Doctoral Theses (2009 - )

Permanent URI for this collectionhttps://hdl.handle.net/1807/9945

The School of Graduate Studies (SGS) requires doctoral and masters graduands to submit a thesis written as a required element of their degree program in electronic format.

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SGS intends to house all available digitized Doctoral and Masters theses by U of T graduate students on this site. The current collection is but a small sample of that scholarly work.

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Chemical Fates of Oils Deposited on Indoor Surfaces
(2023-06) Zhou, Zilin; Abbatt, Jonathan; Chemistry
The COVID-19 pandemic has substantially increased the public awareness of indoor environments. Despite the global effort in reducing primary indoor air pollutants, especially emissions related to solid fuel combustion, there remains insufficient monitoring and understanding of secondary pollutant generation via indoor chemical reactions. In particular, the multiphase oxidation that occurs between airborne species and reactive material on indoor surfaces is an important contributor that can also impact occupant health and wellbeing. Transported from outside, ozone is the most important indoor oxidant. It rapidly reacts with unsaturated lipids that are commonly found on indoor surfaces contaminated by cooking oils and human skin lipids. However, such reactions have not been comprehensively characterized, especially in complex indoor environments. Here, I initially studied the heterogeneous ozonolysis of a pure representative lipid (triolein) in a controlled reactor. With mass spectral and quantitative NMR methods, our results indicate that triolein decays rapidly with ozone exposure, with stable secondary ozonides (SOZs) the major condensed-phase products. Known as the Criegee mechanism, the reaction products are strongly dependent on ambient relative humidity. Specifically, the SOZ molar yield peaks at ~80% under dry conditions, regardless of the ozone mixing ratio, whereas water vapor significantly reduces its formation and favors the release of volatile organic compounds (VOCs). This is due to water scavenging the Criegee Intermediates, and the resulting α-hydroxyhydroperoxides (α-HHPs) decompose into aldehydes and reactive H2O2. A kinetic multilayer model (KM-GAP) which implements this set of chemical reactions accurately simulates the yields of major products under indoor relevant conditions. After studying fundamental mechanisms, I characterized the chemical fate of commercial cooking oil on genuine indoor surfaces. While SOZs are the major products when oils are exposed to air, low-ozone dark locations lead to the slow formation of hydroperoxides, which cannot be explained by the Criegee mechanism. Additionally, indoor direct sunlight drives lipid peroxidation whose products are potential toxins. Overall, my indoor sampling studies indicate that autoxidation and/or photooxidation mechanisms of unsaturated oils also play an important role in indoor oxidative surface chemistry, depending on their rates relative to ozonolysis.
Radical Cloud Chemistry: Oxidation of Organic Compounds in Atmospheric Water
(2020-06) Hems, Rachel; Abbatt, Jonathan; Chemistry
Atmospheric aging of organic compounds by photo-chemical oxidation can occur rapidly in the aqueous phase in aerosol particles, cloud droplets, and fog droplets and can result in changes to their chemical composition and absorptivity. Despite their capacity to significantly alter the properties of organic aerosol, aqueous phase reactions have not gained as much attention as other atmospheric aging pathways. The goal of this thesis was to further our understanding of photoreactions of atmospherically-relevant organic compounds in cloud water. This goal was accomplished through three projects, which investigated factors influencing the production of hydroxyl (OH) radicals in aqueous phase reactions and the lifetime, oxidation mechanism, and fate of light absorbing molecules that contribute to brown carbon. In the first project, the impact of dissolved organic compounds, specifically from α-pinene ozonolysis secondary organic aerosol (SOA) material, on the Fenton reaction, a major source of OH radicals in cloud and fog water, was investigated. OH radical production was significantly suppressed in the presence of SOA material, likely due to complexation of the iron involved in the Fenton reaction. In the second project, aqueous photo-oxidation kinetics of three light-absorbing nitrophenol compounds often associated with wildfire smoke were examined and were found to have an in-cloud lifetime on the order of hours. Photo-oxidation initially caused an absorption enhancement that corresponded to products with added oxygenated functional groups. Further photo-oxidation led to products that do not absorb visible light, reducing their contribution to atmospheric warming. In the third project, representative and chemically complex brown carbon was generated in the laboratory from wood smoke. Light absorption increased from aqueous photoreaction and OH oxidation and was linked to oligomerization and functionalization of aromatic compounds, similar to the nitrophenol chemistry. While extended OH oxidation led to a decrease in absorbance, brown carbon in wood smoke is likely to have a longer lifetime than estimated by individual components. Overall, this work demonstrated key outcomes of organic chemical transformations resulting from aqueous atmospheric aging under relevant cloud conditions and provided implications for their effect on the climate system.

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Browsing Doctoral Theses (2009 - ) by Author "Abbatt, Jonathan"