Rate constants and activation energies for ozonolysis of isoprene methacrolein and methyl-vinyl-ketone in aqueous solution: Significance to the in-cloud ozonation of isoprene

Rate constants and activation energies for the reactions of ozone with isop rene, methacrolein, and methyl-vinyl-ketone in aqueous solution have been d etermined at temperatures from 5 to 30 degreesC, using the stopped-flow-tec hnique and monitoring ozone decay. The rate constants at 25 degreesC and th e activation energies have been found to be 4.1 (+/-0.2) x 10(5) M-1 s(-1) and 19.9 (+/-0.5) kl mol(-1) for isoprene, 2.4 (+/-0.1) x 10(4) M-1 s(-1) a nd 23.9 (+/-0.5) kl mol(-1) for methacrolein, and 4.4 (+/-0.2) x 104 M-1 s( -1) and 18.0 (+/-0.5) kl mol(-1) for methylvinyl-ketone A UV spectrum of a transient intermediate with a lifetime of about 15 s formed during the ozon ation of isoprene was obtained in the range 220 to 300 nm. It rises steadil y toward 220 nm. It is suggested that the spectrum can be attributed to the two unsaturated Criegee-intermediates (carbonyl oxides), which would conce ivably be stabilized by resonance. Lifetime considerations indicate that th e oxidation of isoprene and its first-generation reaction products, methacr olein and methyl-vinyl-ketone, by ozone and OH in the aqueous phase of a cl oud environment play only a minor role compared to homogeneous gas-phase pr ocessing. (C) 2001 John Wiley & Sons, Inc.