For years, scientists theorized that a molecule called ClOOCl in the stratosphere played a key role in destroying ozone. Now, using measurements from a NASA aircraft laboratory flying over the Arctic, Harvard scientist Rick Stimpfle and colleagues observed the molecule for the first time. They report their discovery in the Journal of Geophysical Research-Atmospheres, published by the American Geophysical Union.

"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 ClOOCl before," Stimpfle said in an interview. The common name atmospheric scientists use for ClOOCl, he said, is "chlorine dimer"--two identical chlorine-based molecules, ClO or chlorine monoxide--bonded together. The rare dimer exists only in the particularly cold stratosphere over polar regions where chlorine monoxide levels are relatively high. "Most of the chlorine in the stratosphere," Stimpfle adds, "continues to come from human-induced sources."

ClOOCl triggers ozone destruction, he explains, in three basic steps:

• 1. ClOOCl absorbs sunlight and breaks into two chlorine atoms and an oxygen molecule.
• 2. The two chlorine atoms react with two ozone molecules, forming two chlorine monoxide molecules and two oxygen molecules.
• 3. The two chlorine monoxide molecules then react with each other to reform ClOOCl.

"You are now back to where you started with respect to the ClOOCl molecule," Stimpfle says, "but in the process you have converted two ozone molecules into three oxygen molecules. This is the definition of ozone loss."

These results were acquired during a joint US-European science mission, SOLVE/THESEO-2000, based in Kiruna, Sweden, from November 1999 to March 2000. A NASA ER-2 aircraft--essentially a U2--flew into Russian air space for the first time with the cooperation of Russian authorities, Stimpfle says, for the purpose of collecting scientific data of interest to the world community. The instrument used to measure ClOOCl was designed to detect several important inorganic chlorine species and was housed in a wing pod of the ER-2. This work was funded by the NASA Upper Atmospheric Research Program.

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Contact: Harvey Leifert, hleifert@agu.org, 202-777-7507, American Geophysical Union

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