Formation of Gas-Phase Hydrogen Peroxide via Multiphase Ozonolysis of Unsaturated Lipids
2020
Zhou, Zilin | Abbatt, Jonathan P. D.
Gas-phase H₂O₂ is a prominent oxidant in the atmosphere. It also induces lung tissue damage and forest decline. Although it is known that ozonolysis of gas-phase alkenes leads to H₂O₂ formation at high relative humidities (RHs), little is known about this chemistry via gas-surface heterogeneous oxidation. Here, we quantitatively investigated the formation of H₂O₂ from surface ozonolysis of unsaturated lipids commonly found in human skin oil and atmospheric aerosol. This process is initiated by water reacting with Criegee intermediates (CIs), forming α-hydroxyhydroperoxides (α-HHPs) as the key intermediate products. α-HHPs are thermally stable molecules that act as reservoirs of H₂O₂ under dry conditions, prior to rapid decomposition to release H₂O₂ in the presence of water vapor. The upper limit of the α-HHP lifetime derived from oxidized methyl oleate was roughly 2 min at 50% RH. The molar H₂O₂ yields (relative to ozone consumption) vary from 7% to 35% for triolein, methyl oleate, and squalene, with no significant dependence on relative humidity from 50% to 80%. In contrast, oleic acid has the lowest H₂O₂ yield (∼2%), likely due to a more competitive reaction between CIs and its carboxylic acid headgroup. The implications of this chemistry in atmospheric and indoor environments are discussed.
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