The Fragmentation vs. Functionalization Paths of Molecules during Atmospheric Chemical Aging
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Throughout this thesis we explore the two major mechanisms that organic carbon compounds experience in the atmosphere when reacting with major oxidizers: fragmentation and functionalization. We start by exploring the photo-oxidation of n-aldehydes – including their chemistry and organic aerosol formation – because they are known to fragment strongly in their first-generation chemistry with the OH radical. It is found that this strong fragmentation path suppresses their ability to form OA when compared to similar n-alkanes. n-Aldehydes with fewer than thirteen carbons do not produce OA under atmospheric concentrations. We also study two sequences of molecules with different saturation concentrations that systematically increase in oxygenation. Higher oxygenation was found to suppress the ability of a molecule to form OA. The position of a functional group in a carbon backbone was also found to affect OA formation. Functional groups located in the center of a large carbon backbone suppress OA formation when compared to functional groups on the end (e.g., 7- and 2-tridecanol, respectively). In the sequences mentioned, pinonaldehyde is a key molecule studied since it is an important oxidation product of alpha-pinene which is one of the most emitted molecules from biogenic sources. Production of OA from pinonaldehyde confirms the importance of biogenic aging.
As complement to the experimental work, two different computer simulations are used as prognostic tools of OA formation for the chemical species presented in this thesis. The first simulation is the two-dimensional volatility basis set (2D-VBS) developed by Donahue and coworkers at Carnegie Mellon University. This box model predicts OA evolution of bulk organic aerosol systems by knowing initial oxygenation and saturation concentrations. The second one is GECKO-A developed by Aumont and co-workers from the University of Paris. Its approach consists of following every reaction and species involved in the chemistry of OA formation. Both simulations give reasonable results when compared to the experimental OA formation potential of different molecules presented in the first half of this thesis. This is despite their very two different approaches in predicting OA formation. We conclude that by properly considering fragmentation and functionalization paths, atmospheric OA formation can be reasonably predicted.