Mechanism of HOx formation in the gas-phase ozone-alkene reaction. 2. Prompt versus thermal dissociation of carbonyl oxides to form OH.

In a companion paper (Kroll, J. K.; Clarke, J. S.; Donahue, N. M.; Anderson , J. G.; Demerjian, K. L. J. Phys. Chem. A 2001, 105, 1554) we present dire ct measurements of hydroxyl radical (OH) yields for the gas-phase reaction of ozone with a number of symmetric alkenes. Yields are strongly pressure-d ependent, contrary to the results of prior scavenger studies. Here we prese nt a statistical-dynamical model of OH production from the reaction, utiliz ing RRKM/master equation calculations to determine the fate of the carbonyl oxide intermediate. This model agrees with our experimental results, in th at both theory and observations indicate strongly pressure-dependent OH yie lds. Our calculations also suggest that ethene ozonolysis produces OH via a different channel than the substituted alkenes, though the identity of thi s channel is not clear. This channel may play a role in the ozonolysis of m onosubstituted alkenes as well. Our time-dependent master equation calculat ions show that the discrepancy between OH yields measured in our direct stu dy and those measured in prior scavenger studies may arise from differing e xperimental time scales; on short time scales, OH is formed only from the v ibrationally excited carbonyl oxide intermediate, whereas on longer time sc ales OH formation from thermal dissociation may be significant. To demonstr ate this we present time-dependent measurements of OH yields at 10 Torr and 100 Torr; yields begin increasing after hundreds of milliseconds, an effec t which is much more pronounced at 100 Torr. These results are entirely con sistent with theoretical predictions. In the atmosphere, the thermalized ca rbonyl oxide may be susceptible to bimolecular reactions which, if fast eno ugh, could prevent dissociation to OH; however there is little experimental evidence that any such reactions are important. Thus we conclude that both mechanisms of OK formation (dissociation of vibrationally excited carbonyl oxide and dissociation of thermalized carbonyl oxide) are significant in t he troposphere.