High resolution solar spectra are routinely recorded since more than two decades by the University of Liège at the International Scientific Station of the Jungfraujoch (Swiss Alps, 46.5°N, 8.0°E, 3580 m asl) with Fourier Transform Infrared (FTIR) spectrometers. Over the last years, major improvements have been implemented in the algorithms used to retrieve the abundances of atmospheric constituents accessible to the FTIR technique. Now, in addition to total column, information on the vertical distribution of the target gas is often available, allowing producing partial column data sets. We take advantage of these improvements to generate and characterize long-term total and partial columns time series of some important inorganic fluorinated trace gases deduced from FTIR measurements performed at Jungfraujoch.
First, our investigations on hydrogen fluoride (HF) indicate that the adoption of a Galatry line shape model for this species significantly improves the fitting quality of the retrievals. The sensitivity of our FTIR measurements to HF inversions in three distinct regions that range from the low to the high stratosphere is confirmed thanks to products derived from two satellites and from two numerical models. We further demonstrate that the HF seasonal variations that occur in the low stratosphere are mainly responsible for the seasonal cycle observed in our HF total column time series.
We have also developed an original multi-spectrum multi-window retrieval strategy allowing to derive, for the first time, some information on the vertical distribution of carbonyl fluoride (COF2) from ground-based FTIR measurements. After comparison with model data, our COF2 and HF FTIR datasets are combined to assess the atmospheric inorganic fluorine burden Fy.
A trend analysis of our HF, COF2 and Fy time series is then performed for four different time periods spanning the 1985-2010 time interval. While we observe a recent stabilization for HF, corresponding COF2 data show a significant rise, after a period of significant reduction in its accumulation rate. This is probably ascribable to the combination of the decrease of its main source gas CFC-12 with the increase of the substitute product HCFC-22. However, this increase in the COF2 rate of change does not significantly impact the Fy trend, which is essentially driven by the change in HF. In addition, we show that the partitioning between the two major fluorine reservoirs HF and COF2 has not changed since the beginning of this century. Together, they account for around 95% of total inorganic fluorine in the atmosphere.
Finally, we study the long-term evolution of carbon tetrafluoride (CF4), for the first time from ground-based FTIR measurements. The trend analysis of our time series indicates a slowing, initiated during the nineties, in the CF4 growth rate despite the fact that the absolute loading of this compound is still increasing. Our linear accumulation rates are consistent with those deduced from space or surface measurements.