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Great Supervisor Week Mentions
Allan is an incredibly supportive supervisor. He never makes you feel like you are not smart (even if you ask silly questions!) and always keeps you optimistic about your project when you have doubts about whether things will work out or not. We are very lucky to have him as a supervisor!
Graduate Student Supervision
Doctoral Student Supervision (Jan 2008 - May 2021)
No abstract available.
The diffusion coefficients of large and small molecules in organic-water mixtures are important to atmospheric chemistry, as organic-water mixtures are useful proxies for atmospheric organic aerosol. Diffusion coefficients of large molecules (molecules with radius Rdiff ≥ the radius of the organic matrix molecule, Rmatrix) and small molecules (Rdiff > Rmatrix) are combined with the Stokes-Einstein relation to calculate the viscosity of an organic-water mixture, resolving a discrepancy in the literature between two previously published viscosity data sets.The results presented here increase our ability to quantify the relationship between diffusion of large and small molecules and the viscosity of organic-water mixtures, which are useful proxies for atmospheric organic aerosol.
Ice nucleating particles (INPs) are particles that cause heterogeneous ice nucleation in the atmosphere. INPs affect the formation and properties of ice and mixed-phase clouds and therefore influence the radiative forcing of the Earth-atmosphere system. However, the climatic effect of INPs is poorly understood, in part, because the concentrations, properties, and sources of INPs are not well understood, especially at remote locations.In the following dissertation, the concentrations, properties, and sources of INPs in remote Canadian environments are investigated. The environments studied included three coastal marine sites (two at mid-latitude and one in the Arctic), one ground site in the Arctic boundary layer, and the Arctic free troposphere. The concentrations of INPs at -25 oC were found to range from 0.01 to 3 L-1, and the INP concentrations measured in the Arctic were lower than that at mid-latitude.At the three coastal marine sites, the ice nucleating ability of aerosol particles was found to be dependent on the particle size with larger particles being more efficient at nucleating ice. Mineral dust was likely a major component of the supermicron INPs, and sea spray aerosol was not likely the major source of INPs at these sites. At the ground site in the Arctic boundary layer, INP concentrations at -25 oC were correlated with tracers of mineral dust, anti-correlated with tracers of sea spray aerosols, and not correlated with tracers of anthropogenic aerosols, which suggest that mineral dust was a major contributor to the INP population at this site. The majority of the particles collected in the Arctic free troposphere were mineral dust, and aluminosilicates and silicates were the major mineral types. A large fraction of the mineral dust was internallymixed with inorganic species (e.g., sea salt and sulfates). Particle dispersion modelling suggested iiithat mineral dust particles collected at both ground level and in the free troposphere were transported over long distances from East Asia.The results presented in this dissertation increase our understanding of the concentrations, properties, and sources of atmospheric INPs, and should be useful to constrain models of INPs.
Ice nucleating particles (INPs) in the Arctic can influence climate and precipitation in the region; yet our understanding of the concentrations and sources of INPs in this region remain uncertain. The following dissertation investigates 1) the properties and concentrations of INPs in the sea surface microlayer and bulk seawater samples collected in the Canadian Arctic, and 2) the source region of measured concentrations of INPs in the Canadian Arctic marine boundary layer. All measurements were made in the Canadian Arctic on board the CCGS Amundsen during the summers of 2014 and 2016.INPs were ubiquitous in the microlayer and bulk seawater samples, and were likely heat-labile biological materials between 0.2 and 0.02 μm in diameter. There was a strong negative correlation between salinity and freezing temperatures, and a strong positive correlation between the fraction of meteoric water in each sample and freezing temperatures, possibly due to INPs associated with terrestrial run-off. Spatial patterns of INPs and salinities in 2014 and 2016 were similar. However the concentrations of INPs were higher on average in 2016 compared to 2014, and INP concentrations were enhanced in the microlayer compared to bulk seawater in several samples collected in 2016.Average concentrations of INPs measured in the Canadian Arctic marine boundary layer fell within the range of INP concentrations measured in other marine boundary layer locations. The ratio of measured mineral dust surface area to sea spray surface area ranged from 0.03 to 0.09. Based on these ratios, and the ice active surface site densities of mineral dust and sea spray aerosol determined in previous laboratory studies, mineral dust is a more important contributor to the INP population than sea spray aerosol for the samples analysed. Based on particle dispersion modelling, the source of INPs in the Canadian Arctic marine boundary layer during the summer of 2014 was from continental regions such as the Hudson Bay area, eastern Greenland, or northwestern continental Canada.
