One of the world's biggest exports is invisible, generates absolutely no economic gain and is moved around the globe, continent to continent, by the tons. It's a commodity that's tiny and foreign -- in fact you might be breathing some right now.
It's plain old dust, and enough of it crosses the Atlantic Ocean from Africa to America to eclipse federal government limits in Florida. In a recent storm it was spotted wafting from China to North America, clear across the Pacific.
An instrument aboard NASA's orbiting Terra spacecraft keeps an eye out for this uncontrollable traffic of dust. A recently released set of images compares a typical hazy day in China, on July 9, 2000 with a dust storm on April 7, 2001 and a higher-resolution image of the storm.
The images cover an area from central Manchuria, near the top, to parts of North and South Korea at the bottom.
The high-resolution image at right reveals a number of atmospheric wave features, like the ridges and valleys of a fingerprint, that scientists said are probably induced by changes in surface topography that disturb the wind flow. A few small cumulus clouds are also visible and are casting shadows on the thick lower dust layer.
African dust exceeds EPA limits
A 1999 study showed that African dust makes its way across the Atlantic, filling skies with enough particles to push parts of Florida, at least, over the prescribed air quality limit set by the U.S. Environmental Protection Agency (EPA).
The dust is kicked up by high winds in North Africa and carried 15,000 to 20,000 feet (4,570 to 6.100 meters) into the atmosphere by easterly winds, said Joseph M. Prospero of the University of Miami's Rosenstiel School of Marine and Atmospheric Sciences.
As the dust reaches the west coast of Africa, it's caught up in the trade winds and carried to North America, where it becomes an almost indistinguishable component of what the EPA calls particulate matter -- small bits that can penetrate deep into your airways and react with lung tissue.
Prospero says the African dust probably has a similar effect throughout the southeastern United States.
"Agencies normally think of air quality in terms of pollution that comes from local or regional sources," Prospero said in 1999. "The fact that such large amounts of dust can be carried over such long distances shows that we must also be concerned about distant sources because they can play a significant role in our air quality."
The specific impacts of African dust on health are unknown.
Age-old phenomenon
Wafting dust is nothing new. Ancient evidence of African dust exists in the soil of Bermuda, for example, indicating that it has been kicked up and transported across the ocean for eons.
The movement of dust from Africa to North American peaks during the summer, when several pulses, each lasting several days, bring large quantities of reddish particles across the Atlantic.
Prospero says manmade photochemical pollution in the eastern and southeastern United States is also at its highest in summer. The dust causes a dense haze and often is mistaken for local pollution, he said.
Asian dust
Asia's desert areas are prone to soil erosion, as underground water tables are lowered by prolonged drought and by industrial and agricultural water use, researchers say. Heavy winds blowing eastward across the arid and sparsely vegetated surfaces of Mongolia and western China pick up large quantities of mustard-colored dust.
Airborne dust clouds from the April 2001 storm blew across the Pacific Ocean and were carried as far as North America. But the worst effects are felt in China.
Nearly a million tons of Gobi Desert dust blow into Beijing each year, according to the Xinhua News Agency in China. During a similar dust outbreak last year, the Associated Press reported that the visibility in Beijing had been reduced to the point where buildings were barely visible across city streets and airline schedules were significantly disrupted.
The effects aren't all bad: The minerals transported in this manner are believed to provide nutrients for both oceanic and land ecosystems.
Analyses of images such as these feed data into the Asian-Pacific Regional Aerosol Characterization Experiment, an international campaign to understand how dust and other particles -- including those emitted by human activities and industrial sources -- influence the chemistry of the atmosphere.
Researchers also want to learn how all this affects the amount of solar energy Earth radiates back into space, which researchers say will improve understanding of how future changes in aerosol concentration and composition may influence changes in Earths climate system as a whole.
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