Gas-phase rate coefficients of the reaction of ozone with four sesquiterpenes at 295 ± 2 K

Abstract

The rate coefficients of the reaction of ozone with the four atmospherically relevant sesquiterpenes β-caryophyllene, α-humulene, α-cedrene and isolongifolene were investigated at 295 ± 2 K and atmospheric pressure by at least two independent experimental investigations for each reaction. Relative rate experiments were carried out in a flow tube using two different experimental approaches with GC-MS detection (RR 1) and PTR-MS analysis (RR 2) as the analytical techniques. Absolute rate coefficients were determined in a stopped-flow experiment following the ozone depletion by means of UV spectroscopy. The average rate coefficients from the combined investigations representing the mean values of the different experimental methods are (unit: cm³ molecule⁻¹ s⁻¹): k_{(O3+β-caryophyllene)} = (1.1 ± 0.3) × 10⁻¹⁴ (methods: RR 1, RR 2, absolute), k_{(O3+α-humulene)} = (1.2 ± 0.3) × 10⁻¹⁴ (RR 1, RR 2), k_{(O3+α-cedrene)} = (1.7 ± 0.5) × 10⁻¹⁶ (RR 2, absolute) and k_{(O3+isolongifolene)} = (1.1 ± 0.5) × 10⁻¹⁷ (RR 2, absolute). The high ozonolysis rate coefficients for β-caryophyllene and α-humulene agree well with the results by Shu and Atkinson (Int. J. Chem. Kinet., 1994, 26) and lead to short atmospheric lifetimes of about two minutes with respect to the ozone reaction. The relatively small rate coefficients for α-cedrene and isolongifolene differ from the available literature values by a factor of about 2.5–6. Possible reasons for the deviations are discussed. Finally, calibrated sesquiterpene FT-IR spectra were recorded for the first time.