Lidar Technique Refines Aerosol Data
SEATTLE — Measuring the impact of man-made aerosols on global climate change may become a little more accurate, thanks to a new device termed a 180° backscatter nephelometer. The device, developed by Theodore L. "Tad" Anderson and his colleagues in the department of atmospheric sciences at the University of Washington, measures the light scattered by particles suspended in a gas; it will enable atmospheric scientists to compare the results of satellite-based lidar systems against standardized data gathered on the ground.
The apparatus could provide a reference point for researchers, enabling them to "ground-truth" their lidar results for the atmosphere against a known quantity.
Researchers suspect that aerosols contain particulates that reflect solar radiation out into space before it reaches the Earth's surface, cooling the planet in the process.
Lidar can be used to estimate the amount of light scattered by aerosols if the ratio between extinction and 180° backscatter is known. Extinction is the number of photons scattered or absorbed per unit distance; 180° backscatter refers to light scattered back directly to the laser.
This ratio could only be estimated previously from theoretical models of aerosol properties. Lack of knowledge of this ratio prevented quantitative use of lidar data for studying the aerosol climate problem.
Enter the 180° nephelometer. Like traditional nephelometers, it measures the amount of light lost to scattering by drawing aerosol particles through an enclosed chamber. A portion of the chamber is illuminated, and photomultiplier tubes sense the light. However, where traditional nephelometers use illumination over a broad range of angles, this one uses an Nd:YAG 532-nm laser beam and a set of mirrors designed so that the only illumination to reach the tubes comes from just the 177° angle -- close to the 180° angle at which lidar systems operate.
Satellite-based lidar can give information about broad areas of the atmosphere, but it must be checked against local sampling areas that use nephelometers to measure the amount of light lost to scattering. A new 180° nephelometer measures this scattering at the same angle at which lidar systems operate, allowing for more accurate interpretation of lidar data.
By drawing a known particulate into the nephelometer and measuring the ratio of extinction to backscatter, researchers can validate their remote lidar measurements against a calibrated, on-site technique.
The 180° nephelometer will be commercially available from TSI Inc. of St. Paul, Minn., in approximately one year.
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