HALOE: Tracking Ozone Loss From Space
NASA Langley Research Center's Halogen Occultation Experiment
(HALOE) has been returning critical data about the
Earth's ozone layer since it was launched on NASA's Upper
Atmosphere Research Satellite (UARS) in 1991. As part of NASA's
Mission to Planet Earth program to monitor the health of the
Earth's environment from space, UARS carries 10 instruments for
studying the chemistry and dynamics of the upper atmosphere (15 to
120 km above the surface of the Earth). HALOE's job is to measure
global ozone as well as the chemicals involved in its
Since the discovery in 1985 of the so-called ozone "hole" - a
seasonal loss of ozone in the stratosphere over Antarctica - the
world's attention has been drawn to this urgent environmental
problem. Because the ozone layer protects life on Earth from the
harmful effects of the sun's ultraviolet radiation, it is important
to determine the causes and mechanisms of ozone loss, as well as
the likelihood that populated regions of the world will be
Ozone in the stratosphere is destroyed when it combines with
chlorine, forming oxygen and chlorine monoxide. A single chlorine
molecule can destroy 100,000 ozone molecules in its lifetime. Most
chlorine comes from the decay of human-made compounds known as
chlorofluorocarbons (CFCs). CFCs came into wide use in the 1950s as
refrigerants, blowing agents for creating foam insulation, and as
industrial cleaning agents.
The loss of stratospheric ozone means that more solar
ultraviolet (UV) radiation reaches the Earth's surface. Since UV
radiation has been linked to skin cancer, there is a significant
human health risk posed by ozone depletion. It has been estimated
that a one percent decline in ozone levels can lead to a two
percent rise in human skin cancer cases.
In 1987, the nations of the world took action to halt ozone loss
by signing the Montreal Protocols, which limited the use of CFCs.
Subsequent amendments to the protocols have banned the production
of CFCs and other ozone-depleters beginning in 1996. Because
chlorine lasts for many years in the stratosphere, however, the
positive effects of this ban may not be seen until well into the
Studying the Earth's Ozone Layer
The study of the ozone layer - with instruments based in space,
on the ground, on airborne balloons, and on high-flying aircraft -
is one of the highest priorities for environmental science in the
1990s. The HALOE experiment is one of the leaders in that effort as
the UARS spacecraft continues its ongoing mission to reveal the
secrets of the Earth's upper atmosphere.
How Does HALOE Work?
As UARS orbits the Earth, it experiences 15 sunrises and 15
sunsets each day which last only two to three minutes each. During
each orbital sunrise and sunset, HALOE measures the amount of ozone
and other trace gases in the atmosphere by measuring the amount of
sunlight that comes through the atmosphere at different altitudes.
The less sunlight that gets through at a specific wavelength, the
more a particular gas is absorbing it.
Orbiting at an altitude of nearly 375 miles (600 kilometers),
UARS observes almost the entire globe, from 80 degrees north
latitude to 80 degrees south latitude. Because each sunrise and
sunset occurs over a different point on the Earth, HALOE measures
ozone values over the whole globe in three to four weeks.
Besides measuring ozone, HALOE is "tuned" to detect chemical
compounds that either indicate ozone destruction or play a role in
it. These include hydrogen chloride (HCl), hydrogen fluoride (HF),
methane (CH4), nitric oxide (NO), nitrogen dioxide (NO2), and water
vapor (H2O). HALOE also measures the amount of small particles
(called aerosols) in the upper atmosphere, as well as the
temperature of the upper atmosphere.
One of HALOE's key objectives is to measure the concentration of
HCl, the main reservoir of chlorine in the stratosphere, and HF.
The fluorine-containing HF is produced only by human-made CFCs,
whereas HCl is produced by natural sources as well. A careful
analysis of the ratio between HCl and HF, and how it varies over
time, can reveal how much of the ozone-destroying chlorine in the
stratosphere is due to human activities.
What Has HALOE Learned?
