April 23, 2007
Monday

Vast plumes of fine-grained desert sand and pollution sweep constantly from Asia across the Pacific and over North America, darkening skies along the way and affecting the weather and global warming, but scientists don't yet know to what extent.

This week, researchers plan to launch a flying laboratory they hope will give them an answer.

A team from the University of California-San Diego's Scripps Institution of Oceanography and the National Center for Atmospheric Research in Boulder, Colo., began successful test flights last week aboard the government's newest high-altitude research aircraft. Within a few days, the team's leaders say, the Gulfstream V plane, financed by the National Science Foundation, will head to Anchorage, Alaska, and from there will fly over the ocean to Yokota Air Base near Tokyo on the first leg of its six-week mission capturing plume samples.

The plumes of particles, still poorly understood, are so huge that they are among the largest weather-influencing events on Earth. They are caused by a combination of factors.

Dust storms from fierce winds that whip the Central Asian deserts cast a pall over Korea, rise high into the atmosphere as they speed eastward, and often cause a yellow-brown haze over California, the scientists say. At the same time, millions of tons of dense soot and chemicals rise into the air from China's booming coal-burning industries each year, and they, too, add to the density of the plumes that reach North America.

"Dust from deserts like the Gobi and the Taklimakan have been crossing the Pacific at high altitudes for millennia, but in the last 10 or 15 years it has been mixed with a growing burden of soot and industrial chemicals," said Veerabhadran Ramanathan, an atmospheric scientist at Scripps. "Now we need to know just how those micron-sized particles can change global temperatures and storm clouds and rainfall patterns."

Ramanathan and Jeffrey Stith, a cloud physicist at the National Center for Atmospheric Research, are chief scientists for the airborne mission called PACDEX, the Pacific Dust Experiment. On each flight, one of the two will join other researchers aboard the plane, while the other leads a ground-support team providing updated information from computer models and satellite data.

During six weeks of the mission, the plane will fly wherever the prevailing springtime winds are carrying the dust and soot. The flying laboratory's 6,000-mile range and ability to fly as high as 45,000 feet - or even down to 1,000 feet - will allow it to grab particle samples for analysis on board.

Ground-based scientists in China, Japan and South Korea are already making measurements with instruments beamed skyward. In addition, Kimberly Prather, an atmospheric chemist at Scripps and a member of the PACDEX team, has sent one of her graduate students, Andrew Ault, 23, to Gosan Island off the southern tip of Korea to analyze microscopic dust and pollution particles at ground level.

Scientists like Ramanathan, Stith and Prather believe those particle clouds play an as-yet unrealized role in global warming. Water vapor in high-altitude clouds cannot form raindrops or ice crystals without a single particle as a nucleus for each drop or crystal. When they do form, they trap heat just the way greenhouse gases do.

"Dust from the Asian deserts plays a major, major role in climate change," Prather said, "and that fact hasn't figured fully so far in all the international estimates of the rate of change. It's what we're trying to calculate right now, and it's taking our neat chemistry experiment to know how the atmosphere changes when dust is aloft."

The particle clouds aren't on a one-way street, however, Ramanathan noted. The winds over American industry are also sending pollutants - particularly sulfates and nitric oxides that cause acid rain - far across the Atlantic.

"We send our own brown clouds to Europe and Asia," he said, "so each one of us is polluting each other's backyard."

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