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CORVALLIS, Ore. - Researchers studying the origin of cirrus clouds have found that these thin, wispy trails of ice crystals are formed primarily on dust particles and some unusual combinations of metal particles - both of which may be influenced by human activities.

The findings are important, scientists say, because cirrus clouds cover as much as one-third of the Earth and play an important role in global climate. Depending on altitude and the number and size of ice crystals, cirrus clouds can cool the planet by reflecting incoming solar radiation - or warm it by trapping outgoing heat.

However, what the net effect is, and how humans impact it, is still unclear.

Results of the study, which was funded by NASA and the National Science Foundation, were published this week in the journal Science.

"Cirrus clouds are complicated but the important message is that dust and certain metals provide the seeds for a majority of the ice crystals that form the clouds," said Cynthia Twohy, an Oregon State University atmospheric scientist and co-author on the study. "Other particle types - including bacteria and soot from human-produced combustion or natural sources - don't seem to contribute much to the nuclei of cirrus crystals.

"These biological particles may be important in the formation of lower altitude clouds," added Twohy, who is a professor in OSU's College of Earth, Ocean, and Atmospheric Sciences. "But they were surprisingly absent from the particles we sampled from cirrus clouds."

During the study, led by scientists at the Massachusetts Institute of Technology and the National Oceanic and Atmospheric Administration, the researchers conducted flight missions from 2002 to 2011 over North America and Central America at 20,000 to 50,000 feet elevation, where cirrus clouds often form. As their planes flew through the clouds, researchers captured and heated the ice crystals, which then evaporated, leaving behind a tiny kernel that they analyzed using an onboard mass spectrometer.

Despite the length of the study and its different geographic locations, the researchers found similar outcomes: About 60 percent of the cloud particles they analyzed could be traced to mineral dust blown into the atmosphere, or to metallic aerosols.

"Mineral dust can occur naturally," Twohy said, "or it can be influenced by human activities. Certainly the major deserts like the Sahara and Gobi are enormous sources of mineral dust. But agriculture, over-grazing and climate and land-use changes can also contribute."

Twohy said the scientists have not yet traced the origin of the dust to see how much of it came from natural versus anthropogenic causes. The metallic aerosols, she added, are unusual and may be easier to trace to specific sources. Containing elements like lead, zinc, tin and copper, they appear to be from industrial activities, according to other scientists in the study.

"As the climate warms, it is possible that we will see an expansion of desert lands, which could lead to even more dust entering the atmosphere," Twohy said. "That could create more cirrus clouds, but what that means in terms of warming or cooling is unsure and an area for future research."

An expert in cloud formation, Twohy has been involved in some 30 different aircraft missions over the years to understand the composition and characteristics of clouds and how they are influenced by pollution. She has studied clouds in North America, Central America, South America, Africa, the Southern Ocean and the Indian Ocean.

"At lower altitudes, clouds are known to be influenced by pollution - especially near cities," Twohy said. "They have more droplets, they reflect more light and they rain less. The impacts of cirrus clouds on climate are much more complex. But this gives us a starting point because we now have a better understanding of the particle types and mechanisms that lead to their formation."