Carbon kinetic isotope effect in the reaction of CH₄ with HO

The carbon kinetic isotope effect in the CH₄ + HO reaction can enrich atmospheric methane in ¹³C relative to its sources. In the only previous measurement of the carbon kinetic isotope effect in this reaction, a value of k₁₂/k₁₃ of 1.003 was reported. In the present experimental determination we have found a significantly larger kinetic isotope effect of 1.010±0.007 (95% confidence interval). This report discusses the experimental details of the carbon kinetic isotope effect determination in the CH₄ + HO reaction and the possible atmospheric implications in the use of the present result. Tropospheric methane is known to have a ¹³C/¹²C ratio of −47‰ (parts per thousand) relative to Peedee belemnite, and most known methane sources for which measurements of the stable carbon isotope ratio have been performed are depleted in ¹³C relative to the atmosphere. A k₁₂/k₁₃ ratio of 1.010 leads to an expected value for the weighted average of the ¹³C/¹²C ratio of the methane sources which is about 10‰ more depleted than the atmosphere (approximately −57‰). This result is fundamentally important to those researchers who use stable carbon isotope ratios to study atmospheric methane.