OZONE HOLE OVER ARCTIC SURPRISES RESEARCH TEAM
by David Perlman, Science Editor
San Francisco Chronicle, May 31, 2000
Clouds of ice and acid high in the stratosphere disrupted
the Northern Hemisphere's protective ozone layer this past
winter, and scientists say the ozone loss poses potentially
serious health problems for humans if it repeats in coming
winters.
An international team of researchers detected the unusually
cold ozone-destroying clouds in the stratosphere over the Arctic
last winter, and the team's leaders reported their findings
yesterday at a meeting of the American Geophysical Union in
Washington.
The findings are surprising because scientists had expected
that the protective ozone layer would recover after
ozone-destroying chemicals known as chlorofluorocarbons, or CFCs,
were banned by international treaty four years ago.
But that has not happened, the team of nearly 200 scientists
and technicians found. Instead, the same phenomenon that has
opened a growing "ozone hole" over Antarctica each winter for
more than 10 years is occurring over the Arctic Circle as well.
"Ozone levels over the Arctic have been dropping for the
past four to five years," said Brian Toon, an atmospheric
scientist at the University of Colorado and a leader of the team.
"As a result the stratosphere over the Arctic is beginning to
look a lot like the Antarctic.
"This is the first time, in fact, that we've seen the polar
clouds lasting long enough to affect the ozone layer
significantly," he said.
The ozone loss in the Southern Hemisphere is already causing
health problems for many people in Australia and New Zealand
because the sun's damaging ultra-violet radiation is no longer
blocked by the layer of ozone.
The same could happen in populated areas of the United
States, Canada and Russia if the Northern Hemisphere's ozone
layer continues to thin, the researchers said.
Using an array of satellites, weather balloons and
high-flying aircraft, the scientists found that polar
stratospheric clouds made of ice particles and nitric acid were
larger and lasted longer than researchers had found in
previous years.
The clouds, known as PSCs, generally form about 13 miles
above the poles each winter when temperatures drop to 110 degrees
below zero, Toon said.
This past winter, he reported, instruments on satellites,
balloons and two high-altitude aircraft all showed that ozone
concentrations in some parts of the stratosphere declined as much
as 60 percent between November and March.
The complex chemistry that caused the loss begins with the
presence of chlorine atoms. Chlorine was released into the
atmosphere for many years in the form of CFCs, which were once
widely used for refrigerators, air-conditioning equipment and
many industrial products.
CFCs were banned in 1996 by a treaty known as the Montreal
Protocol, but they are expected to linger in the upper atmosphere
for many more years, scientists say.
'DRIZZLING' NITRIC ACID
Highly reactive chlorine can destroy ozone molecules on
contact, and it is the nitric acid in the polar clouds that
transforms normally inert chlorine into the form that tears the
ozone apart. This past winter, the researchers found, nitric acid
"drizzled" down from clouds high in the stratosphere and turned
even more chlorine than usual into the ozone-destroying form.
Much of what the international team discovered was also
predicted in computer models developed by a separate group of
scientists led by Azadeh Tabazadeh, an atmospheric physicist at
the NASA Ames Research Center in Mountain View.
The arctic atmosphere is not nearly as cold as it is over
Antarctica, but it has been growing colder over the past 20 years
by more than four degrees per decade, Tabazadeh said in an
interview last week. Because of this, she said, this past winter
should in fact have seen colder than normal temperatures over the
Arctic, leading in turn to larger and longer-lasting polar
stratospheric clouds.
That is precisely what happened this past winter, and as a
result the nitric acid compounds high in the stratosphere dropped
down to turn the lingering chlorine atoms into ozone destroyers.
"If our model is correct and if our estimate of the cooling
rate in the stratosphere is correct, then we predict that by 2010
ozone in the (arctic) stratosphere will be decreased by up to 30
percent," Tabazadeh said.
'GREENHOUSE GASES'
Ironically, the situation is made worse by heat-trapping
gases that have been linked to global warming.
Toon said the "greenhouse gases" in Earth's upper atmosphere
that continually trap heat at the Earth's surface, also act like
radiators to lower temperatures in the stratosphere. As a result,
the icy polar clouds this past winter grew larger and colder and
lasted longer than expected, he said.
Although the clouds of ice and nitric acid particles form
high in the stratosphere directly over the Arctic, their
influence on the protective ozone layer extends far south each
spring as they drift over densely populated areas of the United
States, Canada and Russia.
It is these regions that could see increases in skin cancer
rates and other damaging effects from solar ultraviolet radiation
if the ozone layer thins during future winters as it has now, the
researchers said.
Based in Kiruna, Sweden, this past winter, the international
project was the largest in a series of annual efforts known as
the Stratospheric Aerosol and Gas Experiment. The project
included researchers from Canada, Russia, Japan, and the United
States, as well as from several European organizations.
