2/22/91: NASA CONFIRMS 1990 OZONE HOLE EQUALLED PREVIOUS LOWEST RELEASE: 91-32 Continuing observations by the Total Ozone Mapping Spectrometer (TOMS) have confirmed that the depletion of stratospheric ozone over Antarctica in 1990 matched the levels observed in 1987 and 1989, the lowest yearly ozone totals recorded. TOMS, an instrument aboard NASA's NIMBUS-7 satellite, has monitored Antarctic ozone concentrations since 1979. Research by NASA scientists at Goddard Space Flight Center, Greenbelt, Md., indicates that the 1990 Antarctic "ozone hole" matched the record 1987 ozone depletion in depth, duration and area. During the formation of the hole in August, the total ozone values were the lowest yet recorded. The "ozone hole" is a large area of intense ozone depletion over the Antarctic continent that typically occurs between late August and early October and typically breaks up in mid-November. Ozone, a molecule made up of three atoms of oxygen, comprises a thin layer of the upper atmosphere that absorbs harmful ultraviolet radiation from the Sun. In the presence of sunlight, atoms of chlorine and other chemicals can strip an oxygen atom from an ozone molecule, leaving behind an oxygen molecule which does not absorb the radiation. Because of the catalytic nature of the reactions, each chlorine atom can destroy thousands of ozone molecules. In meteorological conditions that are regularly monitored in the Southern Hemisphere, this process creates the annual ozone hole over Antarctica. The 1990 Antarctic ozone hole began developing in the last week of August, about one week earlier than the beginning of depletion in 1987 and 1989, though the timing of the start of the depletion has varied over the years. The rate at which the ozone hole formed approximated the record 1987 decline and reached a minimum of 125 Dobson Units on Oct. 4, 1990. As in 1987, the 1990 total ozone values within the hole slowly and steadily increased from mid-October through November. The ozone hole breakup occurred in early December, the latest date yet recorded, and the December 1990 South Pole average of 287 Dobson Units was the lowest yet recorded. This December value compares to values near 350 Dobson units for the years 1978 to 1982 and 325 Dobson units for 1987. TOMS data is one part of a larger NASA effort to study ozone depletion. Its data is calibrated by the Shuttle Solar Backscatter Ultraviolet instrument, a Space Shuttle payload next scheduled for flight aboard STS-43 in July. The Upper Atmosphere Research Satellite, scheduled for launch in November, will investigate the chemistry, dynamics and energy flows of the upper atmosphere, particularly in regard to ozone depletion. The ATLAS-1/Space Shuttle mission in April 1992 will carry several instruments that will complement TOMS measurements of global total ozone by studying the distribution of ozone at various altitudes. In addition, NASA plans to launch three more TOMS instruments in the next few years: aboard a Soviet Meteor satellite in August, aboard a U.S. expendable rocket in late 1993 and aboard the Japanese Advanced Earth Observing Satellite in 1995. These complementary missions are important to understanding the dynamic processes and feedback mechanisms that can lead to ozone depletion. Though the specific chemical interactions and meteorological conditions associated with ozone depletion are relatively well understood, other areas such as general atmospheric chemistry and physics need further investigation before quantitative predictions can be made about ozone depletion. All NASA's ozone depletion studies are part of the agency's Mission to Planet Earth, NASA's contribution to the multi-agency U.S. Global Change Research Program. To better understand this planet, Mission to Planet Earth programs are designed to observe the Earth as a complete environmental system. Comprehensive observations will be the basis for improving models of global processes to better understand natural environmental change and the effects of human activity on this planet.