UC Davis

Secondary organic aerosol (SOA), formed from atmospheric transformation of primary organic emissions, is an important component of atmospheric aerosol. A great effort has been made to understand SOA sources, composition, and properties over the last decade; however, our understanding of SOA is still limited, and especially, SOA formation in atmospheric aqueous-phase remains poorly interpreted. A thorough characterization of the composition and properties of SOA and a better understanding of the formation and fate of SOA are critical for predicting the impact of SOA on climate and human health. This dissertation takes advantage of advanced techniques such as high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) and aims to understand the formation and aging of SOA generated from phenolic precursors which are emitted in significant quantities from biomass burning, as well as to investigate brown carbon (BrC) formation in phenolic SOA. Chapter 2 presents phenolic aqueous-phase SOA (aqSOA) formation from the photosensitized reaction of guaiacyl acetone (a phenolic carbonyl model compound) with a triplet excited states of organic carbon (3C*). Rapid and efficient aqSOA production were observed with mass yield ~80%. The composition of phenolic aqSOA continuously evolves, with oligomerization and functionalization dominating the initial formation while fragmentation taking over during extended periods. BrC chromophores produced in initial aqSOA formation are identified as oligomers and functionalized monomers of the phenolic precursor. Additionally, significant carboxylic acid formation was observed in phenolic aqSOA formation. In chapter 3, photochemical aging of phenolic aqSOA generated from different oxidants (i.e., •OH and 3C*) were studied. The aqSOA generated from •OH reaction (•OH-aqSOA) is more oxidized and shows lower amounts of oligomeric products than the aqSOA generated from 3C* reaction (3C*-aqSOA), and accordingly, BrC formation is less significant in •OH-aqSOA. Prolonged photochemical aging extended up to 21 days of equivalent atmospheric sunlight results in a significant loss of aqSOA mass and decay of BrC absorption, suggesting fragmentation and evaporation of oxidized volatile species dominates the prolonged photo-aging. Additionally, the effects of increased oxidant concentration during aging were investigated. Accelerated chemical evolution, specifically, promoted fragmentation and evaporation of volatile species and facilitated photobleaching were observed with increased oxidant concentration. Chapter 4 compares phenolic SOA generated from gas-phase reactions (gasSOA) with that from aqueous-phase reactions. Significant chemical differences were observed between phenolic gasSOA and aqSOA. The initially formed gasSOA is more oxidized than the initially formed aqSOA, and there is negligible change of oxidation degree during gasSOA formation, while aqSOA continuously gets more oxidized during formation. Oligomerization plays an important role in the initial formation of phenolic aqSOA, whereas is not observed in phenolic gasSOA. Additionally, aqSOA shows more pronounced carboxylic formation. The chemical differences observed between gasSOA and aqSOA indicate the importance of incorporating both gas-phase and aqueous-phase reaction pathways in models for better prediction of SOA formation. Another important part of this dissertation is to explore the sources, composition, and optical properties of ambient water-soluble BrC (WS-BrC) aerosol in North California. In Chapter 5, the water-soluble components of PM2.5 samples collected in Davis were analyzed by HR-ToF-AMS and UV-vis spectroscopy (UV-vis). Positive matrix factorization (PMF) was applied to the combined AMS and UV-vis data and resolved five water-soluble organic aerosol (WSOA) factors (including a fresh and an aged water-soluble biomass burning organic aerosol (WS-BBOA) and three water-soluble oxygenated organic aerosol (WS-OOA) factors) with distinctive mass spectral and UV-vis spectral profiles. The fresh WS-BBOA is the most light-absorbing and contributes the most to the total light absorption of WS-BrC in Davis. Additionally, the BrC chromophores in WS-BBOA and WS-OOA appear to be important sources of aqueous-phase oxidants (i.e., 3C* and singlet oxygen (1O2*) ) in the atmosphere.