Abstract
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 δ13C 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. The isotopic composition of CH4 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 (reaction with OH, O(1D) 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 insensitivity 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.
Highlights
In the well-mixed troposphere, the CH4 mixing ratio and its isotopic composition are determined by the principal balance between the sources and sinks (Stevens and Rust, 1982; Quay et al, 1999; Miller et al, 2002)
The difference in cmin can lead to a bias when comparing profiles from different regions if kinetic isotope effects (KIEs) increases with decreasing mixing ratio
A method to take into account the residence time in the stratosphere in combination with the observed mixing ratio and isotope trends in the troposphere has been presented
Summary
In the well-mixed troposphere, the CH4 mixing ratio and its isotopic composition are determined by the principal balance between the sources (and their isotopic signatures) and sinks (and the corresponding kinetic fractionation factors) (Stevens and Rust, 1982; Quay et al, 1999; Miller et al, 2002). In the absence of in situ sources CH4 mixing ratios decrease strongly in the stratosphere due to the removal by the three chemical (1st order) reactions with OH, O(1D) and Cl (Wahlen et al, 1989; Wahlen, 1993; Brenninkmeijer et al, 1995; Irion et al, 1996; Sugawara et al, 1997; Ridal and Siskind, 2002; McCarthy et al, 2003; Rice et al, 2003). The CH4 mixing ratio and its isotopic composition in an air parcel is dependent on the reaction with OH, O(1D) and Cl (chemical removal) on the one hand and its pathway through the stratosphere In this paper we present data from 13 stratospheric balloon flights, which increase the number of balloon observations in the literature by more than an order of magnitude and the number of total published data for the stratosphere by a factor of more than 2
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