The heterogeneous reaction of particle-phase methyl esters and ozone elucidated by photoelectron resonance capture ionization: Direct products of ozonolysis and secondary reactions leading to the formation of ketones

Photoelectron resonance capture ionization aerosol mass spectrometry (PERCI-AMS) facilitates the measurement of heterogeneous particle-gas phase reactions of relevance to atmospheric chemistry. This methodology, which has been demonstrated to have analytical merit for single component particles, such as oleic acid, has been extended to heterogeneous reactions of binary and ternary particle systems. Herein is described the direct measurement of unambiguous products formed by the reaction between multicomponent particles composed of fatty acids and corresponding methyl esters and ozone. Reaction products measured include unfragmented acids and aldehydes from ozonolysis, in addition to molecular ions of the substrates. The minimal fragmentation afforded by the low energy and tunable photoelectrons unique to the PERCI methodology allows for the straight-forward analysis presented herein. For example, in a reacted three-component particle consisting of methyl oleate/methyl linolenate/methyl linoleate, 25 of the predicted 36 ozonolysis products are measured directly as their molecular ions.Furthermore, a secondary reaction between in situ-generated Criegee intermediates and unsaturated fatty acids and methyl esters is described leading to the proposed formation of novel ketones. Evidence that this reaction occurs at the carbon–carbon double bond is also presented and a mechanism is proposed. Oxygenation reactions of atmospherically relevant aerosols are of significance in that they may affect particulate properties such as hygroscopicity and ability to act as cloud condensation nuclei.