Sample essay on The Problem of Ozone Depletion

Sun emits radiations at all wavelengths. Of these, radiations important to our planet are: ultraviolet radiations, light rays, and infrared rays or heat waves. Radiations of visible spectrum and infrared rays carry little energy which does not harm living beings.

The energy content of ultraviolet radiations is however, larger than the limits of tolerance of a living cell and hence is harmful or even lethal to a living system. Though these high energy radiations triggered the biochemical process on, which led to the emergence of life on this planet, their continued presence was injurious to the living system. Life as a consequence, had to await the development of an effective shield of atmospheric gases (O₂, N₂, O₃,) which could check the biocidal radiations high up in the atmosphere before it could come out on land.

(1) Ultraviolet Radiations and Ozone:

Ultraviolet radiations are usually grouped, rather arbitrarily, into the following three categories:

1. Ultraviolet-A (UV-A):

With a wavelength range between 3150-4000 A which do not cause much harm to a living system. Only a part of these radiations reaches earth’s surface, which are tolerated by the living beings.

2. Ultraviolet-B (UV-B):

With a wavelength from 2800 A to 3150 A which are more damaging than UV-A.

3. Ultraviolet-C (UV-C):

With a wavelength range from 2000 A to 2800 A which carry larger amount of energy. These are the most damaging radiations for the biosphere. However, they are almost completely absorbed by atmospheric gases.

Much of the harmful high energy solar radiations are absorbed at various heights by the gaseous mantle which surrounds our planet. Though oxygen, nitrogen and a number of other constituents of the atmosphere absorb short wavelength ultraviolet radiations, none of these gases can absorb effectively wavelengths greater than 2200 A.

This leaves a gap which is filled by ozone alone. It absorbs all radiations between 2200 to 2900 A. Radiations above 2900 A are not completely absorbs by this gas, which however are considerably diluted by the ozone layer. A depletion of ozone content of the atmosphere shall result in increased penetration of 2900 A to 3150 A radiations (UV-B) which are very injurious to the biosphere. In extreme cases radiations with wavelengths lower III 2800A may also reach earth’s surface.

Laboratory studies have shown that absorption of ultraviolet radiations within the range of UV-B by a living cell is largely due to the absorption of energy by nucleic acids, the DNA and RNA molecules. The energy thus absorbed breaks up these macromolecules and affects adversely such vital processes as protein synthesis, growth and reproduction. Alterations in DNA molecules could; cause long range genetic effects which could change the very shape of life on this planet.

(2) The Ozone Layer:

High up in the stratosphere, about 15-40 kms above earth’s surface, short wavelength ultraviolet radiations in the range of 1800 A to 2200 A are absorbed by molecular oxygen which splits up into its constituent atoms. These atoms combine with molecular oxygen to produce ozone:

Therefore, ozone is a result of photochemical reactions in which the starting molecule is oxygen. Along with this reaction another photochemical reaction which causes breakdown of ozone molecules due to absorption of 2000-2900 A radiations also occur.

The two reactions, i.e., the formation and destruction of ozone molecule normally balance t each other and ultimately result in effective absorption of short wavelength ultraviolet radiations in the stratospheric region. Life underneath is thus protected from the biocidal solar radiations.

Ozone occurs in the atmosphere because the atmosphere contains oxygen. Without oxygen there would have been no ozone umbrella to protect the living systems on earth’s surface. Evolution of atmospheric oxygen and subsequently the ozone layer has, therefore, been intimately connected with the evolution of life on this planet. Primitive atmosphere contained very little oxygen.

The only process which could have contributed some oxygen is photo-dissociation of water molecules by: 1950 A – 2000 A ultraviolet radiations which are soon cut off as some oxygen accumulates and this stops the photo-dissociation reaction. Large concentrations of oxygen could not be attained by this process alone.

It was only after the evolution of photosynthesis that a gradual accumulation of oxygen occurred. With the accumulation of this vital gas more and more ozone was produced which provided means to cheak 2200-2900 A ultraviolet radiations effectively. With the built up oxygen concentration little 1800-2200 A could reach upto earth’s crust. Ozone formation and the zone of its maximum concentration, therefore, gradually shifted upwards where the ultraviolet light of suitable wavelength was available, i.e., the stratosphere.

Most of the ozone present in the atmosphere, almost 90% is concentrated in the stratosphere at an altitude of about 15-40 kms. If the entire ozone present in the stratosphere is condensed into a single sheet covering the globe, it will form only 2.4 – 2.6 mm thick layer at equator and 3.1 – 4.5 mm thick layer at latitudes about 70°N and S. At ground level ozone occurs in traces only. Though maximum amount of ozone production occurs over equator, its concentration is lowest here as most of the ozone formed is displaced towards the poles with massive movement of atmospheric air.

