Douglas Isbell
Headquarters, Washington, DC October 25, 1996
(Phone: 202/358-1753)
Allen Kenitzer
Goddard Space Flight Center, Greenbelt, MD
(Phone: 301/286-8955)
RELEASE: 96-217
1996 ANTARCTIC OZONE HOLE BELOW RECORD AVERAGE SIZE
Two NASA instruments again have detected substantial
depletion of ozone levels over Antarctica, commonly referred
to as the Antarctic ozone hole.
The average size of the Antarctic ozone hole during 1996
has been almost as large as in the peak year of 1993,
although ozone values are higher than the record lows seen in
September 1994, according to preliminary analysis of
satellite data by scientists at NASA's Goddard Space Flight
Center in Greenbelt, MD. During the current year, the ozone
hole covered a surface area over the South Pole roughly equal
in size to the North American continent.
These data were recorded by two of NASA's Total Ozone
Mapping Spectrometer instruments (TOMS) launched this year,
one on board the NASA Earth Probe satellite and another on
the Japanese Advanced Earth-Observing Satellite (ADEOS)
satellite. Low ozone amounts over the Antarctic continent
consistent with these TOMS data also have been validated by
ground-based instruments and other satellite-based
instruments.
The average size of the ozone hole during this year was
8.3 million square miles, similar to observations in the last
four years. The largest observed average size of the ozone
hole was in 1993, at 8.5 million square miles.
The hole started to form in mid-August of this year and
reached a one-day peak size on Sept. 7, 1996, of about 10
million square miles, then quickly shrunk to values of less
than 8.5 million square miles. The previous largest one-day
peak size hole was 9.4 million square-miles on Sept. 27,
1992. In comparison, the surface area of North America is
8.1 million square-miles while Antarctica has a surface area
of 5.4 million square-miles.
Since the mid-1980s, the region covered by low total
ozone begins to grow each year in early August. This region
reaches its maximum extent in September, while the lowest
ozone values are typically seen in late September and early
October. The ozone hole usually disappears by early
December. The ozone hole in 1996 opened up slightly earlier
than in previous years, but had begun to decrease in surface
area below 7.7 million square-miles by Oct. 16, 1996.
"This ozone hole is very similar to those seen in recent
years," said Dr. Paul Newman, research scientist in the
Laboratory for Atmospheres at Goddard. "Although its area
climbed briefly over that of the previous peak, that is not
as great a concern as the average size, because
meteorological conditions can cause large day-to-day
fluctuations. This is similar to winter temperatures, where
one really cold day is not as important as the average
temperature over the whole winter season."
The ozone amounts measured by TOMS/ADEOS and TOMS/Earth
Probe dropped to 111 Dobson units on Oct. 5 near the center
of the Antarctic continent, with values below 220 Dobson
units measured over a wide area. Total ozone values less
than 100 Dobson units were measured in both 1993 and 1994,
with the record low value of 88 Dobson units measured on
Sept. 28, 1994.
Ozone, a molecule made up of three atoms of oxygen,
comprises a thin layer of the atmosphere which absorbs
harmful ultraviolet radiation from the Sun. A Dobson unit is
related to the physical thickness of the ozone layer if it
were brought to the Earth's surface. The global average
ozone layer thickness is 300 Dobson units, which equals 1/8th
of an inch, approximately the thickness of two stacked
pennies. In contrast, the ozone layer thickness in the ozone
hole is about 100 Dobson units (1/25th of an inch),
approximately the thickness of a single dime.
Scientists at the South Pole from the National Oceanic
and Atmospheric Administration (NOAA), working with balloon-
borne measurements, have found low total ozone values similar
to those seen in 1995. "However, in the central region of
the ozone hole, from 7.5 to 12.5 miles altitude, ozone
depletion was more severe than in the past," said Dr. Dave
Hofmann of the NOAA Climate Monitoring and Diagnostics Lab in
Boulder, CO. The NOAA measurements showed that complete
destruction of ozone at an altitude of 10 miles was observed
over the period from Sept. 24 to Oct. 14. "Total ozone did
not reach record lows because of unusually high ozone above
the ozone hole at 15 miles which compensated for the low
values in the ozone hole," Hofmann said.
"These deep and large ozone holes are likely to continue
to form annually until the stratospheric chlorine amount
drops to its pre-ozone hole values," said Dr. Richard
Stolarski, also a research scientist at Goddard. "The
slightly earlier ozone hole this year probably resulted from
the continued increase of Antarctic stratospheric total
chlorine levels."
Since the discovery of the ozone hole in 1985, TOMS has
been a key instrument for monitoring ozone levels throughout
the southern hemisphere. The first TOMS aboard NASA's
Nimbus-7 satellite measured Antarctic ozone levels from
November 1978 to May 1993, and it helped make ozone a
household word through pictures of the Antarctic ozone hole.
It was followed by a TOMS sensor on a Russian satellite.
TOMS data also provided part of the scientific
underpinning for the Montreal Protocol, under which many of
the world's nations have agreed to phase out the use of
ozone-depleting chemicals. As a result of restrictions in
the Montreal Protocol, chlorine levels have already peaked in
the lower atmosphere, and should peak in the Antarctic
stratosphere in about three to five years.
The size and depth of the ozone hole, and global ozone
levels, depend on meteorological conditions and on the amount
of chlorine present in the atmosphere, and may be affected by
the presence of sulfate aerosols produced by volcanic
eruptions. Scientists speculate that ozone values over
Antarctica were low in 1993 and 1994 because of the enhanced
presence of sulfuric acid aerosols in the stratosphere due to
the June 1991 Mount Pinatubo eruption in the Philippines.
TOMS-Earth Probe, launched in July of this year, is the
third in the series of TOMS instruments. Operating from a
312-mile orbit, TOMS-Earth Probe is principally dedicated to
collecting ozone and aerosol data in the lower atmosphere.
The fourth TOMS instrument was launched in August aboard
ADEOS into a 500-mile orbit. ADEOS is an international
climate change research mission that includes instruments
from the U.S., Japan, and France, with investigators from
many countries around the world.
Both TOMS-EP and ADEOS are key parts of a global
environmental effort which includes NASA's Mission to Planet
Earth, a long term, coordinated research effort to study the
Earth as a global environmental system.
TOMS ozone data and pictures are available to anyone
with a computer connection to the Internet World Wide Web at:
http://jwocky.gsfc.nasa.gov
The TOMS instruments are managed by the Goddard Space
Flight Center for NASA's Office of Mission to Planet Earth,
Washington, DC.
-end-
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