As the most abundant unsaturated constituent of skin lipids, squalene plays an important role for indoor air quality, but also serves as a proxy for unsaturated hydrocarbons relevant in combustion and atmospheric chemistry. Once in contact with air, photochemical transformation readily occurs, e.g. via reaction with O3 or OH, leading to fragmentation, isomerization and/or functionalization. Hence, chemically changed species are formed which have been shown to constitute a health hazard in numerous cases. While ozonolysis of squalene has been studied intensively, little is known about kinetics, reaction pathways and product formation of OH uptake, in particular at environmental conditions. In order to create such conditions, polydisperse liquid squalene particles are introduced into a continuous flow stirred tank reactor. Here, the steady state can be held for an arbitrary duration, allowing for low concentration measurements up to several hours. In combination with a vacuum ultraviolet photoionization aerosol mass spectrometer this set up allows monitoring the kinetics of an OH-initiated reaction and to determine the chemical composition of an aerosol in the course of the reaction. The influence of varying OH concentration [OH] (105 – 108 molec./cm3) on the effective uptake coefficient is investigated, as well as the effect of trace gases on the reaction rate and product formation. The results show that the effective uptake coefficient increases as a function of decreasing [OH]. While at high [OH] (~108 molec./cm3) chain termination reactions involving OH radicals result in an effective uptake coefficient of 1.9, particle-phase secondary chain chemistry prevails at environmental [OH] (~105 molec./cm3) thus increasing the effective uptake to over 100. The reaction appears to form highly oxidized multifunctional reaction products with O/C ratios greater than 0.6. Preliminary analysis based on the identification of masses with different types of mass spectrometry and comparison to a stochastic kinetics model suggest that these products are multifunctional hydroxyperoxyl radicals, formed via inter- or intramolecular H-abstraction, which further react to stable hydroxyperoxides (QOOH). The effects of QOOH on indoor air quality and skin is yet unknown, however the structurally similar squalene peroxide (formed outside via UVA/UVB and photoxidizer) has been shown to cause inflammatory skin diseases such as skin cancer and skin ageing.

Rapid autoxidation of squalene particles forms multifunctional hydroxyperoxides

251st National Meeting (2016)

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