AAAR 31st Annual Conference
October 8-12, 2012
Hyatt Regency Minneapolis
Minneapolis, Minnesota, USA
Abstract View
Selective Depletion of Branched and Higher-Volatility Alkanes in the Heterogeneous OH Oxidation of Motor Oil Particles
Gabriel Isaacman, ARTHUR CHAN, Theodora Nah, Katheryn Kolesar, Chris Ruehl, David Worton, Drew Gentner, Timothy Dallmann, Thomas Kirchstetter, Christopher Cappa, Robert Harley, Kevin Wilson, Allen H. Goldstein, University of California, Berkeley
Abstract Number: 472 Working Group: Aerosol Chemistry
Abstract Motor oil serves as a useful surrogate for anthropogenic particulate emissions due to its compositional similarity to both diesel exhaust and the complex mixture of hydrocarbons observed in typical ambient aerosol samples. However, because of its complexity, detailed analysis of the composition and atmospheric degradation of complex hydrocarbon mixtures is difficult using traditional techniques. In order to improve this understanding, we subjected homogenously nucleated motor oil (15W-40) particles to heterogeneous hydroxyl radical oxidation. Samples were collected and analyzed by gas chromatography using vacuum-ultraviolet ionization and high resolution time-of-flight mass spectrometry (GC/VUV-HRTOFMS). This “soft” ionization technique allows us to classify compounds by carbon number, cyclization, and branching, providing nearly complete resolution of hydrocarbon mixtures. The distribution of branched isomers of alkanes was found to be different between diesel and motor oil, with fresh diesel exhaust shown to have a distribution similar to that of motor oil. In addition, we use high resolution analysis to extract the hydrocarbon fraction of the oxidized motor oil particles and compare relative reaction rates of various compound classes. Branched alkanes are found to have heterogeneous oxidation rates of 1.6 ± 0.3 to 2.1 ± 0.2 times faster than straight-chain alkanes depending on degree of branching, while cyclic compounds oxidize at 0.7 ± 0.2 times the average rate of alkanes. Higher volatility alkanes are observed to oxidize faster, while cyclic compounds do not strongly exhibit volatility dependent oxidation rates. These relative rates are expected to have significant effects on aerosol composition following multi-day atmospheric aging, as cyclization and branching impact ratios of fragmentation and functionalization.