Tu, Peijun2018-05-152018-05-152017http://udspace.udel.edu/handle/19716/23164Secondary organic aerosol (SOA), which is produced by the oxidation of volatile organic compounds (VOCs) emitted from biogenic and anthropogenic sources, has great impact on the environment and human health. In this dissertation, SOA particles derived from biogenic precursors were characterized with various mass spectrometry techniques for molecular level analysis. Differences in the chemical compositions of these particles at different formation stages were used to gain insight into the formation and fate of SOA in the atmosphere. While not pursued in this dissertation, the changes studied here may also provide significant information about SOA toxicity and harm to human health. ☐ SOA derived from ozonolysis of biogenic precursors was generated in a flow tube reactor and then sent into a photo chamber where the OH radicals could be produced to simulate further aging (fresh SOA oxidation with OH radicals to produce aged SOA). The molecular compositions of both fresh and aged SOA were studied with high resolution ESI-MS, and thousands of unique molecular formulas were characterized. Among these, a class of highly oxidized multifunctional (HOM) components, which are believed to contribute significantly to the formation of SOA, were identified and compared with previously reported Extremely Low-Volatility Organic Compounds (ELVOC) detected in the gas phase and Low Volatility Organic Oxygenated Aerosol (LV-OOA) measurements of the particle phase. HOMs in fresh SOA consisted mostly of monomers and dimers, which are consistent with condensation of ELVOCs reported from a separate study. Aging caused an increase in the average number of carbon atoms per molecule of the HOMs, which is consistent with particle phase oxidation of (less oxidized) oligomers already existing in fresh SOA. For the biogenic precursors and experimental conditions studied, HOMs in fresh biogenic SOA have molecular formulas more closely resembling LVOOA than HOMs in aged SOA, suggesting that aging of biogenic SOA is not a good surrogate for ambient LVOOA. ☐ In a separate set of experiments, SOA particles were size-selected in the 30-100 nm range with a Differential Mobility Analyzer (DMA) and analyzed by both on- and off-line mass spectrometry techniques. The chemical composition was found to change significantly with particle size. Both the average oxygen-to-carbon (O/C) ratio and carbon oxidation state (OSc) were found to decrease with increasing particle size, while the change of relative abundance of oligomers was opposite as the particle size increases. These changes allowed the relative contributions of condensation, partitioning, and particle phase oligomerization to be determined at various stages of particle formation and growth. Condensation of non-/low- volatility, highly oxidized species dominates the formation/growth of smaller SOA particles, while the partitioning of semi-volatile, less oxidized species tends to play an important role in the growth of larger SOA particles. The formation of oligomers that primarily takes place in the particle phase (accretion reactions) becomes more favored as the volume to surface area ratio of the particle increases. ☐ Additionally, due to the complex molecular components of atmospheric nanoparticles, Reverse Phase Liquid Chromatography (RPLC) and Ion-Mobility Separation (IMS)- Mass Spectrometry were employed for molecular separation. Compositions partially separated based on their size, shape and polarity were subjected to tandem mass spectrometry for structure elucidation. In some cases, isomers/ isobars were identified and separated with the help of HPLC using gradient elution method.Pure sciencesAnalytical chemistryAtmospheric chemistryEnvironmental scienceMass spectrometryThesis10356327632018-02-20en