Doctoral Theses (2009 - )

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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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Arctic Aerosol Sources and Continental Organic Aerosol Hygroscopicity
(2011-08-29) Chang, Rachel Ying-Wen ; Abbatt, Jonathan P. D. ; Chemistry
Atmospheric particles can affect climate directly, by scattering solar radiation, or indirectly, by acting as the seed upon which cloud droplets form. These clouds can then cool the earth's surface by reflecting incoming sunlight. In order to constrain the large uncertainties in predicting the ultimate effect of aerosol on climate, the sources of atmospheric particles and their subsequent ability to turn into cloud droplets needs to be better understood. This thesis addresses two parts of this issue: the sources of Arctic aerosol and the hygroscopicity of continental organic aerosol. Small particles were observed in Baffin Bay during September 2008 that coincided with high atmospheric and ocean surface dimethyl sulphide (DMS) concentrations suggesting that the aerosol formed from oceanic sources. An aerosol microphysics box model confirmed that local DMS could have produced the observed particles. In addition, the particle chemical composition was measured using aerosol mass spectrometry in the central Arctic Ocean in August 2008 and particles were found to be 43% organic and 46% sulphate. Factor analysis further apportioned the aerosol mass to marine biogenic and continental sources 33% and 36% of the time, respectively, with the source of the remaining mass unidentified. The second part of the study parameterises the hygroscopicity of the ambient organic aerosol fraction (κorg) at Egbert, Ontario and Whistler, British Columbia. This was done using two methods: 1) by assuming that the oxygenated organic component was hygroscopic and that the unoxygenated organic component was non-hygroscopic, κ of the oxygenated component was found to be 0.22 ± 0.04, and 2) by assuming that κorg varied linearly with the atomic oxygen to atomic carbon ratio, it could be parameterised as κorg = (0.29 ± 0.05) × (O/C). Calculations predict that knowing κorg is important in urban, semi-urban, and remote locations whenever the inorganic mass fraction is low.
The Atmospheric Fate of Organic Nitrogen Compounds
(2015-11) Borduas, Nadine; Abbatt, Jonathan P. D.; Murphy, Jennifer G.; Chemistry
Organic nitrogen compounds are present in our atmosphere from biogenic and anthropogenic sources and have impacts on air quality and climate. Due to recent advances in instrumentation, these compounds are being detected in the gas and particle phases, raising questions as to their source, processing and sinks in the environment. With their recently identified role as contributors to aerosol formation and growth, their novel large scale use as solvents in carbon capture and storage (CCS) technology and their emissions from cigarette smoke, it is now important to address the gaps in our understanding of the fate of organic nitrogen. Experimentally and theoretically, I studied the chemical atmospheric fate of specific organic nitrogen compounds in the amine, amide and isocyanate families, yielding information that can be used in chemical transport models to assess the fate of this emerging class of atmospheric molecules. I performed kinetic laboratory studies in a smog chamber to measure the room temperature rate coefficient for reaction with the hydroxyl radical of monoethanolamine, nicotine, and five different amides. I employed online-mass spectrometry techniques to quantify the oxidation products. I found that amines react quickly with OH radicals with lifetimes of a few hours under sunlit conditions, producing amides as oxidation products. My studies on amides revealed that they have much longer lifetimes in the atmosphere, ranging from a few hours to a week. Photo-oxidation of amides produces isocyanates and I investigated these mechanisms in detail using ab initio calculations. Furthermore, I experimentally measured isocyanic acid’s Henry’s Law constant as well as its hydrolysis rate constants to better understand its sinks in the atmosphere. Finally, I re-examined the structure-activity relationship (SAR) of organic nitrogen molecules for improved model parameterizations.
