Atmospheric Structure and Ozone

Ozone is an inorganic molecule formed by the three molecules of oxygen with the chemical formula O$ _3$. It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope O$ _2$ and more reactive molecule.

When oxygen molecules absorb ultraviolet photons and undergo photolysis, ozone is obtained
$3O _2 \xrightarrow [photolysis]{UV : light} 2O _3$.

Ozone shields the surface of the earth from ultraviolet (UV) radiation from the sun. Ozone layer reduces the chances of skin cancer.

40 to 50% depletion in ozone layer occurs in the South polar region.
Ozone is destroyed by collisions with other atoms and molecules.
The large scale destruction of the stratospheric ozone first came into focus in 1987 through the experiments in antarctica.

Depletion of ozone layer is mainly due to Chloroflorocarbons.
Note:
The following describes the destruction of the ozone by chlorofluorocarbons.
Chlorofluorocarbons contains the chlorine molecule which reacts with ozone (O$ _3$) molecule and splits into O$ _2$ and O molecules.
Chlorofluorocarbons are used in refrigerators, air conditioners and aerosol sprays. They damage the ozone layer of the atmosphere.

The presence of CFCs in the atmosphere results in the depletion of the ozone layer. The chemical formula of this substance is CF$ _2$Cl$ _2$. They are used as coolants in refrigerators, coolers, etc. The CFCs causes the breaking of the ozone molecule which forms the oxygen radical and O$ _2$. The depletion of the ozone layer results in the penetration of the harmful UV radiations from the sun into the Earth's atmosphere. 

The correct answer is Stratosphere. Option C

Chlorofluorocarbons or Chlorofluoromethane are also known as freon or aerosols. They are chemicals used as refrigerants, propellants and solid plastic foams. These chemicals are also released as aerosols by jets flying at high altitudes. Along with nitrogen oxides, chlorofluorocarbons react with ozone of ozonosphere and depletes the same. Chlorofluorocarbons release reactive chloride which directly reacts with ozone molecules setting a chain reaction.

CFCs (Chlorofluorocarbons) are the most well-known pollutants responsible for the catalytic destruction of the ozone layer in the stratosphere.

• Nitrous oxide is stable at ground level and breaks down when it reaches the stratosphere to form nitrogen oxides, that trigger ozone-destruction. 
  
• The main cause of ozone depletion are halocarbon refrigerants, such as chlorofluorocarbons (CFCs), HCFCs, halons).
  
• So, the correct option is 'Both B and C'.
  

Ozone layer is formed in the stratosphere of the atmosphere by the action of UV rays on the oxygen molecules. This ozone layer acts as absorbant for the harmful UV radiations and so, these radiation do not cross the troposphere to reach the earth's surface. However, the ozone depleting pollutants like chlorofluorocarbons reach the stratosphere and break when exposed to the UV rays. This process releases the reactive chlorine which further reacts with ozone to form simple oxygen. That is how the ozone layer is depleted. Hence, the correct answer is 'Chlorofluorocarbons'.

Ultraviolet radiations cause photochemical splitting of oxygen molecules in the stratosphere. The nascent oxygen combines with molecular oxygen to form ozone. In troposphere nascent oxygen comes from nitrogen dioxide.

The ozone depleting substances include the chlorofluorocarbons, oxides of nitrogen, methyl bromide, carbon tetrachloride etc because these split up when exposed to UV radiations and release the reactive chlorine and bromine. These reactive species of chlorine and bromine then react with ozone to release the nascent oxygen. Sulphur dioxide and carbon dioxide are greenhouse gases. Carbon monoxide is an air pollutant.