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.

Arctic polar stratospheric clouds
"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."
NASA's
Upper Atmosphere Research Satellite
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 5 to 7 degrees Fahrenheit
(about 3 to 4 degrees Celsius)," she said.

Environment Canada
Nov 30 1999 ozone
image
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.