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

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

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