Heterogeneous reaction of ozone with liquid unsaturated fatty acids: detailed kinetics and gas-phase product studies

Detailed kinetics and product yield studies have been performed for the heterogeneous reaction between gas-phase ozone and three liquid fatty acids using a coated-wall flow tube and chemical ionization mass spectrometry. Gas-surface reaction probabilities for ozone loss of (8.0 +/- 1.0) x 10(-4), (1.3 +/- 0.1) x 10(-3), and (1.8 +/- 0.2) x 10(-3) have been measured at room temperature ( 298 K) for oleic acid, linoleic acid and linolenic acid, respectively. The temperature dependence of the uptake coefficients was found to be small and positive. Comparison of these results to the kinetics of the equivalent gas-phase reactions implies that there is a definite enhancement in the rate for the heterogeneous process due to entropic factors, i.e. due to collisional trapping of ozone in the surface layers of the liquid, and a possible effect on the activation energy of the reaction. For linoleic acid, the reaction probability was found to be independent of relative humidity (up to 55%), to +/-10%, at 263 K. Volatile reaction products were observed using proton-transfer-reaction mass spectrometry. Nonanal was observed with a 0.50 (+/-0.10) yield for the reaction with oleic acid, whereas hexanal and nonenal were observed for linoleic acid with 0.25 (+/-0.05) and 0.29 (+/-0.05) yields, respectively. These results indicate that the primary ozonide formed initially in the reaction can decompose via two equal probability pathways and that a secondary ozonide is not formed in high yield in the aldehydic channel. These reactions represent a source of oxygenates to the atmosphere and will modify the hygroscopic properties of aerosols.