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Insights Into Predicting Secondary Organic Aerosol Formation From Anthropogenic Volatile Organic Compounds: Impact of Molecular Structure and NOx Concentration

  • Author(s): Li, Lijie
  • Advisor(s): Cocker, David R
  • et al.
Abstract

Understanding secondary organic aerosol (SOA) formation is of critical importance to public health and global climate. SOA formation from anthropogenic volatile organic compounds (VOCs) is influenced by NO, precursor molecular structure, oxidation conditions and other factors. This dissertation explores the impact of NO effect and molecular structure for two categories of VOCs at urban atmosphere relevant conditions by utilizing the state of art 90 m3 UCR/CE-CERT chamber facilities.

Monocyclic aromatic hydrocarbons are the dominant anthropogenic SOA precursor in urban areas. The impact of instantaneous NOx source is demonstrated by data mining of m-xylene photooxidation experiments, developing of new reaction scenario and SAPRC model prediction. The relationship of SOA growth rate to NO2/NO ratio, instantaneous HC/NO, absolute NO concentration, peroxy radical reaction branching ratio and hydroxyl radical concentration are illustrated. It is found that NO at sub-ppb level enhances ·OH formation by increasing HO2· and RO2· and therefore promotes SOA formation. Further, innovative SOA composition analysis methods normalizing aerosol yield and chemical composition on an aromatic ring basis are developed and utilized to explore aerosol formation from oxidation of 17 aromatic hydrocarbons. The yield normalization process demonstrates that the aromatic ring is a more significant driver of aerosol formation than alkyl substitute structure. More important, four oxygens per aromatic ring are observed in SOA chemical composition, regardless of the alkyl substitutes attached to the ring. The investigation on SOA formation from aromatic hydrocarbons provides new perspective on the complicated aromatic oxidation mechanisms.

Glycol ethers, from consumer products, are newly identified as SOA precursors in this dissertation. The impact of molecular structure on SOA formation from glycol ethers and relative ethers is investigated. The presence and location of –OH are found to be an important structure in ether precursor to SOA formation. Products with cyclic structure are observed in both gas and particle phase during the oxidation of ethers containing –OH. Further, intermolecular cyclization pathway is considered to determine SOA formation during ether oxidation, especially under atmospherically relevant NOx conditions.

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