HOx Production: Ozonolysis of Alkenes
That the hydroxyl radical is formed in high yields from the reaction of ozone with alkenes is well established. OH yields were first measured in indirect studies by use of OH scavengers [e.g., Atkinson et al.,1992] or tracers [e.g., Paulson et al., 1999]; our laboratory provided the first spectroscopic confirmation of OH production, using LIF [Donahue et al., 1998]. Modeling studies [Paulson and Orlando, 1996; Ariya et al., 2000] have indicated that this class of reactions can be a major, even dominant, source of tropospheric HOX in both urban and rural settings. As this channel of HOX formation is nonphotolytic (“dark”), it may be active throughout the night; this may explain anomalously high nighttime HOX levels observed very recently [Faloona et al., 2001]. Thus, it is important that we fully understand the mechanism of ozone-alkene reactions, particularly the factors controlling HOX yields.
The basic mechanism of OH formation from alkene ozonolysis is now fairly well-constrained. A mechanism consistent with a wide range of results (direct and indirect OH yield measurements, molecular product yield measurements, and theoretical modeling of the reaction) is shown in Figure 3. A key intermediate is the carbonyl oxide, which is initially formed with an excess of vibrational energy and has never been detected in ozonolysis reactions.
Figure 3. Mechanism of alkene ozonolysis, shown for trans-2-butene.
The branching ratios of the different pathways shown are for the most part very poorly constrained; our understanding relies largely on theory and measurements of reaction product yields. While pressure- and time-dependent OH yield measurements [Kroll et al., 2001a,b] offer some of the best insight to date into the fate of carbonyl oxides, direct detection of the species would be vastly preferable. IR CRDS is an ideal technique for the detection of carbonyl oxides, due to its sensitivity and the strong absorption features expected at 1900 cm-1 [Kim and Schaefer, 2000]. Direct measurement of carbonyl oxides will help us address key questions about the reaction mechanism, particularly surrounding branching ratios. Studies will include measurements of stabilized carbonyl oxide yields as a function of pressure and reaction time, direct determination of syn:anti ratios, and measurements of the rate constants of bimolecular reactions.
Direct detection of a wide variety of other radical products would also be very useful for our understanding of alkene ozonolysis. To date, only simple radicals (H, OH, HO2) have been measured, yet they account for just a fraction of HOX produced. Therefore, total HOX yields from ozone-alkene reactions remain largely speculative. Measurement of other organic radicals using CRDS could significantly improve our understanding of total HOX production.
In addition, measurements of hydroxyl radicals using LIF will be continued to address a number of uncertainties in the reaction mechanism. For example, by measuring OH and OD from the ozonolysis of partially deuterated alkenes [Kroll et al. 2002], we have shown that anti carbonyl oxides produce hydroxyl via the “hot acid” channel (bottom half of Figure 3). Further studies of a wider variety of deuterated alkenes should afford additional insight into the importance of this channel, as well as the factors controlling syn:anti ratios. Secondly, measurements of OH yields in the presence of “carbonyl oxide scavengers” (such as water) are necessary to improve our understanding of the importance of bimolecular reaction of the stabilized carbonyl oxides. If bimolecular reactions effectively compete with dissociation to OH, tropospheric production of HOX may be highly dependent on atmospheric conditions (such as temperature and relative humidity), and may be significantly lower than estimated from previous laboratory studies.