Antarctic ozone level reaches new low.

Thanks in part to Mt. Pinatubo's lingering legacy, the seasonal ozone hole over Antarctica set a new record last week. Measurements made by satellite and by balloon-borne and ground-based instruments indicate that ozone concentrations above the frozen continent plummeted to an all-time low.

Instruments flown on balloons from the U.S. research station at the South Pole reveal that from late August to early October, about 70 percent of the ozone disappeared from Antarctic skies, says David J. Hofmann of the National Oceanic and Atmospheric Administration (NOAA) in Boulder, Colo. On Oct. 6, the balloon ozone sensors detected only 90 Dobson units of ozone, a reading corroborated by a ground-based spectrometer.

"This is the lowest value of total ozone ever recorded on Earth," Hoffman told SCIENCE NEWS. The previous record low, set last year, was 105 Dobson units.

The Antarctic ozone hole forms during the southern hemisphere's spring-time, when sunlight returns to the dark polar sky, energizing chlorine and bromine pollutants that break apart ozone molecules in the stratosphere. Such chemical destruction weakens the protective layer that normally absorbs ultraviolet radiation from the sun. The region of ozone-poor air breaks up in late October and November.

Measurements made by a U.S. ozone spectrometer flying on a Russian satellite show that the hole grew to cover an extraordinarily large area for the second year in a row. The region of ozone-depleted air spread over 23 million square kilometers, almost the size of the entire North American continent. The hole measured 15 percent larger than it has in most previous years, says Arlin J. Krueger of NASA's Goddard Space Flight Center in Greenbelt., Md.

Scientists remain unsure why the hole has grown so large in the last two years, but they suspect a link with the eruption of Mt. Pinatubo, which flooded the stratosphere with tiny droplets of sulfuric acid. These aerosols can provide a surface for chemical reactions involved in the destruction of ozone.

Balloon measurements from the South Pole support the Pinatubo theory. In the last two years, ozone loss has occurred lower in the stratosphere than in years past (SN: 10/24/92), p.278). Because this is the same region where Pinatubo aerosols reside, Hofmann suggests they have enhanced the hole's development by enabling chlorine to attack ozone over a greater vertical range.