the flow reactor. The model was applied to data from experiments
using iron citrate alone and to mixed iron and copper citrate experiments. We tested chemical
mechanisms 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 kinetic
parameters 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 citric
acid 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 reactions
between 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 similar
aging processes with SOA such as α-pinene and OA particles containing nitrate and/or iodine
species.