Understanding lidar
How does a lidar measure wind and detect air pollution?
By Jane Palmer
Scientists use LIDARs—LIght Detection and Ranging instruments—to detect aerosols and ozone and measure winds in the atmosphere. Lidars are similar to radars, but instead of using radio waves to make their measurements, they use laser light: They emit laser pulses into the atmosphere that are then scattered by air molecules and aerosols. Some of this scattered light returns to the lidars, where it is analyzed to determine wind velocity, detect aerosol layers and measure ozone concentration at several ranges along the lidar line-of-sight.
Lidars make use of the Doppler effect to determine average velocity of particles from which the light is scattered. Because of the Doppler effect, the wavelength of light scattered from particles moving toward or away from the lidar is slightly changed. By measuring the shift in wavelength of the scattered
“Our ozone instrument is cutting-edge because it incorporates a solid-state laser that directly produces ultraviolet light.”
—CIRES Fellow Mike Hardesty
light collected at the lidar, the velocity of the particles can be estimated. Because the particles are very small and move with the mean wind, measuring the average particle velocity is equivalent to measuring the average wind velocity.
To measure ozone, the differential absorption lidar (known as DIAL) method is used. In the DIAL technique, laser pulses at two or more wavelengths are transmitted into the atmosphere, with the wavelengths chosen so that different portions of the energy from each wavelength are absorbed by the ozone along the paths. By looking at the difference in the amount of light at each wavelength, or color, that comes back to the lidar, scientists can calculate the quantities of ozone present in the atmosphere.
“Our ozone instrument is cutting-edge because it incorporates a solid-state laser that directly produces ultraviolet light,” Hardesty said. The ultraviolet source has the advantage of being rapidly tunable so as to produce at a very high rate the different wavelengths needed for ozone measurements.
