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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Aqueous-phase Organic Chemistry in the Atmosphere
(2015-01) Zhao, Ran; Abbatt, Jonathan PD; Chemistry
Atmospheric aqueous phases (i.e. cloud, fog and aerosol liquid water) are important reaction media for the processing of organic compounds. Quantitative data on aqueous-phase organic chemistry under the atmospheric context are sparse compared to the data from comparable gas-phase processes. A series of studies was conducted to provide fundamental information on this topic. An online analytical technique, Aerosol Chemical Ionization Mass Spectrometry (Aerosol CIMS), was employed to quantitatively monitor aqueous-phase OH oxidation of glyoxal and methylglyoxal. Quantification of all the major reaction products was achieved, permitting complete reaction mechanisms to be presented. An unexpected class of compounds, α-hydroxyhydroperoxide (α-HHP), was observed during the experiments, and the formation of this class of compounds was further quantified. Formation of α-HHP can be potentially important in aerosol liquid water, affecting aerosol toxicity and the gas-particle partitioning of small aldehydes. Aerosol CIMS was further applied to the OH oxidation of levoglucosan, using a high mass resolution CIMS. Unique reaction trends were observed, from which novel mechanism analysis frameworks were introduced. Aqueous-phase photochemical processing of a variety of light-absorbing organic compounds (Brown Carbon (BrC)) was investigated in the laboratory. The light absorptivity of BrC was significantly altered via these processes, indicating that the chemical processing of BrC species needs to be considered for a sound assessment of their atmospheric implications. Finally, cloud-partitioning of a toxic compound, isocyanic acid (HNCO), was investigated in the field, representing the first online measurement of gas-phase compounds dissolved in cloudwater. A secondary source of HNCO in the ambient air was also observed. Overall, aqueous-phase chemistry leads to reaction products which likely contribute to secondary organic aerosol (SOA) formation. At the same time, aqueous-phase chemistry also facilitates the transformation and removal of specific species with atmospheric significance (e.g. tracer compounds, pollutants, BrC).
Natural Sources of Volatile Organic Compounds to the Summer Arctic Troposphere
(2018-03) Mungall, Emma Louise; Abbatt, Jonathan PD; Chemistry
Due to chemistry-climate coupling, observations of chemical processes in the atmosphere are crucial to improving understanding of the Earth system and enabling future climate predictions. Quantifying the effect on climate of current and future anthropogenic influence requires first understanding natural processes. This task is complicated by the interactions between natural and anthropogenic emissions. Because the summer Arctic experiences limited anthropogenic influence, it is considered an analog for some pre-industrial atmospheres. Due to its remoteness, observations in the region are scarce. The goals of this thesis were to 1) make observations of the tropospheric composition of the summer Canadian Arctic Archipelago and 2) improve understanding of the sources of volatile organic compounds (VOCs) in the region. To achieve the first goal, two new datasets were collected by field deployment of chemical ionization mass spectrometers. The second goal was addressed through analysis of the newly collected data sets, modeling experiments, and laboratory experiments. This work confirmed that local marine sources are the major contributors to high levels of dimethyl sulfide (DMS) in the summer Arctic, and supported the hypothesis that DMS emissions from melt ponds on top of the sea ice may be significant. Unexpectedly, we found that heterogeneous chemistry at the sea surface microlayer emits large quantities of formic acid to the atmosphere, as well as smaller amounts of many other OVOCs, some of which may play a role in the formation of secondary organic aerosol. Methane sulfonic acid in aerosol particles, which has traditionally been considered a conservative tracer for DMS, was shown to be degraded by heterogeneous oxidation during atmospheric transport. Finally, the presence of high levels of formic and acetic acids in the summer Arctic calls into question the current understanding of the sources of these acids. In summary, this thesis begins to paint a clearer picture of the chemical composition of the summer Arctic troposphere while emphasizing that further measurements are badly needed to bring that picture into focus.
Pollen Ice Nucleating Particles and Their Response to Atmospheric Processing
(2020-11) Gute, Ellen; Abbatt, Jonathan PD; Physical and Environmental Sciences
Clouds are crucial to life on Earth as they help regulate temperature and distribute water resources. Yet, large uncertainties exist around cloud development and evolution with many open questions about processes spanning from micro- to macro-scales. On the micro-scale, ice formation contributes to precipitation formation and cloud radiative properties; yet our understanding of this process is limited. This thesis assesses the ice nucleating (IN) ability of pollen and subpollen particles (SPPs) under mixed-phase and cold cloud conditions. In a first step, pollen IN activity was investigated under mixed-phase cloud conditions where IN activity was found to vary between pollen types and that submicron SPPs drive pollen IN activity. Additionally, one pollen type, grey alder, was identified to initiate freezing above −10°C, thus contributing to a small set of highly active ice nucleating particles (INPs). In a second step, atmospheric processing of SPPs was investigated, as SPPs can experience atmospheric transport and exposure to varying environmental conditions. Several processing pathways were found to compromise pollen IN activity. The largest impacts were observed for the most IN active pollen by exposure to simulated solar radiation. Under cold cloud conditions, chemical oxidation negatively impacts the IN activity of silver birch and grey alder pollen, occurring on a similar timescale as changes to the molecular structure were observed. The findings from laboratory INP measurements were translated to parametrize INP concentrations from SPPs, indicating INP concentrations below 10-2 m-3 in one modeling scenario. To overcome potential barriers in detecting such low INP concentrations by commonly used INP instruments, a Portable Fine Particle Concentrator was evaluated for INP measurements in the field. Enrichment factors are found to be particle size dependent while an INP enrichment factor of 16±5 was achieved in a mountain-top field study. The insights gained from this thesis will help assess the importance of pollen as INPs and reduce uncertainties that cloud processes currently impose on weather and climate predictions.

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