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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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Volume 6, issue 3 | Copyright

Special issue: Carbon dioxide, other greenhouse gases, and related measurement...

Atmos. Meas. Tech., 6, 837-860, 2013
https://doi.org/10.5194/amt-6-837-2013
© Author(s) 2013. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 27 Mar 2013

Research article | 27 Mar 2013

High accuracy measurements of dry mole fractions of carbon dioxide and methane in humid air

C. W. Rella1, H. Chen2, A. E. Andrews2, A. Filges3, C. Gerbig3, J. Hatakka4, A. Karion2,7, N. L. Miles5, S. J. Richardson5, M. Steinbacher6, C. Sweeney2,7, B. Wastine8, and C. Zellweger6 C. W. Rella et al.
  • 1Picarro, Inc., Santa Clara, CA, USA
  • 2National Oceanic and Atmospheric Administration, Earth System Research Laboratory, Global Monitoring Division, Boulder, CO, USA
  • 3Max Planck Institute for Biogeochemistry, Jena, Germany
  • 4FMI, Finnish Meteorological Institute, Helsinki, Finland
  • 5The Pennsylvania State University, Department of Meteorology, University Park, PA, USA
  • 6Empa, Swiss Federal Laboratories for Materials Testing and Research, Laboratory for Air Pollution/Environmental Technology, Duebendorf, Switzerland
  • 7CIRES, University of Colorado, Boulder, CO, USA
  • 8Laboratoire des Sciences du Climat et l'Environnement, Gif sur-Yvette, France

Abstract. Traditional techniques for measuring the mole fractions of greenhouse gases in the well-mixed atmosphere have required dry sample gas streams (dew point < −25 °C) to achieve the inter-laboratory compatibility goals set forth by the Global Atmosphere Watch programme of the World Meteorological Organisation (WMO/GAW) for carbon dioxide (±0.1 ppm in the Northern Hemisphere and ±0.05 ppm in the Southern Hemisphere) and methane (±2 ppb). Drying the sample gas to low levels of water vapour can be expensive, time-consuming, and/or problematic, especially at remote sites where access is difficult. Recent advances in optical measurement techniques, in particular cavity ring down spectroscopy, have led to the development of greenhouse gas analysers capable of simultaneous measurements of carbon dioxide, methane and water vapour. Unlike many older technologies, which can suffer from significant uncorrected interference from water vapour, these instruments permit accurate and precise greenhouse gas measurements that can meet the WMO/GAW inter-laboratory compatibility goals (WMO, 2011a) without drying the sample gas. In this paper, we present laboratory methodology for empirically deriving the water vapour correction factors, and we summarise a series of in-situ validation experiments comparing the measurements in humid gas streams to well-characterised dry-gas measurements. By using the manufacturer-supplied correction factors, the dry-mole fraction measurements have been demonstrated to be well within the GAW compatibility goals up to a water vapour concentration of at least 1%. By determining the correction factors for individual instruments once at the start of life, this water vapour concentration range can be extended to at least 2% over the life of the instrument, and if the correction factors are determined periodically over time, the evidence suggests that this range can be extended up to and even above 4% water vapour concentrations.

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