Digitization and calibration of historical solar absorption infrared spectra from the Jungfraujoch site

Abstract. This study describes the digitization and calibration of historically significant solar absorption spectra recorded at the Jungfraujoch International Scientific Station in the 1950s. Using a homemade Pfund-type grating spectrometer, these spectra were recorded on paper rolls to study the solar spectrum which was then used to compile a solar atlas between 2.8 and 23.7 µm (421 to 3571 cm−1) that, in particular, later contributed to the development of the HITRAN (High-Resolution Transmission Molecular Absorption Database) database. We have now digitized these analogue recorded spectra to make them available for atmospheric studies. Our approach involves image-processing techniques, including colour masking for digitization and peak detection for accurate wavenumber calibration against a synthetic spectrum. We have also developed a validation method by re-digitizing degraded Fourier transform infrared (FTIR) spectra to the same resolution as the old spectra to evaluate the digitization accuracy. Furthermore, we have studied the influence of line thickness on the digitization error. The number of spectra transformed into a machine-readable format is 106 (freely available for download), with an average digitization error of 1.55 % and a wavenumber shift standard deviation of 0.075 cm −1. These digitized and calibrated spectra now offer a valuable resource for atmospheric studies, providing essential historical data for atmospheric research. This work not only helps to preserve scientific heritage but also enhances the utility of historical data in contemporary research.