NASA Ames Research Center, Moffett Field, CA
University of Colorado, Boulder

May 30, 2000


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.

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.

Color photos of polar stratospheric clouds from SOLVE on the Internet.

May 25, 2000
John Bluck

NASA Ames Research Center, Moffett Field, CA
NASA Jet Propulsion Laboratory, Pasadena, CA

May 25, 2000


The ozone layer that protects life on Earth may not be recovering from the damage it has suffered over the Arctic region as quickly as scientists previously thought, according to a paper published in the May 26 issue of the journal Science. Specifics of the research also will be presented at the annual meeting of the American Geophysical Union in Washington, DC, on May 31.

More polar stratospheric clouds than anticipated are forming high above the North Pole, causing additional ozone loss in the sky over the Arctic, according to Dr. Azadeh Tabazadeh, lead author of the paper and a scientist at NASA's Ames Research Center in California's Silicon Valley. The stratosphere comprises Earth's atmosphere from about 9 to 25 miles (about 15 to 40 kilometers) altitude and includes the ozone layer.

"Polar stratospheric clouds provide a 'double-whammy' to stratospheric ozone. They provide the surfaces which convert benign forms of chlorine into reactive, ozone-destroying forms, and they remove nitrogen compounds that act to moderate the destructive impact of chlorine," said Dr. Phil DeCola, Atmospheric Chemistry Program Manager at NASA Headquarters, Washington, DC.

"The Arctic has become colder and more humid, conditions that promote formation of more polar stratospheric clouds that take part in polar ozone destruction. The main conclusion of our study is that if this trend continues, Arctic clouds will remain longer in the stratosphere in the future," Tabazadeh said.

"An ozone hole forms every spring over the Antarctic in the Southern Hemisphere which is colder than the Arctic," said Tabazadeh. "The Arctic has been getting colder and is becoming more like the Antarctic; this could lead to more dramatic ozone loss in the future over the Northern Hemisphere, where many people live."

Researchers used data from NASA's Upper Atmosphere Research Satellite to analyze cloud data from both the north and south polar regions for the study. "What we found from the satellite was that polar stratospheric clouds currently last twice as long in the Antarctic as compared to the Arctic," Tabazadeh said.

"However, our calculations show that by 2010 the Arctic may become more 'Antarctic-like' if Arctic temperatures drop further by about 37 to 39 degrees Fahrenheit (about 3 to 4 degrees Celsius)," she said.

When Arctic polar stratospheric clouds last longer, they can precipitate, removing nitrogen from the upper atmosphere, which increases the opportunity for chlorine compounds to destroy ozone more efficiently. The polar stratospheric clouds involved in the reactions contain nitric acid and water, according to researchers who discovered these clouds in 1986.

"Data from the Microwave Limb Sounder on UARS have provided the first opportunity to observe nitric acid throughout the Arctic and the Antarctic over a period of many years," said Michelle Santee, a scientist at NASA's Jet Propulsion Laboratory, Pasadena, CA, who is a co-author of the Science paper. "The continued presence of nitric acid in the Arctic winter -- which is not the case in the Antarctic -- helps to moderate ozone loss by reducing the amount of reactive chlorine, but this could change in the future," she added.

More than a decade ago, scientists determined that human-made chlorine and bromine compounds cause most ozone depletion. Manufacturers made the chlorine compounds, chloroflourocarbons or "CFCs," for use as refrigerants, aerosol sprays, solvents and foam-blowing agents. Fire fighters used bromine-containing halogens to put out fires. Manufacture of CFCs ceased in 1996 in signatory countries under the terms of the Montreal Protocol and its amendments.

The Montreal Protocol bans CFC emissions. As a result, the chlorine concentration in the upper atmosphere is already starting to decline, according to Tabazadeh. "Scientists used to believe that as chlorine levels decline in the upper atmosphere, the ozone layer should slowly start to recover. However, greenhouse gas emissions, which provide warming at the Earth's surface, lead to cooling in the upper atmosphere. This cooling promotes formation of the kind of polar stratospheric clouds that contribute to ozone loss," she added. "Several recent studies, including this one, show that ozone recovery is more complex and will take longer than originally thought," she explained.

The Office of Earth Sciences, NASA Headquarters, Washington, DC funded this research.

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