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March 4, 2002

Atmospheric aerosols found to brighten clouds

Higher reflectivity may result in global cooling, offsetting greenhouse effect

WEST LAFAYETTE, Ind. – Atmospheric scientists have long suspected that microscopic aerosol particles emitted from industrial processes increase the brightness of clouds, resulting in greater reflection of sunlight and cooling of Earth's climate. However, this supposition is based on model calculations rather than observations, and the model calculations are uncertain.

Now, scientists at Purdue University and the U.S. Department of Energy's Brookhaven National Laboratory have combined satellite measurements of cloud brightness, water content and other variables with model calculations of atmospheric aerosols to demonstrate the brightening effect.

This effect, described in the Feb. 19 issue of the Proceedings of the National Academy of Sciences, should be accounted for in assessing the magnitude of global climate change, the researchers say.

"We're not saying that aerosols can counteract the greenhouse effect," said lead scientist Stephen Schwartz, an atmospheric chemist at Brookhaven, "but rather that we need to know how much of a cooling effect they have so we have a clearer picture of the greenhouse effect."

Working with Schwartz in the study were Carmen Benkovitz of the Atmospheric Sciences Division at Brookhaven National Laboratory, and Harshvardhan, professor and head of Purdue's Department of Earth and Atmospheric Sciences.

Schwartz's team has been working for more than a decade to develop and refine a "chemical transport model" to calculate aerosol distribution. The model uses archived weather data and weather prediction models to track the distribution of aerosols from industrial sources to various parts of the atmosphere.

"This model is the key to knowing where and when to look for the aerosol effect," he said.

By analyzing data from the model, the Purdue-Brookhaven team identified two one-week episodes during April 1987 when the modeled concentration of sulfate aerosol over the North Atlantic Ocean – far from any local sources of aerosol emissions – increased significantly and then decreased over the course of each week. These large variations in aerosol concentration and the fact that there were no obscuring clouds during these events made them ideal episodes for studying the effect of aerosols on cloud brightening.

The next challenge was to get the data on cloud brightness for that area over the same time period. For this, the Purdue group retrieved satellite measurements of radiance, showing how much light the clouds reflected, and optical depth, a value related to how much light is transmitted through the cloud. They then used the measurements to calculate the size of the cloud droplets and the amount of liquid water in the cloud.

The findings show that, for a given amount of liquid water in the cloud, its reflectivity was indeed higher on the days with higher aerosol content than on the days with lower aerosol levels.

"If the effect is as widespread as we think it is, it would produce quite a substantial cooling effect on climate," Schwartz said. "This new study provides a method of quantifying the phenomenon globally over the past 15 years using archived satellite data. Once this is done, we will have a much better idea of the true magnitude of the aerosol effect, which counteracts the greenhouse effect globally."

Harshvardhan notes the aerosol effect may have a different geographical distribution from the greenhouse effect, and "the consequence of this mismatch is unknown." One key to assessing the overall impact of aerosols, he said, will be further development of the satellite-based measurements. Research in this area is now under way in Purdue's Department of Earth and Atmospheric Sciences.

This work was funded by the U.S. Department of Energy and the National Aeronautics and Space Administration.

Contact: Susan Gaidos, Purdue University News Service, (765) 494-2081,

Writer: Karen McNulty Walsh, Brookhaven National Laboratory, (631) 344-5056,

Sources: Stephen Schwartz, (631) 344-3100,

Harshvardhan, (765)-494-0693 or (765)-494-4753,

NOTE TO JOURNALISTS: This news story, prepared at Brookhaven National Laboratory, is being released jointly by BNL and Purdue University. Harshvardhan, a researcher involved in the following study, uses only a single name.

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