撒哈拉沙尘的移动之谜

Satellite pictures of Saharan dust clouds have been in the news all summer, but to Shankar Chellam, they have just raised more questions. How much impact did the Saharan dust have on Houston's air? Is it more toxic than our home-grown dust? Chellam, a professor in the department of civil and environmental engineering at the University of Houston's Cullen College of Engineering, is searching for answers to those and other questions. Clouds of African dust often migrate across the Atlantic Ocean during summer months, affecting Houston's air quality from mid-June through mid-September. Chellam said it's especially prevalent in late August and early September.

The dust, whipped up（筋疲力尽） by sandstorms in northwest Africa and carried by trade winds across the Atlantic Ocean, takes about 10 days to two weeks to reach the United States and, ultimately, Houston. Chellam said he collaborates on the project with Joseph Prospero, professor emeritus of marine and atmospheric chemistry at the University of Miami. Chellam became interested after he noticed "the most curious coincidence" as he was collecting data on industrial air pollution outside plants along the Houston Ship Channel in 2008. He began the project expecting that the plants would emit a constant amount of pollution, as measured from just beyond their property lines. But he discovered that on some days they emitted relatively little, while emissions were much higher on other days.

"What we quickly realized was that the impact of the refineries varied with time," he said. "We naively had gone into our research expecting that if we took three months of data, we would get an average." A five-day period in July and August 2008 showed a large variation. That coincided with an influx of Saharan dust, Chellam said. To determine the actual impact of the Saharan dust required scientific detective work. Chellam and his team determined the "fingerprint" of the African dust, allowing them to differentiate it from other types of pollutants in their samples: industrial dust, vehicle pollutants and smoke from wildfires, among other things. "There are millions of sources of pollution," he said.

His lab works with African dust collected in Barbados, before it has picked up other contaminants along the route to the United States. Still, he said, the metals in the dust are distinct. That finding allowed him to determine that a spike in pollution levels in the 2008 readings reflected the arrival of Saharan dust. A paper on those findings has been accepted for publication in in the journal Environmental Science and Technology. In addition to Prospero, his co-authors include Ayse Bozlaker, a post-doctoral researcher working with Chellam, and Matthew Fraser of Arizona State University. Work by other scientists has linked the dust migration with coral reef stresses and other environmental problems, but the impact on human health is less clear. Chellam, whose research does not extend into health impact, said he would expect it to affect people with asthma and other respiratory problems. "But clearly more research is needed," he said. "The composition of the dust is not the same" as other industrial and vehicle dust.

"And if the composition is different, the health impact may be different," he said.