This dissertation studies the silica/water interface using sum frequency generation spectroscopy. The effects of alkali chloride ions and temperature on the hydrogen bonding network at the interface are examined. We observed that the structure of water in the Stern layer depends on the identity of the cation. The ability of a cation to displace the hydration water on silica surface is in the order of Mg²⁺ > Ca²⁺ > Li⁺ > Na⁺, consistent with the trend of the acid dissociation constant of the salt. We conclude that ions with a high pKa, such as Mg²⁺ and Ca²⁺, have a local electrostatic field strong enough to polarize water molecules in the hydration shells of the ions. These partially hydrolyzed water molecules form linkages with the negative charges on the silica, forming solvent shared ion pairs. During freezing of pure water, we observed a transient phase of ice at water/mineral interfaces, which had enhanced IR-visible sum frequency generation intensity for several minutes. Most forms of ice are centrosymmetric but a possible explanation of for the transient phase is the formation of stacking-disordered ice during the freezing process. Stacking-disordered ice, which has only been observed in the bulk ice at temperatures lower than -20 °C, is a random mixture of layers of hexagonal ice and cubic ice. The transient phase at the ice/mineral interface was observed at temperatures as high as -1 °C. This observation suggests that the mineral surface may play a role in promoting the formation of the stacking-disordered ice at the interface. The effect of ions during freezing at the silica/water interface was investigated. Ice is the first phase to form. NaCl·2H₂O forms below the eutectic temperature, indicating that the formation and growth of ice does not push the ions out of the interfacial region. We compared the surface freezing diagram with the bulk equilibrium phase diagram of aqueous sodium chloride solutions. Although the concentration of ions is higher at a charged surface, we observe that freezing point depression at the surface is analogous to freezing point depression for homogeneous freezing and bulk equilibrium phase diagram.
Ice nucleating particles (INPs), which are a small fraction of the total aerosol population, are capable of catalyzing ice formation under atmospheric conditions. INPs may therefore influence the development, albedo, and lifetime of mixed-phase and ice clouds, and ultimately indirectly effect climate. As this aerosol indirect effect represents one of the largest sources of uncertainty in our understating of climate processes, measurements that quantify and characterize the atmospheric INP population are needed. The micro-orifice uniform deposit impactor-droplet freezing technique (MOUDI-DFT) was developed to measure INP concentrations in the atmosphere as a function of size and temperature in the immersion mode. The first campaign using the MOUDI-DFT was conducted in a Colorado forest. The concentrations of INPs and bioparticles were increased and correlated during and following rainfall events, and their size distributions were similar. This indicates that rainfall-associated mechanisms of bioparticle release may influence the abundance and efficiency of INPs in this region. The MOUDI-DFT was next used at a coastal site in Western Canada. INP concentrations were strongly correlated with those of fluorescent bioparticles and the size distributions of these particles were similar, suggesting bioparticles were an important source of INPs during this study. Despite the predominance of marine air masses, no evidence of a marine INP source was found. Six parameterizations of ice nucleation were tested and found to be poor predictors of the measured INP concentrations, identifying a need to develop INP parameterizations appropriate for coastal environments. Finally, size-resolved INP measurements from six ground-level sites in North America and one in Europe were presented, covering Arctic, alpine, coastal, marine, agricultural, and suburban environments. On average, 78 % of INPs were supermicron in size and 53 % were in the coarse mode (> 2.5 micrometers). Large particles were therefore a significant component of the ground-level INP in these diverse locations. The results presented in this dissertation increase our understanding of atmospheric INP concentrations, composition, and size. This information can be used to constrain INP sources, improve modeling of their long-distance transport and related indirect climate effects, and determine the ability of existing instrumentation to capture the full INP population.
