AAAR 32nd Annual Conference
September 30 - October 4, 2013
Oregon Convention Center
Portland, Oregon, USA
Abstract View
OH-initiated Heterogeneous Oxidation of Cholestane: A Model System for Understanding the Aging of Cyclic Alkane Aerosols
HAOFEI ZHANG, Chris Ruehl, Arthur Chan, Theodora Nah, David Worton, Gabriel Isaacman, Allen H. Goldstein, Kevin Wilson, Lawrence Berkeley National Laboratory
Abstract Number: 165 Working Group: Aerosol Chemistry
Abstract Aliphatic hydrocarbon aerosols play a substantial role in the urban atmosphere, in which cyclic alkanes make up a large fraction in certain types of particulate matter, such as diesel particles and motor oil particles. In the present study, cholestane (C27H48) was selected as a model compound for cyclic alkanes and OH-initiated heterogeneous oxidation experiments were conducted in a flow tube reactor. OH exposure was varied in the experiments, corresponding to a chemical lifetime in the atmosphere of up to one week. Oxidation products were collected on filters and analyzed using the soft ionization gas chromatography technique. By using this technique, most of the functional isomers can be separated and quantified. The analysis suggests that the first-generation functionalization products (cholestane ketones and alcohols) are the dominant reaction products that account for up to 80% of total speciated compounds. In contrast, fragmentation products and higher-generation functionalization products are much less abundant (less than 5%). Of the fragmentation products, C21H34O, is formed in the largest abundance, suggesting the potential importance of cleaving bonds with tertiary carbon atoms during oxidation. In addition, the concentration ratios of first generation ketones to alcohols were close to one at every oxidation level. The alcohol group was found to be more abundant on the ring (~20%) of cholestane, in contrast to the ketone groups, which form preferentially on the aliphatic side chain (~50%). These results provide new insight into the heterogeneous oxidation mechanism of cyclic alkanes.