Intercomparison of in situ NDIR and column FTIR measurements of CO₂ at Jungfraujoch

Abstract. We compare two CO₂ time series measured at the High Alpine Research Station Jungfraujoch, Switzerland (3580 m a.s.l.), in the period from 2005 to 2013 with an in situ surface measurement system using a nondispersive infrared analyzer (NDIR) and a ground-based remote sensing system using solar absorption Fourier transform infrared (FTIR) spectrometry. Although the two data sets show an absolute shift of about 13 ppm, the slopes of the annual CO₂ increase are in good agreement within their uncertainties. They are 2.04 ± 0.07 and 1.97 ± 0.05 ppm yr⁻¹ for the FTIR and the NDIR systems, respectively. The seasonality of the FTIR and the NDIR systems is 4.46 ± 1.11 and 10.10 ± 0.73 ppm, respectively. The difference is caused by a dampening of the CO₂ signal with increasing altitude due to mixing processes. Whereas the minima of both data series occur in the middle of August, the maxima of the two data sets differ by about 10 weeks; the maximum of the FTIR measurements is in the middle of January, and the maximum of the NDIR measurements is found at the end of March. Sensitivity analyses revealed that the air masses measured by the NDIR system at the surface of Jungfraujoch are mainly influenced by central Europe, whereas the air masses measured by the FTIR system in the column above Jungfraujoch are influenced by regions as far west as the Caribbean and the USA.

The correlation between the hourly averaged CO₂ values of the NDIR system and the individual FTIR CO₂ measurements is 0.820, which is very encouraging given the largely different sampling volumes. Further correlation analyses showed, that the correlation is mainly driven by the annual CO₂ increase and to a lesser degree by the seasonality. Both systems are suitable to monitor the long-term CO₂ increase, because this signal is represented in the whole atmosphere due to mixing.