A significant decline in ozone over the Arctic last winter was due to an increase in the size and longevity of polar stratospheric clouds, according to a group of researchers who participated in a massive, international atmospheric science campaign.
The ozone-destroying clouds are made of ice and nitric acid, said University of Colorado at Boulder Professor Owen B. Toon, one of five project scientists heading up NASA’s SAGE III Ozone Loss and Validation Experiment, or SOLVE. The SOLVE project involved satellites, aircraft, balloons and ground-based instruments operated from December 1999 through March 2000 by more than 200 scientists and support staff from the United States, Canada, Europe, Russia and Japan.
Polar stratospheric clouds generally form about 13 miles above the poles when temperatures drop to minus 110 degrees Fahrenheit and below, said Toon, a professor in CU-Boulder’s Laboratory for Atmospheric and Space Physics. The SOLVE campaign was staged out of Kiruna, Sweden.
In some parts of the Arctic stratosphere -- which is located from about 10 miles to 30 miles above Earth -- ozone concentrations declined as much as 60 percent from November 1999 through March 2000. The fragile stratospheric ozone layer shields life on Earth from the harmful effects of ultraviolet radiation.
Toon is the co-project scientist in charge of NASA’s DC-8 aircraft that made about 25 flights over the region last winter. Six CU faculty members and four graduate students worked with scientists from several other Colorado institutions on the SOLVE campaign, including the National Oceanic and Atmospheric Administration, the National Center for Atmospheric Research and the University of Denver.
Toon will participate in a news briefing on the subject at the spring meeting of the American Geophysical Union to be held May 30 to June 3 in Washington D.C. Other panelists include Eric Jensen of NASA’s Ames Research Center in Moffett Field, Calif.; Edward Browell of NASA’s Langley Research Center in Hampton, Va.; Ken Carslaw of the University of Leeds in the United Kingdom; and Michael Kurylo of NASA’s Upper Atmosphere Research Program in Washington, D.C.
Although seasonal ozone loss is more severe in the Antarctic, the ozone loss in the Arctic presents more serious health threats to humans, said Toon. Ozone-depleted air from the Arctic drifts south toward North America, Europe and Russia each spring, increasing the amounts of UV light reaching Earth’s surface in the highly populated mid-latitudes and causing potential increases in several types of cancer.
Most chlorine compounds pumped into Earth’s atmosphere in recent decades by humans initially were tied up as chlorine nitrate or hydrochloric acid, both of which are non-reactive. But if there is a surface area to attach to like the polar stratospheric cloud ice crystals, the chlorine compounds change into ozone-gobbling chlorine radicals in late winter and early spring after reacting with sunlight.
The greenhouse effect, which warms Earth near its surface, may ironically be cooling the stratosphere enough to cause these clouds to form earlier and persist longer, said Toon. Greenhouse gases may be radiating energy and heat away from the upper stratosphere, creating prime conditions for polar stratospheric cloud formation.
"With the clouds persisting longer, we are seeing greater ozone losses even though the amount of chlorine in the atmosphere has declined slightly," said Toon. The use of CFC’s and other ozone-destroying chemicals were banned worldwide in 1996.
Another troubling phenomenon observed for the first time during the SOLVE campaign last winter was a "denitrification" of some polar stratospheric clouds. Scientists observed nitrogen -- which can act to moderate the destructive activity of reactive chlorine compounds on ozone -- drizzling out of the clouds. "This was a real surprise," said Toon.
The ozone loss over the Arctic has been generaly increasing since the winter of 1995-96, said Toon. If greenhouses gases warming Earth’s atmosphere are shown to be the culprit in lengthening the amount of time the polar stratospheric clouds persist, the recovery of the Arctic ozone layer may be delayed by decades, scientists predict.
Color photos of polar stratospheric clouds from SOLVE are available on the Internet at: http://george.arc.nasa.gov/dx/basket/pix/pscpix/PSCcloudcaps/PSCpix.html.
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