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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

atmospheric chemistry and physics

Comparisons of continuous atmospheric CH&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt;, CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; and N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O measurements – results from a travelling instrument campaign at Mace Head