Water droplets in the atmosphere do not freeze homogeneously until -38ºC. Freezing at warmer temperatures requires heterogeneous ice nuclei (IN). Despite the importance of ice in the atmosphere, little is known about the microscopic mechanisms of heterogeneous ice nucleation. This thesis employs molecular dynamics simulations to investigate ice nucleation by silver iodide, kaolinite, potassium feldspar, gibbsite, and a protein. Silver iodide is one of the best known ice nucleating agents. We examined seven surfaces of silver iodide and observed ice nucleation on three surfaces. The surfaces that nucleated ice organized the first layer of water molecules into a configuration resembling ice, such as chair conformed hexagonal rings. Surfaces that do not nucleate ice do not organize water into icelike configurations, such as planar rings. Results suggest lattice mismatch is insufficient in predicting ice nucleation, and a finer atomistic match is required. Finite silver iodide disks and plates were used to probe the relationship between the size of a nucleating surface and maximum temperature of ice nucleation. Larger disks nucleated ice at warmer temperatures than smaller disks by forming larger initial cluster of ice which could reach the critical size easier than homogeneously formed clusters. Kaolinite is a common clay known to nucleate ice. Our simulations investigated both sides of the (001) surface and found both sides able to nucleate ice. The Al-surface was simulated with varying degrees of freedom of motion. An optimum amount of movement was required to nucleate ice as the surface needs to adapt into a configuration favorable to ice. Ice nucleated on the Si-surface via the formation of a novel composite surface structure which facilitated bulk ice nucleation. Potassium feldspar simulations explored three variations of the two primary cleavage planes. All surfaces failed to nucleate ice and density profiles suggest that the surfaces are unlikely to nucleate ice. We succeeded in nucleating ice on gibbsite with prepared surface conformations compatible with ice. Biological IN, such as ice nucleation proteins, are among the most efficient IN. We attempted to simulate ice nucleation via a protein, but were unable to achieve ice nucleation.
Aerosols are ubiquitous throughout the Earth’s atmosphere, and secondary organic material (SOM), which is produced from the oxidation of volatile organic compounds, is estimated to constitute a significant fraction of atmospheric aerosol mass. Furthermore, particles containing SOM can cause negative health outcomes, and affect Earth’s climate, both directly by scattering solar radiation, and indirectly by acting as nuclei for cloud droplets.Despite the importance of particles containing SOM, their physical properties, such as viscosity, are poorly constrained. To address this knowledge deficit, a technique to measure the viscosity of small samples of material, similar to that produced in environmental simulation chambers, was developed and validated. This technique was then used to measure the viscosity of SOM produced via the ozonolysis of α-pinene in an environmental simulation chamber. The viscosity of this material was found to depend strongly on the relative humidity (RH) used when measuring viscosity and the concentration of SOM mass at which the SOM was produced. A difference between the viscosity of the water-soluble component of SOM and the total SOM (water-soluble and water-insoluble components) was also observed.The viscosity of saccharides and a tetraol were subsequently measured, with these compounds serving as proxies of highly oxidized components of SOM found in the atmosphere. For saccharides, viscosity was determined to increase by at least four orders of magnitude as molar mass doubled. In addition, the tetraol was determined to have a viscosity at least two orders of magnitude lower than that of SOM produced via the oxidation of isoprene, in which the tetraol has been identified.Finally, literature viscosity data for organic compounds was used to demonstrate that saturation vapour concentration, the mass based equivalent of saturation vapour pressure, is a useful parameter for predicting viscosity, and better than elemental oxygen-to-carbon ratio or molar mass, at least for organic compounds containing only one or two functional groups.The results presented in this dissertation increase our knowledge of the viscosity of SOM, and its dependence on RH, the SOM mass concentration at which the SOM is produced, number of hydroxyl functional groups in the organic molecule, and molar mass.
