Fourier Transform (FTIR) spectrometers are Michelson type interferometers. They use the mathematical concept of Fourier transformation as a means of converting a detector signal output – called the interferogram – into a form useful for spectral analysis. The instruments are set up to record interferograms of incoming solar light in the infrared region.
The instruments used in the FTIR ACTRIS central facility are commercially available high resolution spectrometers developed by the Bruker company (125HR or 125M).
The solar absorption spectra are analysed using optimal estimation implemented in the open source retrieval code SFIT. The CREGARS-FTIR central facility retrieves ozone, formaldehyde, nitrous dioxide, ethane and ammonia.
NDACC (Network for the Detection of Atmospheric Composition Change) has contributions from a large international community of scientists measuring with FTIR instruments, called NDACC Infrared Working Group (IRWG). The CREGARS-FTIR central processing is fully compliant to the guidelines set forward by the IRWG and all CREGARS-FTIR data is forwarded from the ACTRIS data center to the NDACC data host. Besides the central processing of the spectra, the CREGARS-FTIR central facility offers training courses, maintenance, development and documentation of the retrieval strategies for the target gases including new candidate targets, calibration services for FTIR instruments and a QA/QC facility for all instruments in CREGARS. Detailed information on the instrument configurations and retrieval implementation for each target species can be found.