Investigating the atmospheric composition from the 1950s on using historical solar absorption infrared spectra

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Studying climate change requires long-term observations. Passive remote sensing has been established as a powerful tool for studying the composition of the atmosphere up to about 60 km altitude. Such observations can be performed from the microwave spectral region through the infrared and into the UV/Vis. Measurements in the infrared spectral region are of high relevance, because many trace gases in the stratosphere and troposphere can be detected. For about three decades infrared observations have been performed using FTIR-spectrometers. The high-alpine site at Jungfraujoch/Switzerland started regular FTIR-observations in 1984. However, pioneering observations using a grating spectrometer were already recorded in the geophysical year 1950/1951 to investigate the Earth’s and solar atmosphere. The corresponding spectra for 1950/1951 are available on ~80 paper rolls, covering altogether the 2.8 – 23.7 micro meter spectral range (corresponding to 422 – 3571 cm-1). More grating spectra are available as electronic files for the 1976-1989 time period, but they cover only narrow spectral ranges selected to include HCl, HF, N2O, CH4 features. Since the instrument works in the same way as current FTIR-spectrometers, looking from the ground towards the sun and measuring in the infrared spectral region, the spectra also contain the atmospheric absorption features of the trace gases, as the current FTIR spectra. The aim of our project is to digitize, conserve, and analyze the historical spectra recorded in 1950 and 1951, together with an analysis of spectra for the 1976-1989 time period with regard to atmospheric composition. This allows allow us to study atmospheric composition, for a few species at their background levels, before mankind began to significantly change it, and study their long-term evolution until the present time. We propose to analyze the historical spectra for several species. Even for those trace gases where a subset of the historical spectra have been analyzed, a new retrieval is extremely important due to two reasons. (i) the spectral data have changed because of new laboratory measurements and new developments in molecular spectroscopy and (ii) the retrieval processes have been further developed significantly due to the availability of fast computers and improvements in the retrieval codes. Trace gases of interest to be retrieved include CH4, N2O, CO, HCN, HNO3, C2H6, OCS and HCl. For a few trace gases, like HCl, the retrieval might be difficult, due to the small signal-to-noise ratio and the lower spectral resolution of the historical spectra. But for these trace gases giving an upper limit is valuable information when studying the long-term change in the atmospheric composition.