AAAR 36th Annual Conference October 16 - October 20, 2017 Raleigh Convention Center Raleigh, North Carolina, USA
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Feedbacks between Atmospheric Aerosol Microphysics and Photochemistry of Iron Complexes
PABLO CORRAL ARROYO, Peter Aaron Alpert, Jing Dou, Beiping Luo, Ulrich Krieger, Markus Ammann, Paul Scherrer Institut
Abstract Number: 700 Working Group: Aerosol Chemistry
Abstract Physical and chemical transformations of atmospheric particles, known as aerosol aging, are key to understand their impact on climate, air quality and health. Photochemistry is one aging process in which chromophores in aerosol particles act as photocatalysts inducing oxidation of non-absorbing molecules. Iron (Fe(III)) carboxylate complexes absorb light below about 500nm, followed by ligand to metal charge transfer (LMCT). This results in the reduction to Fe(II) and oxidation of the carboxylate ligands, which represents an important sink of organic acids in the troposphere, and in the production of reactive oxygen species, HO2, H2O2 and organic peroxides. We investigate the turnover of these photocatalytic processes as it responds to changes in relative humidity, which drives kinetic limitations through the plasticizing effect of water in viscous highly oxidized organic particles. To achieve this scope we study the internal structure and oxidation spatial gradient within single aerosol particles before and after irradiation with UV light (370nm) by using an X-ray microspectroscopic method (Scanning Transmission X-Ray Microspectroscopy, STXM). We use the spatially resolved Fe(II) fraction as an observable for transport limitations of reactants and intermediates as a function of relative humidity. We also measure HO2 radical production and VOC production from irradiated macroscopic films of similar composition in Coated Wall Flow Tube (CWFT) experiments to obtain further information on the chemical mechanism and yields. Additional information comes from experiments in an Electrodynamic Balance (EDB), in which the overall mass loss and radius decrease is measured as a function of the same environmental variables as in the other techniques. A numerical model which includes chemical equilibria, reactions and diffusivities was created to analyze our data for different relative humidities. These data will be used to better understand aerosol chemical/structural evolution due to photochemical oxidation and quantify mechanisms and degradation kinetics under varying environmental conditions.