This Lidar technique is based on the use of a pair of wavelengths close to each other, with a large absorption coefficient difference. Such a pair of wavelengths, chosen for the detection of a specific pollutant, is sent into the atmosphere and backscattered signals are compared. The concentration of the pollutant is retrieved as a function of distance. By scanning in azimuth or zenith direction, 2D or 3D mappings are obtained, like a molecule specific radar.