Wednesday, November 6, 2013

Ozone layer depletion

Ozone layer depletion process
Ozone is a colourless, odorless gas composed of three atoms of oxygen (O3). Ozone has the same chemical structure regardless of where it occurs and can be useful or harmful depending on where it occurs in the atmosphere. Ozone is formed naturally in the upper stratosphere when wavelengths less than 240nm are absorbed by normal oxygen molecules which dissociate to give O atoms. The O atoms in combination with other oxygen molecules produce ozone.
In the stratosphere, about 19 to 30 km above the Earth's surface, ozone is constantly being produced and destroyed naturally. This production and destruction makes stratosphere with ozone layer that filters the Ultra-Violet radiation from the Sun and protects life on Earth. Normally there is a fine balance between production and destruction of ozone thereby safeguarding life on Earth.
Man-made chemicals called Chloro Fluoro Carbons(CFCs) are used as aerosol sprays, refrigerants and coolants etc destroy ozone molecules in  the stratosphere.
The CFCs themselves do not destroy ozone molecules but they decay ozone molecules at low temperatures. A small amount of chlorine atom and chlorine mono-oxide function as catalyst in the process of destruction of ozone. The equations involved are:
Cl  +  O3  =  ClO  + O2
ClO  + O  =  Cl  +  O2
Hence, net effect:
O3  +  O  =  2O2
Chlorine atom in the above reaction functions as a catalyst and is not consumed in the reaction. Chlorine atom used in the reaction remains as chlorine atom even at the end of the reaction. Once chlorine has broken one ozone molecule, it is free to repeat the process until it is removed by another reaction in the atmosphere. Chloro-Fluoro-Carbons are very stable molecules and can live upto 100 years.

Ozone Depletion Potential (ODP)
The ozone depletion potential of a compound is defined as the measure of its ability to destroy stratospheric ozone.
It may be defined as the ratio of total amount of ozone destroyed by a particular agent to the amount  of ozone destroyed by the same mass of CFC-11.
The ODP of CFC-11 is always taken as 1.0
ODP is a relative measure with CFC-11 taken as a standard reference. Therefore, if the ODP of a compound is 0.5, it is roughly speaking half as 'bad'  as CFC-11.

Factors affecting ODP
  1. Nature of  the halogen (Bromine containing halocarbons usually have much higher ODPs than hydrocarbons. This is because Bromine is an effective ozone destruction catalyst than Chlorine)
  2. The number of chlorine or bromine atoms in a molecule.
  3. Molecular mass and
  4. Atmospheric lifetime
Dobson Unit
 Dobson Unit (DU) is the scale for measuring the total amount of ozone occupying a column overhead.
One Dobson Unit (1DU) is defined as 0.01mm at 0C and 1 atmospheric pressure.
If the ozone layer thickness when compressed to 0C and 1 atmosphere pressure is about 5mm, the average amount of ozone would be about 500 DU.

Harmful effects  of ozone layer depletion
Ozone layer protects all life forms on Earth from the Sun's harmful UV radiation. Any significant decrease in the amount of ozone in the stratosphere results in the amount of UV radiation reaching the Earth's surface leading to harmful effects on all living organisms.
Effects on human health 
  1. Reddening of skin in sun shine (Sun burn)
  2. Skin cancer
  3. Reduction in body's immunity to disease
  4. Eye disorders like cataracts and blindness
Other living organisms
  1. UV rays are particularly harmful to small plants and animals living in the sea called 'plankton'. Plankton forms the base of ocean food chain
  2. UV rays damage certain crops like rice which is the staple food for many people in the world
  3. UV radiation can damage polymers used in paint, clothing and other materials.

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