Ozone
Ozone is a simple molecule of three oxygen atoms and it is present in the two lowest layers of the atmosphere - troposphere and stratosphere. The troposphere extends from the surface of the earth upto 12 km and stratosphere extends upto about 50 km from the surface of the earth. Almost ninety percent of all ozone in the atmosphere is in the stratosphere.

Ozone is naturally formed due to the action of sunlight with oxygen molecule. Molecular oxygen is broken down in the stratosphere by solar radiation to yield atomic oxygen, which then combines with molecular oxygen to form ozone. Ozone is also formed in the lower atmosphere due to man-made activities. In presence of sunlight it is formed due to catalytic reaction of nitrogen oxides and hydrocarbons.

Stratosphere ozone acts as a shield thereby preventing the harmful ultra violet radiation from the sun reaching the earth. . Radiation at such wavelength is responsible for sunburn; skin cancer, cataract and can weaken the immune systems that protect us from infection. UV Radiation retards plant growth and damages the genetic structure of plants and animals.

Although the presence of ozone in the upper atmosphere allows life to flourish, it is a toxic gas, which is irritating to the respiratory system while present in the lower tropospheric layer. Ozone is also one of the gases responsible for increasing the temperature of the earth.

Ozone measurement
The concentration of ozone in vertical atmospheric column is measured directly or indirectly by remote system. The indirect system of measurement can be ground based or satellite based, while direct system can be balloon, rocket or air craft based. The concentration of ozone is measured in terms of Dobson Unit. Dobson spectrophotometer is a standard instrument for measuring the ozone concentration. Beside optical methods, electro chemical method is used especially on balloon measurements.


Ozone depletion
Over the past 10-15 years, there has been a large and unexpected loss of ozone in the stratosphere each spring above the Antarctica. The ozone concentration has dropped from 320 Dobson units in 1957-64 periods to about 200 Dobson units. Ozone layer is not only diminishing above Antarctica but also in other parts of northern and southern hemisphere. The rate of degradation of Ozone layer is estimated to be around 1% per 10 years. Above northern Europe, the decrease is estimated to be about 3.5% per 10 years during winter.

Mechanism of ozone depletion
The reason for ozone depletion was earlier explained by various theories. Later on, it was confirmed that the depletion is attributable to chemicals released by human activity. Not so long ago, it was thought that Nitrogen oxides released from supersonic aircraft, industrial process, etc. are the major agents of Ozone depletion. Now, there is strong scientific evidence that man-made chlorine and bromine as well as organic compounds like chlorofluorocarbons (CFCs), halons, etc. are mostly responsible for ozone depletion in the upper atmosphere. In the lower atmosphere the chemically inert CFCs and halons are virtually uncreative while in stratosphere they react with ozone and deplete the ozone level thereby creating so-called 'ozone hole'.

Solar radiations break down many of gases in the stratosphere that constrain chlorine and bromine. Chlorine, in presence of sunlight, reacts with ozone breaking it down to form oxygen and chlorine monoxide. The latter reacts with free oxygen to form chlorine and the cycle starts again. One molecule of chlorine monoxide can destroy 10,000 to 1,00,000 ozone molecules.