Ozone Depletion
In contrast to the harmful ozone formed as a
photochemical oxidant at ground level (tropospheric
ozone), ozone in the stratosphere, between 25 and 40
km above the earth's surface, is the natural filter
that absorbs and blocks the sun's short wavelength
ultraviolet (UV-B) radiation, which is harmful to
life.
Ozone exists in equilibrium in the stratosphere,
balanced between formation from molecular oxygen and
destruction by ultraviolet radiation. The presence of
reactive chemicals in the stratosphere, such as the
oxides of hydrogen, nitrogen and chlorine, can
accelerate the process of ozone destruction and
therefore upset the natural balance, leading to a net
reduction of the amount of ozone. These chemicals can
participate in many ozone-destroying reactions before
they are removed from the stratosphere.
In 1974, it was found that man-made CFCs, although
inert in the lower atmosphere, can survive for many
years and migrate into the stratosphere. There, they
are destroyed by ultraviolet radiation, releasing
atomic chlorine, which attacks the stratospheric
ozone layer. This leads to another reaction that
regenerates atomic chlorine, which in turn destroys
more stratospheric ozone. This chain reaction can
cause the destruction of as many as 100,000 molecules
of ozone per single atom of chlorine.
CFCs are used as propellants and solvents in
aerosol sprays; fluids in refrigeration and air-conditioning
equipment; foam-blowing agents in plastic foam
production; and solvents, mainly in the electronics
industry. Studies in the 1980s showed that emissions
of bromine can also lead to a significant reduction
in stratospheric ozone. Bromofluorocarbons (halons
1211 and 1301) ar e widely used to extinguish fires,
and ethylene dibromide and methyl bromide are used as
fumigants.
The concentration of chlorine in the stratosphere
is set mainly by anthropogenic sources of CFCs,
carbon tetrachloride and methylchloroform. Methyl
chloride is the only natural organo-chlorine compound
found in the atmo sphere. The concentration of
chlorine in the atmosphere due to methyl chloride has
remained unchanged since perhaps 1900. The major
additions of chlorine to the atmosphere have occurred
mainly since 1970 and have been attributed to
anthropogenic sources. At present the total chlorine
in the atmosphere due to organochlorine compounds is
approaching 4.0 parts per billion by volume (ppbv), a
2.6-fold increase in only 20 years.
UV-B radiation is known to have a
multitude of effects on humans, animals, plants and
materials:
- Exposure to increased UV-B radiation can
suppress the body's immune system, which
might lead to an increase in the occurrence
or severity of infectious diseases such as
herpes, leishmaniasis and malaria and a
possible decrease in the effectiveness of
vaccination programmes. Enhanced levels of UV-B
radiation can lead to increased damage to the
eyes, especially cataracts, and to an
increase in the incidence of non-melanoma
skin cancer.
- Plants vary in their sensitivity to UV-B
radiation. Some crop species, such as peanut
and wheat, are fairly resistant, while others,
such as lettuce, tomato, soybean and cotton,
are sensitive. UV-B radiation alters the
reproductive capacity of some plants and also
the quality of harvestable products,
seriously affecting food production in areas
that already suffer acute shortages.
- Increased UV-B radiation has negative effects
on aquatic organisms, especially small ones
such as phytoplankton, zoo plankton, larval
crabs and shrimp, and juvenile fish. Because
many of these small organisms are at the base
of the marine food web, increased UV-B
exposure may have a negative effect on the
productivity of fisheries.
(Source: Unido and Industrial
Sustainable Studies)
