Brian Dunbar                                                                               
April 22, 1993
NASA Headquarters, Washington, D.C.                                               
Embargoed
(Phone:  202/358-1547)                                                                  
until 6 p.m.

Allen Kenitzer
Goddard Space Flight Center, Greenbelt, Md. 
(Phone:  301/286-8955)

RELEASE:  93-74

1992-93 GLOBAL OZONE LEVELS LOWER THAN ANY PREVIOUS YEAR

     In the second half of 1992, global ozone levels were 2 to 3 
percent lower than any previous year and 4 percent lower than 
normal, based on extensive data analysis from NASA's Nimbus-7 
satellite.

     "We are seeing lower global ozone values than we've ever seen 
before," said James F. Gleason, Ph.D., an atmospheric scientist 
with the University Space Research Association working at NASA's 
Goddard Space Flight Center, Greenbelt, Md.  "We predicted lower 
ozone in 1992, but nothing like the values we actually observed."

     The very lowest levels were observed in December 1992 when 
the global average was approximately 280 Dobson units.  By 
comparison, a normal December value is about 293 Dobson units.  
Previously, the lowest level of 286 Dobson units was observed in 
December 1987.

     Ozone, a molecule made up of three atoms of oxygen, is 
located primarily in the upper atmosphere, where it absorbs 
harmful ultraviolet radiation from the sun.  A Dobson unit is the 
physical thickness of the ozone layer if it were brought to the 
Earth's surface (300 Dobson units equals 1/10th of an inch or 3 
millimeters).

     Extensive analysis of independent data from the National 
Oceanic and Atmospheric Administration (NOAA-11) Solar Backscatter 
Ultraviolet spectrometer (SBUV/2) and the Russian Meteor-3 Total 
Ozone Mapping Spectrometer (TOMS) instrument confirm the Nimbus-7 
TOMS data.  Comparison of all systems with the ground-based World 
Standard Dobson Instrument and the Dobson network indicates that 
the satellite instrument measurements are consistent during the 
period.
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Northern Hemisphere Mid-Latitudes

     The 1992 ozone levels were especially low in the 
mid-latitudes of the northern hemisphere.  The December 1992 
mid-latitude ozone levels were 9 percent below normal.  The low 
mid-latitude ozone values continue into 1993.  The January 1993 
ozone levels were 13-14 percent below normal.  Preliminary 
observations of March 1993 mid-latitude ozone show that the levels 
continue to be 11 to 12 percent below normal.

     Preliminary results from the Shuttle Solar Backscatter 
Ultraviolet (SSBUV) instrument, flown as part of the recent ATLAS 
2 mission, also observed low springtime, northern hemisphere ozone 
levels in agreement with Nimbus-7 TOMS.  Only in the equatorial 
region were ozone values well within the range of the previous 
year's data.

     Scientists say they can only speculate on the cause of the 
1992 low ozone values.  While the exact cause is unknown, the low 
ozone may be related to the continuing presence of particles 
produced in the upper atmosphere following the eruption of Mount 
Pinatubo in the Philippines in June 1991.  The results of this 
ozone data analysis will be published in Science magazine.

     The Nimbus-7/TOMS has measured ozone levels since November 
1978 and continues to be the primary monitor of global ozone 
levels.  The NASA TOMS instrument on the Russian Meteor-3 
satellite was launched in August 1991.  The NOAA-11 SBUV/2 has 
measured ozone since January 1989.  The SSBUV has flown annually 
on the Space Shuttle since 1989.

     The TOMS instruments, the Nimbus-7 satellite and the SSBUV 
project are managed by the Goddard Space Flight Center for NASA's 
Office of Mission to Planet Earth, Washington, D.C.

     Mission to Planet Earth is NASA's long term, coordinated 
research effort to study the Earth as a global environmental 
system.  It is comprised of satellites such as Nimbus-7 and UARS, 
Space Shuttle missions such as this month's flight of ATLAS-2 and 
airborne and ground-based studies.

     The NOAA-11 SBUV/2 instrument was launched in December 1988 
and is one of a sequence of operational ozone measuring 
instruments on board the NOAA operational spacecraft series.  The 
spacecraft and instruments are managed by NOAA/National 
Environmental Satellite Data and Information Service.  The data 
processing and evaluation is a cooperative NOAA-NASA effort.

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