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 isoprene, methacrolein, and methyl-vinyl-ketone in aqueous solution have been determined at temperatures from 5 to 30°C, using the stopped-flow-technique and monitoring ozone decay. The rate constants at 25°C and the activation energies have been found to be 4.1 (±0.2) × 105 M−1 s−1 and 19.9 (±0.5) kJ mol−1 for isoprene, 2.4 (±0.1) × 104 M−1 s−1 and 23.9 (±0.5) kJ mol−1 for methacrolein, and 4.4 (±0.2) × 104 M−1 s−1 and 18.0 (±0.5) kJ mol−1 for methyl-vinyl-ketone.

A UV spectrum of a transient intermediate with a lifetime of about 15 s formed during the ozonation of isoprene was obtained in the range 220 to 300 nm. It rises steadily toward 220 nm. It is suggested that the spectrum can be attributed to the two unsaturated Criegee-intermediates (carbonyl oxides), which would conceivably be stabilized by resonance.

Lifetime considerations indicate that the oxidation of isoprene and its first-generation reaction products, methacrolein and methyl-vinyl-ketone, by ozone and OH in the aqueous phase of a cloud environment play only a minor role compared to homogeneous gas-phase processing. © 2001 John Wiley & Sons, Inc.

Int J Chem Kinet 33: 182–190, 2001