Local and remote aerosol measurement techniques compared

December 12, 1999

CHAMPAIGN, Ill. - The environmental effects of man-made atmospheric particles, known as "aerosols," are of growing concern. Aerosols are now thought to significantly affect Earth's energy balance, throwing a wrinkle in climate-change forecasts. But accurate quantification of this effect will require combining remote techniques, such as satellite-based lidar, that can survey vast regions of the atmosphere, with local techniques that can provide detailed information on particle characteristics.

Researchers from the University of Illinois, the University of Washington and the University of Arizona recently met at an aerosol-monitoring site near Bondville, Ill., to study the influence of aerosols on climate change, and to compare the results from two fundamentally different measuring techniques. The program was supported by NASA and the National Oceanic and Atmospheric Administration.

"One of our goals was to perform systematic measurements of the ambient aerosols to better understand their chemistry and physics, and to improve the inputs for global climate models," said Mark Rood, a U. of I. professor of environmental engineering. "Another goal was to make a definitive determination of the uncertainties of the measurement methods under highly controlled conditions to help guide future ground-based, airborne and satellite measurements."

Local measurements were obtained by drawing particles out of the atmosphere and into a battery of instruments to determine aerosol mass concentration, composition, particle size distribution, and light scattering/absorption properties. Remote measurements were obtained by a micro-pulsed lidar deployed four kilometers from the station with its laser aimed directly at the station's sample inlet.

"With this setup, we obtained lidar data from the same air parcels being sampled by the instruments at the station," said Wayne Erxleben, a postdoctoral researcher at Arizona. The direct intercomparison of the two techniques was enabled by a new, 180-degree backscatter nephelometer operated as part of the local measurements at the station.

"The new nephelometer measures the amount of light scattered by particles directly back at the light source," said Tad Anderson, a research meteorologist from Washington and the director of the project. "This measurement is crucial to the comparison because it is the same quantity sensed by the lidar. But it is also a quantity that is very important to climate change, because it is closely related to the energy being re-radiated back to space by atmospheric particles, instead of being absorbed by Earth's surface."

A wide variety of pollution levels were encountered during the experiment, said Anderson, who presented the team's preliminary findings at the American Geophysical Union meeting in San Francisco, Dec. 13-17. "Both methods had high uncertainties during the clean conditions, but during the polluted conditions, our early analysis indicates agreement on key quantities within about 20 percent."

The results will be used to help interpret future measurements, including global aerosol surveys from a NASA satellite scheduled for launch in 2003.

University of Illinois at Urbana-Champaign

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