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September 16, 1999

‘Soapy’ droplets make brighter clouds

News and Information

The organic properties of some particles, such as those from the burning of agricultural waste, have been found to increase the number of cloud droplets in polluted air, allowing more sunlight to be reflected into space than would occur normally.

The phenomenon affects climate locally, and probably regionally, say researchers from the Consilio Nazionale delle Ricerche in Bologna, Italy, and the University of Washington in Seattle, who report their findings in the Sept. 16 edition of the British journal Nature.

Such organic particles bring a decrease in surface tension – commonly observed when a small amount of soap or detergent is added to water – during the formation of cloud droplets, said Robert Charlson, a UW atmospheric chemist. The result is the formation of more droplets than normal, making clouds that are somewhat more reflective of sunlight.

Different organic substances have different effects on surface tension, depending on the molecular nature of the particles, he said.

“Just compare a teaspoonful of sugar to a teaspoonful of detergent in, say, a bucket of water. The sugar does virtually nothing while the soap makes the water sudsy,” Charlson said.

Charlson co-authored the study with Maria Cristina Facchini, Mihaela Mircea and Sandro Fuzzi of Consilio Nazionale delle Ricerche.
To reach their findings, the researchers collected large volumes of water from clouds in the Po River Valley of northern Italy. The water then was evaporated to simulate the reverse process of droplet formation.

“What we found out is that the stuff in the Po valley is more like soap than sugar,” Charlson said. The researchers discovered up to a 30 percent decrease in surface tension in the cloud droplets, which they calculate would result in about 20 percent more droplets than normal and reduce the size of droplets about 6 percent.

At the top of the atmosphere, the increase in the number of droplets results in about 1 percent more sunlight being reflected away from Earth, they said. The effect of the small increase in reflection is opposite the warming associated with man-made carbon dioxide in the atmosphere, but the pollution particles also represent a fundamental change in the chemistry of the atmosphere. Similar effects are likely in other agricultural and industrial regions, Charlson said, but no data are available to make a global estimate.

“It helps to explain the many complex factors that control the amount and properties of clouds on Earth,” Charlson said. “Clouds are a major feature of climate that are essential for precipitation, but they also strongly influence how much sunlight reaches the surface.”

The researchers noted that many organic compounds that turn up as atmospheric particles, or aerosols, often are listed as insoluble but in reality are soluble under the conditions that trigger droplet formation.

In the last 10 years, Charlson has pushed the idea that, in some regions, aerosols from industrial pollution might be countering the atmospheric warming effects of greenhouse gases by reflecting sunlight into space. He is part of the science team for a NASA mission scheduled for launch in 2003, called PICASSO-CENA, which should provide crucial data to help scientists better understand the role of aerosols in human-caused climate change.

However, that work involves particles present in the atmosphere outside of clouds. The current study is among the first to examine how organic pollutants change the reflective properties of clouds.

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For more information, Charlson can be reached through Sept. 21 at the Hotel Commercianti in Bologna, 011 39 051 233052. The hotel’s fax number is 011 39 051 224733. After Sept. 21 he can be reached at Stockholm University, 011 468 162 397.

NOTE: Time difference is six hours later than EDT, nine hours later than PDT.