Ground-based Fourier-Transform InfraRed (FTIR) spectrometers are used to determine zenith column amounts of trace gases like CO2, H2O, HDO, CH4, N2O, CFC-11, CFC-12, O3, NO, NO2, HNO3, ClONO2, HCl, and HF in the atmosphere. High resolution spectrometers are used, often from BRUKER® Optics. Absorption spectra are measured in the infrared spectral domain using the sun or the moon as natural source of radiation. In order to assure high quality of the spectra the spectrometer (hardware) has to be aligned very carefully. See here for detailed instruction.
To couple the solar beam into the spectrometer a solar tracking system is needed. Typically the tracker consists of 2 rotation stages (from Newport® in our setup): one to move the entire setup in azimuth, the other one to move the first mirror (in elevation). The position of the motor stages is calculated by a computer code. To ensure an accurate tracking an active control system is needed. Usually a quadrant detector is implemented to improve and check the accuracy of the tracking. More recently a CCD camera and a image processing software has been developed and is deployed at several sites:
The high spectral resolution of the FTIR is needed to separate lines of different species and to allow profile retrieval for some species. To ensure a trace gas profile retrieval not inferred by instrumental line shape errors the instrument has to be aligned very accurately. To test this, i.e. to make sure that the line shape is reproduced correctly by the instrument cell measurements are made and analysed with
To retrieve trace gas concentration profiles and column amounts the recorded broadband infrared spectra are devided into small spectral regions, so called microwindows. They have been selected for each trace gas to be retrieved. First of all synthetic spectra are calculated by a line-by-line and layer-by-layer algorithm for each microwindow. The synthetic spectra are fitted to the measured ones by the software package PROFFIT (PROFile FIT). PROFFIT allows to retrieve volume mixing ratio (vmr) profiles and to scale partial or total vmr profiles. PROFFIT allows us to fit several species simultaneously in several microwindows. The line shape of the absorption line is used to derive a profile. Please click here for further informations on