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 direct measurements of hydroxyl radical (OH) yields for the gas-phase reaction of ozone with a number of symmetric alkenes. Yields are strongly pressure-dependent, contrary to the results of prior scavenger studies. Here we present a statistical-dynamical model of OH production from the reaction, utilizing RRKM/master equation calculations to determine the fate of the carbonyl oxide intermediate. This model agrees with our experimental results, in that both theory and observations indicate strongly pressure-dependent OH yields. Our calculations also suggest that ethene ozonolysis produces OH via a different channel than the substituted alkenes, though the identity of this channel is not clear. This channel may play a role in the ozonolysis of monosubstituted alkenes as well. Our time-dependent master equation calculations show that the discrepancy between OH yields measured in our direct study and those measured in prior scavenger studies may arise from differing experimental time scales; on short time scales, OH is formed only from the vibrationally excited carbonyl oxide intermediate, whereas on longer time scales OH formation from thermal dissociation may be significant. To demonstrate this we present time-dependent measurements of OH yields at 10 Torr and 100 Torr; yields begin increasing after hundreds of milliseconds, an effect which is much more pronounced at 100 Torr. These results are entirely consistent with theoretical predictions. In the atmosphere, the thermalized carbonyl oxide may be susceptible to bimolecular reactions which, if fast enough, 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 the troposphere.