Pinatubo deepens the Antarctic ozone hole.

When it came time for the annual ozone pool this year, researchers at NASA's Goddard Space Flight Center all missed the mark. Everyone guessed total ozone concentrations would come close to, if not surpass, the record low value because the atmosphere was filled with volcanic acid from last year's eruption of Mt. Pinatubo -- a factor believed to help human-made chemicals in their attack on the ozone layer. Yet satellite measurements from early October showed that ozone levels did not fall as low as expected, so the highest prediction won by default.

Measurements by balloon-borne instruments, however, now threaten to reopen that contest. In contrast to the satellite data -- which indicate that this year's lowest ozone concentration was roughly 126 Dobson units -- balloon measurements made on Oct. 11 show that ozone levels above the South Pole reached an all-time low of 105 Dobson units. The balloon data also suggest that sulfuric acid from Pinatubo did indeed worsen the ozone loss by allowing chlorine chemicals to attack ozone farther down in the stratosphere than normal, says David J. Hofmann of the National Oceanic and Atmospheric Administration in Boulder, Colo.

At this point, the satellite and balloon teams cannot tell which set of measurements lies closer to the truth. In either case, though, Earth's protective ozone shield suffered a particularly severe attack this year. In late September, the satellite-borne detector showed that the ozone hole had reached record proportions in terms of aerial extent (SN: 10/10/92, p.229). Atmospheric scientists are currently debating why its breadth grew so large this year.

At the time, the depth of the hole -- the ozone levels measured where the shield is weakest -- also seemed on the way to a record. But in the last week of September and early October, a huge, ozone-rich patch of air over the South Pacific pushed its way over the edge of the Antarctic continent. The Pacific air distorted the circling vortex of winds that normally encloses the Anarctic stratosphere and helps chlorine destroy ozone there. Weakened by the disturbance, the vortex took on an oblong shape and shifted toward the Atlantic, says Arlin J. Krueger of NASA Goddard in Greenbelt, Md. This event slowed the ozone destruction.

While satellite instruments give a broad aerial view of the hole, balloon measurements can show how much ozone resides at specific levels of the atmosphere. The balloon launches revealed unusually severe ozone loss this year in the lower part of the straosphere, between 10 and 18 kilometers in altitude, Hofmann says.

Chemical reactions destroyed all ozone in a 4-km-thick layer between the altitudes of 14 and 18 km. This contrasts with the pattern seen in previous years, when total ozone loss occurred in a region only 1 to 2 km thick. Moreover, the chemical attack in years past spared ozone between 10 and 13 km high. This year, that layer lost about a third of its ozone, says Hofmann.

Normally this low region is too warm for the formation of frozen cloud particles that hasten the ozone destruction process. But this year, ground-based and balloon-borne instruments detected a significant number of small particles in the lower stratosphere. Hofmann suggests the particles were droplets of volcanic sulfuric acid that helped chlorine chemicals extend their destructive power even lower in the polar stratosphere than normally possible.

In the Sept. 24 NATURE, Hofmann and colleagues reported seeing a similar -- but less severe -- phenomenon last year, when the lower stratosphere contained sulfuric acid from the eruption of Mt. Hudson in Chile. These particles should have dropped out of the stratosphere by now, but the droplets from Pinatubo's much higher cloud should still remain up there, says Hofmann.

While the Antarctic ozone hole comes during its spring, balloon launches in May and June suggest that this region also suffered some ozone destruction during the fall season as a result of the volcanic particles, says Hofmann.