Atmospheric particulate matter (PM) is a component of air pollution that negatively impacts human health and welfare and has strong impacts on global climate. The organic fraction of PM, organic aerosol (OA), is often a dominant fraction of PM mass. Organic aerosol can be emitted directly into the atmosphere as primary OA (POA) or can be produced in the atmosphere from processes such as gas phase reactions of volatile organic compounds (VOCs) with oxidants or aqueous phase reactions of dissolved organics both of which form secondary OA (SOA). The formation and evolution of SOA, as well as the interaction between SOA and POA, are poorly characterized, which leads to uncertainties in the prediction of their concentrations and impacts in the atmosphere. This dissertation addresses processes associated with gas phase SOA formation as well as chemical and physical processing of SOA and POA through experimental studies investigating: 1) the volatility of SOA, 2) the influence of SOA on the heterogeneous oxidation of POA, and 3) the chemical mechanisms of POA oxidation.
In the first set of studies (Chapters 2 3), it is experimentally demonstrated that there has been a fundamental disconnect between the properties of SOA as derived from SOA formation and growth experiments and those derived from evaporation experiments, which has implications for the representation of SOA within air quality and climate models. Specifically, SOA is experimentally determined to be less volatile than predicted based on formation studies through the measurement of the extent of evaporation with temperature change along with concurrent measurement of the particle composition. Volatility measurements were made as a function of mass concentration for α-pinene+O3 SOA and with accompanying particle composition measurements for different SOA types. It was found that SOA volatility was independent of mass concentration and that nine types of SOA had similar volatilities. Furthermore, SOA composition remained constant as particles evaporated. When compared to results from a detailed, physically based model of evaporation, these observations suggest that there are condensed-phase chemical reactions that rapidly produce oligomers and that oligomers are likely the majority of the SOA mass.
In a second study (Chapter 4), a detailed experimental and model assessment of the chemical pathways associated with OH-driven heterogeneous chemistry of two model POA compounds is discussed. The chemical pathways for oxidation of POA are often assumed to be the same as gas phase reactions, yet the higher density of molecules in the condensed phase may increase the dominance of alternate mechanisms. The heterogeneous oxidation of squalane and BES are used as model POA compounds to investigate structure-dependent chemical mechanisms of oxidation. The oxidation of squalane is dominated by the formation of products with added ketone or alcohol functionality whereas the oxidation of BES is dominated by the addition of ketone moieties with minor contributions from pathways forming alcohol substituted products. These differences are shown to be linked directly to differences in the dominant chemical pathways available to the different precursor molecules that result from structural differences in the molecules.
In Chapter 5, the influence of an SOA “coating” on the OH initiated heterogeneous oxidation of model POA particles, comprised of squalane, is considered. Previous studies have shown that SOA can protect buried compounds from reaction with O3 but the extent to which such protection extends to other oxidants had not been established. Here, it is shown that when OH is the oxidant SOA does not block squalane from oxidation and, in fact, the rate of oxidation with OH exposure is enhanced. Although it is clear that an enhancement occurs, the exact extent of enhancement is dependent on the assumed particle morphology, i.e. whether the particle is well mixed, partially engulfed or there is an SOA coating on the squalane core.
|Advisor:||Cappa, Christopher D.|
|Commitee:||Anastasio, Cort, Wexler, Anthony S.|
|School:||University of California, Davis|
|Department:||Civil and Environmental Engineering|
|School Location:||United States -- California|
|Source:||DAI-B 77/04(E), Dissertation Abstracts International|
|Keywords:||Heterogeneous oxidation, Isomer-specific oxidation, Mixing state, Oligomers, Secondary organic aerosol, Volatility|
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