Laboratory Studies of Deposition Mode Heterogeneous Ice Nucleation: Effect of Ice Nuclei Composition, Size and Surface Area
(2010-02-18T20:48:29Z) Kanji, Zaminhussein Abdulali ; Abbatt, Jonathan P. D. ; Chemistry
The indirect aerosol effect contributes to major uncertainties in determining the radiation budget of the earth. A large uncertainty is due to the formation of ice clouds onto natural or anthropogenic aerosols. Field studies have shown that mineral type particles are often associated with ice crystals in the mid-upper troposphere and given the long residence time in the atmosphere of dust particles (~2 weeks in the absence of precipitation), their contribution to ice formation processes is not fully defined. In order to probe ice formation onto natural mineral dust in a setting where it could be suspended as aerosol, a new continuous flow diffusion chamber (CFDC) was built. This allowed investigations of the effects of total aerosol surface area and particle size. The CFDC was also used in an international inter-comparison of ice nucleation instrumentation to compare efficiencies of soot, biological aerosol (bacteria) and samples of natural desert dusts from different regions of the world. The laboratory observations were parameterized using nucleation rates (Jhet) and contact angles () as described by classical nucleation theory. For both this experimental technique and a static one developed during the candidate’s Masters degree, mineral dust particulate proved to be the most efficient ice nuclei (IN) activating at RH with respect to ice (RHi) as low as 105% at T = 233 K. The efficiency varied with particle size and aerosol surface area (SA). Large particles or higher SA activated at lower RHi than small particles or lower SA. The static chamber was sensitive to the first ice event out of a large SA and therefore gave true onset RHi, which was lower than the onset defined by the CFDC studies, which was not sensitive to a single ice event. In addition the static chamber used a broader size range of particulate matter, including super micron particles while the CFDC particles sizes were restricted to below 0.5 µm. Soot and organic coated dust particles were inefficient IN compared to pure dust. Soot aerosols showed some efficiency at T < 233K where deposition ice formation was apparent. The hygroscopic organics had intermediate ice activity between dusts and alkyl-organics and soot. Bacteria aerosols were active in the deposition mode for T as high as 247 K. Contact angles () computed for ice germs forming onto natural mineral dust were small, 7<  < 29, at 223 K for RH ranging from ice to water saturation. It was concluded that there is no single value for the onset of ice formation in the atmosphere via deposition freezing. The associated contact angles show that there is a distribution of active sites on IN and that not all active sites have the same affinity for initiation of ice formation even within the same aerosol type. This work provides evidence that deposition mode nucleation can be an alternate pathway to homogeneous nucleation when mineral aerosols are present in the troposphere since the high T - low RH conditions required for deposition mode nucleation are more easily encountered in the atmosphere than the low T - high RH required for homogeneous nucleation.
A Mechanistic Examination of Redox Cycling Activity in Carbonaceous Particulate Matter
(2013-08-09) McWhinney, Robert ; Abbatt, Jonathan P. D. ; Chemistry
Mechanistic aspects of carbonaceous aerosol toxicity were examined with respect to the ability of particles to catalyse reactive oxygen species-generating redox cycling reactions. To investigate the role of secondary organic material, we examined two systems. In the first, two-stroke engine exhaust particles were found to increase their ability to catalyse redox cycling in the presence of a reducing agent, dithiothreitol (DTT), when the exhaust was exposed to ozone. This occurred through deposition of redox-active secondary organic aerosol (SOA) onto the particle that was ten times more redox active per microgram than the primary engine particle. In the second system, naphthalene SOA formed highly redox active particles. Activity was strongly correlated to the amount of the 1,4- and 1,2-naphthoquinone measured in the particle phase. However, these species and the newly quantified naphthalene oxidation product 5-hydroxy-1,4-naphthoquinone accounted for only 30% of the observed DTT decay from the particles. Gas-particle partitioning coefficients suggest 1,4- and 1,2-naphthoquinone are not strong contributors to ambient particle redox activity at 25°C. However, a large number of redox active species are unidentified. Some of these may be highly oxidised products of sufficiently low vapour pressure to be atmospherically relevant. DTT activity of diesel particles was found to be high per unit mass. The activity was found to be associated with the insoluble fraction as filtration of the particles nearly eliminated DTT decay. Neither methanol nor dichloromethane extracts of diesel particles exhibited redox activity, indicating that the redox active species are associated with the black carbon portion of the particles. Examination of particle concentration techniques found that use of water condensation to grow and concentrate particles introduced a large organic artefact to the particles. Experiments with concentrated inorganic particles suggest that the source of this artefact is from irreversible uptake of water-soluble volatile organic compounds. Overall, carbonaceous redox active species can be thought of as a continuum from small, water-soluble species to redox active functionalities on elemental carbon backbones. In addition to clearly defined, quantifiable species, future research may need to consider examining broader chemical classes or redox-active chemical functionalities to overcome the inherent complexity of these constituents.
