Despite the increasing interest in biogenic secondary organic aerosols (SOAs), their role in the climate system remains the greatest source of uncertainty in global models. Cloud formation, critical for the net cooling effect provided by cloud cover, is dependent on the abundance of SOA particles and their ability to activate the cloud formation process. This propensity of an SOA particle to form a cloud is driven by particle size, chemical composition and chirality, and particle morphology. Due to the heterogeneous nature of these particles, there is still a substantial need for more information about the possible structures and physical properties of individual constituents within SOAs. Although the most abundant monoterpene precursor compound, α-pinene, exists in the atmosphere as an enantiomeric pair, there is little to no research into the impacts of stereochemistry on the atmospherically relevant properties of subsequent particles. Particle formation, growth, decomposition, and cloud formation capabilities all depend on interfacial processes. Given that, the work outlined in this thesis strives to probe the interfacial properties of SOAs by targeting synthetic standards. Dimers derived from α-pinene have been putatively identified in environmental samples and provide the target structures for those compounds synthesized in this work. Oligomeric species in SOA particles are believed to constitute a significant portion of the particle mass and have the propensity to increase the cloud formation capabilities of particles through the depression of their surface tension. In this work ten dimer standards are synthesized via the ozonolysis of α-pinene. Five are synthesized from two monomers from the same enantiomer of α-pinene, while the other five utilize one monomer from each enantiomer. The surface tension depression capabilities of the dimer standards are evaluated using pendant drop tensiometry, which establishes a dependence of surface activity on backbone stereochemistry. The relative hydrophobicity of the dimers is evaluated using LCMS to determine their octanol water and octanol ammonium sulfate partitioning coefficients. Prompted by an interest in the surface specific orientation and properties of the dimers in SOA particles, SFG spectroscopy is used to evaluate the samples. ESI-LCMS and ToF-SIMS are utilized to compare the composition of laboratory-generated secondary organic material (SOM) and our standards, identifying possible matches to elucidate common dimer formation mechanisms. This work also seeks to develop new methodologies by which to probe liquid liquid phase separation (LLPS), a process critical to determining particle morphology and surface activity. SFG spectroscopy is utilized to study the LLPS process in 100 nm particles with atmospheric proxy compounds. This method is established as a powerful tool by which to study LLPS under atmospheric conditions in relevant size ranges and concentrations. Future work will constitute the use of this method to probe the LLPS properties of the dimer standards synthesized here. Overall, the research described here works towards deepening our understanding of oligomeric species in SOAs with specific regard to their structure and chirality.

Creator

• Bellcross, Aleia

DOI

• https://doi.org/10.21985/n2-m753-r676

Subject

• Physical chemistry
• Atmospheric chemistry
• Chemistry

Language

• en

Alternate Identifier

• http://dissertations.umi.com/northwestern:16628
• etdadmin_upload_993914

Date created

• 2023-01-01

Resource type

• Dissertation

Rights statement

• In Copyright