Ozone gas is continuously formed by the action of UV rays on molecular oxygen, and also degraded into molecular oxygen in stratosphere. There should be a balance between production and degradation of ozone in stratosphere.
Of late, the balance has been disrupted due to enhancement of ozone degradation of chlorofluorocarbons(CFCs). CFCs are formed by anthropogenic activities like use of refrigerators, pesticides, artificial solvents, foam products. UV rays react with CFCs releasing Cl atoms. These Cl atoms degrades ozone. It is only under certain meteorological conditions that ozone hole is formed. The conditions are:
- Cold temperatures during polar winter
- Ice cloud formation
- Special meteorological conditions to form the polar vortex.
- Followed by the polar sun rise in spring.
However recently scientists have discovered that hole size is decreasing due to Montreal Protocol, 1987 to control the emission of ozone depleting substances. However there is another reason for shrinking of hole, i.e.; due to rise of temperatures due to global warming, which is not suitable for ozone hole formation
Antarctic Ozone Hole
The seasonal thinning of the ozone layer of the earth’s atmosphere above Antarctica, so allowing abnormal amounts of ultra-violet light to reach the earth’s surface in those regions. The ozone hole is caused by the effect of pollutants in the atmosphere destroying stratospheric ozone. During the Antarctic winter something special happens to the Antarctic weather.
Firstly, strong circular winds form that blow around the whole continent, this is known as the “polar vortex” – this isolates the air over Antarctica from the rest of the world. Secondly, special clouds form called Polar Stratospheric Clouds. Clouds don’t normally form in the stratosphere and these turn out to have the effect of concentrating the pollutants that break down the ozone, so speeding the process up.
Artic Ozone Hole
An Arctic Ozone Hole, if similar in size to the Antarctic Ozone Hole, could expose over 700+ million people, wildlife and plants to dangerous UV ray levels. The likely hood of this happening seems inevitable based on the deterioration of ozone layer caused by the effects of global warming on the upper atmosphere.
Upper atmospheric conditions in the Northern Hemisphere are becoming similar to those of the Antarctic. The result of this could be the formation of an “Arctic Ozone Hole” or “low ozone event”. The alarming difference is that there are millions of people that live in the area that will be exposed to this deadly radiation. These conditions could expose large numbers of people and animals to more ultraviolet radiation, which can cause skin cancer and disrupt reproduction of some animals and destroy plant life.
Dobson units in context with Ozone Depletion
The Dobson Unit is the most common unit for measuring ozone concentration. One Dobson Unit is the number of molecules of ozone that would be required to create a layer of pure ozone 0.01 millimeters thick at a temperature of 0 degrees Celsius and a pressure of 1 atmosphere (the air pressure at the surface of the Earth).
Expressed another way, a column of air with an ozone concentration of 1 Dobson Unit would contain about 2.69x1016ozone molecules for every square centimeter of area at the base of the column. Over the Earth’s surface, the ozone layer’s average thickness is about 300 Dobson Units or a layer that is 3 millimeters thick.