Ozone Hole and Ozone Depleting Substances
Ozone Hole and Ozone Depleting Substances
- In the presence of sunlight, ozone forms and breaks continuously. During winters at poles, there is absence of sunlight.
- Thus, ozone cannot be formed. However, due to human activities, the ozone layer gets depleted.
- Ozone depleting substances have the tendency to concentrate at poles because the gravitational pull of Earth is the highest at poles due to the flattening of Earth at the poles.
- The significant decrease in concentration of ozone in a particular region of stratosphere is called ‘ozone hole’.
- Ozone hole was first observed over Antarctica in 1985. Ozone hole has also developed over the Arctic region.
- In the recent years, due to ban on the use of chlorofluorocarbons, ozone has replugged to an extent.
Ozone Depleting Substances
CFCs are molecules made up of chlorine, fluorine and carbon. The CFCs are used as:
- Refrigerants in refrigerators, air conditioners, etc.
- Propellants in foam making, fire extinguishers, cleaning electronic and metallic components.
The use of CFCs is so popular because they possess the following properties:
- Low toxicity.
- Chemical stability.
Due to high thermal stability, the CFCs, after evaporation pass through the lower atmosphere to reach stratosphere. In the stratosphere, the CFCs are exposed to UV radiations which lead to the disintegration of the CFCs.
Disintegration of CFCs produces free chlorine atoms which are highly reactive. Free chlorine atoms react with ozone molecules to form chlorine monoxide and oxygen.
Cl + O3 ClO + O2
Chlorine monoxide further combines with an atom of oxygen to form oxygen molecule and free chlorine atom.
ClO + O Cl + O2
Chlorine is again produced at the end of the cycle. A single chlorine atom can destroy thousands of ozone molecules before it encounters reactive hydrogen or nitrogen to form a stable compound.
Replacement for CFCs
- The CFCs are considered as the most potent threat to the ozone layer.
- Thus, various compounds are used as replacement for CFCs. Ozone Hole and Ozone Depleting Substances
Hydrochlorofluorocarbons (HCFCs) or Hydrofluorocarbons (HFCs)
- These are considered as effective replacement for CFCs. The HFCs are even more reactive than the CFCs.
- They disintegrate at the surface of the earth; thus free atoms hardly reach the upper atmosphere. However, HFCs are greenhouse gases.
Nitrous Oxide (N2O)
- Nitrous oxide is a thermally stable gas that reaches the stratosphere, where it is destroyed due to UV rays.
- Nitrous oxide reacts with oxygen atom to form nitric oxide and diatomic oxygen (O2).
N2O + 3O 2NO + O2
- Nitric oxide (NO) further reacts with the ozone molecule to form nitrous oxide and oxygen atom.
NO + O3 N2O + O2
- Nitrous oxide, which initiates the process of destruction of ozone is produced at the end of the cycle, which in turn destroys the ozone molecules.
- Hydrobromofluorocarbons (HBFCs) and methyl bromide can also result in the depletion of ozone.
- The HBFCs are used as fire extinguishers. Methyl bromide is used as a pesticide.
- Bromine atom destroys the ozone molecules 100 times more than the chlorine atom.
- Bromine combines with ozone forming bromine oxide and oxygen.
Br + O3 BrO + O2
- Bromine oxide further reacts with chlorine oxide (C10) to form free chlorine atoms.
BrO + CIO Br + Cl + O2
Role of Polar Stratospheric Clouds in Ozone Depletion
- Polar stratospheric clouds are formed at very low temperature and at the height below 20 km. Such conditions are possible only at the poles and that too during winters.
- In summer, polar stratospheric clouds disappear due to increase in the temperature.
- These clouds provide substrates for chemical reaction between hydrochloric acid (HC1) and chlorine nitrate (C1ONO2).
- Hydrochloric acid (HC1) and chlorine nitrate (C1ONO2) are stable compounds and act as reservoirs of chlorine.
- Usually, the reaction between these two compounds is very slow. Ozone Hole and Ozone Depleting Substances
- However, in the presence of a suitable substrate provided by the polar stratospheric clouds, the reaction occurs at a faster pace leading to free chlorine atoms.
Cl + CH4 HC1 + CH3
C1O + NO2 C1ONO2 (stable)