Ice nucleation occurs throughout the atmosphere. Some atmospheric ice particles are formed through nucleation on insoluble atmospheric aerosols known as ice nuclei (IN). The abundance and chemical composition of these IN affect the properties of clouds and in turn the radiative balance of the Earth through the indirect effect of IN on climate. The indirect effect of IN on climate is one of the least understood topics in climate change. A better understanding of ice nucleation and better capabilities to parameterize ice nucleation are needed to improve the predictions of the effect of IN on climate. Using a temperature and humidity controlled flow cell coupled to an optical microscope, the ice nucleation properties of three different mineral dust particles are examined in two different freezing modes. Results showed that the freezing ability of supermicron dust particles is lower than that of submicron dust particles of the same type. These freezing results along with literature freezing results of nine biological aerosol particles are used to evaluate different schemes used to parameterize ice nucleation in atmospheric models. These schemes are evaluated based on the ability to reproduce the laboratory freezing results. It was found that a single parameter scheme based on classical nucleation theory was unable to reproduce the freezing results of all particles studied. However, more complex schemes were able to reproduce the freezing results.The results in this thesis can be used by atmospheric modellers to improve predictions of mixed-phase and ice clouds and climate change.
Black carbon is a subset of the total atmospheric aerosol population that is formed in the incomplete combustion of fossil fuels, biofuels, and biomass. This research focused on the properties of black carbon particles measured in the boundary layer and free troposphere, as well as the activation of black carbon particles by cloud droplets. The primary motivation for this research is to increase our understanding of the properties of black carbon under these different atmospheric conditions.A single particle soot photometer was used to study properties of black carbon particles incorporated into cloud droplets at two field locations: 1) a marine boundary layer site, and 2) a high elevation mountain site. At both sites, a size dependence on the fraction of black carbon incorporated into cloud droplets was observed; and for small (
Aerosol particles containing both organic material and inorganic salts are abundant in the atmosphere. These particles may undergo phase transitions when the relative humidity fluctuates between high and low values in the atmosphere. This dissertation focuses on liquid-liquid phase separation in atmospherically relevant mixed organic-inorganic salt particles. Liquid-liquid phase separation has potentially important implications in chemical and physical processes in the atmosphere. A humidity and temperature controlled flow cell coupled to either an optical, fluorescence, or Raman microscope was used to study the occurrence of liquid-liquid phase separation and the phase separation relative humidity (SRH) of particles containing atmospherically relevant organic species mixed with inorganic salts. Organic species in the particles studied include single organic species, such as carboxylic acids, alcohols, and oxidized aromatic compounds, as well as complex laboratory-produced secondary organic material. Material directly collected from the atmospheric environment was also studied. In this dissertation, the effects of oxygen-to-carbon elemental ratio (O:C) of the organic species, salt types, molecular weight of the organic species, and temperature on the occurrence of liquid-liquid phase separation and SRH were studies. The oxygenic-to-carbon elemental ratio was a useful parameter for predicting liquid-liquid phase separation and SRH. Liquid-liquid phase separation did not depend strongly on the molecular weight of the organic species or temperature. The correlation between SRH and O:C in particles containing organic species mixed with different salts were qualitatively similar. Results of this research will help improve the understanding of liquid-liquid phase separation in the atmospheric aerosols, and may, in turn, improve simulations and predictions of atmospheric chemistry and climate. Supplementary materials: http://hdl.handle.net/2429/50970
In the atmosphere, ice can form on solid aerosol particles called ice nuclei. This research focuses on the ice nucleation properties of mineral dusts and biological particles. The motivation for this research is two-fold. First, ice nucleation on these aerosols may influence cloud formation, cloud reflectivity and precipitation patterns in what is an indirect climate effect. This effect is one of the largest uncertainties in current climate models. Second, ice nucleation may be an important removal mechanism for these particles from the atmosphere and may influence their long-distance transport. Currently, ice nucleation is represented in a simplistic manner or not at all in models used to predict the long-distance transport of aerosols.A temperature and humidity controlled flow cell coupled to an optical microscope was used to study the ice nucleation properties of four mineral dusts, eighteen fungal spores, and six bacteria. It was found that acidic coatings reduce the ice nucleating ability of the mineral dusts. The fungal spores showed a wide range of ice nucleating properties and there was no inherent difference in the ice nucleation ability of spores belonging to different taxonomic groups. Four of the bacteria studied were very poor ice nuclei and the fifth bacterium was an excellent ice nucleus.The results from the flow cell experiments on fungal spores were used to describe ice nucleation in two modeling studies that simulated atmospheric transport. One study found that a significant fraction of large fungal spores (20 micrometers in diameter) can reach high altitudes where they could act as ice nuclei. The other study focused on smaller spores (3 to 8 micrometers) and found that ice nucleation on these spores effects their long-distance transport to polar and marine regions.The laboratory results were used to show that mineral dusts are more important than the fungal spores or bacteria that were studied on a global annual scale. These results can be used to improve parameterizations of ice nucleation on mineral dusts and biological particles in future modeling studies investigating the indirect effect of aerosols on climate or the long-distance transport of aerosols in the atmosphere.
