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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 50 cm 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 100 sccm, corresponding to a 4 second response time, this ensured a fully controlled pressure in the sample cell of 186.65 hPa (140 Torr) throughout the aircraft altitude operating range up to 12.5 km 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 600 ppm. 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 0.4 Hz (2.5 seconds measurement interval) of 4 ppm or 5 % (relative) (whichever is greater) for H2O < 100 ppm, and 5 % (relative) or 30 ppm (whichever is smaller) for H2O > 100 ppm. 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 50 ppm water vapor. Accuracy at water vapor mole fractions below 50 ppm was difficult to assess, as the reference systems suffered from lack of data availability.
