Date of Award

Spring 2008

Project Type

Dissertation

Program or Major

Earth and Environmental Sciences

Degree Name

Doctor of Philosophy

Abstract

The chlorine atom (Cl) is a potential oxidant of volatile organic compounds (VOCs) in the atmosphere and is hypothesized to lead to secondary organic aerosol (SOA) formation in coastal areas. The purpose of this dissertation is to test this hypothesis and quantify the SOA formation potentials of some representative biogenic and anthropogenic hydrocarbons when oxidized by Cl in laboratory chamber experiments. The chosen model compounds for biogenic and anthropogenic hydrocarbons in this study are three monoterpenes (alpha-pinene, beta-pinene, and d-limonene) and two aromatics (m-xylene and toluene), respectively. Results indicate that the oxidation of these monoterpenes and aromatics generates significant amounts of aerosol. The SOA yields of alpha-pinene, beta-pinene, and d-limonene obtained in this study are comparable to those when they are oxidized by ozone, by nitrate radical, and in photooxidation scenarios. For aerosol mass up to 30.0 mug m-3, their yields reach approximately 0.20, 0.20, and 0.30, respectively. The SOA yields for m-xylene and toluene are found to be in the range of 0.035 to 0.12 for aerosol concentrations up to 19 mug m-3. For d-limonene and toluene, data indicate two yield curves that depend on the initial concentration ratios of Cl precursor to hydrocarbon hydrocarbon. Zero-dimensional calculations based on these yields show that SOA formation from the five model compounds when oxidized by Cl in the marine boundary layer could be a significant source of SOA in the early morning.

In addition, the mechanistic reaction pathways for Cl oxidation of alpha-pinene, beta-pinene, d-limonene, and toluene with Cl have been developed within the framework of the Caltech Atmospheric Chemistry Mechanisms (CACM). Output from the developed mechanisms is combined with an absorptive partitioning model to predict precursor decay curves and time-dependent SOA concentrations in experiments. Model calculations are able to match (in general within general +/- 50%) final measured SOA concentrations. Species predicted to dominate SOA composition include carboxylic acids and organic peroxides.

Finally, the influence of surface tension on the formation of SOA is investigated using a size-dependent absorptive partitioning model that accounts for the influence of surface tension on the gas/particle partitioning of semi-volatile organic compounds (the Kelvin effect). Results from numerical simulations indicate that if non-polar organic species constitute a significant fraction of pre-existing aerosol (PA), the Kelvin effect on SOA formation may be negligible. However, if PA is dominated by polar organic compounds, the Kelvin effect on SUA formation is significant when the PA initial diameter is smaller than approximately 100 nm (decreasing SUA formation from specific compounds by as much as a factor of 2.5). If the PA is an aqueous aerosol, the Kelvin effect on SOA formation is most important (decreasing SOA formation from specific compounds by as much as a factor of 10).

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