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.

Vast clouds of dust, soot, and other tiny particles called aerosols migrate over the Pacific from eastern Asia to North America. Now a team of American, Chinese, Japanese, and South Korean scientists is in the midst of a two-month effort to conduct the most detailed study yet of this region's air-pollution plumes.

The goal is to help provide a reality check on climate models, which poorly represent the effect these particles have on the global and regional climate. The results of these field measurements could well feed into current efforts by the World Meteorological Organization and the European Center for Medium-Range Weather Forecasts in Britain to build the effects of airborne particles into weather forecasts.

By any measure, the Asian plumes represent some of the largest pollution events on Earth, researchers say. While air pollution also migrates from North America to Europe, and from Europe across Eurasia, those amounts pale in comparison to Asia's eastbound freight.

Soot from Asia that reaches the West Coast accounts for 80 percent of the black-carbon soot in the skies over the United States, notes Veerabhadran Ramanathan, director of the Center for Clouds, Chemistry, and Climate at the Scripps Institution of Oceanography in La Jolla, Calif. More generally, natural and man-made particles in the plumes represent the single most vexing problem atmospheric scientists face as they strive to understand the handful of outside factors, or "forcings," that affect Earth's climate system.

Aerosols, soot, and dust collectively "are the big gorilla at the table," Dr. Ramanathan says.

These particles have a direct effect on global and regional climate by intercepting sunlight and radiating it back into space. Over the Pacific on a clear day, the plumes can cut sunlight reaching the ocean surface by 10 to 15 percent, scientists say. Globally they may be concealing as much as half the warming effect of the carbon dioxide that human industrial processes have pumped into the air since the beginning of the Industrial Revolution, researchers add.

Moreover, these particles have an indirect effect on climate and weather through their complex effects on cloud formation. And they represent a significant source of airborne gunk that can make it difficult for some cities in the western United States to meet air-quality standards.

Many of these effects are still poorly understood and quantified. To help fill the gap, the team, led by Ramanathan and Jeff Stith of the National Center for Atmospheric Research in Boulder, Colo., is flying an instrument-laden Gulfstream jet through the plumes as they migrate across the Pacific. Although the team also is using satellite measurements and data from ground stations, the jet holds the key to the project. Its 6,000-mile range and its ability to fly from just above the sea surface to 50,000 feet allows the team to get samples from the plumes – and from the clouds they affect – at a range of altitudes.

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.

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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