Implications of the large carbon kinetic isotope effect in the reaction CH₄ + Cl for the ¹³C/¹²C ratio of stratospheric CH₄

Recent investigations of the carbon kinetic isotope effect (KIE) of the reaction CH 4 + Cl yielded KIE Cl = 1.066±0.002 at 297 K (increasing to 1.075±0.005 at 223 K) [ Saueressig et al. , 1995]. In order to assess the effect of the exceptionally large KIE cl on δ 13 C of stratospheric CH 4 we applied a two‐dimensional, time dependent chemical transport model. The model results demonstrate the strong influence of the CH 4 + Cl reaction on δ 13 CH 4 in particular in the middle and upper stratosphere, where this reaction contributes several tens of percent to the total CH 4 sink. The Cl sink helps to explain the relatively large overall isotope fractionation of 1.010–1.012 observed in the lower stratosphere [ Brenninkmeijer et al. , 1995; Brenninkmeijer et al. , 1996], even though the model results predict a smaller effect than observed.