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 δ¹³C in CH₄. The δ¹³C value increased from about -47.5 %̊ at the tropopause to about -38.9 %̊ at 34.7 km, accompanied by a rapid decrease of the CH₄ concentration with height. From the measured values of the CH₄ concentration and δ¹³C, the apparent carbon isotopic fractionation factor was estimated to be 1.0131±0.0006. The contributions of the chemical CH₄ reactions with OH, O(¹D) 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 δ¹³C in the stratosphere.