Ground-based CO observations at the Jungfraujoch from 1997 till 2007: Comparison between FTIR and NDIR measurements

Here we will discuss CO measurements taken at the Swiss Alpine Jungfraujoch station (46.5º N, 8.0° E, 3580m a.s.l.) over the 1997-2007 time period. Results from two distinct techniques, namely Fourier Transform Infrared solar absorption spectrometry (FTIR), and the in situ
Nondispersive Infrared technique (NDIR) have been compared. While the in situ NDIR measurements detect local CO concentrations at the site, the FTIR technique provides integrated measurements along the line-of-sight.
Nevertheless, the pressure broadening of the spectral absorption lines recorded at high resolution enables retrieving information on the vertical distribution of CO, mainly in the troposphere, including its concentration near the surface. To provide enough information content the averaged vmr between 3.58 and 7 km is derived from the FTIR profile data.
Both datasets show a significant negative trend over the time period. However, the NDIR dataset's negative trend is much stronger. Pettitt change point tests reveal that the NDIR-FTIR bias changes substantially from 1997 till 2004 after which the bias stabilizes. Possible causes for
these observations will be critically discussed.