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

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 δ 13 C 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 δ 13 C = −14 ‰ and δD = +190 ‰, the largest enrichments observed for atmospheric CH 4 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 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 (reaction 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 insensitivity of the measurements to the kinetic fractionation in the lower stratosphere. Full quantitative interpretation of the CH 4 isotope data in terms of the three sink reactions requires a global model.

Interlaboratory comparison of <i>δ</i><sup>13</sup>C and <i>δ</i>D measurements of atmospheric CH<sub>4</sub> for combined use of data sets from different laboratories

“…This effect had not been recognized for more than a decade since the early years of GC-IRMS measurements (Merritt et al, 1995) and thus has not been taken into account in many data sets of atmospheric δ 13 C-CH 4 reported in the meantime (e.g. Miller et al, 2002;Morimoto et al, 2006;Fisher et al, 2011;Röckmann et al, 2011;Umezawa et al, 2012a, b). Furthermore, because the Kr effect is system dependent and variable with time , applying plausible corrections to past data may not be feasible.…”

LISA: a lightweight stratospheric air sampler

“…25 (Brenninkmeijer et al, 1995) studied the correlation of CO2, CH4 and CO mole fractions and their isotopic composition measurements, and found a high correlation between mole fractions and isotopic compositions. It was shown that the 13 C and 2 H isotopic composition of stratospheric CH4 is strongly correlated with its mole fractions (Röckmann et al, 2011) and similar findings were presented for N2O (Kaiser et al, 2006). A theoretical explanation to the tracer-tracer relations due to rapid mixing of air along isentropic surfaces in the stratosphere was presented by (Plumb, 2007).…”

“…Although the Tedlar bags perform slightly better than the MLF bags, both are satisfactory for CH4 measurements when considering its large gradient in the stratosphere (500-2000 ppb e.g. (Röckmann et al, 2011)). …”

“…Since the mole fractions of methane are significantly lower in the stratosphere than a typical value of around 2000 ppb in the free troposphere (e.g. Rice et al 2003;Röckmann et al 2011), we prepared two samples (nos. 6&7) with low mole fractions (~120 ppm CO2, ~600 ppb CH4 and ~75 ppb CO) by diluting air from a cylinder with nitrogen.…”

“…Long-term monitoring of CO2, CH4, N2O and various halocarbons and isotopic analysis of CO2, CH4, N2O have been performed annually for more than a decade (Aoki et al, 2003;Nakazawa et al, 1995Nakazawa et al, , 2002. The stratospheric distribution of methane and its stable isotopes have been performed in order to understand the stratospheric methane sink (Rice, 2003;Röckmann et al, 2011;Sugawara et al, 1997). The stratospheric distribution of N2O and their position dependent isotopic compositions were determined (Kaiser et al, 2006) and 20 were subsequently used to validate transport models.…”