Carbon 13 and D kinetic isotope effects in the reactions of CH4 with O(1D) and OH:: New laboratory measurements and their implications for the isotopic composition of stratospheric methane

Measurements of the C-13 and D kinetic isotope effects (KIE) in methane, (KIE)-K-13C = k((CH4)-C-12)/k((CH4)-C-13) and (KIE)-K-D = k((CH4)-C-12)/k((CH3D)-C-12), in the reactions of these atmospherically important methane isotopomers; with O(D-1) and OH have been undertaken using mass spectrometry and tunable diode laser absorption spectroscopy to determine isotopic composition. For the carbon kinetic isotope effect in the reaction with the OH radical, (KIEOH)-K-13C = 1.0039 (+/-0.0004, 2 sigma) was determined at 296 K, which is significantly smaller than the presently accepted value of 1.0054 (+/-0.0009, 2 sigma). For (KIEOH)-K-D we found 1.294 (+/-0.018, 2 sigma) at 296 K, consistent with earlier observations. The carbon kinetic isotope effect in the reaction with O(D-1), (KIEO(1D))-K-13C, was determined to be 1.013, whereas the deuterium kinetic isotope effect is given by (KIEO(1D))-K-D = 1.06. Both values are approximately independent of temperature between 223 and 295 K. The room temperature fractionation effect 1000(KIE-1) in the reaction of O(D-1) with (CH4)-C-12 versus (CH4)-C-13 is thus approximate to 13 parts per thousand, which is an order of magnitude greater than the previous value of 1 parts per thousand. In combination with recent results from our laboratory on (KIE)-K-13C and (KIE)-K-D for the reaction of CH4 with Cl, these new measurements were used to simulate the effective kinetic isotope effect for the stratosphere with a two-dimensional, time dependent chemical transport model. The model results show reasonable agreement with field observations of the (CH4)-C-13/(CH4)-C-12 ratio in the lowermost stratosphere, and also reproduce the observed CH3D/CH4 ratio.