Brian Dunbar Headquarters, Washington, D.C. April 30, 1992 (Phone: 202/453-1547) Dane Konop National Oceanic and Atmospheric Administration, Silver Spring, Md. (Phone: 301/713-2465) RELEASE: 92-55 U.S. STUDY ENHANCES CONCERN FOR NORTHERN OZONE DEPLETION Results of a recently completed U.S. multi-agency aircraft study indicate that the ozone shield of the Northern Hemisphere is increasingly vulnerable to depletion by man-made chemicals, said Dr. James Anderson of Harvard University, the study's Mission Scientist. This 7-month study indicates the magnitude of human-caused ozone losses in the Arctic depends on the severity and length of the polar winter, which varies from year to year. The research team found that during this winter, the Arctic was primed chemically for ozone loss by mid-January, but the early warming of that region that occurred in late January precluded major ozone depletion. The study also revealed important evidence supporting the conclusion that ozone decreases at mid-latitudes are associated with increased levels in chlorine and bromine in the stratosphere. Since concentrations of human-made, ozone-depleting chemicals will continue to rise during the 1990s despite current regulations, it is increasingly likely in coming years that substantial Arctic ozone losses will occur during particularly cold, protracted winters and that mid-latitude ozone trends will continue to decline. The 80 scientists of the second Airborne Arctic Stratospheric Expedition (AASE II) used two NASA aircraft to examine the ozone-related chemistry and air motions of the lower stratosphere from early October 1991 through late March 1992. The flights, largely based out of Bangor, Maine, covered latitudes from south of the equator to the North Pole and extended from Norway to Alaska. NASA's Dr. Michael Kurylo, the AASE II Program Manager, said these extensive measurements, combined with satellite and meteorological data, provide the first detailed picture of the factors that drive changes in the Arctic ozone layer from fall, through winter, and into spring. - more - - 2 - Stratospheric ozone is destroyed by chemical reactions when exposed to chlorine monoxide (ClO) and bromine monoxide (BrO) in the presence of sunlight. The chlorine monoxide arises primarily from chlorofluorocarbons (CFCs), and the bromine monoxide is produced in part by halons. CFCs and halons are industrially produced chemicals that have many common uses, including propellants, refrigerants and fire suppressants. Ground-based and airborne experiments in 1986 and 1987 demonstrated that these chemicals are the cause of the large seasonal ozone losses in the Antarctic spring. In January-February 1989, flights out of Stavanger, Norway, found that ClO concentrations could be as high in the Arctic winter as in the Antarctic spring. Yet ozone losses observed in the Arctic winter in 1989 were much smaller than those of the Antarctic in spring, suggesting the need to better understand the seasonal changes in ozone and the gases that affect it. The current aircraft campaign with its extended seasonal coverage, refined abilities to separate chemical and air-motion effects, and several critical new instruments have taken a much closer look at how such chemical species, against the backdrop of complex air motions, can cause ozone losses in the Arctic and Northern latitudes. Over the past several years, the results from the suite of instruments on the high-flying ER-2 aircraft and the far-ranging DC-8 aircraft have demonstrated that the ClO abundances are greatly elevated in polar regions by the presence of small particle polar stratospheric clouds (PSCs). Chemical reactions that occur on the surfaces of these particles cause more of the chlorine from CFCs to appear as ClO, rather than as the less-reactive forms such as chlorine nitrate and hydrochloric acid. Rapid ozone loss occurs when air perturbed by these surface reactions is exposed to sunlight. The AASE II observations within the Arctic region revealed how the ClO abundances changed with season. The unique sequence of aircraft flights demonstrated that the ClO concentrations grew during the fall-to-winter months. By mid-January, the ClO abundances had reached values that were higher than any seen previously, either in the Arctic or the Antarctic. Though large ozone losses were not observed then only because much of the Arctic region was still dark, the study revealed an unprecedented potential for ozone loss if this air were to be exposed to extensive sunlight. The somewhat early warming of the stratosphere in late January reduced the ClO abundances considerably before the sunlight returned, averting extensive Arctic ozone losses this year. The scientists have concluded that the ClO growth seen in the Arctic during AASE I in 1989 and AASE II this year is likely to occur each winter. In years for which these high levels of Arctic ClO persist into the sunlit months of spring (February and March), chemical models predict that ozone depletion will occur. The temperature record of the Arctic shows that winters with cold stratospheric temperatures in February are not uncommon. - more - - 3 - While ClO was still high in January, there was some exposure to Arctic sunlight, apparently resulting in 10 to 20 percent ozone loss at flight altitudes. High-ClO air could be traced to regions of high PSCs. Less ozone-depleting conditions were found to be linked to air that had not been exposed to PSCs. Thus, the AASE II mission has established in detail how and when ozone losses can occur in the Arctic. Because the atmosphere of the Arctic region is so variable, the full extent of overall ozone losses this year will require further assimilation of the extensive data set of AASE II and other sources. Higher-than-expected ClO amounts also were found on flights southward to mid latitudes. The data indicated that these elevated levels also were due to chemistry occurring on the surfaces of particles, which at these latitudes were not PSCs, but particles of sulfate in the lower stratosphere. The aircraft studies also showed that the chemistry occurring on these sulfate particles lowers the abundances of the nitrogen oxides in the region, said Dr. David Fahey, one of the scientists from the National Oceanic and Atmospheric Administration (NOAA). Like ClO, nitrogen oxides destroy stratospheric ozone. Unlike ClO, stratospheric nitrogen oxides are produced largely by natural processes. At these sunlit mid latitudes, elevated abundances of ClO, coupled with observed BrO abundances and reduced levels of nitrogen oxides, suggest that a larger portion of the ozone destruction taking place there is likely to be due to human influences rather than to natural ones. The AASE II observations are consistent with the view expressed in the recent United Nations Environment Programme report that implicates ClO and BrO in the downward ozone trends observed over mid latitudes by satellites and ground-based measurements over the past several years. The mission results also underscore the need to include such additional ozone-depleting processes in models used to predict for governments the future ozone losses associated with regulatory strategies regarding CFCs, their substitutes, other chlorine-containing gases and the bromine-containing halons. The eruption of Mount Pinatubo in the Philippines has increased the abundance of natural sulfate particles, potentially enhancing ozone losses due to chemical reactions that occur on particle surfaces. Ozone levels were reduced within layers of Mount Pinatubo aerosols in the tropics, but further analyses are needed before the eruption's effect on the ozone layer can be assessed quantitatively. However, since volcanic particles settle out of the stratosphere in 3 to 5 years, any ozone loss caused by Mount Pinatubo will be short lived in contrast to the chronic effect of the long-lived CFCs and halons. The researchers noted that within the Arctic region, the particles from Mount Pinatubo likely play a minor role in perturbing the ozone-depleting chemistry compared to the role of the abundant PSCs. No direct injection of chlorine itself by the volcano was observed at any latitudes. - more - - 4 - The AASE II mission was a multi-agency effort involving scientists and support from NASA, NOAA and the National Science Foundation, scientists from several universities and support from the chemical industry's Alternative Fluorocarbon Environmental Acceptability Study. The NASA aircraft are managed by the Ames Research Center, Moffett Field, Calif., for NASA's Office of Space Science and Applications. - end -