The presented system is based upon a differential imaging radiometry technique, differential in both spectral and spatial domains: The observation of a scene, acting as an IR complex source is realized through a set of wide band filters: 1) a 'reference' filter, the transmission of which is located outside the absorption features of the gas of interest: the resulting picture characterizes the scene background. 2) A 'measure' filter, the transmission of which includes the absorption or emission bands of the gas of interest while remaining as close as possible to the 'reference' filter. The use of such filters, characterized by a high IR transmission over a wide wavelength band unlike the conventional radiometric methods, preserves the imaging function of the thermal imager. A specific treatment is applied to the resulting pictures. Its originality consists in that gas concentration is not directly computed from the received IR flux, but from the image contrast variations between the two filtered images. In this way, the emission flux of gas cloud can be eliminated. Finally, the comparison of the 'contrast' images gives an absolute and quantitative measurement of gas concentration of the cloud integrated along the line of sight, whatever the respective temperatures of the gas cloud and the background scene are. A first generation demonstrator base don 8-12 micrometers commercial radiometric camera, and integrating a quasi-real time processing software, was tested on several chemical agents, both in closed chamber and in open field. This paper presents an overview of the measurement technique and gas cloud evolution experiments in urban atmosphere.