Subject: Stratospheric Ozone Depletion
Apparently-To: cestvr@ces.iisc.ernet.in
Status: R

STRATOSPHERIC OZONE DEPLETION

Background

Ozone exists through all levels of the atmosphere, from the surface
to about 100 kilometers (km) altitude.  The majority of ozone is
concentrated in the stratosphere, between about 10 and 30 km, in a
region known as the ozone layer.  The ozone layer is critical to
life on Earth because it absorbs biologically-damaging solar
ultraviolet radiation.  Depletion of stratospheric ozone increases
the amount of this ultraviolet radiation received at the Earth's
surface, which would likely increase the incidence of skin cancer
and melanoma, as well as possibly impairing the human immune
system.  Damage to terrestrial and aquatic ecosystems may also
occur.

The concentration of stratospheric ozone has been observed to be
decreasing throughout much of the globe.  The most dramatic
decrease is observed over Antarctica each austral spring. 
Observations have demonstrated that the Antarctic ozone depletion
is due to man-made chemicals, and the weight of evidence suggests
that these chemicals likely cause much of the mid-latitude
depletion as well.  Heterogeneous chemistry (mixed-phase
reactions), involving increased amounts of chlorine and bromine in
the stratosphere, are key to the ozone decline.  The sources of
chlorine are largely chlorofluorocarbons, human-produced chemicals
that are used as refrigerant, foaming, and cleaning agents. 
Bromine also has a large anthropogenic source.  It is found in
halons that are used in various types of fire extinguishers and in
some agricultural fumigation.  An international treaty, the
Montreal Protocol on Substances that Deplete Stratospheric Ozone,
has been signed by many countries, including the United States. 
This treaty calls for the phase-out of chlorofluorocarbons by the
year 2000, although there are provisions for a faster phase-out if
the science warrants.  Because new advance in scientific
understanding are constantly occurring, major scientific
assessments are conducted periodically to determine if stricter
provisions are necessary.  The most recent assessment was conducted
in 1991; some of the results are described below.

Issues

The Antarctic Ozone Hole:  In the high-latitude stratosphere, the
typical process of ozone production is through photochemical (with
sunlight) reactions and of ozone loss is through transport into the
troposphere (lower atmosphere).  However, the huge decreases in
stratospheric ozone discovered in the mid-1980s during the austral
spring over Antarctica could not be explained by transport
processes.  Several scientific expeditions were mounted to
investigate the cause of this "ozone hole."   These expeditions
showed that the Antarctic ozone hole is very likely caused by a
combination of special meteorology and the presence of human-
produced chlorine and bromine.  The Antarctic stratosphere is
extremely cold in the winter, allowing ice particles to form
spontaneously, creating Polar Stratospheric Clouds (PSC's).  The
PSC ice crystals chemically change chlorine from a passive to an
active form.  

Nitrogen attaches to the ice crystals and is transported away from
the region, removing the compound chlorine normally reacts with to
return to its passive state.  The active chlorine acts as a
catalyst to destroy ozone, when sunlight returns in the spring. 
This heterogenous chemistry terminates as the stratosphere warms
and nitrogen-rich air is again transported into the Antarctic and
allowing chlorine to switch back to its passive state.  Ozone-rich
can than be transported into the regions, enough to largely "fill"
the hole.

Global Stratospheric Ozone:  Stratospheric ozone is measured from
ground-based and satellite-based instruments.  Data from these two
sources continue to show a decline in the total amount of ozone in
the atmosphere during the Northern Hemisphere winter.  New evidence
indicates that significant ozone decreases are also occurring in
the spring and summer in both hemispheres and during the Southern
Hemisphere winter.  These decreases are observed mainly in the
lower stratosphere, below 25 km, at middle and high latitudes,
where heterogeneous chemistry occurs as in the Antarctic.  The
increased abundance of chlorine and bromine in the stratosphere is
likely at the root of the ozone depletion.  Evidence suggests that
heterogeneous chemical reactions, similar to those involving ice
crystals over Antarctica, can occur on the surface of sulphate
aerosol particles which reside in the stratosphere.  Another cause
for part of the observed decreased ozone could be the transport of
ozone-depleted air from polar regions into the middle latitudes.

An Arctic Ozone Hole?  The Arctic stratosphere rarely gets as cold
as the Antarctic; however, PSC's have been observed there.  Also,
high concentrations of chlorine-containing chemicals have been
observed in the Arctic stratosphere.  While significant ozone
losses have been observed in the Arctic, they are, thus far, much
smaller than those in the Antarctic and therefore do not develop
into an ozone "hole."  NOAA and NASA are conducting an expedition
during the winter of 1991-92 to learn more about the chemistry of
the Arctic stratosphere.

What can be done?:  The ozone-depleting chemicals are being phased
out of production in most countries, under the terms of the
Montreal Protocol.  Several countries, including the United States,
have sped up the timetable for ceasing production to mid-1990's. 
Because of the important functions these ozone-depleting substances
perform, substitutes are being developed.  Some of the most likely
substitutes do contain chlorine, but are more apt to react in the
lower atmosphere so less chlorine would enter the stratosphere. 
Much research remains to be done to develop completely ozone-safe
substitutes.

NOAA's Contribution

     Contributes to the international debate on stratospheric ozone
     depletion, including the U.S. delegation, the Montreal
     Protocol, and several ozone scientific assessments.

     Conducts studies to better define the chlorine and bromine
     chemistry which destroys ozone and has been a major
     participant in Antarctic and Arctic research campaigns.
     
     Develops theoretical models to understand and predict the
     interaction of meteorology and chemistry.

     Monitors stratospheric ozone from ground- and satellite-based
     instruments.

     Evaluates the chemical and physical properties of proposed CFC
     substitutes and determines their impact on stratospheric    
     ozone.


TROPOSPHERIC OZONE