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Surface Ocean and Lower Atmosphere Study—Air-Sea interactions and their climatic and environmental impacts
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Iron Reoxidation in Photochemical Cycling
Wednesday 9th @ 1050-1110, Conference Room 7
Jing Dou* , ETH Zurich, Switzerland
Beiping Luo, ETH Zurich, Switzerland
Peter A. Alpert, Paul Scherrer Institute, Switzerland
Pablo Corral Arroyo, Paul Scherrer Institute, Switzerland
Markus Ammann, Paul Scherrer Institute, Switzerland
Ulrich K. Krieger, ETH Zurich, Switzerland
Thomas Peter, ETH Zurich, Switzerland
Presenter Email: jing.dou@env.ethz.ch |
| Iron (Fe(III)) carboxylate complexes in aerosol particles absorb light below about 500 nm followed by ligand to metal charge transfer (LMCT) which reduces Fe(III) to Fe(II) and oxidizes carboxylate ligands[1]. When O2 is present, production of radicals, peroxides and oxygenated volatile organic compounds (OVOC) ensues. Importantly, radicals (e.g., OH-, HO2- and RO2- reoxidize Fe(II) to Fe(III) and can then complex with neighboring acid groups closing a photocatalytic cycle. We investigated iron carboxylate catalyzed photochemistry by tracking mass and size changes of single, levitated organic aerosol particles in an electrodynamic balance (EDB) under visible (473 nm) light irradiation as a function of relative humidity (RH). Particle had an around 10 μm radius and contained Fe(III)-citrate in aqueous citric acid with a mole ratio of 0.05. A mass loss was observed during Fe(III)-citrate photochemistry due to the evaporation of volatile (e.g., CO2) and semi-volatile (e.g., ketones) products. To quantify Fe(II) to Fe(III) reoxidation we first exposed the particle to N2 and light until all Fe(III) reduced to Fe(II), and then switched off light and introduced O2. At 48% RH, we found that 10 hours exposure to O2 was sufficient for all Fe(II) to be reoxidized while at 24% RH, complete reoxidation after 25 hours was not yet achieved. We attribute the differences in recovery time to the O2 diffusion limitations (i.e., limited O2 availability for reoxidation) at lower RH. To better understand the interplay of photochemical reaction pathways and molecular diffusion in our system, we developed a numerical model to simulate Fe(III)-citrate photochemical cycling in single particles. Molecular diffusion coefficients of CO2 and O2 as a function of RH, and the oxidation rate of Fe(II) by O2 directly were derived using our model. With these well-defined and physically constrained parameters, we predict the evolution of products as well as organic acids degradation in the condensed phase under atmospheric conditions. |
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