The Thinning of the Ozone Layer

Prepared by Rutheren

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.

Where is it exactly ?

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.[

Words make it tough but pictures are easy.

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.

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]

Ozone HoleAntarctic 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.

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

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

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.

In other words…

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.

Conclusion