AAAR 32nd Annual Conference
September 30 - October 4, 2013
Oregon Convention Center
Portland, Oregon, USA
Abstract View
The Influence of Molecular Structure and Chemical Functionality on the Heterogeneous OH-initiated Oxidation of Unsaturated Organic Particles
THEODORA NAH, Sean Kessler, Kelly Daumit, Jesse Kroll, Stephen R. Leone, Kevin Wilson, University of California, Berkeley
Abstract Number: 641 Working Group: Aerosol Chemistry
Abstract Recent studies on the oxidation of organic particles suggest that the heterogeneous oxidation rate and reaction mechanism depend heavily on the molecular structure of the hydrocarbons in the particle. In this work, two model reaction systems (OH + squalene (C30H50) and OH + unsaturated fatty acid (oleic acid (C18H34O2), linoleic acid (C18H32O2) and linolenic acid (C18H30O2)) are analyzed to investigate the effect of molecular structure and chemical functionality on the reaction rates and mechanism in the heterogeneous OH-initiated oxidation of unsaturated organic particles. Squalene is a branched alkene with six C=C double bonds, while oleic acid, linoleic acid and linolenic acid are linear acids with one, two and three C=C double bonds, respectively.
The effective uptake coefficient for squalene measured at 10 % [O2] is 2.34 ± 0.07, while the effective uptake coefficients measured for oleic acid, linoleic acid and linolenic acid at 10 % [O2] are 1.72 ± 0.08, 3.75 ± 0.18 and 5.73 ± 0.14, respectively. The effective uptake coefficients are larger than unity, providing clear evidence for particle-phase secondary chain chemistry. The effective uptake coefficient for squalene is smaller than that measured for linoleic acid and linolenic acid when O2 is present in reaction. Additionally, the effective uptake coefficient for squalene increases with the [O2] in the reactor, whereas the effective uptake coefficients for the unsaturated fatty acids decrease with increasing [O2]. This suggests that the chain propagation chemistry in squalene oxidation is different from the unsaturated fatty acids. Elemental composition analysis of squalene particles shows that the average particle mass and particulate carbon content decreases immediately when oxidation is initiated at 10 % [O2]. In contrast, elemental composition analysis of linoleic acid particles shows that O2 does not influence the rate of particle volatilization in the OH + linoleic acid reaction. This suggest that while fragmentation reactions becomes more important when O2 is present in the OH oxidation of branched unsaturated organic aerosol, O2 does not alter the relative importance of fragmentation reactions in the OH oxidation of linear unsaturated organic aerosol.