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

Abstract

Measurements of the 13 C and D kinetic isotope effects (KIE) in methane, 13C KIE = k ( 12 CH 4 )/ k ( 13 CH 4 ) and D KIE = k ( 12 CH 4 )/ k ( 12 CH 3 D), in the reactions of these atmospherically important methane isotopomers with O( 1 D) 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, 13C KIE OH = 1.0039 (±0.0004, 2σ) was determined at 296 K, which is significantly smaller than the presently accepted value of 1.0054 (±0.0009, 2 σ). For D KIE OH we found 1.294 (± 0.018, 2σ) at 296 K, consistent with earlier observations. The carbon kinetic isotope effect in the reaction with O( 1 D ) 13C KIE O(1 D ) , was determined to be 1.013, whereas the deuterium kinetic isotope effect is given by D KIE O(1 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( 1 D ) with 12 CH 4 versus CH 4 is thus ≈ 13‰, which is an order of magnitude greater than the previous value of 1‰. In combination with recent results from our laboratory on 13C KIE and D KIE for the reaction of CH 4 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 13 CH 4 / 12 CH 4 ratio in the lowermost stratosphere, and also reproduce the observed CH 3 D/CH 4 ratio.