10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Heterogeneous Ozonolysis of Squalene Particles: Gas Phase Products Depend on Water Vapor Concentration
CALEB ARATA, Nadja Heine, Pawel Misztal, Kevin Wilson, Allen H. Goldstein, University of California, Berkeley
Abstract Number: 180 Working Group: Indoor Aerosols
Abstract The oxidation of skin lipids plays an important role in the composition of indoor air. Squalene, C30H50, constitutes 10% of the mass of human skin lipids, and contributes 50% of the unsaturated carbon bonds in skin lipids. When exposed to ozone, squalene is quickly oxidized to products spanning a wide range of volatility with some products remaining in the condensed phase while many fragmentation products are released to the gas phase.
Previous work examining the condensed phase products of pure squalene particle ozonolysis in a flow tube reactor found that the rate of squalene ozonolysis was independent of water vapor concentration. However, an increase in water vapor concentration led to lower concentrations of secondary ozonides, increased concentrations of carbonyls, and reduction of particle diameter suggesting water changes the fate of the Crieege intermediate.1
To determine if this loss of volume corresponds to an increase in gas phase products, we measured gas phase volatile organic compound (VOC) concentration via proton transfer reaction time-of-flight mass spectrometry (PTR-TOF-MS), a soft chemical ionization technique that allows for quantification of the full suite of oxygenated VOCs which might be expected from these reactions. Our studies were conducted in a flow tube reactor at atmospherically relevant ozone exposure levels (10-30 ppb hr) with pure squalene particles. Along with VOC measurements, the condensed phase composition was measured with vacuum ultraviolet photoionization aerosol mass spectrometry (VUV-AMS) and particle size and number were detected by scanning mobility particle sizer (SMPS).
PTR-TOF-MS measurements indicate that an increase in water vapor concentration leads to a strong enhancement of gas phase oxidation products at all levels of ozone exposure. At high ozone exposure (30 ppb hr), all early generation gas phase products with an unsaturated carbon bond are consumed and known terminal products (acetone, 4-oxopentanal (4-OPA), and succinaldehyde) dominate the gas phase VOC composition across all levels of water vapor concentration. An increase in water vapor from ~3% RH to 70% RH at this high ozone exposure effectively doubles the mass concentration of gas phase products. Even under modest ozone exposure (10 ppb hr), changing water vapor concentration enhances VOC mass concentration by ~70%. At all levels of ozone exposure and water vapor concentration the commonly identified squalene oxidation products (terminal products + 6-methyl-5-hepten-2-one (6-MHO)) account for greater than 75% of the observed mass in the gas phase, and we also detect smaller yields of a complex array of products presumably accounting for the remainder of the mass.
The ability of water concentration to dictate the fate of the Criegee intermediate, and therefore the volatility of the oxidation products, suggests that indoor humidity will affect the concentration of gas phase VOCs emitted from ozonolysis of skin oil. This work also strongly suggests that future heterogeneous alkene ozonolysis experiments should consider the effects of humidity on the products of the reactions.
1) Nadja Heine, Frances A. Houle, and Kevin R. Wilson, Environmental Science & Technology 2017 51 (23), 13740-13748.