But a University of Washington researcher has found it doesn't always work that way.

The atmosphere often is highly layered, and such layering is most pronounced when a temperature inversion blocks intruding air from above and below, said Peter Hobbs, a UW atmospheric sciences professor. The result can be a lower layer of polluted air, followed by 500 to 1,000 feet of pristine air -- what Hobbs calls a clean-air slot -- topped by another layer of pollution.

"It's like the cream filling in a layer cake, with the cream representing the clean air," Hobbs said.

The UW's Cloud and Aerosol Research Group, which Hobbs heads, encountered many clean-air slots during research flights over southern Africa in the summer of 2000. The findings are published in today's edition of the journal Nature.

Temperatures generally tend to decrease with altitude. Clean-air slots occur in temperature inversions, where the air temperature suddenly increases with altitude gain over a small segment of the atmosphere and then begins to decrease again.

Temperature inversions typically occur in the lower atmosphere, at altitudes between about 5,000 and 10,000 feet. The upper and lower boundaries of an inversion form a barrier that hinders pollution from entering the inversion from above or below. As a result, clean air can become sandwiched between colder layers of polluted air above and below.

In southern Africa, the UW research group found clean-air slots sandwiched between pollution rising directly from the surface and smog from more distant sources that had risen high into the atmosphere before drifting over the region.

"We've seen clean-air slots in other places around the world, but they were much more pronounced and more common in southern Africa," Hobbs said. "Because of the heavy pollution both above and below, clean-air slots stand out much more in southern Africa than they do in most other places."

Hobbs' team measured pollution at 5,000 particles per cubic centimeter in the polluted layers below and above the clean-air slots but at only 1,000 parts per cubic centimeter in the clean-air slots.

"What's remarkable is that clean-air slots can persist for days on end, over thousands of square miles," Hobbs said. "If you were a bird, you could fly endlessly in wonderfully clear air, provided you stayed within a clean-air slot."

In southern Africa, Hobbs' team often flew beneath the tracks of satellites so that measurements taken from the aircraft could be compared with those from the satellites.

The research was paid for in part by the National Aeronautics and Space Administration (NASA). - By Vince Stricherz


[Contact: Peter Hobbs, Vince Stricherz, Sandra Hines]

21-Feb-2002