On discrepancies between different laboratory measurements of kinetic parameters for the reaction of the hydroxyl radical with halocarbons

This paper discusses laboratory-measured rate coefficients for the reaction of hydroxyl radicals with halogenated organic compounds. A knowledge of such rate coefficients is vital in assessing the tropospheric lifetimes of the compounds. There appear to be significant differences between measurements from different laboratories. Possible sources of the discrepancies are discussed critically, and the possible influence of the technique employed (flash-photolysis or discharge-flow) is considered. Reactive impurities, secondary losses of OH and unanticipated heterogeneous processes might each invalidate the experimental measurements. The effects of these influences are analysed in turn. The conclusion is drawn that the most probable source of error derives from surface phenomena, and some experimental evidence is given that supports this idea. In these circumstances, it may turn out that the most reliable data are those obtained at high temperatures and that, contrary to received wisdom, it may be advantageous to extrapolate high-temperature data to calculate rate coefficients at tropospheric temperatures.