IMPLICATIONS OF THE LARGE CARBON KINETIC ISOTOPE EFFECT IN THE REACTION CH4+CL FOR THE C-13/C-12 RATIO OF STRATOSPHERIC CH4

摘要

Recent investigations of the carbon kinetic isotope effect (KIE) of the reaction CH4 + Cl yielded KIE(Cl) = 1.066 +/- 0.002 at 297 K (increasing to 1.075 +/- 0.005 at 223 K) [Saueressig el al., 1995]. In order to assess the effect of the exceptionally large KIE(Cl) on delta(13)C of stratospheric CH4 we applied a two-dimensional, time dependent chemical transport model. The model results demonstrate the strong influence of the CH4 + Cl reaction on delta(13)CH(4) in particular in the middle and upper stratosphere, where this reaction contributes several tens of percent to the total CH4 sink. The Cl sink helps to explain the relatively large overall isotope fractionation of 1.010-1.012 observed in the lower stratosphere [Brenninkmeijer el al., 1995; Brenninkmeijer el al., 1996], even though the model results predict a smaller effect than observed. [References: 30]