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.