10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Molecular Diffusion Limitations Coupled with Aerosol Aging Initiated by Iron Citrate Photochemistry
PABLO CORRAL ARROYO, Peter Aaron Alpert, Jing Dou, Beiping Luo, Ulrich Krieger, Markus Ammann, Paul Scherrer Institut
Abstract Number: 691 Working Group: Aerosol Chemistry
Abstract Physical and chemical transformations that change the properties of atmospheric particles are known as aerosol aging. Understanding of these is essential to evaluate their impact on climate, air quality and health. Photochemistry may start aging processes by the presence of chromophores in aerosol particles that act as photocatalysts inducing oxidation of non-absorbing molecules [1]. Iron (Fe(III)) carboxylate complexes absorb light below about 500 nm followed by ligand to metal charge transfer (LMCT), the reduction of iron to Fe(II) and oxidation of the carboxylate ligands, a process that is recognized as an important sink of organic acids in the troposphere [2]. We investigate the feedbacks between these photocatalytic processes and molecular diffusion within aerosol particles. To achieve this scope, iron oxidation within mixed iron citrate and citric acid (1:1 molar ratio) particles is spatially mapped before and after irradiation with UV light (370 nm) using scanning transmission X-ray microscopy with near edge X-ray absorption fine structure spectroscopy (STXM/NEXAFS). This yields 2D-projected Fe(II) and Fe(III) fractions with 35x35 nm resolution. We observed gradients in the composition of single aerosol particles which indicates oxygen diffusion limitation. Additionally, we measure the release of HO2 radicals and volatile organic compounds (VOCs) to the gas phase when films loaded with iron citrate and citric acid in Coated Wall Flow Tube (CWFT) are irradiated with light. The nonvolatile compounds produced after irradiation are analyzed by HPLC-MS. Finally, we present a numerical photochemical and molecular diffusion model to explain our results. We have found that particles can be devoid of oxygen due to reacto-diffusive limitation and thus, maintain reduced particle cores despite being surrounded to the oxygen rich atmosphere. This is counter to a typical concept that oxygen saturates atmospheric aerosol.
[1] George G., Ammann M., D’Anna B., Donaldson D. J., Nizkorodov S. A., Heterogeneous photochemistry in the Atmosphere, Chem. Rev., 2015, 115 (10), pp 4218-4258. [2] Weller, C., Horn, S., and Herrmann, H.: Photolysis of Fe(III) carboxylate complexes: Fe(II) quantum yields and reaction mechanisms, Photochemistry and Photobiology A: Chemistry, 268, 24-36, 2013.