Reaction of methyl radicals with ozone

摘要

The rate constant for the reacton of methyl radicals with ozone has been examined. The methyl radicals were generated by the reaction of oxygen atoms with ethylene in a fast flow reactor, and were detected using a photoionization mass spectrometer. For a sufficiently high concentration of oxygen atoms, the methyl radicals were observed to be at their steady state concentration. Addition of the O3ndash;O2mixture to this system caused a decrease in the steady state CH3concentration, due to the reactions, CH3+O3rarr;products (5) and CH3+O2rarr;products (4). Signals from the CH3radical were measured in the presence and absence of ozone. In the case of no ozone, the O3ndash;O2mixture was passed through a heated quartz tube to convert the ozone to molecular oxygen. Since the rate constants for the reaction of CH3+O and CH3+O2were determined previously, the absolute rate constant for reaction (5) could be obtained from the competition among these reactions. The rate constant determined for reaction (5) is (7.0plusmn;2.7)times;10minus;13cm3moleculeminus;1secminus;1, and is independent of the total pressure (2sim;6 Torr of helium). This rate constant is about 200 times less than that for the reaction of CH3+O and is nearly equal to the high pressure limit rate constant for the CH3+O2reaction. The reaction mechanism and relevance to reactions in the stratospheric ozone layer are discussed.