γ-Lactones in Canadian high arctic aerosols: Tracers for the oxidation of unsaturated fatty acids and intramolecular esterification of γ-hydroxyacids during polar sunrise

Abstract

In this study, we detected for the first time a series of γ-lactones (C7-C16) isolated from Canadian high arctic aerosols collected from late February to June as the sun rises from below the horizon to the maximum. γ-Lactones were identified and quantified by GC-MS utilizing the fragment ion (m/z 85) of [C4H5O2]+. Their molecular distributions showed the predominance of C9 species (on average, 25 pg/m3) followed by C10, C8, C11, C12, C16, C15, and C7. The odd-carbon numbered γ-lactones such as C9 and C11 are produced by the oxidation of oleic (Δ9C18:1) and eicosanoic (Δ9C20:1) acids, respectively, to monocarboxylic acids in the atmosphere, followed by the subsequent oxidation to positional isomers of hydroxy acids and intramolecular esterification of γ-hydroxy acids. Unsaturated fatty acids, plausible sources of the γ-lactones, can originate from marine phytoplankton and terrestrial vegetation via long-range atmospheric transport to the Arctic. Series of ω-oxoacids were also detected with the predominance of C9 species, supporting the photochemical oxidation of biogenic unsaturated fatty acids in the Arctic atmosphere after polar sunrise and/or during the long-range transport to the Arctic.