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Secondary Organic Aerosol Formation From Select Volatile Organic Compounds

  • Author(s): Chen, Chia-Li
  • Advisor(s): Cocker III, David R.
  • et al.
Abstract

This thesis enhances our understanding of secondary organic aerosol (SOA) formation from select anthropogenic sources including polycyclic aromatic hydrocarbons (PAHs), PAHs mixed with m-xylene and an atmospheric surrogate, and unburned whole gasoline vapors. Major SOA chemical characteristics and physical properties were explored along with SOA formation within the UCR CE-CERT environmental chamber.

SOA formation was significant for all three PAHs precursors during photooxidation under high NOx, low NOx and extremely low NOx conditions with 1-methylnaphthalene forming the most SOA followed by 2-methylnaphthalene and naphthalene. SOA yields greater than 1.0 were observed for extremely low NOx (H2O2) conditions. The atmospheric reactivity influenced by H2O2, NOx levels, initial VOCs/NO ratios, and all impacted the SOA formation from the PAH precursors. Fractal SOA particles were observed for 1-methylnaphthalene or 2-methylnaphthalene high NOx photooxidation, indicating that SOA in these experiments were solid particles. SOA growth rates (aerosol mass concentration (ΔM0) versus hydrocarbon reacted (ΔHC)) from different PAHs-m-xylene mixtures are correlated with initial m-xylene/NO, PAHs/NO, [OH]/[HO2] ratio, [NO]/[HO2] ratio and [HO2]/[RO2] ratio. Addition of m-xylene to PAHs experiments suppressed SOA formation from the PAH precursor. The chemical composition characteristics such as f44 versus f43 , H/C ratio, O/C ratio, and the oxidation state of the carbon (OSc) show that PAHs-m-xylene SOA continuously ages and the SOA exhibits characteristics of both individual precursors.

Finally, the SOA formation from photooxidation of whole gasoline vapor under varying NOx conditions was found to range from 1.7% to 5.2%. Further, addition of ethanol suppressed the SOA formation. This work shows that the traditional two-product SOA formation model was unable to explain the total SOA formation from the complex gasoline mixture using the actual formation observed for individual aromatic SOA precursors.

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