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The isotopic composition of methane in the stratosphere: high-altitude balloon sample measurementsT. Röckmann1,2, M. Brass1,2, R. Borchers3, and A. Engel41Institute 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.
 Notice on Discussion StatusThe requested discussion paper has a corresponding peer-reviewed
final revised paper in the journal Atmospheric Chemistry and Physics (ACP). You are
encouraged to refer to the final revised version. Final Revised Paper (ACP) Discussion Paper (PDF, 3926 KB)
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. Bibtex EndNote Reference Manager XML
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