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
Received: 31 Jul 2012 – Published in Atmos. Meas. Tech. Discuss.: 21 Aug 2012
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.
Revised: 27 Feb 2013 – Accepted: 27 Feb 2013 – Published: 27 Mar 2013
Rella, C. W., Chen, H., Andrews, A. E., Filges, A., Gerbig, C., Hatakka, J., Karion, A., Miles, N. L., Richardson, S. J., Steinbacher, M., Sweeney, C., Wastine, B., and Zellweger, C.: High accuracy measurements of dry mole fractions of carbon dioxide and methane in humid air, Atmos. Meas. Tech., 6, 837-860, doi:10.5194/amt-6-837-2013, 2013.