A method for the passive remote monitoring of chemical vapors by differential Fourier-transform infrared radiometry is presented to determine the characteristics of a chemical vapor plume from a stack located at a distance of more than 1 km from the sensor. This measurement technique is based on the use of a double-beam Fourier-transform infrared spectrometer that is optimized for optical subtraction. A description of the interferometer (compact atmospheric sounding interferometer) is given along with the algorithm (GASEM) that has been developed for the on-line detection, identification, and quantification of chemical vapor plumes. The detection method is described with a particular emphasis placed on its monitoring capabilities. The analysis focuses on the experimental results obtained at a recent open-air experiment for vapor plume mixtures of dimethyl methyl phosphonate and SF6 probed at a distance of 1.5 km. The accuracy of a simplified plume radiance model implemented in the detection algorithm is specifically addressed. The measurement technique has been successfully used to detect and identify low, medium, and high concentrations of vapor mixtures but appears to have limited quantification capabilities in its present form.
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