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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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Volume 11, issue 9 | Copyright
Atmos. Meas. Tech., 11, 5279-5297, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 19 Sep 2018

Research article | 19 Sep 2018

Evaluation of the IAGOS-Core GHG package H2O measurements during the DENCHAR airborne inter-comparison campaign in 2011

Annette Filges1, Christoph Gerbig1, Chris W. Rella2, John Hoffnagle2, Herman Smit3, Martina Krämer4, Nicole Spelten4, Christian Rolf4, Zoltán Bozóki5, Bernhard Buchholz6, and Volker Ebert6 Annette Filges et al.
  • 1Max Planck Institute for Biogeochemistry (MPI-BGC), 07747 Jena, Germany
  • 2Picarro, Inc., Santa Clara, CA 95054, USA
  • 3Research Centre Jülich, Institute for Energy and Climate Research Troposphere (IEK-8), 52428 Jülich, Germany
  • 4Research Centre Jülich, Institute for Energy and Climate Research Stratosphere (IEK-7), 52428 Jülich, Germany
  • 5Hungarian Academy of Sciences (MTA) and University of Szeged (SZTE) Research Group on Photoacoustic Spectroscopy, Szeged, 6720, Hungary
  • 6Physikalisch-Technische Bundesanstalt (PTB), 38116 Braunschweig, Germany

Abstract. As part of the DENCHAR (Development and Evaluation of Novel Compact Hygrometer for Airborne Research) inter-comparison campaign in northern Germany in 2011, a commercial cavity ring-down spectroscopy (CRDS) based gas analyzer (G2401-m, Picarro Inc., US) was installed on a Learjet to measure atmospheric water vapor, CO2, CH4, and CO. The CRDS components were identical to those chosen for integration aboard commercial airliners within the IAGOS (In-service Aircraft for a Global Observing System) project. Since the quantitative capabilities of the CRDS water vapor measurements were never evaluated and reviewed in detail in a publication before, the campaign allowed for an initial assessment of the long-term IAGOS water vapor measurements by CRDS against reference instruments with a long performance record (Fast In-situ Stratospheric Hygrometer (FISH) and CR-2 frost point hygrometer (Buck Research Instruments L.L.C., US), both operated by Research Centre Jülich).

For the initial water calibration of the instrument it was compared against a dew point mirror (Dewmet TDH, Michell Instruments Ltd., UK) in the range from 70000 to 25000ppm water vapor mole fraction. During the inter-comparison campaign the analyzer was compared on the ground over the range from 2 to 600ppm against the dew point hygrometer used for calibration of the FISH reference instrument. A new, independent calibration method based on the dilution effect of water vapor on CO2 was evaluated.

Comparison of the in-flight data against the reference instruments showed that the analyzer is reliable and has a good long-term stability. The flight data suggest a conservative precision estimate for measurements made at 0.4Hz (2.5s measurement interval) of 4ppm for H2O<10ppm, 20% or 10ppm (whichever is smaller) for 10ppm<H2O<100ppm, and 5% or 30ppm (whichever is smaller) for H2O>100ppm. Accuracy of the CRDS instrument was estimated, based on laboratory calibrations, as 1% for the water vapor range from 25000ppm down to 7000ppm, increasing to 5% at 50ppm water vapor. Accuracy at water vapor mole fractions below 50ppm was difficult to assess, as the reference systems suffered from lack of data availability.

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