Antarctic ozone hole reaches record size.

Chemical pollutants combined with unusually low temperatures high in the atmosphere to gnaw a record-breaking hole in Earth's protective ozone layer this year, according to measurements made in the frigid skies over Antarctica.

The stratosphere above both poles has grown colder in recent years for reasons not clear to researchers. The recent Antarctica temperatures, the lowest in 2 decades of measurements, raise concern that the ozone layer will not heal as quickly as scientists had predicted, even though nations are curbing the use of ozone-depleting chemicals.

"It may take longer to recover," says Paul Newman of NASA's Goddard Space Flight Center in Greenbelt, Md.

One fear among scientists is that greenhouse gases could be playing a role in lowering stratospheric temperatures. Carbon dioxide and other forms of pollution trap heat in the lower atmosphere, but they cool off the stratosphere--the layer between 10 and 50 kilometers in altitude.

The ozone hole above Antarctica starts forming in September, when springtime sunlight returns to the polar skies. The light energizes chlorine- and bromine-containing chemicals that break apart ozone molecules in the stratosphere. A key part of the chemical chain reaction takes place on the surface of frozen cloud particles, so cold temperatures worsen ozone destruction.

According to measurements by a NASA satellite, the ozone hole grew to 27.3 million square kilometers on September 19, larger than the North American Continent. The biggest previous ozone hole reached 26 million k[sup 2m] in 1996.

The satellite also showed that ozone concentrations in the worst section of the hole bottomed out at 90 Dobson units, only one-third of what should normally be there this time of year.

Balloon measurements over the South Pole recorded a value of 92 Dobson units, in good agreement with the satellite measurements, says David J. Hofmann of the National Oceanic and Atmospheric Administration in Boulder, Colo.

The cold temperatures this year helped the ozone hole reach new heights, according to Hofmann. "We saw some ozone loss all the way up to 24 km, which is higher than usual." Normally, temperatures are too warm at that altitude to allow the formation of frozen cloud particles.

Scientists trace some of the stratospheric cooling in recent years to the loss of ozone molecules, which absorb sunlight and heat up the surrounding air. But this process cannot explain the extremely low temperatures detected in August and September above Antarctica, before much sunlight had returned to the polar skies, says William J. Randel of the National Center for Atmospheric Research in Boulder.

One cause could be natural weather conditions in the lower atmosphere, which can sometimes send pressure disturbances rippling up into the stratosphere. These so-called planetary waves warm the polar stratosphere and slow ozone destruction. In recent years, however, few planetary waves have buffeted the Arctic and Antarctica during the critical season of springtime ozone loss.

Earlier this year, a computer model suggested that greenhouse warming would reduce the number of planetary waves hitting the Arctic and Antarctica (SN: 4/11/98, p. 228). Other computer models have come to the opposite conclusion, raising questions about the validity of this prediction, says Newman.

The recent cooling, he says, "increases our worry about this potential problem. But we certainly can't say that this is evidence for it."