Model of a Photochemically Induced Iron-Copper Cycling in Viscous Organic Aerosol

KEVIN KILCHHOFER, Ashmi Mishra, Peter A. Alpert, Thomas Berkemeier, Allan K. Bertram, Markus Ammann, Laboratory of Atmospheric Chemistry, Paul Scherrer Institute

     Abstract Number: 159
     Working Group: Aerosol Chemistry

Abstract
Photochemical aging of organic aerosol (OA) particles is an important pathway to generate oxidants and alter atmospheric chemistry. Photolysis of redox-active transition metals such as iron complexed with organic material contributes to aging and changes the oxidative potential (OP) of OA as well as their atmospheric fate. Based on this, we evaluate poorly characterized iron chemistry interacting with other transition metals such as copper. We conducted photochemical aging experiments in a coated-wall flow tube with a mixture of citric acid, iron citrate, and copper citrate as a proxy for an atmospherically-relevant OA. Citric acid is considered a proxy compound of secondary organic aerosol material. The aging was performed under irradiation with UV light (λ = 345 nm) as a function of relative humidity (RH). We measure volatilized CO2 as the first decarboxylation product of citric acid to quantify the rate of photochemical cycling and to constrain the kinetic multi-layer model of aerosol surface and bulk chemistry (KM-SUB¹). The model is employed to gain an understanding of the interplay of physical and chemical processes, i.e., reaction and diffusion, in the multiphase chemical system. The model results show that bulk diffusion of oxygen is a very sensitive parameter implying a strong limitation of the reaction kinetics by molecular transport, suggesting a highly-viscous organic matrix. We implemented an iron-copper catalytic redox cycling mechanism^2–4 intending to better understand the influence of trace metals in the radical chemistry of OA. To support the kinetic model calculations, we aim for calculating bulk diffusion coefficients of different species as a function of RH based on viscosity measurements of our aerosol proxies with poke-flow and fluorescence recovery after bleaching experiments^5,6. This work uniquely includes possible redox reactions between iron and copper complexes, which may elucidate the role of photochemically active OA in the atmosphere.