Background information about the Antarctic ozone hole
by Andreas Fischer
Formation of the ozone hole
Ozone is a form of oxygen and is destroyed by ultraviolet radiation (UV) and by chemical reactions.
Certain manmade chemicals when released into the atmosphere, ultimately result in the chemical
destruction of ozone in the stratosphere, including many compounds that contain chlorine (e.g.
CFCs) and bromine (e.g. Halons). Although these compounds are not themselves reactive, when
they reach the stratosphere, they are broken down into various molecules by intense UV, including
reactive ones. Under some conditions, the presence of a single reactive molecule can result in the
repeated destruction of ozone molecules. However for this repeated “catalytic” process to occur,
other photochemical reactions must also occur.
The Antarctic ozone hole was discovered in the mid-1980s and quickly opened a field of intense
research. At the start, both dynamical and chemical theories were proposed to explain this
phenomenon, but measurements over Antarctica showed that the hole was a result of unusual
chemistry on the surfaces of polar stratospheric clouds (PSCs) that activated chemical processes
necessary for the catalytic destruction of ozone in the stratosphere.
The development of the Antarctic ozone hole is strongly influenced by meteorological conditions in
the stratosphere. The low temperatures during winter over Antarctica lead to the formation of the
polar vortex, a region with high velocity winds in the stratosphere that generally circle the Antarctic
continent. This vortex somewhat limits the exchange of air between its interior and its exterior and
leads to extremely low temperatures. It is these low temperatures that form the PSCs that activate
chemical processes which in the presence of sunlight, result in the rapid ozone depletion that
annually produces the ozone hole. Temperatures colder than -78°C can produce PSCs consisting
of water and nitric acid and temperatures lower than -85°C can produce PSCs of nearly pure
water-crystals. Both types of PSCs provide surfaces where reactions occur that convert less
reactive molecules into much more reactive forms that readily destroy ozone.
Due to these requirements for the formation of the ozone hole (polar vortex with low temperatures,
PSC formation and sunlight) the hole begins to appear in August or early September as the sun
rises over Antarctica after the total darkness and extreme cold of winter. Maximum ozone loss
occurs during September and October and the hole dissipates when the vortex disappears, usually
in November or early December. The year to year changes in the area, depth and persistence of
the ozone hole are dependent upon the meteorological conditions present during each ozone hole
season. Often there are also rapid ozone decreases appearing over the edge of Antarctica in early
August. Due to their small horizontal scales they are called mini-holes being a dynamical rather
than a chemical phenomen.
Ozone measurements
Ozone is measured from satellites, ground based stations, balloons and aircrafts. Ground based
stations and satellites measure the total column ozone, a measurement of the total amount of
ozone in a column extending vertically from Earth's surface to the top of the atmosphere and
reported in Dobson Units (DU). While ground based stations measure from a single location,
satellites provide maps of the ozone distribution on global scales. Balloons, ground based stations
and satellites all provide measurements of the detailed vertical distribution of ozone over a broad
range of altitudes.
Ozone data over Antarctica are supplied by the WMO Global Atmosphere Watch network of
ground based stations and by satellites such as TOMS, SBUV, GOME and TOVS.
Assessing the strength of the ozone hole
An evaluation of size, depth and persistence of the ozone hole is often useful for making year-toyear comparisons. The usual measure of the size of the ozone hole is the area where total column
amounts are less than 220 DU, while the ozone mass deficit (OMD) has been used as a daily
indicator of accumulated ozone depletion and considered a measure of the depth of the ozone
hole. To evaluate the accumulated ozone depletion, it is necessary to compare the daily measured
ozone values at a particular location with its historical pre-ozone hole values, usually considered to
be the daily average column ozone at a particular location during the pre-ozone hole years 1964 –
1976. The OMD is then calculated by summing up the amount of ozone loss within the area more
than 10% below pre-ozone hole norms and is expressed in millions of tons of ozone. The dates of
the beginning and end of the ozone hole period each year is used to indicate the persistence.
To fully assess the Antarctic ozone hole it is also important to consider a variety of meteorological
parameters such as the area and shape of the vortex, the minimum temperatures and the area of
temperatures sufficiently low to produce PSCs. The meteorological data vary considerably with
altitude and are therefore needed at a variety of levels.
To help the general public evaluating possible exposure to damaging UV radiation where they live,
a value called the UV Index was defined that is related to the erythemal effects of solar UV on
human skin. Index values of 1-3 mean low exposure, 4-6 medium, 7-9 high and 10 and more
extreme exposure. This index is also used to evaluate daily UV exposure in the Antarctic and
nearby
populated
regions.
Effects of the ozone hole
Since ozone is an effective absorber of UV, ozone depletion leads, on the average, to an increase
in average UV at the ground. At any particular time, the amount of UV increase depends upon the
elevation of the sun above the horizon, the amount of ozone overhead (column ozone) and local
cloudiness and pollution. A portion of the UV spectrum can cause skin cancer and eye damage in
humans and has the greatest impact on animals, marine organisms and plant life. Therefore the
ozone hole has become a concern for the regions around Antarctica.
Ozone depletion is linked to greenhouse warming and therefore climate change in a number of
ways. First, many of the gases related to ozone depletion are also greenhouse gases, such as
CFCs. Second, both ozone depletion and greenhouse warming can reduce temperatures in the
ozone layer and therefore increase the intensity of the ozone hole. Measurements have shown
that over the past two decades there has been a global and annual-mean cooling of the
stratosphere and it is believed to be due to stratospheric ozone depletion and increases in
greenhouse gases and water vapor in the stratosphere. In addition, the lower temperatures in the
stratosphere are believed to be partially responsible for the Antarctic polar vortex and the
associated
ozone
hole
persisting
longer
in
recent
years.
International Agreements
In 1987 the Montreal Protocol on Substances that Deplete the Ozone Layer was signed to reduce
the global production of ozone-depleting substances. Since then several amendments have been
made such as the London and Copenhagen Amendments. Without the Montreal Protocol and its
Amendments, the continuing use of CFCs and other ozone-depleting substances could have
increased the total stratospheric abundances of chlorine and bromine many times by now if
compared with 1987 values. Although the total amount of chlorine and bromine containing
compounds in the lower atmosphere peaked in 1995 and is beginning to decline, the stratosphere
will recover more slowly. The stratospheric ozone depletion caused by human-produced chlorine
and bromine compounds is expected to disappear by about the middle of this century if there is
continued adherence to the international agreements that are presently in place.
Related links
http://www.atm.ch.cam.ac.uk/tour/
Summaries
to the Ozone Bulletins
2003
2002
2001
2000
1999
1998
1997
1996
1995