Online technique for isotope and mixing ratios of CH<sub>4</sub>, N<sub>2</sub>O, Xe and mixing ratios of organic trace gases on a single ice core sample

Abstract

Abstract. Firn and polar ice cores enclosing trace gas species offer a unique archive to study changes in the past atmosphere and in terrestrial/marine source regions. Here we present a new online technique for ice core and air samples to measure a suite of isotope ratios and mixing ratios of trace gas species on a single sample. Isotope ratios are determined on methane, nitrous oxide and xenon with reproducibilities for ice core samples of 0.15 ‰ for δ 13 C-CH 4 , 0.22 ‰ for δ 15 N-N 2 O, 0.34 ‰ for δ 18 O-N 2 O, and 0.05 ‰ per mass difference for δ 136 Xe for typical concentrations of glacial ice. Mixing ratios are determined on methane, nitrous oxide, xenon, ethane, propane, methyl chloride and dichlorodifluoromethane with reproducibilities of 7 ppb for CH 4 , 3 ppb for N 2 O, 70 ppt for C 2 H 6 , 70 ppt for C 3 H 8 , 20 ppt for CH 3 Cl, and 2 ppt for CCl 2 F 2 . However, the blank contribution for C 2 H 6 and C 3 H 8 is large in view of the measured values for Antarctic ice samples. The system consists of a vacuum extraction device, a preconcentration unit and a gas chromatograph coupled to an isotope ratio mass spectrometer. CH 4 is combusted to CO 2 prior to detection while we bypass the oven for all other species. The highly automated system uses only ∼ 160 g of ice, equivalent to ∼ 16 mL air, which is less than previous methods. The measurement of this large suite of parameters on a single ice sample is new and key to understanding phase relationships of parameters which are usually not measured together. A multi-parameter data set is also key to understand in situ production processes of organic species in the ice, a critical issue observed in many organic trace gases. Novel is the determination of xenon isotope ratios using doubly charged Xe ions. The attained precision for δ 136 Xe is suitable to correct the isotopic ratios and mixing ratios for gravitational firn diffusion effects, with the benefit that this information is derived from the same sample. Lastly, anomalies in the Xe mixing ratio, δXe/air, can be used to detect melt layers.

Real-time analysis of <i>δ</i><sup>13</sup>C- and <i>δ</i>D-CH<sub>4</sub> in ambient air with laser spectroscopy: method development and first intercomparison results

“…A critical requirement for such an observing system is the availability of a suitable high-precision measurement technique. Currently, IRMS is the standard technique to perform high-precision analysis of δ 13 C-and δD-CH 4 in ambient air (Bock et al, 2010Brass and Röckmann, 2010;Fischer et al, 2008;Sapart et al, 2012;Schmitt et al, 2014). Being a laboratory-based technique, it relies on flask sampling, which severely limits its temporal and spatial resolution capability.…”

“…Before use, the biogenic CH 4 was purified from major contaminants, mainly CO 2 and H 2 O, by flushing it through an Ascarite/Mg(ClO 4 ) 2 trap. The δ 13 C and δD-CH 4 values of the reference gases CG 1 and CG 2, as well as of a cylinder with pressurized air used as the target gas were calibrated against the calibration scales of the Stable Isotope Laboratory of the Max Planck Institute (MPI) for Biogeochemistry in Jena, Germany (Sperlich et al, , 2013P. Sperlich, personal communication, 2016).…”

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

“…The UB measurements are referenced to a whole-air working standard with a CH 4 mole fraction of 1508.2 ppb and an assigned δ 13 C-CH 4 value of −47.34 ± 0.02 ‰ (named "Boulder, CA08289" in Schmitt et al, 2014). This value is anchored to the standard scale used at INSTAAR (Sect.…”

“…Later, a method based on a continuous-flow gas chromatography isotope ratio mass spectrometry (GC-IRMS) technique combined with combustion and pyrolysis furnaces became available (Merritt et al, 1995;Burgoyne and Hayes, 1998;Hilkert et al, 1999), which dramatically reduced time and effort in the laboratory and likewise the amount of sample air required (now typically 100 mL STP ). Such systems are now used in most laboratories worldwide to acquire δ 13 C-CH 4 and δD-CH 4 data in the current and past atmosphere (Rice et al, 2001;Miller et al, 2002;Sowers et al, 2005;Ferretti et al, 2005;Morimoto et al, 2006;Fisher et al, 2006;Umezawa et al, 2009;Brass and Röckmann, 2010;Sperlich et al, 2013;Schmitt et al, 2014;Bock et al, 2014;Brand et al, 2016;Röckmann et al, 2016). Although these systems use a similar measurement principle, they vary in the use of pre-concentration of CH 4 in sample air, GC separation and combustion/pyrolysis, data corrections and in the specific IRMS instrument among laboratories (see Schmitt et al, 2013, Sect.…”

Climatic and insolation control on the high-resolution total air content in the NGRIP ice core

“…These data are referred to as vacuum-melt data. Schmitt et al (2014) gauged their instrument in an intercalibration exercise with the Laboratoire de Glaciologie et Géophysique de l'Environnement (LGGE) in Grenoble on EPICA Dome C (EDC) ice. In short, two time intervals of the EDC ice core, which were previously measured at the LGGE , were remeasured with the vacuum-melt device.…”