Passive remote sensing with a Fourier transform IR (FTIR) spectrometer allows the detection and identification of pollutant clouds in the atmosphere. In this work the measurement technique and a data analysis method that does not require a previously measured background spectrum are described. Recent experimental results obtained with anew high sensitive FTIR remote sensor are presented. Many situations do not allow the measurement of a background spectrum prior to the measurement of pollutants in order to perform background removal. After a radiometric calibration of the FTIR spectrometer with IR reference sources the spectral radiance of the environment can be measured. With the inverse function of Planck's radiation law the brightness temperature is computed. The temperature spectrum has a constant baseline for many natural materials that serve as the background in field measurements because their emittance is high and almost constant in the spectral range 800-1200 cm-2. The influence of environmental and instrumental parameters on the sensitivity of the method are discussed. Experimental results are presented to illustrate the enhancement of the signal to noise ratio that can be achieved by the alignment of the spectrometer to backgrounds with a high temperature difference to the environment.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.