Vertical profile of the carbon isotopic ratio of stratospheric methane over Japan

Stratospheric air samples were collected over Japan on August 31, 1994 using a balloon-borne cryogenic Sampler and then analyzed for the CH4 concentration and delta(13)C in CH4. The delta(13)C value increased from about -47.5 parts per thousand at the tropopause to about -38.9 parts per thousand at 34.7 km, accompanied by a rapid decrease of the CH4 concentration with height. From the measured values of the CH4 concentration and delta(13)C, th, apparent carbon isotopic fractionation factor was estimated to be 1.0131+/-0.0006. The contributions of the chemical CH4 reactions with OH, O(D-1) and Cl were also examined by using an one-dimensional photochemical-diffusion model. It is suggested from this examination that the reaction of CH4 with Cl is especially important for delta(13)C in the stratosphere.