From: Sarah Yang, University of California - Berkeley
Published May 19, 2009 10:10 AM

Summer haze has a cooling effect in southeastern United States, says new study

Global warming may include some periods of local cooling, according to a new study by researchers at the University of California, Berkeley. Results from satellite and ground-based sensor data show that sweltering summers can, paradoxically, lead to the temporary formation of a cooling haze in the southeastern United States.

The study, to be published the week of May 18 in the journal Proceedings of the National Academy of Sciences, found that when manmade pollutants mix with the natural compounds emitted from forests and vegetation during the hot summer months, they form secondary aerosols that reflect light from the sun. Such aerosols may also contribute to the formation of clouds, which also reflect sunlight.

The results of this study suggest that climate models need to better account for the effects of organic aerosols, the authors said.

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The researchers conducted observations of aerosols throughout the earth's atmosphere using space-based satellites in combination with ground-based sunphotometers between March 2000 and February 2007.

"This is the first time a study has shown that the aerosols formed from the combination of manmade and natural emissions observed from space are relevant for understanding earth's climate," said study lead author Allen Goldstein, UC Berkeley professor and chair of the Department of Environmental Science, Policy and Management.

The study began when former UC Berkeley Ph.D. student and study co-author Charles Koven showed Goldstein satellite data that indicated a summertime spike in aerosol haze in the southeastern United States. Goldstein noticed that the increased haziness, which could not be explained by human activities alone, coincided with the known regional pattern of biogenic volatile organic compounds. The emission of these compounds - natural hydrocarbons from plants and trees - increases exponentially when the temperature rises, said Goldstein.

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