Formation and evolution of atmospheric organic matter from radical intermediates

Abstract

Atmospheric particulate matter (or "aerosol") has important implications for public health, climate change, and visibility. Our ability to predict its formation and fate is hindered by uncertainties associated with one type in particular, organic aerosol (OA). The study of the chemistry underlying OA formation is complicated by the large number of reaction pathways and oxidation generations for a given precursor species. This thesis describes a series of experiments in which the chemistry is simplified to that of a single alkoxy radical (RO) isomer generated from the direct photolysis of alkyl nitrites (RONO). First, OA was generated from eleven different C10 RO isomers to determine the role of radical molecular structure in the formation of low-volatility species. Here, measured aerosol yields and chemical composition were explained by two major effects: (1) the relative importance of isomerization and fragmentation pathways, which control the distribution of products, and (2) differences in saturation vapor pressure of individual isomers. Next, we developed a method to investigate the reactivity of alkoxy radicals in the condensed phase. The long chain C20 RO radical was generated in hexane solvent to identify possible intermolecular (bimolecular) reactions with the condensed-phase. The lack of formation of the C20 alcohol, the expected product of the bimolecular reaction of RO with hexane indicates that these intermolecular reactions are unable to compete with available unimolecular isomerization processes. Finally, a molecular-level study of this same condensed-phase system with a soft ionization technique permitted the observation of molecular ions that are assigned to specific oxidation products. This approach enables the determination of the extent of branching for another important intermediate, the alkylperoxy radical. The results from this thesis highlight the role of radical structure in the formation of low-volatility products in the atmosphere, in addition to identifying the major reaction pathways responsible for particle-phase oxidative processing.