The key role of CFCs in ozone loss
HALOE's measurements have settled a crucial scientific issue by
confirming that CFCs are responsible for the elevated chlorine
levels in the stratosphere that lead to ozone loss. Because
chlorine also enters the atmosphere naturally - from sources such
as the decay of algae in the oceans and from volcanoes - there had
been some uncertainty as to how much of the total amount was due to
human activities. Natural chlorine sources are believed to account
for only 0.6 parts-per-billion by volume (ppbv), whereas HALOE
measured the total amount of chlorine to be 3.2 ppbv at an altitude
of 50 kilometers. This measurement showed that most of the chlorine
in the stratosphere -- more than 80 percent of it -- is due to
human activities. HALOE is the first instrument that has been able
to confirm the influence of human activities on the amount of
ozone-destroying chlorine in the stratosphere.
A new transport mechanism in the stratosphere
During the Antarctic winter, an air circulation pattern known as
the polar vortex forms in the stratosphere, with high-speed winds
surrounding a quiet central region (similar to the structure of
hurricanes that form in the lower, tropical atmosphere). Normally,
air in the stratosphere remains layered and does not mix
vertically, or mixes very slowly. HALOE data show, however, a
surprising phenomenon occurring in the center of the Antarctic
vortex. Air from very high altitudes descends vertically through
the center of the vortex, moving air to lower altitudes over
several months. One possibility, that will require more study, is
that this downward movement of air may represent a way in which the
stratosphere can cleanse itself of ozone-depleted air. This
important HALOE observation is expected to be a promising area for
HALOE Confirms CFCs Cause Antarctic Ozone Hole
Measurements from the Halogen Occultation Experiment (HALOE)
showed high values of hydrogen fluoride (HF) (Fig. 1a) and low
values of ozone (O3) (Fig. 1b) over the southern polar region and
across large areas of the Southern Hemisphere. Low levels of ozone
over the Antarctic (Fig. 1b) are indicated by darker gray regions,
and represent the region known as the Antarctic ozone "hole." High
values of ozone are indicated by the lighter gray regions.The data
in the figures was collected during the 1994 Southern Hemisphere
spring, from September 11 to October 11. The white circular area
over the South Pole is an area where no HALOE measurements were
The ozone hole is in the center of a spiraling mass of air over
the Antarctic that is called the polar vortex. The vortex is not
stationary and sometimes moves as far north as the southern half of
South America, taking the ozone hole with it. Air currents in the
upper atmosphere can further extend the effects of the ozone hole
into the mid-latitudes and tropics.
Effects on lower latitudes
HALOE also has returned evidence that the effects of the
Antarctic stratospheric vortex extend well beyond the South Pole
during the Antarctic spring, when the vortex is most vigorous. The
HALOE data show that Antarctic-type air, identified by low levels
of ozone and other trace chemicals, reaches as far north as 40
degrees south latitude, covering part of the South American
continent (Fig. 1b). The effects of the vortex are also extended by
air currents into the tropics, to 25 degrees south latitude.
During the 1992 Antarctic Spring, air with low levels of ozone,
which also contained chemicals that help ozone destruction, spread
northward into latitudes well beyond the Antarctic continent (Figs.
1a and 1b).
These measurements and scientific analyses show that the
Antarctic vortex is clearly an important mechanism in global
stratospheric chemistry. HALOE data will continue to be used by
observers and theoreticians as they study such occurrences in the
Evidence of ozone depletion in the Arctic
Stratospheric ozone depletion is not as severe in the Northern
Hemisphere polar region because winter temperatures are not as cold
as in the Antarctic. Also, the Arctic polar vortex does not remain
intact as long as the Antarctic vortex.
HALOE did, however, see evidence of chemical ozone depletion
over the Arctic in 1993. During the 1993 Arctic winter, the polar
vortex remained intact much longer than in other winters observed
by HALOE. While this measured depletion did not constitute an ozone
"hole," it was most likely caused by chemical and physical
processes similar to those seen in the Antarctic.
Scientists will continue to monitor stratospheric ozone levels
in both the Arctic and Antarctic regions using the continuous
global measurements provided by HALOE.
For more information contact:
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