Aerosol particles are ubiquitous throughout the atmosphere and play an important role in human health, climate, and the chemistry of the atmosphere. A significant mass fraction of these particles is composed of organic species, which remain poorly characterized due to the number and diversity of species present. This thesis describes the development and characterization of two versions of a new single particle mass spectrometer with a 3D ion trap for organic aerosol studies.Version I combines CO₂ laser desorption and electron impact ionization in an ion trap. Mass spectra obtained for four species are comparable to NIST EI spectra. Tandem mass spectrometry studies are also demonstrated. The effects of vaporization energy, ionization delay time, and electron pulse width on the mass spectra and fragmentation patterns are examined. The detection limit of the instrument is found to be ~1x10⁸ molecules (350 nm diameter particle) for 2,4-dihydroxybenzoic acid. Version II integrates CO₂ laser desorption and tunable VUV ionization in an ion trap and was used for a detailed study of oleyl alcohol, oleic acid and mixtures thereof. Both the degree of fragmentation in the mass spectra and the translational energy of the vaporized molecules are found to vary as a function of desorption energy in the pure particles and as a function of composition in the mixed particles. These changes can be described by the energy absorbed per particle during desorption. We show that these effects hinder the quantitative response of the instrument and have important implications for other two step laser desorption/ionization systems.The final part of this thesis presents preliminary results from atmospherically relevant particles. Mass spectra of cigarette sidestream smoke, fulvic acid, meat cooking, and ammonium bisulfate aerosols are collected using both versions of the instrument. The two step desorption/ionization process only worked for two types of aerosols, while CO₂ only mass spectra were obtained for all four aerosol types. The suitability of CO₂ desorption strongly depended on particle composition, which will affect the applicability of the technique to atmospherically realistic aerosols. The results also suggest that CO₂ only laser desorption/ionization may be useful for field studies.
Grand canonical Monte Carlo calculations are used to determine water adsorption and structure on kaolinite surfaces, with and without the presence of trench-like defects, as a function of relative humidity (RH), at 235 K and 298 K. Both basal planes (the Al- and Si-surfaces), as well as two edge-like, defect free surfaces are considered. The trenches simulated are rectangular in geometry, and have a fixed depth and varying width. Thegeneral force field CLAYFF is used together with the SPC/E and TIP5P-E models for water. At both 235 K and 298 K, the edges, Al-surface, and trenches adsorb water at sub-saturation, in the atmospherically relevant pressure range. The Si-surface remains dry up to saturation. Both edges and the Al-surface adsorb water up to monolayercoverage. Adsorption on the Al-surface exhibits properties of a first-order process with evidence of collective behavior, whereas adsorption on the edges is essentially continuous and appears dominated by strong water lattice interactions. Only next to the Al-surface, were hexagonal rings observed in the water layer. However, they did not match hexagonal ice Ih. The results obtained using trenches show that the granularity of the surfaces can play a major role in the adsorption of multiple layers of water over a large range of RH. Our calculations suggest that water adsorption in trenches, and possibly in other similar defects, can offer an explanation of the large water coverages reported experimentally. Related to ice, the very dense, proton ordered, ferroelectric structures found in the trenches at235 K do not correspond to any recognizable form of bulk ice. We speculate how these structures might aid ice nucleation and growth, and suggest how this possibility could be further explored with simulations and experiments.
No abstract available.
