Accounting for missing airborne ozone.

Far above Earth's surface, some 35 to 80 kilometers high, lies the ozone layer, an oxygen blanket that shields life from hazardous ultraviolet rays. In recent years, the ozone layer has garnered attention because of its reported thinning, brought on by what scientists believe are the effects of chlorofluorocarbons and other industrial pollutants in the upper atmosphere.

Yet some mysteries about the ozone layer persist.

Why, for example, do the best models for figuring high-altitude ozone production and destruction estimate 10 percent less ozone (O[sub.3]) enveloping the globe than scientists actually observe? This puzzling 10-percent difference between what the models predict and what exists has come to be known as the ozone deficit problem.

To explain this conundrum, scientists posit three possibilities. Ozone may be generated faster or destroyed more slowly than is now thought. "Or," says Tom G. Slanger, a molecular physicist at SRI International in Menlo Park, Calif., "a source of ozone production has been overlooked."

To account for the models' shortfall, Paul L. Houston, a chemist at Cornell University, and his colleagues propose an additional mechanism for stratospheric ozone production. Reporting in the Sept. 23 SCIENCE, they describe a chemical pathway by which two oxygen molecules (O[sub.2]), one highly energized and one ordinary, can combine in the upper atmosphere to yield extra ozone.

The energized oxygen arises by photodissociation, a process whereby the sun's energy breaks up high-altitude ozone and other oxygenated molecules, Houston says. The energetic oxygen then reenters high-altitude circulation and picks up an ordinary oxygen molecule. This reaction yields another ozone molecule and single oxygen atom as part of an ongoing cycle.

The scientists estimate that if the new pathway were used to figure ozone production at 43 km above sea level, the altitude at which the ozone deficit is greatest, it would account for all the missing ozone there.

The researchers base their theory on two experiments. In one, a "photo fragment imaging" test, they merged a beam of ozone molecules with a laser of 226 nanometers, taking electronic snapshots of the event. In the second, they used laser-induced fluorescence to measure the distribution of highly energized oxygen. Their results show "a substantial yield" of highly energized oxygen, enough to account for much of the ozone missing from many widely used atmospheric models.

Although the new pathway does not explain all the missing ozone at every altitude, it does account for a tenth of the deficit overall, they state. It also offers a possible route for the creation of additional, heavy ozone isotopes in the stratosphere that so far have remained unexplained.