Scientific References

Peer Reviewed Literature: Evaluation of the IAGOS-Core GHG package H2O measurements during the DENCHAR airborne inter-comparison campaign in 2011

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
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 intercomparison
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