OH-initiated oxidation of sub-micron unsaturated fatty acid particles

Abstract

The heterogeneous reaction of OH radicals with sub-micron unsaturated fatty acid particles in the presence of H2O2 and O2 is studied to explore how surface OH addition reactions initiate chain reactions that rapidly transform the chemical composition of an organic particle. In the presence of 20.7 ppm [H2O2] in a 10% mixture of O2 in N2, the effective uptake coefficients of oleic acid, linoleic acid and linolenic acid are found to be 1.72 ± 0.08, 3.75 ± 0.18 and 5.73 ± 0.14, respectively. These effective uptake coefficients are larger than unity, providing clear evidence for particle-phase secondary chain chemistry. The effective uptake coefficient increases linearly with the number of C=C double bonds in the unsaturated fatty acid molecule. Elemental composition analysis reveals that there is an addition of, on average, 0.57 ± 0.02, 0.61 ± 0.01 and 0.73 ± 0.04 O atoms per reactive loss of oleic acid, linoleic acid and linolenic acid, respectively, which suggests that OH addition to the C=C double bond is not the sole reaction pathway that consumes the molecular species. These results suggest the potential presence of secondary reactions that consume the unsaturated fatty acid molecular species without increasing the particulate oxygen content. As the unsaturated fatty acid particles become more oxygenated, volatilization also becomes significant. The magnitudes of the effective uptake coefficients are found to be dependent on the concentrations of OH, O2 and H2O2 in the flow reactor. A plausible reaction mechanism is presented to show how surface OH addition reactions initiate chain reactions that rapidly transform an unsaturated organic particle's physicochemical properties.

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

“…50,117,125,126,132,135 These reactive uptake coefficients are generally large, but vary significantly between 0.19 and 2.0, 50,117,132,135 although even higher values have been reported. 125 It should be noted that values above 1.0 are only possible for those coefficients determined by measuring particle loss 117,125,132,135 as opposed to OH loss. 126 That is, secondary reactions, such as propagation steps in radical polymerization may result in the elimination of multiple particles for a single OH radical.…”

“…126 That is, secondary reactions, such as propagation steps in radical polymerization may result in the elimination of multiple particles for a single OH radical. 117,125,135,136 Small variations in uptake coefficient have also been attributed to differences in parametric fit of loss data, presence of O 2 in the reaction environment, and variations in the kinetic energy of OH. 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.…”

“…The authors hypothesized the formation of radicals at the particle surface that are able to migrate through the SOA matrix to the squalane support where further reactions occur. Flow-tube studies of heterogeneous reactions of hydroxyl radicals with organic aerosols ranging from those present in soot 50,117,[123][124][125] and biomass-burning aerosols, [126][127][128] to those from herbicides, 129 pesticides, 130,131 and flame retardants, 132,133 have been performed to obtain information such as product structure, 117,123,125,130,134 yield and volatilization, 123,126 changes in hygroscopic properties 124 and particle size, 124,131 as well as kinetic information such as reactive and degradative rate constants 50,126,127,[129][130][131][132][133][134] and reactive uptake coefficients. 50,117,125,126,132,135 These reactive uptake coefficients are generally large, but vary significantly between 0.19 and 2.0, 50,117,132,135 although even higher values have been reported.…”