Signs of early ozone loss.

Researchers monitoring ozone levels over the South Pole report signs that the ozone hole may have started developing earlier than usual this year.

Balloon-borne sensors measured ozone values as low as 195 Dobson units during the second week of September, 1 week earlier than usual, says Samuel J. Oltmans of the National Oceanic and Atmospheric Administration in Boulder, Colo. He adds that the timing of the ozone depletion does not necessarily mean that this year's hole will be more severe than previous ones.

The ozone hole opens in September when sunlight returns to the extreme south, energizing reactions in which chlorine pollution breaks down ozone molecules in the lower stratosphere--between about 12 and 23 kilometers in altitude. The depletion progresses until early October, when warm winds invade the region and shut down the cycle of chemical destruction.

Although the balloon measurements hint that the hole might be opening early this year, these sensors probe only the region directly over the pole and cannot track the development of the entire hole. Indeed, satellite observations over a broad region show the hole forming essentially on schedule, says Arlin J. Krueger of NASA's Goddard Space Flight Center in Greenbelt, Md.