Evaluation of the IAGOS-Core GHG package Hsub2/subO measurements during the DENCHAR airborne inter-comparison campaign in 2011

摘要

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, COsub2/sub , CHsub4/sub , 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 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 COsub2/sub 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 Hsub2/subO 2/subO 2/subO ?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.