Atmospheric particles play a crucial role in climate, visibility, air pollution, and human health. Reactions between gas-phase molecules and particles (heterogeneous reactions) affect not only the particle composition and morphology, but also the composition of the atmosphere. This thesis investigates the heterogeneous chemistry of organic mixtures and inorganic solutions coated with organic monolayers as proxies for atmospheric particles. The first topic of interest was the reaction between N₂0₅ and aqueous inorganic solutions coated with organic monolayers. The goal of this work was to better understand how organic monolayers on aqueous particles affect the mass transport and kinetics of N₂0₅ uptake by aqueous aerosols, and consequently what effectthe monolayer can have on predicted concentrations of N₂0₅ in the atmosphere.To investigate heterogeneous reactions of inorganic solutions coated with an organic monolayer a new rectangular channel flow reactor was developed. This newly developed flow reactor was described in detail and validated. Subsequently, the new flow reactor was used to study the reactive uptake of N₂0₅ on sulfuric acid solutions in the presence of a variety of 1- and 2-component monolayers with varying functional groups, solubilities, chain lengths, surface pressures, and molecular surface areas. Reactive uptake of N₂0₅ on aqueous sulfuric acid solutions was found to correlate most strongly with the molecular surface area or packing density of the monolayer. These results provide a good foundation for determining the influence of monolayers on heterogeneous reactions in the atmosphere, and highlight the need for characterization of monolayer surface properties of organic monolayers present on atmospheric particles.The second topic of interest was reactions between 0₃ and proxies for meat cooking aerosols with the goal to better understand the effect of the phase and microstructure of the mixtures on the lifetime of oleic acid (OA) in atmospheric particles. The reactive uptake of 0₃ was approximately 1 order of magnitude slower on binarysolid-liquid mixtures and multicomponent mixtures that closely represent compositions of meat-cooking aerosols compared to the liquid solutions. Lifetimes up to 75 min were obtained for these mixtures.
Master's Student Supervision (2010 - 2020)
Atmospheric particles play a significant role in the atmosphere and climate. To be able to better predict their rate of growth and reactivity, information on diffusion coefficients of organic molecules in these particles is needed.Diffusion coefficients of organic tracer molecules were measured in sucrose water solutions, which served as proxies for atmospheric particles. Diffusion coefficients of rhodamine 6G (hydrodynamic radius = 5.89 Å), an organic tracer molecule, ranged from 1.50∙10-¹⁴ to 4.73∙10-⁹ cm²/s as the water activity ranged from 0.38 to 0.8, respectively, in sucrose water solutions. The measured diffusion coefficients were compared to diffusion coefficients calculated using the Stokes-Einstein equation, which has often been used in the past to estimate diffusion coefficients in atmospheric particles. A breakdown of the Stokes-Einstein equation was observed at a water activity of 0.38 or a viscosity of 3.3∙10⁶ Pa s or Tg/T of 0.94 (Tg being the glass transition temperature and T the temperature of the measurement).In addition, diffusion coefficients of intrinsic fluorophores were measured in brown limonene secondary organic material (SOM) which is thought to be important in the atmosphere. The SOM was generated by the oxidation of limonene followed by aging with NH₃ vapour. The diffusion coefficients of the intrinsic fluorophores in brown limonene SOM ranged from 3.82∙10-¹² to 8.32∙10-⁹ cm²/s as the water activity ranged from 0.38 to 0.9. From the diffusion coefficients mixing times of large organics in brown limonene SOM were calculated, which ranged from 0.001 to 2.6 s across water activities of 0.9 and 0.38. Hence, mixing times of large organics are short and atmospheric particles consisting of brown limonene SOM are well mixed in the atmosphere when water activities range from 0.38 to 0.9.Furthermore, the hydrodynamic radius of the intrinsic fluorophores within brown limonene SOM was determined to be 7.34 ± 1.47 Å. This hydrodynamic radius is consistent with suggested structures for the intrinsic fluorophores.
