Escalating Threat of Ozone Depletion

Escalating Threat of Ozone Depletion 

Dr. Arvind Singh

The stratospheric ozone layer serves as a shield against the harmful ultraviolet (UV) radiations emanating from the sun. The UV radiations are low wavelength radiations extremely harmful to living beings on the earth. Ozone plays a major role in the climatology and biology of the Earth. It filters out all radiations below 300 nanometers. About 90% of ozone is found in stratosphere (extending from 16 km up to 50 km above the earth surface), while only 10% of it is found in the troposphere (extending up to 16 km above the Earth surface). Contrary to stratospheric ozone, the tropospheric ozone serves as a gaseous pollutant.

Although the ozone is present up to 50 km, it is densest in the region between 20-25 km from the Earth’s surface. The ozone layer does not consist solely of ozone but also has a mixture of other common atmospheric gases. The total amount of ozone in a column of air from Earth’s surface up to an altitude of 50 km is the total column zone. This is recorded in Dobson Units (DU), a measure of the thickness of the ozone layers by an equivalent layer of pure ozone gas at normal temperature and pressure at sea level. The average thickness of the ozone layer around the tropics is in the range 240-260 DU, whereas it is in excess of 350 DU in mid-latitudes where the thickness has a seasonal variation also, being highest in the spring around 450 DU, and minimum in autumn. The global average thickness of the ozone layer is 300 DU. Chemically ozone consists of 3 atoms of oxygen and is formed by the action of UV radiations on oxygen (O2). Oxygen molecules absorb UV light and as a result split into oxygen atoms. The oxygen atoms then react with unsplit oxygen molecules to produce ozone (O3). 

Causes and mechanisms of ozone depletion:

Chlorofluorocarbons (CFCs) used as refrigerant are the chief culprit chemicals, that causes the depletion of ozone layer. Besides, chlorofluoro- carbons, halons and nitrous oxide (N2O) are the other chemicals that cause the thinning of the stratospheric ozone. The most common chlorofluorocarbons are CFCl 3 (which is used primarily as a propellant in spray cans) and CF 2Cl 2 (which is known as Freon and is used as refrigerant). Other CFCs are used as the blowing agent for soft polyurethane foams, such as in seat cushions, and as agents for metal cleaning and drying, industrial sterilization of medical equipments and fast freezing of foods. The chlorofluorocarbons are very stable chemicals in the atmosphere.

Halons used as fire-extinguishing agents are also responsible for the thinning of stratospheric ozone. Halons are similar in structure to chlorofluorocarbons but contain bromine atom instead of chlorine. Comparatively they are more harmful to ozone layer than chlorofluorocarbons. The chlorofluorocarbons and the halons migrate into the upper atmosphere after they are released. Since they are heavier than air, therefore they are slowly carried by air to just above the lower atmosphere and then they slowly diffuse into the upper atmosphere. This is a slow process and can take as long as5-15 years.

Nitrous oxide facilitating the ozone depletion is released in the atmosphere as a result of fossil fuel combustion, biomass burning and changing land use patterns. It is also released by microbial action on the nitrogenous fertilizers in the soil.

Some more gases have now been recognized as ozone destroyers. For instance, carbon tetrachloride used in dry cleaning also provides chlorine to the ozone layer.

Besides man-made sources like chlorofluorocarbons, fertilizers, biomass burning, airplanes and use of fossil fuels, there are natural sources like volcanoes, lighting and natural decomposition under anaerobic conditions. Supersonic jet planes flying high are also important sources of destroying the ozone layer.

Chlorine (Cl) atoms are the cause of ozone depletion which comes out from the breakdown of CFCs. The chlorine atoms (Cl) released into the stratosphere combine with ozone and strip away the oxygen atoms one by one. One atom of chlorine has the ability to destroy up to 100,000 molecules of ozone. 
Chlorine thus acts as a catalyst and just one chlorine atom can destroy several thousand of ozone molecules before the released chlorine gets converted into dilute hydrochloric acid (HCl) and comes down as acid rain. The similar catalytic action of breaking ozone into oxygen is also performed by bromine (Br) and nitrogen oxide (NO). The nitrogen system of ozone breakdown is very important. Nitrous oxide (N2O) produced by biological denitrification slowly reaches the stratosphere where it gets oxidized into nitrogen oxide (NO).

The nitrogen oxide (NO) catalyzes ozone dissociation in which oxygen is formed and nitrogen oxide is regenerated until the nitrogen dioxide (NO2) an intermediary compound is eventually acted by water and acid precipitation in form of rain.

