Comparisons of continuous atmospheric CH₄, CO₂ and N₂O measurements – results from a travelling instrument campaign at Mace Head

Abstract. A 2-month measurement campaign with a Fourier transform infrared analyser as a travelling comparison instrument (TCI) was performed at the Advanced Global Atmospheric Gases Experiment (AGAGE) and World Meteorological Organization (WMO) Global Atmosphere Watch (GAW) station at Mace Head, Ireland. The aim was to evaluate the compatibility of atmospheric methane (CH₄), carbon dioxide (CO₂) and nitrous oxide (N₂O) measurements of the routine station instrumentation, consisting of a gas chromatograph (GC) 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. For initial quality and performance control, the TCI was run in parallel with the Heidelberg GC before and after the measurement campaign at Mace Head. Median differences between the Heidelberg GC and the TCI were well within the WMO inter-laboratory compatibility target for all three greenhouse gases. At Mace Head, the median difference between the station GC and the TCI were −0.04 nmol mol⁻¹ for CH₄ and −0.37 nmol mol⁻¹ for N₂O (GC-TCI). For N₂O, a similar difference (−0.40 nmol mol⁻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 reference scale used for the TCI and the Scripps Institution of Oceanography (SIO-1998) scale used at Mace Head and in the whole AGAGE network. Median differences between the CRDS G1301 and the TCI at Mace Head were 0.12 nmol mol⁻¹ for CH₄ and 0.14 μmol mol⁻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 and the CRDS G1301 at Mace Head are much smaller than the WMO inter-laboratory compatibility target, while this is not the case for CO₂ and N₂O.