Reactive Uptake of O3 by Multicomponent and Multiphase Mixtures Containing Oleic Acid

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Abstract
The heterogeneous reaction of O3 with lauric acid/oleic acid (LA/OA) mixtures and myristic acid/oleic acid (MA/OA) mixtures were studied as a function of composition, physical state, and microstructure at 298 K. Lauric acid and myristic acid are both alkanoic acids, whereas oleic acid is an alkenoic acid. Additionally, we investigated the uptake of O3 by multicomponent mixtures that closely represent the composition of meat-cooking aerosols. These measurements were performed with a rotating-wall flow-tube reactor coupled to a chemical ionization mass spectrometer. The reactive uptake coefficients (γ) of O3 on liquid LA/OA and MA/OA solutions range from 4 × 10-4 to 7.2 × 10-4. The γ values measured for solid−liquid LA/OA and MA/OA mixtures (which consist of solid LA or solid MA in equilibrium with a liquid) range from 2 × 10-5 to 1.7 × 10-4. These experiments show that only 7% solid by mass in the solid−liquid mixture can decrease γ by an order of magnitude compared to the liquid mixtures. The γ values for solid−liquid mixtures that closely represent the composition of meat-cooking aerosols range from 1.6 × 10-5 to 6.9 × 10-5. We found that γ of solid−liquid mixtures depends on the microstructure of the mixtures, which in turn depends on the method of preparing the films. Furthermore, experiments employing solid−liquid mixtures show an increase in γ with increasing film age. This can be explained either by the formation of a nonequilibrium phase followed by its relaxation to the stable phase or by Ostwald's ripening, which refers to a change in the solid microstructure due to a tendency to minimize the total surface free energy of the solid. We used the obtained γ values to estimate OA lifetimes for polluted atmospheric conditions. For liquid solutions, the lifetimes were on the order of a few minutes. The lifetimes derived for solid−liquid mixtures are up to 75 min, significantly longer than for liquid solutions. Our study emphasizes the effect of the physical state and microstructure of multicomponent mixtures on the heterogeneous chemistry.

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