OZONOLYSIS AT VEGETATION SURFACES - A SOURCE OF ACETONE, 4-OXOPENTANAL, 6-METHYL-5-HEPTEN-2-ONE, AND GERANYL ACETONE IN THE TROPOSPHERE

The present study gives a possible explanation for the ubiquitous occu rrence of 6-methyl-5-hepten-2-one and acetone in ambient air and repor ts for the first time on a widespread occurrence of geranyl acetone an d 4-oxopentanal. We have conducted a series of laboratory experiments in which it is demonstrated that significant amounts of geranyl aceton e, 6-methyl-5-hepten-2-one (6-MHO), 4-oxopentanal (4-OPA), and acetone are formed by the reaction of ozone with foliage of common vegetation in the Mediterranean area (Quercus ilex > Citrus sinensis > Quercus s uber > Quercus freinetto > Pinus pinea). In order to rule out biologic al formation, epicuticular waxes were extracted from the leaves, dispe rsed on glass wool and allowed to react with a flow of artificial air. Significant amounts of 6-MHO and 4-OPA were formed at ozone concentra tions of 50-100 ppbv, but not at zero ozone. A number of terpenoids co mmon in vegetation contain the structural element necessary for ozonol ytic formation of 6-MHO. Two sesquiterpenes (nerolidol; farnesol), and a triterpene (squalene) selected as representative test compounds wer e demonstrated to be strong precursors for acetone, 4-OPA, and 6-MHO. Squalene was also a strong precursor for geranyl acetone. The atmosphe ric lifetime of geranyl acetone and 6-MHO is less than Ih under typica l conditions. For the present study, we have synthesized 4-OPA and inv estigated the kinetics of its gas-phase reaction with OH, NO3, and O-3 . A tropospheric lifetime longer than 17 h under typical conditions wa s calculated from the measured reaction rate constants, which explains the tropospheric occurrence of 4-OPA. It is concluded that future atm ospheric chemistry investigations should included geranyl acetone, 6-M HO, and 4-OPA. In a separate experiment it was demonstrated that human skin lipid which contains squalene as a major component is a strong p recursor for the four above-mentioned compounds plus nonanal and decan al. The accidental touching of material which later comes into contact with ozone can lead to strong artifact formation of these carbonyl co mpounds. Previously published results on these compounds must be seen in this new light. (C) 1998 Elsevier Science Ltd. All rights reserved.