2. What is the ozone layer ?
The ozone layer or ozone shield refers to a region of Earth's
stratosphere that absorbs most of the Sun's UV radiation. It
contains high concentrations of ozone (O3) relative to other
parts of the atmosphere, although it is still very small relative
to other gases in the stratosphere. The ozone layer contains
less than ten parts per million of ozone, while the average
ozone concentration in Earth's atmosphere as a whole is only
about 0.3 parts per million. The ozone layer is mainly found in
the lower portion of the stratosphere, from approximately 20 to
30 kilometres above Earth, though the thickness varies
seasonally and geographically.
4. The Ozone Layer
Ozone in the Earth's stratosphere is created by ultraviolet light striking
oxygen molecules containing two oxygen atoms (O2), splitting them into
individual oxygen atoms (atomic oxygen);
The atomic oxygen then combines with unbroken O2 to create ozone, O3.
The ozone molecule is unstable (although, in the stratosphere, long-lived) and
when ultraviolet light hits ozone it splits into a molecule of O2 and an atom of
atomic oxygen, a continuing process called the ozone-oxygen cycle.
Chemically, this can be described as:
O₂ + ℎνuv → 2O
O + O₂↔ O₃
About 90% of the ozone in our atmosphere is contained in the stratosphere.
Ozone concentrations are greatest between about 20 and 40 kilometres
(66,000 and 131,000 ft), where they range from about 2 to 8 parts per
million.
If all of the ozone were compressed to the pressure of the air at sea level, it
would be only 3 millimeters thick.[
6. Why is the Ozone Layer important ?
The 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.
7. Ozone Depletion
The ozone layer can be depleted by free radical catalysts, including nitric oxide (NO), nitrous
oxide (N2O), hydroxyl (OH), atomic chlorine (Cl), and atomic bromine (Br). While there are
natural sources for all of these species, the concentrations of chlorine and bromine have
increased markedly in recent years due to the release of large quantities of man-made
organohalogen compounds, especially chlorofluorocarbons (CFCs) and bromofluorocarbons.[8]
These highly stable compounds are capable of surviving the rise to the stratosphere, where Cl
and Br radicals are liberated by the action of ultraviolet light. Each radical is then free to
initiate and catalyze a chain reaction capable of breaking down over 100,000 ozone
molecules.
The breakdown of ozone in the stratosphere results in a reduction of the absorption of
ultraviolet radiation. Consequently, unabsorbed and dangerous ultraviolet radiation is able to
reach the Earth’s surface.
Ozone levels over the northern hemisphere have been dropping by 4% per decade. Over
approximately 5% of the Earth's surface, around the north and south poles, much larger
seasonal declines have been seen, and are described as ozone holes.
The ozone hole is an annual thinning of the ozone layer over Antarctica, caused by
stratospheric chlorine.[2][10] Other more moderate thinnings have also been called "ozone
holes", such as the one over the North Pole during certain weather conditions.
The discovery of the annual depletion of ozone above the Antarctic was first announced in a
paper by Joe Farman, Brian Gardiner and Jonathan Shanklin which appeared in Nature on May
16, 1985.[11]
8. Ozone Hole
Antarctic ozone hole is an area of the
Antarctic stratosphere in which the recent
ozone levels have dropped to as low as 33% of
their pre-1975 values. The ozone hole occurs
during the Antarctic spring, from September
to early December, as strong westerly winds
start to circulate around the continent and
create an atmospheric container. Within this
polar vortex, over 50% of the lower
stratospheric ozone is destroyed during the
Antarctic spring.[21]
As explained above, the primary cause of
ozone depletion is the presence of chlorine-
containing source gases (primarily CFCs and
related halocarbons). In the presence of UV
light, these gases dissociate, releasing
chlorine atoms, which then go on to catalyze
ozone destruction.
9. What is Chlorofluorocarbon (CFC)?
A chlorofluorocarbon (CFC) is an
organic compound that contains
only carbon, chlorine, and fluorine,
produced as a volatile derivative of
methane, ethane, and propane.
They are also commonly known by
the DuPont brand name Freon
Characteristics of CFC
• Low boiling point
• Not reactive
• Not caustic
• Not corrosive
• Not easily combustible
10. Where can CFC be found ?
Many CFCs have been widely used
in
• Refrigerators
• Air conditioners
• Fire extinguishers
• Aerosol sprays
• Automobile coolers and heaters
• Electrical equipment
• Shaving cream
11. How CFC affects the ozone layer
The characteristics of CFC is that it is not reactive and this makes it possible
for it to reach the stratosphere. There the ultraviolet radiation strikes it and
causes a chlorine atom to break away.
CFCl₃ Cl + CFCl₂
The chlorine atom then reacts with the ozone to produce chlorine monoxide
and oxygen.
Cl + O ClO + O₂
Chlorine monoxide reacts with atomic oxygen in the stratosphere to produce
another molecule of oxygen and frees itself
ClO + O Cl + O₂
The free chlorine repeats the above cycle and destroys the ozone layer
changing it to oxygen. Each atom of chlorine has the potential to destroy tens
of thousands of ozone molecules.
14. Steps to overcome ozone depletion
To overcome the problem of the thinning of the ozone layer, many countries
have signed the Montreal protocol in 1987 after identifying the substance that
is destroying the ozone layer.
Among the conditions agreed upon are:
• To stop the use of CFC before the year 2000
• Hydrochlorofluorocarbon (HCFC) that has a less potentiality to destroy the
ozone layer has been suggested as a substitute for CFC.
