AAAR 35th Annual Conference October 17 - October 21, 2016 Oregon Convention Center Portland, Oregon, USA
Abstract View
Rapid Autoxidation of Squalene Aerosol by Hydroxyl Radicals Forms Multifunctional Hydroxyperoxides
NADJA HEINE, Kevin Wilson, Lawrence Berkeley National Laboratory
Abstract Number: 275 Working Group: Aerosol Chemistry
Abstract As the most abundant unsaturated constituent of skin lipids, squalene plays an important role in indoor air quality, and serves as a proxy for unsaturated hydrocarbons in the atmosphere. Once in contact with oxidants in air, photochemical transformations readily occur, leading to fragmentation, isomerization and/or functionalization. The products of these transformations may pose significant risks to human health due to the oxidative stress following inhalation or due to contact with skin. While ozonolysis of squalene has been studied intensively, little is known about the kinetics, reaction pathways and product formation following reactive uptake of hydroxyl radicals (OH), in particular under environmental conditions.
Aerosol continuous flow reactor measurements in combination with vacuum ultraviolet photoionization aerosol mass spectrometry and high resolution mass spectrometry allow the chemical composition of an aerosol to be precisely determined and monitored over the course of chemical processing. In this study, the dependence of OH concentration ([OH]=10$^5– 10$^8 molec./cm$^3) on the oxidation of squalene aerosol is explored, and the impact of relative humidity (RH) on product formation is investigated. The reaction mechanism is constrained by a stochastic kinetics model.
With decreasing [OH], an unexpectedly large acceleration in the effective heterogeneous OH reaction rate was found. This acceleration originates from free radical chain reactions, propagated by activated alkoxy radicals, and formation of OH inside the particle. At high [OH], however, chain termination reactions prevail. Analysis of reaction products with high resolution mass spectrometry reveals the formation of highly oxidized multifunctional reaction products. Comparison to a stochastic kinetics model suggests that these products are multifunctional hydroxyperoxides (QOOH), formed via inter- or intramolecular H-abstraction. Their quantity strongly depends upon relative humidity, and is highest under dry conditions.
The effects of QOOH on indoor air quality and skin are 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. The rapid chemical transformation under low [OH] conditions might suggest that oxidation of unsaturated organic particles may play a more prominent role than expected.