In this paper we present results derived from sky measurements in the thermal infrared (TIR) with the 4-channel ground-based radiometer CLIMAT. The measurements were performed during the Sahelian campaign NIGER-98, in the dry season of 1998, in the dusty region of Niamey (Niger). Cloud contamination is identified with the help of satellite data. Effects due to water vapor and mineral dust can be separated by means of their different spectral signatures with the available radiometric channels. The resulting TIR radiance of dust corrected from the water vapor changes is closely correlated to the photometric dust optical depth. The spectral signature of mineral dust derived from the campaign is compared with calculated signatures. It is shown to be strongly dependent on dust composition. The measured signature is in good agreement with simulations based on independent determinations of dust properties, including particle size distribution derived from AERONET photometric network and Sahelian dust composition from the literature.
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