Sulphur hexafluoride (SF6): comparison of FTIR-measurements at three sites and determination of its trend in the northern hemisphere

Fourier transform infrared spectrometry has been used to retrieve the total column abundances of SF6 at three locations in the northern hemisphere, i.e., the Ny- Ålesund site in Spitsbergen/Norway at 79∘N, the Jungfraujoch observatory in Switzerland at 47∘N and the Kitt Peak observatory in Arizona, USA, at 32∘N. The total column results have been converted to average tropospheric mixing ratios. The mean increases in these mixing ratios have been found to be equal to 0.31±0.08pptvyr-1 at Ny Ålesund, 0.24±0.01pptvyr-1 at the Jungfraujoch and 0.28±0.09pptvyr-1 at Kitt Peak for the common period March 1993 to March 2002, in agreement with corresponding CMDL data (0.21±0.0002pptvyr-1) at the surface. The limited accuracy of the Ny Ålesund and Kitt Peak data results from strong tropospheric water vapour interferences at these lower altitude sites. Observations at all three locations show that SF6 is still accumulating in the atmosphere. Extrapolations of linear and second-order fits to the Jungfraujoch data predict tropospheric mixing ratios of SF6, respectively equal to 16.4±0.5 and 14.7±0.6 in 2050, and 28.2±0.9 and 22.2±0.8pptv in 2100, significantly lower than those reported in the literature so far.