III.3. MODERN TECHNIQUES FOR AIR POLLUTANTS

III.3. MODERN TECHNIQUES FOR AIR POLLUTANTS

LIDAR is the optical equivalent of the radar, and so is often referred to as laser radar. In a radar, radio waves are transmitted into the atmosphere, which scatters some of the power back to the radar's receiver. A LIDAR also transmits and receives electromagnetic radiation, but at a higher frequency. LIDARs operate in the ultraviolet, visible and infrared region of the electromagnetic spectrum.

LIDAR is an acronym for light direction and ranging, and is a laser remote sensing technique used in both science and industry. LIDARs are used to precisely measure distances and properties of far-away objects.
III.3.1.1. LIDAR Design
A block diagram for a basic LIDAR system is illustrated in Figure III.3.1.

A simplified block diagram of a LIDAR contains a transmitter (laser), receiver (an optical telescope) and a detector system.

In LIDAR a powerful laser transmits a short and intense pulse of light. The pulse is expanded to minimize its divergence, and is directed by a mirror into the atmosphere. As the pulse travels upward it is scattered by atmospheric constituents (mostly nitrogen) and aerosol particulates. Light that is backscattered and into the field-of-view of the telescope is collected and channelled toward detectors by an optical fiber or other optics. Filters are used to eliminate light away from the laser's wavelength, and a mechanical shutter blocks the intense low-level returns when required. The amount of light received is measured as a function of time (or distance) using sensitive photo-detectors, and the signals are digitized for storage on a computer's hard drive.

LIDARs typically use extremely sensitive detectors called photomultiplier tubes to detect the backscattered light. Photomultiplier tubes convert the individual quanta of light and photons first into electric currents and then into digital photo-counts, which can be stored and processed on a computer. The photo-counts received are recorded for fixed time intervals during the return pulse.

In the last years different kinds of lasers were used depending on the power and wavelength required. The lasers may be both cw (continuous wave, on continuous like a light bulb) or pulsed (like a strobe light). Gain mediums for the lasers include, gases (e.g. HeNe = Helium Neon or Xenon Fluoride), solid state diodes, dyes and crystals (e.g. Nd:YAG = Neodymium:Yttrium Aluminum Garnet).

LIDARs are valuable instruments for atmospheric research because they provide an active remote sensing technique that can probe atmospheric regions inaccessible to other instruments, and at high spatial and temporal resolution. The technique has made possible the spatially resolved measurement of atmospheric constituents, as well as various atmospheric parameters, such as temperature, winds, clouds, etc. LIDARs operating from ground and space provide complementary information: where satellite-borne LIDARS provide global coverage but at lower horizontal resolution, ground-based instruments reveal the fine detail required for atmospheric process research.