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Abstract:
A two-month measurement campaign with a Fourier Transform InfraRed (FTIR) analyser
as a Travelling Comparison Instrument (TCI) was performed at the AGAGE and
WMO GAW station Mace Head, Ireland. The aim was to evaluate the compatibility
5 of atmospheric CH4, CO2 and N2O measurements of the routine station instrumentation,
consisting of a gas chromatograph (GC-MD) for CH4 and N2O as well as a cavity
ring-down spectroscopy (CRDS) system for CH4 and CO2. The advantage of a TCI
approach for quality control is that the comparison covers the entire ambient air measurement
system, including the sample intake system and the data evaluation process.
10 For initial quality and performance control the TCI was run in parallel to the Heidelberg
GC (GC-HEI) before and after the measurement campaign at Mace Head. Median
differences between the GC-HEI and the TCI were well within the WMO Inter Laboratory
Compatibility (ILC) target for all three greenhouse gases. At Mace Head, the
median difference between the GC-MD and the TCI were −0.04 nmolmol−1 for CH4
15 and −0.37 nmolmol−1 for N2O. For N2O a similar difference (−0.40 nmolmol−1) was
found when measuring surveillance or working gas cylinders with both instruments.
This suggests that the difference observed in ambient air originates from a calibration
offset that could partly be due to a difference between the WMO N2O X2006a scale
used for the TCI and the SIO-1998 scale used at Mace Head and in the whole AGAGE
20 network. Median differences between the CRDS G1301 and the TCI at Mace Head
were 0.12 nmolmol−1 for CH4 and 0.14 μmolmol−1 for CO2 (CRDS G1301−TCI). The
difference between both instruments for CO2 could not be explained, as direct measurements
of calibration gases show no such difference. The CH4 differences between
the TCI, the GC-MD and the CRDS G1301 at Mace Head are smaller than the WMO In25
ter Laboratory compatibility (ILC) target (WMO, 2009), while this is not the case for CO2
and N2O. During the campaign it was also demonstrated that the new optical instrumentation allows detection of very small vertical CO2 and CH4 gradients, here between 10ma.g.l. and 25ma.g.l. This provides a new opportunity of evaluating greenhouse gases gradients in terms of regional fluxes.