The depletion of the ozone layer within the stratosphere in recent years is a matter of serious concern. Worldwide depletion of the ozone layer had occurred, however, the depletion is more severe over Antarctica. Severe depletion of the ozone layer is commonly called as "Ozone holes". The ozone hole was first discovered in 1982 by the British Antarctic Survey, an institute of the National Environmental Research Council. It was obvious by 1985 that the ozone layer is destroyed over the Antarctic every year during the spring season in September – October (about 40-50% every spring) having a hole in the stratosphere through which harmful UV radiations can enter into the Earth's atmosphere.

During winter, vortex of very cold air blows over the pole. So neither sunlight nor warm air from low latitude enters in this vortex. The concentration of chlorine monoxide (ClO) is usually very high in this vortex of cold air which is responsible for the ozone destruction, but it cannot destroy ozone during this time since ozone breakdown requires light and Antarctic winters are dark. However, with the onset of spring, sunlight returns and chlorine monoxide (ClO) reacts with ozone breaking it down to oxygen (O2).

The nitrogen oxide is the chemical present in the stratosphere which destroys the ozone eater chlorine monoxide (ClO). It is evident that the rate of ozone depletion in polar region is far greater than tropics and sub-tropics. In fact, at a low temperatures of the stratosphere above the polar region, the nitrogen oxides freeze to form ice clouds as a result of which chlorine monoxide (ClO) accumulates causing ozone depletion. 

Probable consequences of ozone depletion:

Depletion of stratospheric ozone layer would facilitate the increased rate of UV radiations on the Earth's surface consequently there would be an increased incidence in cases of skin cancer, sun burn and premature aging of the skin in human beings especially in white skin races. A 10% decrease in stratospheric ozone appears likely to lead a 20-30% increase in skin cancer. About 6,000 people die of such cancers in the United States of America each year. Such cases increased by 7% in Australia and New Zealand.

The UV radiations would weaken the immune system by decreasing the viability of lymphocytes in human beings. This would cause an increase in susceptibility to certain infections like herpes, measles, chicken pox and other viral diseases that elicit rash and parasitic diseases such as malaria introduced through the skin in human beings. Increased UV radiations would enhance the cases of eye cataracts, blindness and other eye diseases. UV radiations can damage several parts of the eye including the lens, cornea, retina and conjunctiva. Cataracts are the major cause of blindness in the world. A report by United Nations Environment Programme predicts a 26% rise in cataracts for every 10% drop in the ozone level.

The increased level of UV radiations would cause a decline in productivity of agricultural crops due to reduced leaf size, stunted growth and increased susceptibility to weeds, diseases, and pests. The damaging effect may be through chloroplasts, DNA and enzyme systems. Wheat, rice, barley, corn, tomatoes, cabbage, pea, soybeans, cucurbits, cauliflower, broccoli and carrots are the sensitive plants to UV radiations.

The increased influx of UV radiation would destroy the larval forms of some marine life and induce mutations in micro-organisms. The UV radiations would also damage the plant and animal planktons. In zooplanktons (microscopic animals) the breeding period may be shortened. As the microscopic plant and animals form the basis of the marine food chain, therefore any alteration in their productivity would affect the global production of fish.

Increases in solar UV radiations could affect terrestrial and aquatic biogeochemical cycles, thus altering both sources and sinks of green house and chemically important trace gases e.g. carbon dioxide (CO2), carbon monoxide (CO) carbonyl sulfide (COS) and possibly other gases including ozone. These potential changes would contribute to biosphere-atmosphere feedbacks that attenuate or reinforce the atmospheric buildup of these gases.

Synthetic polymers, naturally occurring biopolymers as well as some other materials of commercial interest are adversely affected by UV radiation. Hence, elevated UV levels would therefore accelerate their breakdown, limiting the length of time for which they are useful outdoors.

The ozone depletion may also lead to temperature changes and rainfall failures on Earth.


It can be concluded that the depletion of ozone layer is a threat to life on the Earth as it would cause an increase in the incidence of skin cancer and eye cataracts and would also weaken the immune system in human beings. The depletion of stratospheric ozone layer would adversely affect crop productivity and global fish production. Therefore, it is extremely essential to minimize the use of chlorofluorocarbons, halons and other chemicals responsible for the thinning of protective ozone layer.
Dr. Arvind Singh is M. Sc. and Ph. D. in Botany with area of specialization in Ecology. He is an dedicated Researcher having more than four dozen published Research Papers in the Journals of National and International repute. His main area of Research is Restoration of Mined Lands. However, he has also conducted Research on the Vascular Flora of Banaras Hindu University Main Campus, India.

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