LAUSR

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 Julich). 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 70 000 to 25 000 ppm water vapor mole fraction. During the inter-comparison campaign the analyzer was compared on the ground over the range from 2 to 600 ppm 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 0.4 Hz (2.5 s measurement interval) of 4 ppm for H2O  < 10 ppm, 20 % or 10 ppm (whichever is smaller) for 10 ppm  H2O  < 100 ppm, and 5 % or 30 ppm (whichever is smaller) for H2O  > 100 ppm. Accuracy of the CRDS instrument was estimated, based on laboratory calibrations, as 1 % for the water vapor range from 25 000 ppm down to 7000 ppm, increasing to 5 % 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.