Atmos. Chem. Phys. Discuss., 11, 12039-12102, 2011
© Author(s) 2011. This work is distributed
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The isotopic composition of methane in the stratosphere: high-altitude balloon sample measurements
T. Röckmann1,2, M. Brass1,2, R. Borchers3, and A. Engel4
1Institute for Marine and Atmospheric Research Utrecht, Utrecht University, Utrecht, The Netherlands
2Atmospheric Physics Division, Max Planck Institute for Nuclear Physics, Heidelberg, Germany
3Planets and Comets Department, Max Planck Institute for Solar System Research, Katlenburg-Lindau, Germany
4Institute for Atmospheric and Environmental Sciences, J. W. Goethe University, Frankfurt, Germany

Abstract. The isotopic composition of stratospheric methane has been determined on a large suite of air samples from stratospheric balloon flights covering subtropical to polar latitudes and a time period of 16 yr. 154 samples were analyzed for δC and 119 samples for δD, increasing the previously published dataset for balloon borne samples by an order of magnitude, and more than doubling the total 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 processes accordingly increase the isotopic composition up to δ13C=−14‰ and δD= +190‰, the largest enrichments observed for atmospheric CH4 so far. When analyzing and comparing kinetic isotope effects (KIEs) derived from single 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 profile. Temporal isotope trends can also be determined in the stratosphere and compare reasonably well with the tropospheric trends. The effects of chemical and dynamical processes on the isotopic composition of CH4 in the stratosphere are discussed in detail. Different ways to interpret the data in terms of the relative fractions of the three important sink mechanisms (reaction with OH, O(1D)) and Cl, respectively), and their limitations, are investigated. The classical approach of using global mean KIE values can be strongly biased when profiles with different minimum mixing ratios are compared. Approaches for more local KIE investigations are suggested. It is shown that any approach for a formal sink partitioning from the measured data severely underestimates the fraction removed by OH, which is likely due to the insensitivity of the measurements to the kinetic fractionation in the lower stratosphere. Attempts can be made to correct for the lower stratospheric sink bias, but full quantitative interpretation of the CH4 isotope data in terms of the three sink reactions requires a global model.

Citation: Röckmann, T., Brass, M., Borchers, R., and Engel, A.: The isotopic composition of methane in the stratosphere: high-altitude balloon sample measurements, Atmos. Chem. Phys. Discuss., 11, 12039-12102, doi:10.5194/acpd-11-12039-2011, 2011.
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