Recent observations by NASA's Upper Atmosphere Research Satellite (UARS) have shown exceptionally high levels of chlorine monoxide (ClO) at high northern latitudes, raising the possibility of enhanced ozone depletion over populated areas of the Earth, according to a UARS scientist.

The Microwave Limb Sounder (MLS) detected elevated levels of ClO -- a key constituent in the chemical processes that lead to ozone depletion -- over large populated areas of Europe and Asia, north of about 50 degrees latitude, said MLS Principal Investigator Dr. Joe W. Waters of the Jet Propulsion Laboratory. For example, high ClO levels were observed on January 11, 1992, over Scandinavia, Northern Eurasia and the cities of London, Moscow and Amsterdam.

These ClO levels, approximately 1 part per billion volume (ppbv) are comparable to levels observed within the Antarctic ozone hole. Stratospheric ClO molecules, which result primarily from industrial chemicals such as chlorofluorocarbons (CFCs) released in the lower atmosphere, are the dominant form of chlorine that destroys ozone in a process that starts when sunlight breaks up the CFCs.

Sustained levels of ClO could lead to significant ozone destruction over the Northern Hemisphere, perhaps even to an ozone hole over the Arctic, Waters said. Whether an ozone hole actually develops will depend on how long the elevated ClO levels persist.

MLS data also show very low ozone levels in the tropical stratosphere over an area roughly coinciding with the plume from the Mount Pinatubo volcanic eruption in mid-1991. Observation of reduced ozone in the tropics, linked to volcanic plumes, raises the possibility that volcanic eruptions may trigger ozone depletion processes similar to those occurring within the Antarctic ozone hole.

In the tropics, preliminary results show ozone levels at an altitude of 21 kilometers (about 69,000 feet) about 50 percent less than typical pre-eruption levels observed by other means. (UARS was launched September 12, 1991, three months after Mount Pinatubo erupted.) MLS total ozone levels in the tropics appear to be about 10 percent lower than typical. This area of low ozone extends roughly from 10 degrees South latitude to about 20 degrees North. In addition, the MLS observed transient areas of low ozone across the western United States, findings that were corroborated by independent ground-based measurements in Boulder, Colorado.

Computer models have predicted that aerosols from Mount Pinatubo would deplete the ozone layer at a greater rate than previous eruptions because of the additional chlorine. These models assumed that chemical reactions would occur on the surfaces of the stratospheric sulfur compounds within the volcanic cloud that are similar to reactions than occur on the surfaces of stratospheric ice crystals above Antarctica. The possibility also exists, Waters said, that the low tropical ozone is due to atmospheric dynamics rather than chemistry.

UARS is providing the first opportunity to study these processes from a global perspective. During the satellite's primary mission, scientists will have the chance to monitor ozone depletion through two northern winters. UARS scientists will combine data from the 10 instruments to develop a long-term three-dimensional profile of the chemistry, dynamics, and energetics of the Earth's upper atmosphere. UARS data will also be combined with data collected from ground-based, aircraft, and balloon campaigns.

The MLS team announced their results in five scientific papers given at the recent meeting of the American Meteorological Society in Atlanta.

The Goddard Space Flight Center manages the UARS project for NASA's Office if Space Flight and Applications. JPL developed and operates the MLS instrument on UARS with collaboration from Heriot-Watt University, Edinburgh University and Rutherford Appleton Laboratory in the United Kingdom.

##### NOTE TO EDITORS: Black-and-white and color computer plots are available to illustrate this story from JPL Public Information (Phone 818-354-5011). They show ozone distribution for December 6, 1991 and January 11, 1992, and chlorine monoxide distribution for January 11.

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