Laboratory Spectroscopy of atmospheric constituents from the far infrared to the UV range
Remote sensing for determining the distribution of atmospheric trace gases relies on line parameter spectroscopic databases (e.g., HITRAN, GEISA, or JPL) for line positions, line strengths, pressure broadening parameters or absorption cross sections. The database is mandatory for linking the characteristic spectral signature obtained by remote sensing to the trace gas distribution in the atmosphere. Remote sensing of the atmosphere is carried out from ground-, airplane-, balloon-, or satellite-based platforms. The spectroscopic database is mostly obtained from laboratory experiments.
High accuracy intensity measurements
The extension of the spectroscopic database of atmospheric trace gases requires accurate measurement of spectral intensities and line shapes. Whereas the accurate determination of line positions from Fourier Transform (FT) spectra can be achieved easily, the measurement of intensities is hampered by numerous systematic error sources which have to be characterized and minimized. At DLR, for the first time, line strengths were experimentally determined with FT-spectroscopy with an accuracy better than 0.15%. This was achieved by extending the line model in combination with multispectrum fitting and other improvements.
The FT spectrometer at DLR is a commercial high-resolution BRUKER IFS 125 HR instrument covering the spectral range from 10 to 50000 cm-1 with a maximum resolution of 0.001 cm-1 (30 MHz). The instrument can be evacuated to avoid absorption of radiation by water and carbon dioxide in the ambient air. The spectrometer is equipped with a number of highly sophisticated absorption cells covering absorption paths from 0.15 m to 200 m and temperatures from 200 K to 350 K. Several sensitive detectors cover the entire electromagnetic spectrum from Far Infrared to UV. For the Mid Infrared region the detector optics was optimized to yield sensitivities up to 10 times higher than standard commercial detectors.
Several gas cells have been developed:
Technical details of the lab infrastructure can be found in the flyer (see link at right).