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Calibration of Boreas: a new laser-based instrument for in-situ automated measurement of δ13C and δ2H isotope ratios in methane
by Dr. Chris Rennick, Dr. Emmal Safi, Mrs./Ms. Aimee Hillier, Mr. Eric Mussel-Webber, Dr. Tim Arnold, Dr. Ruth Hill-Pearce, Dr. Paul Brewer, Dr. Joachim Mohn, Dr. Ivan Prokhorov, Dr. Heiko Moossen, Dr. Thomas Hausmaninger, Dr. Stefan Persijn, Dr. Javis Nwaboh


Methane is an attractive target for greenhouse gas reduction as it has a relatively short atmospheric lifetime and is emitted by anthropogenic sources such as agriculture, landfill, and fossil fuel use. The sources emit methane with distinct carbon and hydrogen isotope ratios, so continuous monitoring of the isotopic composition of atmospheric methane, together with existing amount fraction monitoring, will provide information to disaggregate emissions fluxes by source sector. We present a new instrument capable of automated continuous measurement of the isotopic ratios of carbon and hydrogen in methane - δ 13C (CH_4) and δ 2H (CH_4). Boreas is a cryogenic preconcentrator instrument that extracts methane from an ambient air (around 1.9 μmol/mol) for measurement by infrared diode laser absorption spectroscopy. The methane sample is quantitatively delivered to the laser spectrometer at a higher amount fraction (around 550 μmol/mol) in a nominally pure nitrogen matrix. Current limitations to optical measurements of δ 13C (CH_4) and δ 2H (CH_4) are the traceability of reference standards to the VPDB and VSMOW scales, respectively, and quantifying the role of residual interfering matrix gases, such as oxygen and argon, on the measurement precision. We will present our calibration strategy, based on high amount fraction synthetic reference materials prepared gravimetrically from a pure methane parent source with a single isotopic composition. The calibration is validated with different methane gas mixtures, and we quantify the contributions to the measurement uncertainty over the amount fraction and isotopic composition ranges of the instrument. A second set of target gas mixtures have been prepared with species known to interfere with the methane measurement and have similar boiling points so may be preconcentrated along with methane. Analysis of these mixtures quantify the potential bias in the preconcentrator and spectrometer measurements of whole air. This work is part of the 19ENV05 STELLAR project and received funding from the EMPIR programme co-financed by the participating states and from the European Union Horizon 2020 Research and Innovation programme.

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Topic : Theme 2: Accuracy requirements for atmospheric composition measurements across economic sectors, and temporal and spatial scales.
Reference : T2-A7

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