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Small particles' big impact on climate

Dust and soot from Asia create air pollution in California, but also temper global warming and may stymie hurricane formation. Scientists are taking a look.

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Of particular interest is the effect aerosols have on conditions in the middle and upper layers of the troposphere. While it will take years to make full sense of the data, even now the team is gaining a deeper appreciation for the challenges aerosols present. Dr. Stith notes, for example, that the team has found pollution layers a few hundred feet thick sandwiched between other pollution layers – each layer with its own humidity-related tipping point for forming cloud droplets. These differences present challenges to global and regional climate models, which have a tough time capturing processes that happen on such small scales.

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Ramanathan, who took part in a similar, larger-scale experiment over the Indian Ocean in 1999, notes that this time black-carbon soot is appearing at far higher altitudes than it did over the Indian Ocean.

"That worries me greatly," he says, because the higher the soot, the longer it remains in the atmosphere. Soot at six miles up has two to three times the warming effect of soot at half a mile up, he says, because of its persistence at higher altitudes.

Moreover, high in the troposphere, winds can carry aerosols and soot around the globe in under two weeks, affecting cloud formation far from the aerosols' sources. In addition, wispy cirrus clouds form at those altitudes from ice crystals and can amplify the greenhouse effect.

Researchers are unclear about how different mixes of aerosols might affect cirrus clouds. Some combinations might reinforce these clouds' warming effect; others might undercut their blanketlike traits.

It's been a long road to get this far. Over the past 15 to 20 years, atmospheric scientists have grown to appreciate the role soot, dust, and aerosols play as thermostats and potential weather modifiers.

More recently, scientists have begun to include the effects aerosols have on regional weather patterns and even on individual storm systems.

Last fall, for example, scientists from the University of Wisconsin and the National Oceanic and Atmospheric Administration published a study noting that when large dust plumes blow off Africa and over the tropical Atlantic, where hurricanes form, fewer hurricanes seem to occur. Based on earlier work, they noted that the sun-warmed dust at high altitudes could keep nighttime temperatures there warm enough to help stabilize the atmosphere and prevent the development of collections of towering thunderheads that can evolve into tropical cyclones. In March, another team found that Asian plumes appeared to strengthen winter storms in the North Pacific.

The current field project, known as PACDEX, should help bridge the gap be­­tween modeling results and actual conditions. Instruments aboard the Gulfstream jet not only analyze the composition of aerosols in the plumes they encounter, they also intercept cloud droplets and ice crystals and extract the aerosol particles around which they grew. US ground stations help researchers track the evolution and destination of the particles as they reach North America.

To feel more confident about how climate and future weather-forecasting models handle these particles, "we need to understand and observe the interactions of the relevant aerosols in cloud systems," says Greg Charmichael, an atmospheric scientist at the University of Iowa and a member of the PACDEX team.

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