Brian Dunbar
Headquarters, Washington, D.C.                              February 22, 1991
(Phone:  202/453-1547)

Dolores Beasley
Goddard Space Flight Center, Greenbelt, Md.
(Phone:  301/286-2806)  

RELEASE:  91-32

NASA CONFIRMS 1990 OZONE HOLE EQUALLED PREVIOUS LOWEST

	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.