The isotopic composition of methane in the stratosphere: high-altitude balloon sample measurements

The isotopic composition of stratospheric methane has been determined on a large suite of air sam- ples from stratospheric balloon flights covering subtropical to polar latitudes and a time period of 16 yr. 154 samples were analyzed for 13 C and 119 samples for D, increasing the previously published dataset for balloon borne samples by an order of magnitude, and more than doubling the to- tal available stratospheric data (including aircraft samples) published to date. The samples also cover a large range in mixing ratio from tropospheric values near 1800 ppb down to only 250 ppb, and the strong isotope fractionation pro- cesses accordingly increase the isotopic composition up to 13 C= 14 ‰ and D= +190 ‰, the largest enrichments observed for atmospheric CH4 so far. When analyzing and comparing kinetic isotope effects (KIEs) derived from sin- gle balloon profiles, it is necessary to take into account the residence time in the stratosphere in combination with the observed mixing ratio and isotope trends in the troposphere, and the range of isotope values covered by the individual pro- file. The isotopic composition of CH 4 in the stratosphere is affected by both chemical and dynamical processes. This severely hampers interpretation of the data in terms of the relative fractions of the three important sink mechanisms (re- action with OH, O( 1 D) and Cl). It is shown that a formal sink partitioning using the measured data severely underestimates the fraction removed by OH, which is likely due to the insen- sitivity of the measurements to the kinetic fractionation in the lower stratosphere. Full quantitative interpretation of the CH4 isotope data in terms of the three sink reactions requires a global model.