Stratospheric Ozone Layer
Depletion
Previous Page
Page 7
The Earth's
Atmosphere
Before discussing the CFC problem, it is
necessary to understand how the Earth's atmosphere is broken into layers. There
are four distinct areas of air surrounding the Earth. Each has distinctive
characteristics that change as the distance from the Earth's surface increases.
The area humans live in is known as the troposphere. It begins at the surface
of the earth and extends for 7-10 miles (11-16 km). The temperature and
pressure decrease rapidly with altitude until the tropopause is reached. The
tropopause is an area at which a temperature inversion occurs. This area acts
as a nearly impervious barrier to most of the water trying to rise out of the
atmosphere. This is important because what little water passes this point can
escapes into space.
Above
the troposphere is the stratosphere that extends to a point approximately 31
miles (50 km) above the Earth's surface. Temperature remains fairly constant
near the tropopause but begins to increase near the upper bound of the
stratosphere as solar radiation increases. The water that passes the tropopause
forms clouds here. The most important molecule in the stratosphere is ozone.
Next
comes the mesosphere. It extends from the top of the stratosphere to an
altitude of 50 miles (80 km). Finally, the top of the atmosphere is known as
the thermosphere. It begins at 50 miles and continues to outer space. The
temperature continues to increase through the mesosphere and the thermosphere.
Ozone
is of concern to us when it is in the lower two levels of the atmosphere, the
troposphere and the stratosphere. No matter where it is found, ozone is a
relatively unstable molecule. An ozone molecule consists of three atoms of
oxygen bound together in a triangular fashion. Although it represents only a
tiny fraction of the atmosphere, ozone is crucial for life on Earth.
Depending
on where ozone is located, it can either protect or harm life on Earth. In the
stratosphere, ozone acts as a shield to protect Earth's surface from the sun's
harmful ultraviolet radiation. Without this shield, ultraviolet levels at the
Earth's surface would be higher and humans would be more susceptible to skin
cancer, cataracts, and impaired immune systems. In the troposphere, however,
this same ozone molecule is a harmful pollutant that causes damage to lung
tissue and to plants.
The
amounts of helpful and harmful ozone in the atmosphere depend on a balance
between processes that create it and those that destroy it. An upset in the
ozone balance can have serious consequences for life on Earth. Scientists are
finding evidence that changes are occurring in ozone levels. The harmful ozone
is increasing in the air we breathe, while the helpful ozone is decreasing in
our protective ozone shield. In the next few pages, the processes that create
and destroy the helpful ozone will be described. Also, the way that humans
affect these processes will be discussed.
At
the top of the stratosphere ozone is created and destroyed primarily by
ultraviolet radiation. The air in the stratosphere is bombarded continuously
with radiation from the sun. The ultraviolet rays, which are part of this
light, strike molecules of ordinary oxygen (O2) causing them to
split into two single oxygen atoms, known as atomic oxygen or oxygen radicals.
A freed oxygen atom then can collide with an oxygen molecule (O2),
and form a molecule of ozone (O3).
This
process absorbs much of the ultraviolet radiation that would otherwise reach
the Earth's surface. Ironically, this same ultraviolet radiation also causes
the destruction of ozone. When an ozone molecule (O3) absorbs even
low energy ultraviolet radiation, it splits into an ordinary oxygen molecule (O2)
and a free oxygen atom (O). The free oxygen atom then may bond with an oxygen
molecule to make another ozone molecule, or it may steal an oxygen atom from an
ozone molecule to make two ordinary oxygen molecules. Some scientists call
these processes of ozone production and destruction, initiated by ultraviolet
radiation, the "Chapman Cycle."
Natural forces other than the Chapman Reactions
also affect the concentration of ozone in the stratosphere. Since ozone is such
a highly unstable molecule, it reacts very easily, readily donating an oxygen
molecule to nitrogen, hydrogen, or chlorine found in natural compounds. These
elements always have existed in the stratosphere, released from sources such as
soil, water vapor, and the oceans.
In
addition, ozone levels can change periodically as part of regular natural
cycles such as the changing seasons, sun cycles and winds. Moreover, volcanic
eruptions may inject materials into the stratosphere that can destroy ozone.
Over
the Earth's lifetime, natural processes have regulated the balance of ozone in
the stratosphere. An easy way to think about the ozone balance is to imagine a
plastic bag being filled with water. As the bag fills, a hole is punched in it
to allow water to escape. As long as water escapes at the same rate that water
is being poured in, the amount of water in the bag will remain the same.
Likewise, as long as ozone is being created and destroyed at the same rate, the
total amount of ozone will remain the same.
Human
Activity and the Atmosphere In the past two decades, however, scientists
have found evidence that human activities are disrupting the ozone balance.
Human production of chlorine-containing chemicals such as chlorofluorocarbons
(CFCs) has added an additional force that destroys ozone. (CFCs are compounds
composed of carbon atoms bonded to chlorine, fluorine.) As was seen earlier,
CFCs are stable and thus do not react easily with other chemicals in the lower
atmosphere. One of the few forces that can break apart CFC molecules is
ultraviolet radiation in a process called photochemical decomposition. In the
lower atmosphere, however, CFCs are protected from this radiation by the ozone
layer. So, CFC molecules can migrate intact into the stratosphere where they
then are photo-decomposed. At first, scientists thought CFCs were too heavy to
make their way into the upper atmosphere. Although the CFC molecules are
heavier than air, the mixing processes of the atmosphere lift them into the
stratosphere. The mixing process takes many years, up to fifty, and so the
problem is not easily noticed. The ozone in the stratosphere today is being
destroyed by CFCs released many years ago.
