The FTIR–group at the Institute of Environmental Physics at the University of Bremen is focussing on atmospheric trace gas retrievals from ground-based solar absorption spectra recorded by Fourier Transform InfraRed (FTIR) spectrometers. Solar absorption spectra contain information about the composition of the atmosphere because each atmospheric molecule absorbs solar radiation at characteristic wavelengths, leaving characteristic absorption features for each molecule in the spectra. Column densities (total number of molecules above the observer) of several tropospheric and stratospheric gases can be retrieved from the spectra.
Tropospheric gases include CO2, CH4, CO, N2O, C2H2, C2H6, CH2O, OCS and CFCs. Important measurable stratospheric species are O3, HC1 , HNO3, NO, NO2 and C1ONO2. The retrieval algorithms for the total columns are based on a nonlinear least squares fitting procedure in which one scaling factor applied to the whole a priori profile is adjusted until the best agreement is found between the measured and the calculated spectra. For some species (e.g. CO, CH4, HCl, HF, N2O) a vertical profile can be determined from the pressure broadening of the absorption lineshapes. The upper limit for height information is determined by the Doppler width of the absorption lines, which is almost altitude independent. This width being inversely proportional to the wavelength and in the infrared spectral region profile retrieval is limited to below 35–40 km. The vertical resolution of the profiles is about 4km. The algorithms used for the profile retrieval are based on the Optimal Estimation Method (OEM). This retrieval method was commonly used in the microwave spectral domain. In both the microwave and the infrared spectral domains, profile retrievals are possible because of the altitude dependence of the absorption lineshapes, resulting from pressure broadening and temperature sensitivity. Therefore, the physical upper limit for height information is determined by the Doppler width of the absorption lines, which is almost altitude independent. This width being inversely proportional to the wavelength, profiles can be retrieved from microwave spectra up to 70 km altitude while in the infrared, profiles are limited to below 35–40 km.
The solar radiation is collected by the solar tracker and directed to FTIR-spectrometer (Michelson-Interferometer). Inside the spectrometer, the radiation is split into two beams, which travel separate paths through the instrument and are then recombined. The length of one path is varied with respect to the other. At equal path length (ZPD = zero-path difference) constructive interference occurs and the highest intensity is measured. When the optical path difference is changed, interference causes the intensity to fluctuate. This interference-modulated signal as a function of optical path difference is an interferogram. The interferogram is the Fourier transform of the spectrum of the incoming radiation and the spectrum can be reconstructed from the recorded interferogram by the inverse Fourier transform. The information about the trace gases is retrieved by fitting a computer-simulated spectrum to the recorded one.
