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Owen B. Toon, University of Colorado, Boulder, CO 303/492-1534
Jim Scott, University of Colorado, Boulder, CO 303/492-3114
The AGU pressroom
Washington Convention Center, Washington, DC
ARCTIC OZONE DEPLETION LINKED TO LONGEVITY OF POLAR STRATOSPHERIC CLOUDS
A significant decline in ozone over the Arctic last winter was due to an increase in the area and longevity of polar stratospheric clouds (PSCs), according to a group of researchers who participated in a large, 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 massive 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.
"Even very small numbers of particles in PSCs can efficiently remove nitrogen from the stratosphere," said Eric Jensen, a scientist at NASA Ames Research Center, located in California's Silicon Valley. "We found that the clouds lasted longer during the 1999-2000 winter than during past winters, allowing greater ozone depletion over the Arctic."
Polar stratospheric clouds generally form about 13 miles above the poles where temperatures can 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.
Click here to follow links to pictures of polar stratospheric clouds in larger .jpg image format and at high enough resolution for some publications.
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 was the co-project scientist in charge of NASA's DC-8 aircraft that made about 25 flights over the region last winter. He 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 DC. Other panelists include Eric Jensen of NASA's Ames Research Center, Moffett Field, CA.; Edward Browell of NASA's Langley Research Center, Hampton, VA; Ken Carslaw of the University of Leeds in the United Kingdom; and Michael Kurylo of NASA's Upper Atmosphere Research Program, NASA Headquarters, Washington, DC.
Although seasonal ozone loss is more severe in the Antarctic, the ozone loss in the Arctic presents potentially more serious health problems to human beings, said Toon. Ozone-depleted air from the Arctic drifts south toward North America, Europe and Russia each spring, increasing the amounts of ultraviolet light reaching Earth's surface in the highly populated mid-latitudes and potentially causing increases in several types of cancer.
Most chlorine compounds pumped into Earth's atmosphere in recent decades by human activity 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. Greenhouse gases are 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. Manufacture of chlorofluorocarbons ceased in 1996 in signatory countries under the terms of the Montreal Protocol and its amendments.
Downlink information for obtaining video footage of polar stratospheric clouds can be obtained on NASA-TV's Internet site at: http://www.nasa.gov/ntv. NASA TV video footage will be available starting May 30 at noon EDT on GE-2, transponder 9C at 85 degrees West longitude, with vertical polarization. Frequency is on 3880.0 megahertz with audio on 6.8 megahertz.
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