Ozone is a gas the molecules of which are made up of three atoms of oxygen unlike the ordinary oxygen molecule, which is made up of two oxygen atoms. It is an important chemical species present in the stratosphere.

Our earth is enveloped by an atmosphere, which is divided into Troposphere, Stratosphere, Mesosphere and Thermosphere.

Troposphere extends from earth surface to an altitude of about 10 km. at the equator to about 20 km. at the poles. Over this exists the stratosphere, which extends to an altitude of about 60Km. Ozone makes up only a minor fraction of the Earth’s atmosphere and is found in varying concentrations between sea level and a height of some 60 km? Most of the atmosphere’s ozone is found between 20 and 50 km. above the ground, in the region between the troposphere and at stratosphere. The highest concentration occurs between 20 and 25 km., but even here, only about one molecule in 100,000 is ozone. In the stratosphere, ozone is being formed and destroyed but its concentration is maintained fairly constant at around 6 ppm.

Formation of Ozone


(1) In the stratosphere (Upper atmosphere): In lower mesosphere, the atmospheric on (02) absorbs UV radiation and photo dissociates into two oxygen atoms (0).r oxygen atoms combine with molecular oxygen of upper atmosphere, thereby prod ozone (03).

(2) In the troposphere (Lower atmosphere): Automobile exhaust in the street and high and exhaust of other internal combustion engines in the factories and in houses probably the most important source of ozone.

Thousands of tons of incomplete burned hydrocarbon and N02 (Nitrogen oxide) are released in the atmosphere daily’ exhaust, in the presence of UV from sun. This NO reacts with these instead of oz and therefore ozone concentrations build up.

Ozone is formed by photochemical reactions, followed by a three-body reaction: (Third body, Such as N, Or 02)


The role of third body (M) is to absorb the excess energy liberated by the above reaction and thereby 03 molecules are stabilized.


A layer of ozone in the stratosphere is a boon as it shields the passage of the dangerous ultraviolet rays to the earth. Ozone is acting as a protective radiation shield or natural sun for all living organisms on the earth. It strongly absorbs ultraviolet radiations from the sun in the region 220-330 nm and thereby protects the life on earth from severe radiation damage, such as DNA mutation and skin cancer.

Thus only a small fraction of U.V. radiation reaches the low atmosphere and the earth’s surface. Where the ozone shield not there, the consequences could be fatal for humankind. So, if humankind is to continue being able to walk around in the sunshine, then the stability of the ozone layer is essential.


It is in this background that the appearance of the ozone hole becomes a matter of great concern. Studies with some 200 plant species have shown almost two-third of them to be susceptible.

They include members of the bean, pea and’ cabbage families. Among the visible harmful effects of exposure to high doses of ultraviolet radiation were reduced leaf size, stunted growth, poor seed quality and increased susceptibility to weeds disease and pests. A study by marine biologists in the seas around Antarctica showed that higher levels of ultraviolet radiation reduce algal productivity by 6-12 per cent. It also causes damage to various forms of aquatic larvae and other organisms.

Ozone in the Troposphere

Ozone (03) in the Troposphere is a green house gas and is considered to be a pollutant as it can rapidly remove vitamin E (which protects the skin) from the uppermost layer of skin and can cause corrosion problems, damage to vegetations and mutation in DNA, etc. Reports reveal, when Troposphere Ozone level reaches 50 ppb, plant growth reduces by about 15 per cent.