Recently, rates of molecular diffusion of organic species within organic-water particles of atmospheric relevance have become an area of intense research. This is because molecular diffusion rates are required for predicting rates of growth and reactivity of organic-water particles in the atmosphere. Due to the shortage of information on the topic, fluorescence recovery after photobleaching (FRAP) was used to measure the diffusion coefficients of three organic tracer dyes in sucrose-water aqueous solutions that serve as proxies for organic-water particles in the atmosphere. Organic tracer dyes used were fluorescein isothiocyanate (FITC)–dextran (molecular weight (MW) of 1.50x10⁵ g/mol, hydrodynamic radius (RH) of 83.1 Å), calcein (MW of 622 g/mol, RH = 7.4 Å) and fluorescein sodium salt (fluorescein) (MW of 376 g/mol, RH = 5.02 Å). For FITC-dextran, diffusion coefficients ranging from 12.6-1.53x10-² µm²/s were measured for water activities (aw) ranging from 0.99 to 0.75. For calcein, diffusion coefficients ranging from 4.10-1.65x10-³ µm²/s were measured for aw from 0.88 to 0.65. For fluorescein, diffusion coefficients ranging from 7.09-2.51x10-⁴ µm²/s for aw ranging from 0.88 to 0.50. The results in this dissertation showed that Stokes-Einstein equation is still valid for molecules at the size scale of fluorescein in sucrose-water mixtures when the aw ≥ 0.50. This corresponds to viscosities ≤ 10⁴ Pa·s and Tg/T ≤ 0.87. This is consistent with the previous studies by Champion et al. (1997) who also observed consistency between the Stokes-Einstein equation and measurements when Tg/T ≤ 0.86 when studying diffusion of fluorescein in sucrose- water mixtures. However, the results are inconsistent with the studies by Corti et al. (2008a) who showed decoupling between the Stokes-Einstein equation and viscosity measurements when Tg/T > 0.65.
Atmospheric heterogeneous reaction could modify the physical properties and chemical composition of aerosol particles, thereby affecting their roles in climate, air pollution, and human health. The current knowledge of heterogeneous reactions between nighttime gas-phase oxidants and organic aerosols is very limited. The goal of this thesis was to contribute to the understanding of such reactions, particularly to the kinetics of NO₃ and N₂O₅ reactions with a variety of organic substrates and mixtures that can serve as proxies for aerosol particles.In the first series of studies, we investigated the reactive uptake of NO₃ with binary organic mixtures of an unsaturated organic (methyl oleate) and saturated molecules as “matrix molecules”. For liquid mixtures, the uptake coefficients (γ) of the matrices were significantly increased with the addition of small amounts of methyl oleate. The increase can vary by a factor of 5 depending on the type of matrix used. For solid-liquid mixtures, the uptake coefficient decreased by a factor of 10 after exposure to NO₃ for 90 minutes. For all liquid mixtures, the atmospheric lifetime was estimated on the order of a few minutes (with an upper limit of 35 mins), whereas, for liquid-solid mixtures, a lower limit to the lifetime was estimated to be 1-2 hours.In the second series of studies, we investigated NO₃ uptake on solid tridecanal and the uptake on liquid binary mixtures containing tridecanal and saturated organics as matrices. Uptake on the solid tridecanal was shown to be efficient, with γ = (1.6 ± 0.8) ×10–². For liquid binary mixtures, the uptake coefficient also depended on the matrix molecule. The atmospheric lifetime of aldehydes was estimated to range from 1.9–7.5 h due to NO₃ oxidation.In the third series of studies, we investigated the N₂O₅ uptake kinetics on alcohols. The uptake coefficients of N₂O₅ for five different alcohols at 293 K varied by 2 orders of magnitude, ranging from 3×10–⁴ to 1.8×10–². The atmospheric lifetime of alcohols was calculated to range from 0.6–130 h, depending on the physical and chemical properties of the organic liquid.
Atmospheric aerosols are very small liquid or solid droplets which are suspended in a gas. They are present in our atmosphere in great numbers (10³ particles per cm³ in unpolluted rural environments). They represent a substantial source of uncertainty in climate models and influence human health, atmospheric reactivity and visibility. Field studies have shown that mixed organic-inorganic aerosols are abundant in the atmosphere. Mixed aerosols undergo phase transitions between aqueous and crystalline phases, called efflorescence and deliquescence. By examining laboratory prepared mixed droplets in a temperature and humidity controlled flow cell, these phase transitions were observed optically. We studied the efflorescence properties of mixed citric acid-ammonium sulfate particles as a function of temperature to better understand the efflorescence properties of mixed organic-inorganic particles in the middle and upper troposphere. Data at 293 K illustrates that the addition of citric acid decreases the efflorescence relative humidity (ERH) of ammonium sulfate, which is consistent with the trends observed with other systems containing highly oxygenated organic compounds. At low temperatures the trend is qualitatively the same, but efflorescence can be inhibited by smaller concentrations of citric acid. For example at temperatures