UC Riverside

Secondary organic aerosol (SOA) is formed via the oxidation of volatile organic compounds emitted to the atmosphere from both biogenic and anthropogenic sources. Due to the complexity of atmospheric composition and range of ambient conditions, aerosol models, which are mostly based off observed yields from controlled laboratory chamber experiments, greatly underestimate global SOA formation. To increase the understanding of the formation and properties of ambient SOA, it is imperative to explore ways to improve the complexity of chamber studies while still maintaining a level of control not found outside of the laboratory.

A surrogate mixture of reactive organic gases (ROG) was developed to mimic atmospheric reactivity in an urban environment such as the Los Angeles basin. The ROG mixture controlled the reactivity of the chamber system such that all gas phase species were not heavily affected by the addition of an aerosol forming precursor. The ROG mixture was modified to represent an urban environment with a strong biogenic influence by the addition of isoprene. It was found that isoprene’s behavior in the mixture yielded high aerosol formation compared to previous NOX photo-oxidation studies. Incremental aerosol formation was then defined in the different ROG systems from two aromatic compounds, a monoterpene, and a polyaromatic hydrocarbon. Slightly higher incremental yields were seen from each compound in the biogenic influenced ROG mixture than in the anthropogenic ROG mixture. Furthermore, it was found that the aerosol physical and chemical properties were dictated by the added precursor and were comparable to properties seen in single precursor experiments.

The effect of ambient temperature (5°C to 40°C) on aerosol formation was also explored for α-pinene ozonolysis, m-xylene/NOX photo-oxidation, cyclohexene ozonolysis, and vehicle exhaust photo-oxidation with hydroxyl radical. In all systems except the complex vehicle exhaust mixture, severe hysteresis effects were seen in aerosol formation, with the cold temperature systems forming up to 5 times more aerosol mass. These findings do not support traditional gas/particle partitioning theory which assumes temperature effects are reversible. Physical and chemical properties of the aerosol tended to remain fairly consistent, despite changes in ambient temperature.