Biogenic C5 VOCs: release from leaves after freeze–thaw wounding and occurrence in air at a high mountain observatory

During investigations of the formation of volatile organic compounds (VOCs) in leaves, we observed C5 VOCs during leaf drying, senescence, and following freeze–thaw damage. VOCs were quantified by proton-transfer-reaction mass spectrometry (PTR-MS). In freeze-damaged leaves, VOC products were verified with a gas chromatography PTR-MS system, showing that a variety of plants produced 1-penten-3-ol and 1-penten-3-one with smaller amounts of 2(Z)-penten-1-ol and pentenals; similar VOCs have been detected in soybean seed homogenates (Gardner et al., J. Agric. Food Chem. 44 (1996) 882). Most plants wounded in this way also released hexenals and hexanal, and clover also released methylbutanals. The formation of the C5 products was oxygen–dependent, consistent with the involvement of the enzyme lipoxygenase, and pentenone appeared to form independent of an alcohol dehydrogenase reaction; the latter is apparently disrupted by the freeze–thaw treatment. In parallel with these laboratory experiments, on-line PTR-MS measurements of ambient air were conducted at the Sonnblick Observatory in the Austrian Alps (3106 m a.s.l.). Following a hard freeze in central Austria, substantial amounts of C5 VOCs, ranging from 300 pptv to 6 ppbv and including 1-penten-3-ol, methylbutanals and probably pentenone, were detected at this site for several days peaking after midnight. Factor analysis supported their biogenic origin. We speculate that these VOCs were derived from freeze-damaged local vegetation by processes similar to those seen in laboratory freezing studies. If confirmed, these results suggest that leaf-freezing events in forests will give rise to the release of substantial levels of reactive C5 and C6 VOCs that can contribute to regional tropospheric chemistry.