Archive | 2019
Photochemical Aging Processes in Iron Containing Aerosols
Abstract
Aerosol aging refers to the multitude of physical and chemical transformations that atmospheric particles undergo, which plays an important role in the impact on climate, air quality and public health. Out of many different aerosol types in the atmosphere, aqueous particles are of particular interest as photochemistry and radical reactions in the liquid phase are the main drivers of aerosol chemical conversions. Compared to the uptake of oxidants (e.g., •OH, O3) from the gas phase, which are often reacting at or near the surface, photochemically active radiation penetrates the entire volume of aerosol particles, generating radicals throughout their bulk. In the lower troposphere, where UV light intensity with sufficiently low wavelength to directly photolyze aerosol components is low, indirect photochemistry (catalyzing redox processes of non-absorbing molecules) is especially relevant. Important indirect photochemical processes are initiated by either transition metal complexes or photosensitizers, both acting as photocatalysts for aerosol oxidation. We choose iron(III)-citrate (FeIII(Cit)) induced citric acid (CA) degradation as a model system due to the abundance of carboxylate complexes and reactivity of FeIII(Cit) in the atmosphere, and because CA is a good proxy for carboxylic acids in secondary organic aerosol (SOA) and the thermodynamic properties of CA have been studied in detail. FeIII(Cit) absorption extends far into the blue spectral range, inducing the reduction of iron(III) to iron(II) and the oxidation of carboxylate ligands. In the presence of O2, ensuing production of •OH and HO2 will lead to more decarboxylation, production of oxygenated volatile organic compounds, as well as re-oxidation of iron(II) back to iron(III), closing this photocatalytic cycle, which is an important sink of organic acids in the troposphere. It has recently been recognized that aqueous aerosol particles may attain highly viscous, semi-solid, or even glassy states under a wide range of atmospheric conditions. While the impact of reduced molecular mobility in highly viscous particles on dark chemistry has received substantial attention over the last decade, systematic studies on the effect of high viscosity on photochemical processes are scarce. In order to fill the gap, we developed a multilayered photochemical reaction and diffusivity (PRAD) model targeted at the FeIII(Cit)/CA photochemistry system, to assess how viscosity and diffusivity influence photochemical processes. While FeIII(Cit) photochemistry is reasonably well established, there are still a number of ill-constrained parameters in the model, such as the diffusivity of CO2 and O2 in the citric acid solution, the re-oxidation pathways and their reaction constants of iron(II) to iron(III). Therefore, we conducted experiments with single aqueous FeIII(Cit)/CA particles levitated in electrodynamic balance to determine some of these