(3) Stratospheric Ozone Depletion:

The concern over stratospheric ozone depletion was first aroused by the U.S. chemist Harold Johnson in 1971, when he pointed out that Supersonic Aircrafts then under fabrication would introduce large quantities of nitric oxide at precisely those altitudes where ozone content is maximum. Nitric oxide shall catalytically attack ozone molecules and convert them to oxygen. Large-scale use of supersonic aircrafts shall weaken the protective ozone layer on global scale and could result in a variety of adverse consequences such as massive increase in number of skin cancers, modification of flora, fauna and climatic conditions.

Another sensation was created in May 1985, when Farmen and his co-workers showed that the total average quantity of ozone measured over the South Pole in the month of October (the beginning of southern spring) was gradually diminishing. From 1979 to 1985 a reduction of about 40% in ozone content over Antarctica was recorded (Farmen et al, 1985). These observations were later confirmed by other workers as well.

It was soon realized that it was not only nitric oxide but there were other hazards as well which could bring about catalytic destruction of ozone content. Depletion of stratospheric ozone has been found to occur mainly due to the three following constituents of the stratosphere.

1. Nitric Oxide Molecules:

Nitric oxide usually present in stratosphere reacts with ozone to form nitrogen dioxide which in turn reacts with atomic oxygen to produce nitric oxide again.

Chlorine Atoms:

Chlorine atoms react with ozone to yield chlorine monoxide which reacts with atomic oxygen to regenerate chlorine atoms again.

Hydroxyl Ions:

Hydroxyl ions produced by photodissociation of water molecules in the stratosphere, react with ozone molecules to produce HO₂ which reacts with another ozone molecule to yield hydroxyl ions again.

These reactions as given above are not so simple. The chemical processes which occur under conditions of low temperatures, low pressures and plenty of high energy ultraviolet radiations are exceedingly complex in nature and involve almost a hundred and fifty independent reactions. One of the most important features to note in the three set of reactions above is that the principal constituent which starts the reactions is regenerated at the end of cycle and is available again to cause further ozone breakdown.

Under normal conditions nitrogen dioxide combines with chlorine monoxide to form chlorine nitrate which is further converted to nitric and hydrochloric acids – products which are harmless. Similarly methane normally present in the system in the stratosphere has a protective role as it reacts with chlorine atoms to produce the harmless hydrochloric acid.

These reactions are very important as they serve as a means of elimination of both oxides of nitrogen as well as chlorine atoms from the system. Recent data collected by NASA’s Upper Atmospheric Research Satellite indicates that fine particulate material present in the atmosphere plays a vital role in destruction of stratospheric ozone.

Reactions catalysed by sulphuric acid aerosols immobilize nitrogen dioxide and block its protective functions. These aerosols also retard the reaction between methane and chlorine, thus damaging another important defensive reaction which protects the ozone umbrella by locking of chlorine atoms in harmless hydrochloric acid molecules.

(4) The Destruction of Ozone Molecules over Poles :

Much of the stratospheric ozone is formed at equator and is displaced over to Polar Regions so that the concentration of this gas is maximum at the poles. Incidentally, it is at the poles only, the Antarctic and Arctic regions that the destruction of ozone appears to be fastest.

Strong westerly winds around the poles, which often reach a speed of 90-100 metres per second isolate a column of cold air inside a fast moving vortex. Cut off from the warmer air around the temperatures in this column of air drop forming clouds upto an altitude of 25-30 kms. Sub-zero temperatures cause the formation of ice crystals which provide suitable loci for atmospheric reactions to occur.

Oxides of nitrogen are locked in these fine crystals before they can immobilize free chlorine atoms which are held over the long polar winters on fine aerosols as chlorine nitrate and hydrochta acid. With the return of sun and its high energy radiations hydrochloric acid and chlorine nitrate react to release free chlorine atoms which destroy ozone molecules catalytically.

Thus instead of dissociation into harmless acids, weather conditions promote the reaction of chlorine nitrate with hydrochloric acid causing the regeneration of chlorine atoms. By the beginning of summers, when the polar vortex begins to break up the ozone concentration over the Polar Regions has been reduce appreciably.

Ozone hole was first discovered over Antarctica and soon similar ozone depletion was observed over Arctic also. The process of thinning out of this protective ozone umbrella is not confined to Polar Regions alone. Observations conducted from high flying research planes tot shown that blobs of chlorine monoxide rich air slip off the poles and hang over Northern Europe from London to Moscow, high up in the stratosphere. Global ozone monitoring programmes have pointed out a reduction of about 3.0% in mean annual ozone content between the latitudes 53° and 40° North, which covers much of Europe, Great Lake areas, the Black Sea, the Caspian, part of Mongolia, Manchuria and Japan.