A team of researchers from Harvard University in Cambridge, Mass., and from NASA's Jet Propulsion Laboratory in Pasadena, Calif., has employed ultraviolet resonance fluorescence to identify an ozone-destroying molecule in the stratosphere for the first time. The findings will enable atmospheric scientists to better predict variations in the ozone layer and subsequent changes in ultraviolet exposure at the surface.Scientists used an airborne instrument to detect the presence of the chlorine monoxide dimer in the polar atmosphere by UV resonance fluorescence. The instrument was carried aloft on a NASA ER-2 aircraft flown from Kiruna, Sweden, during the winter of 1999-2000. Photo by Ross J. Salawitch, Jet Propulsion Laboratory.The observation of elevated concentrations of chlorine monoxide in polar regions experiencing ozone loss has led atmospheric scientists to theorize that the photodissociation of the chlorine monoxide dimer plays an important role in the decomposition of stratospheric ozone. The dimer, ClOOCl, was released into the atmosphere from halocarbon refrigerants, which were banned under the Montreal Protocol in 1987 but which are expected to continue to exist in the atmo-sphere for decades. Until now, however, scientists had not detected the molecule in the stratosphere, directly or indirectly.The dimer is believed to work as a catalyst, breaking down ozone into oxygen, particularly in late winter and early spring at the poles. Upon absorbing sunlight, it splits into atomic chlorine and an oxygen molecule. The two chlorine atoms each react with a molecule of ozone, yielding two chlorine monoxide and two oxygen molecules. The molecules of chlorine monoxide subsequently react to form ClOOCl again, and the process can begin anew.In the experiments over the winter of 1999-2000, the UV resonance fluorescence instrument took to the air into or bordering the Arctic polar vortex on 11 flights from Kiruna, Sweden, aboard a NASA ER-2 aircraft, a variant of the Lockheed Martin U-2 spy plane. The instrument pyrolyzed the collected ClONO2 and ClOOCl into chlorine monoxide and nitrogen dioxide or chlorine monoxide, respectively, using a grid of resistively heated silicon strips in each of a pair of sampling ducts. The concentration of chlorine atoms was determined by quantitatively measuring the fluorescent response of the cleaved molecular fragments to 118.9-nm ultraviolet radiation. The detection limit was 10 parts per trillion by volume, with an estimated accuracy of ±17 percent.To distinguish which of the collected gases had produced the chlorine monoxide, the researchers relied on the different bond dissociation energies of ClONO2 and ClOOCl, 113 and 71 kJ/mol, respectively. In one duct, the temperature of the silicon heating element was stepped among three settings: 273 K, at which neither molecule dissociates; 363 K, at which ClOOCl dissociates; and 493 K, at which both molecules dissociate. In the other, the temperature was increased from 0 to 503 K in 25-K increments over a period of 700 seconds. Plots of the maximum fluorescence signal for each scan as a function of temperature thus indicated the collection of neither molecule, ClOOCl alone, or both ClOOCl and ClONO2. The researchers expect that the ability to directly measure chlorine monoxide dimers in the stratosphere will enable atmospheric scientists to better understand ozone depletion. They note that photochemical models alone have tended to underestimate the rate of loss attributable to such substances. Combining in situ studies with remote detection and modeling will lead to more accurate forecasts of how global ozone levels will evolve.