Influence of Molecular Structure and Chemical Functionality on the Heterogeneous OH-Initiated Oxidation of Unsaturated Organic Particles

Abstract

The kinetics and products of the heterogeneous OH-initiated oxidation of squalene (C30H50, a branched alkene with 6 C═C double bonds) particles are measured. These results are compared to previous measurements of the OH-initiated oxidation of linoleic acid (C18H32O2, a linear carboxylic acid with 2 C═C double bonds) particles to understand how molecular structure and chemical functionality influence reaction rates and mechanisms. In a 10% mixture of O2 in N2 in the flow reactor, the effective uptake coefficients (γeff) for squalene and linoleic acid are larger than unity, providing clear evidence for particle-phase secondary chain chemistry. γeff for squalene is 2.34 ± 0.07, which is smaller than γeff for linoleic acid (3.75 ± 0.18) despite the larger number of C═C double bonds in squalene. γeff for squalene increases with [O2] in the reactor, whereas γeff for linoleic acid decreases with increasing [O2]. This suggests that the chain cycling mechanism in these two systems is different since O2 promotes chain propagation in the OH + squalene reaction but promotes chain termination in the OH + linoleic acid reaction. Elemental analysis of squalene aerosol shows that an average of 1.06 ± 0.12 O atoms are added per reactive loss of squalene prior to the onset of particle volatilization. O2 promotes particle volatilization in the OH + squalene reaction, suggesting that fragmentation reactions are important when O2 is present in the OH oxidation of branched unsaturated organic aerosol. In contrast, O2 does not influence the rate of particle volatilization in the OH + linoleic acid reaction. This indicates that O2 does not alter the relative importance of fragmentation reactions in the OH oxidation of linear unsaturated organic aerosol.

Time resolved study of hydroxyl radical oxidation of oleic acid at the air-water interface

“…Our result is different from the OH oxidation of unsaturated fatty acid solid particles where reactions between two adjacent molecules were observed, 18,19 this might be because the OA molecules are more densely packed in a particle than at the air-water interface. An alternative to the process proposed for formation of the m/z 297 product would be the direct abstraction of the alpha hydrogen by O 2 to yield the enol product.…”

“…To date, the oxidation of OA by ozone at the air-water interface has been studied by field-induced droplet ionization mass spectrometry (FIDI-MS), 13 sum frequency generation vibrational spectroscopy, 14 and by monitoring other surface properties such as surface tension and surface pressure. 15,16 The NO 3 , OH and ozone oxidation studies of oleic acid particles were also performed using flow tube reactors, [17][18][19][20] chamber studies, 21 attenuated total reflectance infrared spectroscopy, 22 as well as aerosol mass spectrometry. 23 In these studies, the products, mechanism, and kinetics of oleic acid oxidation by various oxidizers have been examined.…”

Reaction of Chlorine Molecules with Unsaturated Submicron Organic Particles

“…Thus, the determined values of the effective reactive uptake coefficients are: system in the gas-phase and provides strong evidence that Cl 2 preferentially attacks the C=C double bond of these unsaturated molecules. However, this trend is in contrast to some recent studies where the reactive effective uptake coefficient of OH and NO 3 by unsaturated fatty acids has been found to be larger than the reactive effective uptake coefficient measured for Sqe [31,32,34]. In the case of the OH oxidation reaction, the value of the Sqe reactive effective uptake coefficient is much larger and has been found to be smaller than the one calculated in the case of LA and LNA particles.…”

“…In the case of the OH oxidation reaction, the value of the Sqe reactive effective uptake coefficient is much larger and has been found to be smaller than the one calculated in the case of LA and LNA particles. This was attributed to the differences in the reactive uptakes of OH radicals at the particle surface and particle-phase secondary reactions, and O 2 was suggested to promote chain prop- [31,32].…”

Heterogeneous chemistry and reaction dynamics of the atmospheric oxidants, O3, NO3, and OH, on organic surfaces

“…116 Recently, flow-tube studies by Wilson and coworkers have added insight into the role of molecular structure and O 2 concentration on the mechanism of organic aerosol oxidation by OH. 137,138 Using mass-spectrometric data, this group compared the reaction mechanisms for both branched squalene and linear linonenic acid (Fig. 11).…”