Aerosols composed primarily of organic constituents are unique chemical environments due to their high surface to volume ratio, their range of chemical complexity, and their variable phase and mixing behavior. In the atmosphere, organic aerosols can have significant impacts on climate, such as when they act as cloud condensation nuclei, as well as impacts on air quality and human health. Additionally, organic aerosols have many applications in industrial processes, such as paints, coatings, and chemical synthesis. An understanding of the mechanisms of chemical transformation in organic aerosol particles is therefore key to understanding how these particles affect both environmental and industrial processes. In oxidation reactions, the particle-phase chemistry and kinetics are often determined by the behavior of reactive intermediates formed after an initiating reaction step. These reactive intermediates can include the stabilized Criegee Intermediate (sCI) formed during ozonolysis, or hydroxyl (OH), alkoxy (RO), or alkylperoxy (RO2) radicals formed during radical chain reactions. The reaction kinetics of these various intermediate species are diverse, often interconnected, and can be affected by the properties of the particle matrix, making them difficult to measure.
The first chapter of this thesis introduces the effective reactive uptake coefficient (?eff), which is used to quantify heterogeneous reaction kinetics. Then experimental results measuring the reaction kinetics of the ozonolysis of alkene-containing aerosol using various forms of aerosol mass spectrometry are presented. In Chapter Two, Atmospheric Pressure Chemical Ionization Mass Spectrometry (APCI-MS) is used to measure the reaction of aerosols composed of a mixture of an alkene and a saturated acid. The decay kinetics of the alkene, as captured by ?eff are used to determine the extent of chain cycling during ozonolysis. The decay kinetics of the acid are used to measure the particle-phase reaction rate between stabilized Criegee Intermediates (sCI) and carboxylic acids, which is found to be six orders of magnitude slower than the corresponding gas- phase reaction rate.
In Chapter Three, temperature-dependent flow tube experiments are conducted using vacuum ultraviolet (VUV) photoionization aerosol mass spectrometry (AMS) to reveal how the energetics of various reaction steps change with temperature. The ?eff of neat alkene aerosol is found to double as temperature is lowered from 293 K to 263 K, which is attributed to a buildup of particle-phase sCI at low temperatures that enhance chain cycling chemistry. The branching ratio between the unimolecular reactions of the sCI and the bimolecular reaction with the acid is investigated, suggesting an activation energy of at least 40 kJ mol-1.
In Chapter Four, progress is described toward an aerosol delivery system for organocerium photocatalysts, which have the potential to perform chemoselective redox chemistry on target molecules for organic synthesis. An apparatus for generating and detecting aerosol containing a Ce (IV) catalyst and target reactants in submicron (~100 nm diameter) organic solvent droplets is developed, allowing particle-phase Ce-ligand complexes to be detected via nanospray ionization (NSI) mass spectrometry. The structures of these complexes in the presence of two different carboxylic acid ligands are discussed and identified as primarily 4-coordinate complexes with 1 or 2 Ce metal centers. These results have implications for the design of future catalysts and experimental apparatus that make use of the particle surface to enhance catalytic performance. Finally, in the fifth chapter, the key findings presented in this thesis are briefly summarized and discussed.
