Kinetics and Products from Heterogeneous Oxidation of Squalene with Ozone

Abstract

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 and (4.0 ± 2.2) × 10 for ozone mixing ratios (MR) of 50 and 25 ppb, respectively, on squalene films deposited on glass surfaces. At an MR of 25 ppb, the lifetime for oxidation was the same as that in an indoor office with an MR between 22 and 32 ppb, suggesting that O was the dominant oxidant in this indoor setting. While the heterogeneous kinetics of squalene and O 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.

Gas-Phase Carboxylic Acids in a University Classroom: Abundance, Variability, and Sources

“…Squalene contains 6 C=C bonds so it is highly reactive. Zhou et al 50 found that levulinic acid, succinic acid, and 4-oxobutanoic are the major condensed-phase products from heterogeneous ozonolysis of squalene. These acids can partition into the gas phase and were identified in the gas phase during this study, with their indoor concentration substantially higher than outdoors (Table S1).…”

Indoor ozone/human chemistry and ventilation strategies

“…In addition to the volatile products, the ozonolysis of squalene also generates a number of long-chained polyunsaturated aldehydes and carboxylic acids such as C27-pentaenal, C22-tetraenal, C17-trienal, C27-pentaenoic acid, C22-tetraenoic acid, C17-trienoic acid, that remain bound to the surface. 13 Other studies looked at ozone removal and product formation on clothes, 20 human hair 8 , pieces of clothes soaked with methanol extracts of human skin lipids, 21 on the heterogeneous mechanism of squalene/ozone reaction, 22 and on modeling of the processes with relevance to indoor environments. 18 The human surface has been shown to be very effective at removing ozone.…”

“…Proton transfer reaction mass spectrometer (PTR-MS) was used, for example, in the above-mentioned simulated aircraft cabin 11,12 and office experiments. 13 Other studies looked at ozone removal and product formation on clothes, 20 human hair 8 , pieces of clothes soaked with methanol extracts of human skin lipids, 21 on the heterogeneous mechanism of squalene/ozone reaction, 22 and on modeling of the processes with relevance to indoor environments. 18,19 Building ventilation is used to control the thermal environment and the concentrations of indoor air pollutants.…”

Ozone in urban China: Impact on mortalities and approaches for establishing indoor guideline concentrations

“…When indoors, humans are exposed not only to ozone, but also to products of ozone‐initiated chemistry . Some of these products (eg, hydroxyl radicals, hydrogen peroxide, organic peroxides, formaldehyde, dicarbonyls, secondary organic aerosols) potentially contribute to the total mortality associated with long‐term exposure to ozone as evaluated using measured or modeled outdoor ozone concentrations.…”

“…50 When indoors, humans are exposed not only to ozone, but also to products of ozone-initiated chemistry. 20,23,36,[51][52][53][54][55][56][57][58][59][60][61][62][63][64] Some of these products (eg, hydroxyl radicals, hydrogen peroxide, organic peroxides, formaldehyde, dicarbonyls, secondary organic aerosols) potentially contribute to the total mortality associated with long-term exposure to ozone as evaluated using measured or modeled outdoor ozone concentrations. Indeed, some of the same products generated when ozone is inhaled are generated indoors and subsequently inhaled (albeit at much lower "intake ratios").…”