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Dec. 7, 2001

John Bluck

NASA Ames Research Center, Moffett Field, Calif.

(Phone: 650/604-5026 or 604-9000)

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AGU Moscone Center press room, rm. 111, San Francisco, Calif.

(Phone: 415/905-1007, general AGU information during meetings)



For her outstanding contributions to geophysical sciences, a young NASA scientist will receive a prestigious medal from the American Geophysical Union (AGU) during its fall meeting in San Francisco.

Azadeh Tabazadeh, a scientist at NASA's Ames Research Center in California's Silicon Valley, will receive the James B. Macelwane medal during an AGU ceremony that begins at 5:30 p.m. PST on Dec. 12 in the San Francisco Marriott Hotel's Yerba Buena Ballroom, Salon 7. When awarded to a member, the medal carries with it automatic designation as an AGU Fellow.

"Azadeh Tabazadeh is one of the finest new talents we have in our Earth Science efforts at Ames, " said Estelle Condon, Ames Acting Deputy Director of Astrobiology and Space Research.

"Azadeh Tabazadeh is noted for her many applications of physical chemistry to problems of importance in atmospheric science," said Brain Toon of the University of Colorado, Boulder, Colo. "She initially explored the role of volcanic eruptions in injecting chlorine compounds into the stratosphere. . . . Azadeh showed that the chlorine would be removed by precipitation in the moist rising volcanic column before it could enter the stratosphere. This paper ended 20 years of debate about this subject through its thorough analysis and plausibility," Toon explained.

Large volcanoes emit much more chlorine than all human-made chlorine emissions, but because volcanic chlorine compounds are precipitated, they do not deplete the ozone layer substantially, according to Tabazadeh. She also made significant contributions in the study of polar stratospheric clouds (PSC)s that are responsible for the formation of the Antarctic ozone hole, according to Toon.

Earlier this year Tabazadeh and colleagues discovered that narrow rings of cold air over the Earth's poles help to form colorful clouds that destroy ozone. The results of this study are published in the March 30, 2001, issue of the journal Science. Tabazadeh was the lead author.

The ozone layer protects life on Earth from the sun’s harmful ultraviolet radiation that could cause skin cancer in human beings and biological damage to living things. The authors of the Science paper believe they solved a decade-old mystery of how glowing, ozone-destroying clouds that contain nitric acid and water form road-dust-size particles that later spread to decompose ozone.

"Large polar stratospheric cloud particles are born inside narrow temperature rings around Earth’s poles in absolute darkness," Tabazadeh said. "Strong winds blow these special clouds away from the cold rings to fill the polar air with ozone-destroying particles. The areal extent of these clouds is often larger than the United States, despite the fact that the clouds initially form inside a narrow temperature ring," she said.

"Scientists used to believe that as chlorine levels decline in the upper atmosphere, the ozone layer should slowly start to recover. However, greenhouse gas and soot emissions, which provide warming at the Earth's surface, lead to cooling in the upper atmosphere. This cooling promotes formation of more clouds that destroy ozone," according to a paper published by Tabazadeh and colleagues in the June 2000 issue of Science magazine. The other authors of the paper are Eric Jensen and Katja Drdla, both of Ames; Toon; and Mark Schoeberl of NASA’s Goddard Space Flight Center, Greenbelt, Md.

More than a decade ago, scientists determined that human-made chlorine and bromine compounds cause most ozone depletion. Manufacturers made the chlorine compounds, chlorofluorocarbons 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 James B. Macelwane Medal was established in 1961 and renamed in 1986 in honor of James B. Macelwane. Outstanding scientists, less than 36 years of age, who contribute to geophysics, are eligible to receive the award. Macelwane, the thirteenth president of the AGU (1953-1956), was renowned not only for his contributions to geophysics but also for his deep interest in teaching and encouraging young scientists. The first recipient of this medal was James N. Brune. As many as three medals may be given annually.

Past Macelwane medal winners include Don L. Anderson (1966), professor of geophysics at the California Institute of Technology, Pasadena, Calif. He also was the AGU president from 1988 -1990. Another past winner for1982 is Steven C. Wofsy. He is a professor of atmospheric and environmental science at Harvard University, Cambridge, Mass.

Tabazadeh also was the recipient of a Presidential Early Career Award for Scientists and Engineers in 1998, which resulted in a visit to the White House.

The annual fall meeting of the American Geophysical Union takes place this year at San Francisco's Moscone Convention Center from Dec. 10 -14. AGU fall meeting details are on the Internet at

Journalists may search the AGU website ( ) for more details about Ames' AGU participation. Use the keyword '' or the researcher's e-mail address to locate abstracts and session information. To arrange interviews at AGU, reporters may contact Harvey Leifert in the AGU Press Room, MC 111, 415/905-1007. Journalists also can arrange interviews at AGU by using the message board located outside Moscone’s main exhibition hall.




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