Online technique for isotope and mixing ratios of CH₄, N₂O, Xe and mixing ratios of organic trace gases on a single ice core sample

Abstract

Abstract. 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 small sample. Isotope ratios are determined on methane, nitrous oxide and xenon with reproducibilities for ice core samples of 0.15‰ for δ13C-CH4, 0.22‰ for δ15N-N2O, 0.34 ‰ for δ18O-N2O, and 0.05‰ for δ136Xe. Mixing ratios are determined on methane, nitrous oxide, xenon, ethane, propane, methyl chloride and dichloro-difluoromethane with reproducibilities of 7 ppb for CH4, 3 ppb for N2O, 50 ppt for 136Xe, 70 ppt for C2H6, 70 ppt for C3H8, 20 ppt for CH3Cl, and 2 ppt for CCl2F2. The system consists of a vacuum extraction device, a preconcentration unit and a gas chromatograph coupled to an isotope ratio mass spectrometer. CH4 is combusted to CO2 prior to detection while we bypassed the oven for all other species. The highly automated system uses only ~160 g ice, equivalent to ~16 mL air, which is less than previous methods. This large suite of parameters on a single ice sample is new and helpful to study phase relationships of parameters which are usually not measured together. A multi-parameter dataset is also key to understand in situ production processes of organic species in the ice, a critical issue observable in many organic trace gases. Novel is the determination of xenon isotope ratios using doubly charged Xe ions. The attained precision for δ136Xe is suitable to correct the isotopic ratios and mixing ratios for gravitational firn 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.