Where is ozone found in the
Ozone is mainly found in two regions of the
Earth's atmosphere. Most ozone (about 90%) resides in a layer
that begins between 6 and 10 miles (10 and 17 kilometers) above
the Earth's surface and extends up to about 30 miles (50 kilometers).
This region of the atmosphere is called the stratosphere. The
ozone in this region is commonly known as the ozone layer. The
remaining ozone is in the lower region of the atmosphere, which
is commonly called the troposphere. The figure (above) shows
an example of how ozone is distributed in the atmosphere.
What roles does ozone play
in the atmosphere and how are humans affected?
The ozone molecules in the upper atmosphere
(stratosphere) and the lower atmosphere (troposphere) are chemically
identical, because they all consist of three oxygen atoms and
have the chemical formula O3. However,
they have very different roles in the atmosphere and very different
effects on humans and other living beings. Stratospheric ozone
(sometimes referred to as "good ozone") plays a beneficial
role by absorbing most of the biologically damaging ultraviolet
sunlight (called UV-B), allowing only a small amount to reach
the Earth's surface. The absorption of ultraviolet radiation
by ozone creates a source of heat, which actually forms the
stratosphere itself (a region in which the temperature rises
as one goes to higher altitudes). Ozone thus plays a key role
in the temperature structure of the Earth's atmosphere. Without
the filtering action of the ozone layer, more of the Sun's UV-B
radiation would penetrate the atmosphere and would reach the
Earth's surface. Many experimental studies of plants and animals
and clinical studies of humans have shown the harmful effects
of excessive exposure to UV-B radiation.
At the Earth's surface, ozone comes into
direct contact with life-forms and displays its destructive
side (hence, it is often called "bad ozone"). Because
ozone reacts strongly with other molecules, high levels of ozone
are toxic to living systems. Several studies have documented
the harmful effects of ozone on crop production, forest growth,
and human health. The substantial negative effects of surface-level
tropospheric ozone from this direct toxicity contrast with the
benefits of the additional filtering of UV-B radiation that
What are the environmental
issues associated with ozone?
The dual role of ozone leads to two separate
environmental issues. There is concern about increases in ozone
in the troposphere. Near-surface ozone is a key component of
photochemical "smog," a familiar problem in the atmosphere
of many cities around the world. Higher amounts of surface-level
ozone are increasingly being observed in rural areas as well.
There is also widespread scientific and public
interest and concern about losses of ozone in the stratosphere.
Ground-based and satellite instruments have measured decreases
in the amount of stratospheric ozone in our atmosphere. Over
some parts of Antarctica, up to 60% of the total overhead amount
of ozone (known as the column ozone) is depleted during Antarctic
spring (September-November). This phenomenon is known as the
Antarctic ozone hole. In the Arctic polar regions, similar processes
occur that have also led to significant chemical depletion of
the column ozone during late winter and spring in 7 out of the
last 11 years. The ozone loss from January through late March
has been typically 20-25%, and shorter-period losses have been
higher, depending on the meteorological conditions encountered
in the Arctic stratosphere. Smaller, but still significant,
stratospheric decreases have been seen at other, more-populated
regions of the Earth. Increases in surface UV-B radiation have
been observed in association with local decreases in stratospheric
ozone, from both ground-based and satellite-borne instruments.
What human activities affect
upper-atmospheric ozone (the stratospheric ozone layer)?
The scientific evidence, accumulated over
more than two decades of study by the international research
community, has shown that human-produced chemicals are responsible
for the observed depletions of the ozone layer. The ozone-depleting
compounds contain various combinations of the chemical elements
chlorine, fluorine, bromine, carbon, and hydrogen and are often
described by the general term halocarbons. The compounds that
contain only chlorine, fluorine, and carbon are called chlorofluorocarbons,
usually abbreviated as CFCs. CFCs, carbon tetrachloride, and
methyl chloroform are important human-produced ozone-depleting
gases that have been used in many applications including refrigeration,
air conditioning, foam blowing, cleaning of electronics components,
and as solvents. Another important group of human-produced halocarbons
is the halons, which contain carbon, bromine, fluorine, and
(in some cases) chlorine and have been mainly used as fire extinguishants.
What actions have been taken
to protect the ozone layer?
Through an international agreement known
as the Montreal Protocol on Substances that Deplete the Ozone
Layer, governments have decided to eventually discontinue production
of CFCs, halons, carbon tetrachloride, and methyl chloroform
(except for a few special uses), and industry has developed
more "ozone-friendly" substitutes. All other things
being equal, and with adherence to the international agreements,
the ozone layer is expected to recover over the next 50 years
* Text and figure are adapted from the Introduction
to "Frequently Asked Questions" of the World Meteorological
Organization/United Nations Environment Programme report, Scientific
Assessment of Ozone Depletion: 1998 (WMO Global Ozone Research
and Monitoring Project-Report No. 44, Geneva, 1999).