Kinetics and Products from Heterogeneous Oxidation of Squalene with Ozone

Motivated by the importance of the heterogeneous chemistry of squalene contained within skin oil to indoor air chemistry, the surface reaction of squalene with gas-phase ozone has been investigated. Using direct analysis in real time mass spectrometry (DART-MS) to monitor squalene, the reactive uptake coefficients were determined to be (4.3 ± 2.2) × 10^–4 and (4.0 ± 2.2) × 10^–4 for ozone mixing ratios (MR_O3) of 50 and 25 ppb, respectively, on squalene films deposited on glass surfaces. At an MR_O3 of 25 ppb, the lifetime for oxidation was the same as that in an indoor office with an MR_O3 between 22 and 32 ppb, suggesting that O₃ was the dominant oxidant in this indoor setting. While the heterogeneous kinetics of squalene and O₃ were independent of relative humidity (RH), the RH significantly affected the reaction products. Under dry conditions (<5% RH), in addition to several products between m/z 300 and 350, the major condensed-phase end products were levulinic acid (LLA) and succinic acid (SCA). Under humid conditions (50% RH), the major end products were 4-oxopentanal, 4-oxobutanoic acid, and LLA. The molar yields of LLA and SCA were quantified as 230 ± 43% and 110 ± 31%, respectively, under dry conditions and 91 ± 15% and <5%, respectively, at 50% RH. Moreover, high-molecular weight (molecular weight of >450 Da) products were observed under dry conditions with indications that LLA was involved in their formation. The mechanism of squalene oxidation is discussed in light of these observations, with indications of an important role played by Criegee intermediates.