As mentioned above, the rate-limiting step of the tropospheric HOX cycle is the initial attack of a hydrocarbon by the hydroxyl radical; therefore, it is crucial that we know the rates of OH-hydrocarbon reactions to high accuracy (10% or better) over a the full range of pressures and temperatures in the atmosphere.
While rates of OH-alkane reactions are well-constrained [Atkinson, 1997a], it is also important that we understand OH-alkene rate constants. Alkenes are emitted in large quantities from both biogenic and anthropogenic sources, and constitute a significant portion of the total amount of tropospheric hydrocarbons [Guenther et al., 1995]. Moreover, OH-alkene reaction rates are significantly faster than the analogous OH-alkane rates because the primary reaction mechanism is an addition rather than an abstraction (Figure 4). Alkenes can thus act as a major, often dominant, sink of OH in the troposphere. Rate data, however, are sparse, with the vast majority of measurements from relative rate studies at room temperature only [Calvert et al., 2000].
Addition of OH to an alkene (propene), forming an OH-alkene pre-reactive complex. Abstraction of H atoms (not shown) may also occur.
Those reactions that have been well studied (OH + simple alkenes, such as ethene and propene) exhibit highly unusual pressure and temperature dependence [Atkinson et al., 1997a]. These probably arise from complexities in the OH-alkene potential energy surface. In addition to the main reaction pathway (involving addition of OH to the double bond), hydrogen abstraction may also occur; the importance of this channel is poorly constrained. Furthermore, the addition channel may not be a simple barrierless addition but rather may proceed over a barrier that is lower in energy than the reactants, due to the existence of a pre-reactive van der Waals complex [Sims et al., 1994]. This results in highly non-Arrhenius behavior, and makes extrapolation to other temperatures and pressures difficult.
Therefore, we propose to measure pressure- and temperature-dependent OH-alkene rate constants, using the HPF technique coupled with laser-induced fluorescence detection of OH, to reduce uncertainties in rates as well as to improve our understanding of the reaction dynamics. A long-standing problem with the measurement of such rate constants using direct techniques (spectroscopic detection of OH) is hydroxyl regeneration. When NOX is present in the reaction mixture (from the H + NO2 radical source), regeneration is rapid due to formation of the hydroxyalkyl intermediate (Figure 2). Sensitive LIF measurements of OH (requiring less NOX) and/or generation of OH from H2O2 photolysis using an excimer lamp should allow for the circumvention of this problem. Because of the wide range of pressures and temperatures available to the HPF systems, we are in a position to study these reactions over far greater range of conditions than has been previously attainable. Further, measurement of HOD production from OD-alkene reactions using FTIR will allow us to better constrain the branching between the abstraction and addition pathways. Detailed RRKM/master equation calculations may also help constrain the reaction mechanism significantly.