Lidar is an acronym for Light Detection and Ranging, an ORS technique that uses laser light pulses to detect contaminants or aerososls in much the same way that sonar uses sound pulses, or radar uses radio waves. With LIDAR a pulse of laser light is sent into the sky, and the instrument measures the amount of light returned due to backscatter from the atmosphere as a function of time. Since the speed of light is a known constant, the time is converted into a distance measurement. The amount of light returned at each distance tells us the atmospheric density of the contaminants.
Lidar can be applied to measure a variety of things in the atmosphere, including temperature profiles, clouds, and atmospheric aerosols. If two laser pulses at appropriate wavelengths are employed, gaseous contaminants such as ozone or NOx can be observed with a technique know as DIAL, which stands for Differential Absorption Lidar.
Lidar has several advantages over traditional methods, but the main advantage is that it can map the location if contamintants over a wide region. Lidars can perform measurements up to 80 km, which is a verly large scale. Due to the rapid nature of laser pulses, the time resolution is very critical (a few nanoseconds) to get good spatial resolution.
The differential absoprtion lidar (DIAL) is a tunable two wavelength transmitter which is used to measure the constituents of the atmosphere. One wavelength is set to match the maximum absorption line of the gas of interest while the second wavelength is calibrated to a nearby wavelength of low absorption. The two wavelengths travel through the atmosphere and are scattered back to a dual channel receiver. The signals are compared and a direct measure of the concentration of the absorbing gas as well as its position in space can be determined.
RAYLEIGH SCATTERING
Rayleigh scattering is simply the theory of elastic (elastic meaning retains wavelength) light scattering by molecules. The Rayleigh Lidar technique is used to study the thermal structure of the middle atmosphere through the creation of density and temperature profiles.
Below 30km, the received signal will contain a significant amount of aerosoal scattering. However, above 30km, lidar measurements consist primarily of elastic molecular backscattering. In the absense of aerosals and when scaled by the square of the altitude, the backscattered signal is proportional to the atmospheric density. Manipulation of the equation of hydrostatic equilibrium and the ideal gas law leads to a measure of temperature.
[This section created from material from the ISTS Lidar laboratory]
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