Ozone depletion in Stratosphere

 Ozone layer 

Ozone is an allotropic form of oxygen which forms a layer in stratosphere (about 50km from Earth's surface) that protects us from Ultraviolet radiations coming from sun. 

Occurrence 

Most of ozone concentration is present in polar region because there is less U.V B (280-320nm) and less air currents are present. Besides, rate of decomposition is equal to rate of formation of ozone. Ozone is present in two forms 

• Good Ozone 
• Bad Ozone 

Good Ozone 

Ozone layer which is present in stratosphere that protects Earth's surface from Ultraviolet radiations. UV radiation are of three types 

UV A ..........(320nm to 400nm)  

UV B ..........(280nm to 320nm)

UV C ...........( 200nm to 280nm) 

Bad Ozone 

Ozone present in troposphere which act as a pollutant , therefore it's called as bad Ozone. 

Formation of ozone 

Ozone is formed in equatorial region then moves to polar region and then towards lower altitude. Ozone is formed by different methods are as follows: 

1. Photochemical reactions 

O2 + hv --------> O + O 

O + O2 + M -----------> O3 + M

"M" is the third body which absorbs energy and stabilize ozone.  

2. Nitrogen dioxide Dissociation

NO2 ---------> NO + O

O + O2 ---------> O3 

3. Electrical discharge 

Electrical discharge used in motor cycles have enough energy to break oxygen molecules. 

Depletion of Ozone 

Ozone depletion is actually reduction in Concentration of ozone due to some harmful chemicals or pollutants. 

Ozone hole 

In 1980's  scientist observed an area where ozone is reduced severely form a hole in ozone layer. Ozone hole is actually reduction in ozone concentration above spring of south Antarctica. 

Mechanism of ozone depletion 

Ozone depletion takes places by both 

• Natural process 
• Anthropogenic process 

1. Natural process 

Ozone is destroyed by natural process as gases or harmful chemicals released from volcanic activity. 

🔸From Oxygen 

In this process, rate of formation and decomposition of ozone takes place by reaction of ultra Violet radiations. As, 

• Formation 

O2 ---------> O + O 

O + O2 ----------> O3 

• Decomposition 

O3 ---------> O2 + O 

🔸From Nitrogen dioxide

NO2 ----------> NO + O 

NO + O3 --------> NO2 + O2 

🔸From Hydroxide 

O3 + OH -------> HOO + O2 

HOO + O ------> OH + O2 

(a) Ozone layer , (b) depleted Ozone layer

Anthropogenic process 

Man made producing chemicals cause ozone depletion; 

X + O3 -------> XO + O2 

XO + O --------> X + O2 

O + O3 --------> O2 + O2 

✓ Chlorofluorocarbons 

It comes from refrigerator, air conditioner, propellent in aerosol spray, foaming agents, electric and metal cleaning etc. 

CFCl3 + hv ---------> CFCl2 + Cl.

Cl + O3 --------> ClO + O2 

ClO + O --------> Cl + O2 

O + O3 ----------> O2 + O2 

Similarly, 

CFCl3 --------> CFCl2 + Cl 

Cl + O3 -------> ClO + O2 

ClO + O3 --------> Cl + 2O2 

O + O3 ---------> O2 + O2 

✓ Water 

H2O + hv ---------> H + OH 

H2O + O ----------> 2OH 

When OH reacts with Ozone as: 

OH + O3 --------> HOO + O2 

HOO + O ---------> OH + O2 

O3 + O -----------> O2 + O2 

When H reacts with Ozone as 

H + O3 ----------> HO + O2 

OH + O ----------> H + O2 

O + O3 ------------> O2 + O2 

✓ Nitrogen dioxide

When NO2 reacts with Ozone as 

NO2 + O3 --------> NO3 + O2 

NO3 + O ---------> NO2 + O2 

O + O3 --------> O2 + O2 

When NO reacts with Ozone as 

NO + O3 -----------> NO2 +  O2 

NO2 + O -----------> NO + O2 

O + O3 ---------> O2 + O2 

✓ Chlorine catalytic cycle 

CF2Cl2 + hv ---------> CF2Cl + Cl 

Cl + O3 -------> ClO + O2 

ClO + O -------> Cl + O2 

O + O3 ------> O2 + O2 

✓ Bromine contribution 

CBrF3 + O3 ------> O2 + BrO + CF3 

BrO + ClO --------> O2 + Cl + Br 

Polar Vertex 

It is a huge ring of circulating air covering almost whole Antarctic region and confining all ozone depletion reaction in it. It is stable throughout winter and half of summer. After that it breaks down by warm air currents. 

Harmful effects 

1. Human health 

• Irritation to eyes 
• Hemorrhage 
• Destroy cell 
• Damage protein and nuclieic acid 
• Eye disease
• Immunological disease 
• Skin disease including skin cancer 
• Photoallergic reaction 

2. Vegetation 

Damage to potato and tobacco crops 

3. Animals 

Bronchial inflammation and irritation of fibrous tissues in animals. It also damage aquatic life and microorganisms. 

4. Economical damage 

It harden rubber types.