OH-initiated Heterogeneous Oxidation of Atmospheric Organic Aerosols
(2010-03-03T17:38:30Z) George, Ingrid Jennifer ; Abbatt, Jonathan P. D. ; Chemistry
The chemical aging of organic aerosols by OH-initiated heterogeneous oxidation was investigated using both model organic and ambient aerosol particles. Organic aerosol particles were exposed to OH radicals in an aerosol flow tube and the modification of their chemical composition and particle properties was studied. Overall, this work has shown that OH-initiated heterogeneous oxidation enhanced the degree of oxidation and the Cloud Condensation Nucleus (CCN) activity of organic aerosol particles for equivalent OH exposure timescales of a few days to a week. Aerosol Mass Spectrometer (AMS) measurements showed that the heterogeneous uptake kinetics of OH radicals onto model primary organic aerosols was efficient. The heterogeneous reaction of organic aerosols with OH led to the production of high molecular weight particle-phase species with the addition of multiple oxygenated functional groups. These results were consistent with the observed increase in particle density with OH exposure. With the exception of solid organic aerosols, the particle volume and mass of organic particles were reduced by less than 20% from OH oxidation at high OH exposures due to volatilization of particle-phase reaction products. The degree of oxidation of the organic fraction of urban ambient aerosols was significantly enhanced for an equivalent atmospheric OH exposure time of 4 days for a daily average atmospheric OH concentration of 2×106 cm-3. Ambient aerosol particles sampled from a sparsely populated, forested region were initially more oxygenated than the urban aerosol particles and did not become more oxidized from reaction with OH radicals. The modification of the hygroscopicity of model primary and secondary organic aerosols from chemical aging was investigated by measuring the CCN activity of organic aerosols exposed to OH radicals. Primary organic aerosols, initially CCN inactive, became as CCN active as secondary organic aerosols due to heterogeneous reaction, where surface tension reduction played a major role. The CCN activity for model secondary organic aerosols was also enhanced due to OH oxidation, but changes were less dramatic than for the model primary organic aerosols.
Role of Water in the Formation, Transformation and Fate of Secondary Organic Aerosol
(2015-06) Wong, Jenny Pui Shan; Abbatt, Jonathan P. D.; Chemistry
Particle-phase water is the most abundant atmospheric aerosol constituent, yet the importance of water for the formation, transformation and fate of secondary organic aerosol (SOA) remains not fully characterized. In order to address this knowledge gap, this thesis explores the role of water in SOA formation, chemical aging and fate as cloud condensation nuclei. The formation of SOA from the photooxidation of isoprene in the presence of various sulfate seed particles was investigated using a flow tube reactor. Under constant environmental conditions, particle-phase water was found to have the largest effect on the amount of SOA formed where this additional organic material was highly oxidized, likely arising from enhanced uptake of organic acids due to their high water solubility. The amount of high molecular weight compounds increases with acidity, suggesting the role of acidity in governing organic composition. The relative humidity (RH) dependence of SOA aging by photolysis was examined using particles containing water-soluble α-pinene SOA material in an environmental chamber at three RH conditions (5, 45 and 85 %). Photolysis led to substantial mass loss where the rate of mass loss increased with increasing RH, suggesting that moisture-induced changes in SOA phase have implications to particle reactivity. Aging of ambient SOA sampled at Whistler, British Columbia found that aging by both gas and aqueous-phase OH increased the degree of oxygenation and CCN activity of the organic material, confirming the hypothesis that there is a simple relationship between the hygroscopicity of organic aerosol and its oxygen-to-carbon ratio. Addition of various types of organic material onto sulfate particles resulted in the suppression of water uptake during droplet growth. Experiments using sulfate particles with different acidity suggest that high molecular weight compounds, formed via acid-catalyzed condensed phase reactions, are the species affecting water uptake kinetics.

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