Small particles' big impact on climate

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.

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