Differential Absorption Lidar Techniques in the Determination of Trace Pollutants and Physical Parameters of the Atmosphere

Abstract Optical remote sensing, remote analysis by means of light, is a two-step analysis: the first step consists in a proper excitation of the medium under study by sending into it radiation at optical frequencies; the second step consists in collecting from it a suitable light signal which has the property of being a direct or indirect carrier of the information needed about the interaction. Therefore, we need a suitable light source as a light transmitter. It can be a natural source, for example, the sun, in the passive techniques or it can be a lamp or a laser, in the active techniques. The receiving antenna is a lens or a telescope. It has the task to collect the light on a detector for the conversion of the optical signal in an electrical signal which is much easier to handle. If the light source is able to emit short pulses, then this configuration is identical to that of radio detection and ranging (radar), whose principles, applications, and utility are well known. This instrument is able to remotely detect an object and evaluate its distance by measuring the time delay between the emission of the electromagnetic pulse and the return of the echo radiation. Radar was utilized very early in the meteorological field, but in this particular application, the radar principle was first utilized at optical frequencies. In fact, in 1938, a group of French researchers was able to measure the height of the cloud base by utilizing tight pulses generated by electric sparks. I Their device, subsequently utilized for routine measurements, can be considered the ancestor of the actual light detection and ranging (lidar) i.e., the optical analog of radar.