The measurement relies on the unique "fingerprint" absorption spectrum of each molecule. An absorption measurement is made with laser light, at a peak of absorption (lambda-on) and at a trough (lambda-off), giving a differential signal. The differential nature of the signal simplifies the measurement process.
A pulsed laser beam is sent out into the atmosphere (Figure 1) and small proportions of the light are backscattered by particles along the beam path to a sensitive detector (Figure 2). In this sense dust particles and aerosols are being used as reflectors, albeit rather weak ones. The laser light is in short pulses and time resolution of the backscattered light (along with the speed of light) gives range resolution as in a simple LIDAR (light detection and ranging).
This is an important advance on the more conventional monitoring systems in which a retro-reflector, which must be re-positioned after each measurement, is used to return the laser beam to the detector. In these systems an average concentration of the species to be measured is obtained and range resolution is not possible, which is a severe limitation.
For concentration measurement the DIAL system relies on a differential return from two closely spaced wavelengths, only one of which is absorbed strongly by the target gas. The size of the differential return signal at different distances along the laser beam path indicates concentration.