Cold Polar Rings Help Form Clouds That Destroy Ozone

Newly discovered, narrow rings of cold air over Earth’s poles help form
colorful clouds that destroy ozone, according to a technical paper that will
appear in the March 30 issue of the journal Science.

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 paper’s authors believe they have 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 (PSC) particles are born inside narrow
temperature rings around Earth’s poles in absolute darkness,” according to
Azadeh Tabazadeh, lead author of the paper and a scientist at NASA’s Ames
Research Center in California’s Silicon Valley. “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.

The other authors of the paper are Eric Jensen and Katja Drdla, both of
Ames; Brian Toon from the University of Colorado, Boulder; and Mark
Schoeberl of NASA’s Goddard Space Flight Center, Greenbelt, MD.

The PSCs form in the stratosphere, the part of Earth’s atmosphere between
about 9 and 30 miles (about 12 to 40 kilometers) altitude, which includes
the ozone layer. The cold rings (about minus120 degrees F., or minus 83
degrees C.) where PSCs form, circle both poles at an altitude of 12 to 20
miles (15 to 26 kilometers). Known for their colorful glows, PSCs provide
surfaces that convert benign forms of chlorine into reactive,
ozone-destroying forms.

The large particles in PSCs also remove nitrogen compounds from the air, a
process called “denitrification.” Nitrogen compounds in the atmosphere
normally moderate the destructive impact of chlorine on ozone. “An ozone
hole forms every spring over the Antarctic in the Southern Hemisphere which
is colder than the Arctic,” said Tabazadeh.

Increased denitrification over the Antarctic can cause the area of the
“ozone hole” there to increase, according to the authors. Last year, a NASA
satellite measured the largest ozone hole ever over the Antarctic, Tabazadeh
said. “It is possible that the area of the Antarctic ozone hole may spread
even farther than that measured last winter before the hole recedes to what
it was in the 1970s,” she added.

“On the other hand, the warmer Arctic climate in the north is becoming
colder, more like the Antarctic in the south. This could lead to more
dramatic ozone loss in the future over the Northern Hemisphere, where many
people live,” Tabazadeh said. The authors report that NASA satellite
observations for the first time showed widespread denitrification as high as
20 percent to 50 percent in the Arctic stratosphere during the 1999-2000
winter.

“Increased denitrification in the Arctic can delay the recovery of the ozone
layer despite the fact that ozone-destroying chlorofluorocarbon (CFC) levels
are declining in the atmosphere due to international agreements,” Tabazadeh
said.

More than a decade ago, scientists determined that human-made chlorine and
bromine compounds cause most ozone depletion. Manufacturers made the
chlorine compounds, “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.

“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,” Tabazadeh added.

NASA’s Office of Earth Sciences, Washington, DC funded this research.