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Discussion papers | Copyright
https://doi.org/10.5194/amt-2018-36
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 11 Apr 2018

Research article | 11 Apr 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Measurement Techniques (AMT).

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), Jena, 07747, Germany
  • 2Picarro, Inc., Santa Clara, CA 95054, USA
  • 3Research Centre Jülich, Institute for Energy and Climate Research Troposphere (IEK-8), Jülich, 52428, Germany
  • 4Research Centre Jülich, Institute for Energy and Climate Research Stratosphere (IEK-7), Jülich, 52428, Germany
  • 5Hungarian Academy of Sciences (MTA) and University of Szeged (SZTE) Research Group on Photoacoustic Spectroscopy, Szeged, H-6720, Hungary
  • 6Physikalisch-Technische Bundesanstalt (PTB), Braunschweig, 38116, Germany

Abstract. As part of the DENCHAR (Development and Evaluation of Novel Compact Hygrometer for Airborne Research) inter-comparison campaign in North-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. Thus, the campaign allowed for an initial assessment validation of the long-term IAGOS water vapor measurements by CRDS against reference instruments with a long performance record (Fast In-situ Stratospheric Hygrometer (FISH), CR-2 frost point hygrometer (Buck Research Instruments L.L.C., US), both operated by research centre Juelich).

The inlet system, a 50cm long 1/8" FEP-tube connected to a Rosemount TAT housing (model 102BX, deiced) installed on a window plate of the aircraft, was designed to eliminate sampling of larger aerosols, ice particles, and water droplets, and provided additional ram-pressure. In combination with a low sample flow of 100sccm, corresponding to a 4 second response time, this ensured a fully controlled pressure in the sample cell of 186.65hPa (140 Torr) throughout the aircraft altitude operating range up to 12.5km without the need of an upstream sampling pump. This setup ensures full compatibility with the deployment of the analyzer within IAGOS.

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 0.7 to 2.5% water vapor mole fraction. During the inter-comparison campaign the analyzer was compared on ground against a dew point hygrometer, which is used for calibrating the reference instrument FISH, in the range from 2 to 600ppm. Furthermore, a new independent calibration method, based on the dilution effect of water vapor on CO2, was tested.

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 (2.5 seconds measurement interval) of 4ppm or 5% (relative) (whichever is greater) for H2O<100ppm, and 5% (relative) or 30ppm (whichever is smaller) for H2O>100ppm. Accuracy of the CRDS instrument was estimated, based on laboratory calibrations, as 1% (relative) for the water vapor range from 2.5% down to 0.7%, than increasing to 5% (relative) 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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