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 CompactHygrometer for Airborne Research) inter-comparison campaign in northernGermany in 2011, a commercial cavity ring-down spectroscopy (CRDS) based gasanalyzer (G2401-m, Picarro Inc., US) was installed on a Learjet to measureatmospheric water vapor, CO2, CH4, and CO. The CRDScomponents were identical to those chosen for integration aboard commercialairliners within the IAGOS (In-service Aircraft for a Global ObservingSystem) project. Since the quantitative capabilities of the CRDS water vapormeasurements were never evaluated and reviewed in detail in a publicationbefore, the campaign allowed for an initial assessment of the long-term IAGOSwater vapor measurements by CRDS against reference instruments with a longperformance record (Fast In-situ Stratospheric Hygrometer (FISH) and CR-2frost point hygrometer (Buck Research Instruments L.L.C., US), both operatedby Research Centre Jülich).For the initial water calibration of the instrument it was compared against adew point mirror (Dewmet TDH, Michell Instruments Ltd., UK) in the range from70 000 to 25 000 ppm water vapor mole fraction. During theinter-comparison campaign the analyzer was compared on the ground over therange from 2 to 600 ppm against the dew point hygrometer used forcalibration of the FISH reference instrument. A new, independent calibrationmethod based on the dilution effect of water vapor on CO2 wasevaluated.Comparison of the in-flight data against the reference instruments showedthat the analyzer is reliable and has a good long-term stability. The flightdata suggest a conservative precision estimate for measurements made at0.4 Hz (2.5 s measurement interval) of 4 ppm forH2O < 10 ppm, 20 % or 10 ppm (whichever is smaller)for 10 ppm < H2O < 100 ppm, and 5 % or30 ppm (whichever is smaller) for H2O > 100 ppm.Accuracy of the CRDS instrument was estimated, based on laboratorycalibrations, as 1 % for the water vapor range from 25 000 ppm down to7000 ppm, increasing to 5 % at 50 ppm water vapor. Accuracy at watervapor mole fractions below 50 ppm was difficult to assess, as the referencesystems suffered from lack of data availability.