Carbon-bearing particles like tar balls in the lower atmosphere are a concern, they say, because they may affect global climate change, as well as air quality.

The team, headed by Mihály Pósfai, an Earth and environmental science professor at the University of Veszprém in Hungary, completed the first comprehensive study of tar balls and report their findings this month in the Journal of Geophysical Research--Atmospheres, published by the American Geophysical Union.

"They are especially abundant in slightly aged--minutes to hours old--biomass [vegetal] smoke," says co-author Peter Buseck, a geochemist at Arizona State University. That means they probably formed from gases in smoke plumes, he says, and contain organic compounds that absorb sunlight. "Tar balls occur in a variety of atmospheric environments that are affected by human activities," he says.

At first glance, tar balls may look like soot, a common form of carbon pollution in the air, but when observed through an electron microscope, the differences become clear, the researchers say.

"Soot forms in the flame and consists of spheres," Pósfai says. Each soot sphere is made of graphitic layers that are concentrically wrapped like the layers of an onion and, with other soot spheres, forms chains or grape-like clusters, he says. Tar balls, on the other hand, are just individual spheres and do not form chains or clusters. They lack any internal structure and don't have onion-like layers in them.

"The internal structure affects the optical properties of the particle--the more ordered the graphitic structure, the darker the color," Pósfai explains. "Dark particles absorb sunlight and thereby heat the atmosphere."

While black soot is the major absorber of sunlight in the atmosphere, tar balls may also be absorbing sunlight. "And this is significant," he says.

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