air pollution

Why black carbon’s in the climate hotseat

U. IOWA (US)—Increasing the ratio of black carbon to sulphate in the atmosphere increases climate warming, finds a new study published in the journal Nature Geoscience.

Black carbons—arising from such sources as diesel engine exhaust and cooking fires—are widely considered a factor in global warming and are an important component of air pollution around the world, according to Greg Carmichael, a professor of chemical and biochemical engineering at the University of Iowa.

Sulfates occur in the atmosphere largely as a result of various industrial processes.

To conduct their study, Carmichael and colleagues made ground-level studies of air samples at Cheju Island, South Korea, and then sampled the air at altitudes between 100 and 15,000 feet above the ground using unmanned aircrafts.

They found that the amount of solar radiation absorbed increased as the black carbon to sulphate ratio rose.

Also, black carbon plumes derived from fossil fuels were 100 percent more efficient at warming than were plumes arising from biomass burning.

“These results had been indicated by theory but not verified by observations before this work,” Carmichael says. “There is currently great interest in developing strategies to reduce black carbon as it offers the opportunity to reduce air pollution and global warming at the same time.”

The study authors suggest that climate mitigation policies should aim to reduce the ratio of black carbon to sulphate in emissions, as well as the total amount of black carbon released.

In a paper published in May 2008 in Nature Geoscience, Carmichael and collaborators found that black carbon soot from diesel engine exhaust and cooking fires—widely used in Asia—may play a larger role than previously thought in global warming.

They note that coal and cow dung-fueled cooking fires in China and India produce about one-third of black carbon; the rest is largely due to diesel exhaust in Europe and other regions relying on diesel transport.

The paper also notes that soot and other forms of black carbon could equal up to 60 percent of the current global warming effect of carbon dioxide, the leading greenhouse gas.

Researchers from the Scripps Institution of Oceanography, La Jolla, Calif.; Seoul National University, South Korea; and the University of Wisconsin, Madison, contributed to the latest study, which was funded by the National Science Foundation.

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