About 90 per cent of the atmospheric ozone is in the stratosphere from 10 to 50 km and is known as ozone layer.
The rest 10 per cent of ozone is in the troposphere. Ozone is beneficial when it is in the stratosphere by protecting us from harmful UV rays and harmful as it helps in formation of photo-chemical smog in the troposphere.
It needs to be mentioned here, that the photo-chemical smog formation is mainly due to human activities. Thus, ozone is mainly beneficial.
Ozone absorbs all the solar ultraviolet radiations of wavelength less than 290 nm and negligibly absorbs those in between 290 to 350 nm.
Ozone is continuously created in the stratosphere and at the same time continuously removed. Thus, there is an apparent equilibrium in the ozone region with the concentration of ozone remaining constant.
The formation of ozone can be described as below. In the first step, photolytic decomposition of diatomic oxygen produces atomic oxygen; the atomic oxygen reacts rapidly with diatomic oxygen in presence of third body (M, most abundant N2 or 02) to form ozone (03).
The principal effect of the overall reaction is that most of the potentially damaging short wavelength ultraviolet radiation is absorbed as it tries to pass through stratosphere. Further, as the absorption of this radiation heats the stratosphere, stable atmospheric conditions are achieved.
The satellite data however, indicated damage to the stratospheric ozone layer over Antarctica, a hole of the size of a big continent.
The main chemicals responsible for depletion of ozone layer were found to be chlorofluorocarbons (CFCs) and specially CFC12, CF2C12, C2F3C13 and chloro flouro bromine (CF3Br). These gases are very stable (residence time: 75-185 years).
They are inert in lower atmosphere but are destroyed by the UV radiation (k< 220 nm) in the ozonosphere and release atomic CI. This atomic CI subsequently destroys the ozone layer through the following processes.
Part of the atomic CI however, can be removed through reaction with potent greenhouse gas CH4 and harmful N02, producing HC1 and CIO N02 which can then be removed by rain. Under these two circumstances, these gases become part of the cure. CI + CH4 HC1 + CH3 CI + N02 -> CIO N02
Another potent ozone depleting gas is methyl bromide (CH3Br), which is used in agriculture to sterilize soil and fumigate crops after harvesting. The released atomic bromine acts in the same way as chlorine and in the same way can be removed through reaction with CH4 and N02.
Greater concentration of N02 if present in the upper atmosphere (main source, jet planes) may also destroy ozone.
In the Arctic, the combination of land and ocean maintains warmer temperature and atmospheric conditions are not as in Antarctica and thus, there is lesser thinning of ozone layer over the Arctic in spring.
However, it is of great concern that due to troposphere warming by greenhouse gases, there is a possibility of stratospheric cooling which ultimately might lead to same atmospheric condition in Arctic like the Antarctic and cause dramatic ozone layer depletion.
Depletion of ozone layer will lead to increase in the flux of UV radiation over the earth’s biosphere. This ultimately leads to:
i. Skin cancer
ii. Eye and lung irritation
iii. Reduced photosynthesis
iv. Affects crop productivity
v. Affects weather patterns through interference with oxygen.