What aircraft leave behind.

As the volume of air traffic soars, scientists have grown concerned about the pollution spewing out the back ends of jets. One long-term study of conditions over Wyoming suggests that airplane exhaust has added substantially to the number of microscopic particles found at high altitudes, possibly helping to cloud the skies.

David J. Hofmann of the National Oceanic and Atmospheric Administration in Boulder, Colo., began investigating this problem in 1976, when one of his meteorological balloons passed through an unusual layer of tiny particles and droplets called condensation nuclei, 75,000 feet above Earth's surface. Because sonic booms occasionally echoed near his research site in Laramie, Wyo., Hofmann suspected that a military jet might have left the trail of particles. He eventually determined that a high-flying reconnaissance aircraft had flown upwind of the region 18 hours earlier.

In the years since, Hofmann and his colleagues have detected hundreds of similar layers between 29,000 and 41,000 feet in altitude, where most jets fly. It is difficult to attribute these bands of condensation nuclei to particular planes because the Federal Aviation Administration doesn't keep records long enough to be useful to the researchers, says Hofmann. On March 31, 1997, however, a balloon passed through a distinct nuclei layer, which they were able to trace to a Delta Airlines flight from Seattle to Dallas-Ft. Worth. The flight had passed upwind of the balloon about 3 hours earlier, report Hofmann and his coworkers in the July 1 GEOPHYSICAL RESEARCH LETTERS.

From studies of aircraft engine exhaust, atmospheric chemists know that sulfurous gases emitted by planes quickly convert to microscopic droplets of sulfuric acid (SN: 7/6/96, p. 12). Natural sources, such as volcanoes, can also produce sulfuric acid droplets and other minute particles in the atmosphere, but these do not form the thin, concentrated layers that aircraft create, says Hofmann.

Looking back over balloon measurements since 1973, Hofmann and his colleagues found 432 discrete instances of condensation nuclei layers. Unlike the natural condensation nuclei, whose numbers rise in summer, these concentrated bands appear with the same frequency in each season, as do aircraft flights. The steadily rising number of nuclei layers has kept pace with the increasing number of jet flights over the years.

The researchers estimate that aircraft have increased the concentration of natural condensation nuclei over Laramie by about 10 percent. This would have little effect if the tiny droplets from aircraft join up with larger natural ones. On the other hand, plane f exhaust could stimulate the growth of cirrus clouds, says Hofmann.

Some researchers have observed an increase in cirrus clouds associated with aircraft contrails, although they have had difficulty estimating the effect of jets on general cloudiness, says Patrick Minnis of NASA's Langley Research Center in Hampton, Va. Cirrus clouds can warm Earth's surface, he notes, and preliminary calculations suggest that the increase in cirrus clouds caused by jets since the 1960s could account for a warming of 0.1 [degrees] C to 0.3 [degrees] C in the United States.