Layer & Ozone Depletion
Filtration and the Ozone Layer
Let us look
in detail at the first protective mechanism afforded by the fact that
O3 and O2 both absorb ultraviolet but at slightly
different wavelengths. O3 absorbs in a region from 240 - 280
nm and O2 absorbs wavelengths shorter than 175 nm. The energy
absorbed in both cases is used to effect chemical change rather than re-emitted.
The UV radiation
absorbed by O2 in the stratosphere actually splits the O2
into oxygen atoms. Each of these oxygen atoms combine with other oxygen
atoms to form O2 or with O2 to form O3.
O3 absorbs UV at the higher wavelengths (240-280 nm) to split
into O and O2. The O released by O3 may recombine
with an O to form O2 or with water to form 2OH radicals. These
changes may be outlined in terms of the following equations:
+ UV radiation (< 175 nm)
O + O
+ O2 O3
O + O O2
+ UV radiation (240-280 nm) O*
+ O2 gas
+ O O2
O* + H2O 2OH
repeats and, over millions of years, has reached an equilibrium state.
The net result of the above reactions is that O2 and O3
are constantly changing into each other, and each cycle takes up energy
in the form of ultraviolet radiation, resulting in a large reduction of
the amount of ultraviolet radiation reaching the troposphere. These reactions
also result in there being a higher concentration of ozone gas in the
lower region of the stratosphere with a maximum of O3 occurring
between 20 and 26 km above the Earth's surface. This area is called the
ultraviolet radiation of the smaller wavelength damages the skin, and
can initiate the process of skin cancer. The stratospheric ozone layer
forms a protective shield protecting us from receiving large amounts of
UV. Note however that some ultraviolet does get through and is responsible
for sunburn, and skin cancer with excessive exposure.
A absorbed by the skin can actually damage
our DNA. Most of us have repair genes that can repair this damage, however
when we are exposed to large amounts of UV, the repair is not enough to
keep up with the damage and this damage can result in skin cancer. People
who can not produce skin pigment (referred to as "albino") have
a genetic condition known as xeroderma pigmentosum, which is accompanied
by a lack of the UV repair gene. These people are therefore several hundred
times as likely as the average person to contract skin cancer.
introduced many compounds into the atmosphere that are capable of disrupting
the cycle of creation and destruction of ozone molecules in the stratosphere.
A family of compounds known as chlorofluorocarbons (CFC's) have had the
most significant effect on the ozone layer by far. This discussion will
focus primarily on CFC's, although the basic process of ozone depletion
is very similar for any of the ozone-depleting substances (ODS).
varying compositions, but all of them contain different proportions of
three elements: carbon (C), Chlorine (Cl) and fluorine (F). Two of the
CFC's that were in common use are: CFC-11 (CFCl3) and CFC-12
produced and used extensively as refrigerants starting in the early 1930's.
They were discovered by a scientist named Medgley who was searching for
a more ideal cooling compound to replace the unsafe chemicals that were
being used at that time, including ammonia and sulfur dioxide. Ammonia
was most widely used, but was undesirable because it is a strong eye and
were seen then as the ideal compounds because they are extremely non-reactive,
and were therefore thought to be harmless. They are chemical inert, non-toxic,
and insoluble in water. For close to fifty years, they were hailed as
miracle substances, and were used extensively in aerosols, refrigerants,
did not know then was that because of their non-reactive nature, CFC's
are able to rise undisturbed into the atmosphere. They are not destroyed
by reactions or removed by precipitation in the tropospheric layer of
the atmosphere, and migrate over several years, eventually reaching as
high as the stratosphere.
of Ozone Cycle
migrate high enough and are hit by enough ultraviolet radiation, they
are broken down and release chlorine atoms. The chlorine atoms react with
O3 gas and the following chain of reactions results:
Cl + O3
ClO + O2
ClO + O
Cl + O2
make ozone molecules unavailable for the vital reactions that absorb incoming
ultraviolet, and are the main source of ozone depletion. One chlorine
atom can destroy over 100,000 molecules of ozone, and the result of this
disruption is a markedly lower than expected concentration of stratospheric
ozone at various points around the world.
of ozone depletion in the stratosphere was predicted in the 1970s by two
scientists named Roland and Molina. They based their prediction on the
action of CFC's on the atmosphere. Although stratospheric ozone depletion
is often referred to as the "ozone hole," that term is misleading.
What we call a hole is actually a sharp reduction in expected ozone concentrations.
Scientists have defined an ozone hole as an area having less than 220
dobson units (DU) of ozone in the overhead column (i.e., between the ground
concentration means that less incoming ultraviolet radiation is absorbed
by the reactions described earlier, and more reaches the troposphere and
the Earth's surface. Humans and other forms of life are exposed to higher
levels of ultraviolet, which can cause more damage to skin cells and sensitive
tissues of the eye than they are capable of repairing.
or the concentration of stratospheric ozone, varies seasonally and latitudinally.
There tends to be more ozone depletion in the winter with more depletion
at the polar regions. The science behind this is somewhat uncertain but
is related to the reaction surfaces that are caused by cold cloud formations
near the poles.
impacts from ozone depletion are related to the effects on ecosystems
by ultraviolet radiation. The exact cause and effect relationship for
many of these impacts is uncertain. The impacts are:
the first substantial international environmental treaty was passed. It
is known as the Montreal Protocol and includes agreements to reduce the
worldwide production of CFCs. The Protocol was precedent-setting in that
it included funds to the developing countries to compensate for the higher
costs of using alternate technologies.
Protocol has been effective in lowering the production of CFCs in the
U.S., although many developing countries have a longer time period for
compliance. However, the CFC molecule is so stable (lasting 1700 years
or more in the atmosphere) that previously produced CFC's will be entering
the stratosphere continuously and we will feel their impacts for many
years to come.
substitutes for CFC's are being developed. The desirable property of CFC--its
chemical inertness--is also the reason it is able to reach the stratosphere.
To engineer a a substitute, one must design a compound that has the desirable
properties but will not contribute to stratospheric ozone depletion. The
new compounds being considered have less chlorine and fluorine. The general
replacements are HCFCs in which one chlorine is replaced by hydrogen,
and HFCs in which chlorine is altogether replaced by hydrogen. Examples
are CHClF2 and CH2F2. The lowered chlorine
compounds are also banned in the U.S. after 2000 by the Clean Air Act.