February 9, 2004
Scientists Find Ozone-Destroying
Molecule
Using measurements from a NASA aircraft
flying over the Arctic, Harvard University
scientists have made the first observations of a
molecule that researchers have long theorized
plays a key role in destroying stratospheric
ozone, chlorine peroxide.
Analysis of these measurements was conducted
using a computer simulation of atmospheric
chemistry developed by scientists at NASA’s
Jet Propulsion Laboratory (JPL), Pasadena,
Calif.
The common name atmospheric scientists use for
the molecule is “chlorine monoxide
dimer” since it is made up of two identical
chlorine-based molecules of chlorine monoxide,
bonded together. The dimer has been created and
detected in the laboratory; in the atmosphere it
is thought to exist only in the particularly cold
stratosphere over Polar Regions when chlorine
monoxide levels are relatively high.
“We knew, from observations dating from
1987, that the high ozone loss was linked with
high levels of chlorine monoxide, but we had
never actually detected the chlorine peroxide
before,” said Harvard scientist and lead
author of the paper, Rick Stimpfle.
The atmospheric abundance of chlorine peroxide
was quantified using a novel arrangement of an
ultraviolet, resonance fluorescence-detection
instrument that had previously been used to
quantify levels of chlorine monoxide in the
Antarctic and Arctic stratosphere.
We’ve observed chlorine monoxide in the
Arctic and Antarctic for years and from that
inferred that this dimer molecule must exist and
it must exist in large quantities, but until now
we had never been able to see it,” said
Ross Salawitch, a co-author on the paper and a
researcher at JPL.
Chlorine monoxide and its dimer originate
primarily from halocarbons, molecules created by
humans for industrial uses like refrigeration.
Use of halocarbons has been banned by the
Montreal Protocol, but they persist in the
atmosphere for decades. “Most of the
chlorine in the stratosphere continues to come
from human-induced sources,” Stimpfle
added.
Chlorine peroxide triggers ozone destruction
when the molecule absorbs sunlight and breaks
into two chlorine atoms and an oxygen molecule.
Free chlorine atoms are highly reactive with
ozone molecules, thereby breaking them up, and
reducing ozone. Within the process of breaking
down ozone, chlorine peroxide forms again,
restarting the process of ozone destruction.
“You are now back to where you started
with respect to the chlorine peroxide molecule.
But in the process you have converted two ozone
molecules into three oxygen molecules. This is
the definition of ozone loss,” Stimpfle
concluded.
“Direct measurements of chlorine
peroxide enable us to better quantify ozone loss
processes that occur in the polar winter
stratosphere,” said Mike Kurylo, NASA Upper
Atmosphere Research Program manager, NASA
Headquarters, Washington.
“By integrating our knowledge about
chemistry over the polar regions, which we get
from aircraft-based in situ measurements, with
the global pictures of ozone and other
atmospheric molecules, which we get from research
satellites, NASA can improve the models that
scientists use to forecast the future evolution
of ozone amounts and how they will respond to the
decreasing atmospheric levels of halocarbons,
resulting from the implementation of the Montreal
Protocol,” Kurylo added.
These results were acquired during a joint
U.S.-European science mission, the Stratospheric
Aerosol and Gas Experiment III Ozone Loss and
Validation Experiment/Third European
Stratospheric Experiment on Ozone 2000. The
mission was conducted in Kiruna, Sweden, from
November 1999 to March 2000.
During the campaign, scientists used computer
models for stratospheric meteorology and
chemistry to direct the ER-2 aircraft to the
regions of the atmosphere where chlorine peroxide
was expected to be present. The flexibility of
the ER-2 enabled these interesting regions of the
atmosphere to be sampled.
For information and images on the Internet,
visit:
http://www.gsfc.nasa.gov/topstory/2004/ 0205dimers.html
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Contacts:
David E. Steitz
Headquarters, Washington
(Phone: 202/358-1730)
Alan Buis
Jet Propulsion Laboratory, Pasadena, Calif.
(Phone: 818/354-0474)
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NASA’s ER-2 Aircraft
The NASA ER-2 aircraft, prior to takeoff for a
SOLVE/THESEO-2000 research flight, at the Arena
Arctica research facility in Kiruna, Sweden
(68N). The photo was taken around noon local
time. The instrument used to measure ClOOCl is
housed in the pod below the wing in the
foreground. Credit: Ross J. Salawitch High-Resolution
Image
Loading NASA’s ER-2
Instruments being loaded onto the NASA ER-2
aircraft, inside the Arena Arctica research
facility at Kiruna, Sweden (68N), prior to a
SOLVE/THESEO-2000 research flight. The instrument
used to measure ClOOCl is housed in the pod below
the wing on the right. Credit: Ross J. Salawitch
High-Resolution
Image
Animation of Ozone Production and
Loss
Ozone is produced by intense ultraviolet
radiation in the upper stratosphere. This
radiation breaks typical oxygen molecules (O2)
into free oxygen atoms. Those free atoms of
oxygen (O) then join with molecular oxygen (O2)
and form molecules of ozone (O3). The ozone
molecule generally absorbs ultra-violet
radiation.
Animation of Ozone Production and
Loss
Ozone is destroyed when it reacts with one of a
variety of chemicals in the stratosphere such as
chlorine, nitrogen, bromine or hydrogen. The
process happens essentially in three steps. In
step one, an ozone molecule is cracked by
sunlight to form an oxygen atom and an oxygen
molecule. In step two, a catalyst, in this case
chlorine, reacts with another ozone molecule to
form ClO and a second oxygen molecule. Finally,
the ClO molecule reacts with the oxygen atom to
form a third oxygen molecule, and reconstitute
the original catalyst. The catalyst converts two
ozone molecules into three oxygen molecules
without being affected itself. A typical chlorine
atom can destroy a large number of ozone
molecules in this fashion. “This is the
definition of ozone loss,” said Harvard
researcher Rick Stimpfle. Credit: Scientific
Visualization Studio/NASA Goddard Space Flight
Center
The Polar Vortex
During winter in the Northern Hemisphere,
stratospheric winds tend to form a vortex around
the pole. Measured ozone losses in the winter of
1999-2000 were unusually severe, propelled by
cold temperatures and the commensurate formation
of Polar Stratospheric Clouds. The atmospheric
vortex essentially forms a container for high
altitude air to lose ozone due to chemical
changes. Measurements of total atmospheric ozone
were taken by NASA’s high altitude ER-2
aircraft, and the space agency’s DC-8.
Readings from NASA’s Total Ozone Mapping
Spectrometer (TOMS) Earth Probe showed a clear
ozone minimum over the polar region during
February and March. Credit: Scientific
Visualization Studio/NASA Goddard Space Flight
Center
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