In collaboration with partners (see 'sites'), the IMK operates four ground-based Fourier-Transform Infrared Spectrometers (FTIR) in Europe and Africa: At Kiruna (North of Sweden), in Karlsruhe (Germany), at Izaña (Tenerife Island, Spain) and at Addis Ababa (Ethiopia). They are used to measure the concentration of trace gases in the atmosphere. Absorption spectra are recorded in the infrared spectral region using the sun (or the moon) as natural sources of radiation. Profiles of H2O, CH4, N2O, O3, NO, HNO3, HCl, and HF and column amounts of CFC-11, CFC-12, NO2, ClO, and ClONO2 are derived from these spectra. These measurements are made within the framework of the NDACC (Network for the Detection of Atmospheric Composition Change) and TCCON (Total Column Carbon Observation Network). Their aim is to record long-term trends of ozone related species and greenhouse gases, and to validate satellite data.