The role of aqueous-phase oxidation in the formation of highly-oxidized organic aerosol

Abstract

Atmospheric particulate matter (or "aerosol") is known to have important implications for climate change, air quality, and human health. Our ability to predict its formation and fate is hindered by uncertainties associated with one type in particular, organic aerosol (OA). Ambient OA measurements indicate that it can become highly oxidized in short timescales, but this is generally not reproduced well in laboratory studies or models, suggesting the importance of formation processes that are not fully understood at present. In this thesis, I focus on the potential for chemistry within aqueous aerosol to produce highly oxidized OA. I first use a retrosynthetic modeling approach to constrain the viable precursors and formation pathways of highly oxidized OA, starting with a target oxidized product and considering possible reverse reactions. Results suggest three general formation mechanisms are possible: (1) functionalization reactions that add multiple functional groups per oxidation step, (2) oligomerization of highly oxidized precursors, or (3) fast aging within the condensed phase, such as oxidation within aqueous particles. The focus of the remainder of the thesis involves experiments designed to study this third pathway. To examine the importance of the formation of highly oxidized OA in the aqueous phase (wet particles or cloud droplets), I investigate aqueous oxidation of polyols within submicron particles in an environmental chamber, allowing for significant gas-particle partitioning of reactants, intermediates, and products. Results are compared to those from analogous oxidation reactions carried out in bulk solution (the phase in which most previous studies were carried out). Both sets of experiments result in rapid oxidation, but substantially more carbon is lost from the submicron particles, likely due to differences in partitioning of early-generation products. Finally, OA is formed from the gas-phase ozonolysis of biogenic precursors in the presence of reactive aqueous particles, showing that oxidation within the condensed phase can generate highly oxidized products. The overall results of this thesis demonstrate that aqueous-phase oxidation can contribute to the rapid formation of highly oxidized OA and therefore its inclusion in atmospheric models should be considered, but that experiments to constrain such pathways must be carried out under atmospherically relevant conditions.