Question

Explain the stratospheric pollution.

Answer 1

Hint – Here we will proceed by explaining the meaning of stratosphere. Then we will explain the formation of the ozone layer in the stratosphere.

Step-By-Step solution:
Stratospheric pollution means depletion of the ozone layer (ozone hole) by certain compounds like oxides of nitrogen, chlorofluorocarbons(CFCs)
The Stratosphere is the layer of earth’s atmosphere just above the troposphere and below the mesosphere. The upper stratosphere consists of a considerable amount of ozone $\left( {{O_3}} \right)$, which protects us from the harmful ultraviolet (UV) radiations coming from the sun. These radiations cause skin cancer in humans.
Formation of the ozone layer in the stratosphere.
In the stratosphere (10-15 km from the earth surface), the ${O_2}$ is partially converted into ${O_3}$. It is a two step process.

Step 1.
The UV radiation coming from the sun has sufficient energy to split the ${O_2}$ into 2 oxygen radicals.
${O_2} + UV \to 2O$

Step 2.
The oxygen radicals react with more oxygen molecules to form ozone.
${O_2} + O + UV \to {O_3}$
Ozone absorbs U.V radiation and again breaks into oxygen molecules and oxygen radicals.
${O_3} + UV \to {O_2} + O$
Heat is given out in this process which warms up the stratosphere. For this reason, the stratosphere is a zone of increasing temperature.
${O_2} + UV \to {O_2} + O + xkcal$
In this way, ${O_3}$ cycle is completed in this region.
Depletion of the ozone layer in the stratosphere.
Due to human activity two compounds NO and CFC, are responsible for depletion of the ozone layer.
NO Nitric Oxide)
$NO + {O_3} \to N{O_2} + {O_2}$
$N{O_2} + O \to NO + {O_2}$
The NO reacts with ${O_3}$causing decrease in the amount of ${O_3}$ and forms $N{O_2}$ which reacts the amount of ${O_3}$and forms $N{O_2}$ which reacts with oxygen radicals for regeneration of NO. Thus there is no net decrease in amount of NO while amount of ${O_3}$decreases.

(b) Chlorofluorocarbons (CFC) or freons
The freons decompose in the presence of U.V. radiation coming from the sun.
$C{F_2}C{l_2} + hv \to C{F_2}Cl + Cl$
$CFC{l_3} + hv \to Cl + CFC{l_2}$
The reactive chlorine atoms then destroy the ozone layer through the following sequence of reactions which are repeated because chlorine atoms are regenerated in the second reaction.
 $
  Cl + {O_3} \to ClO + {O_2} \\
  ClO + O \to Cl + {O_2} \\
 $
One molecule of CFC can destroy more than one thousand ${O_3}$ molecules in the stratosphere.
Effects of Depletion of ozone layer in stratosphere
Due to depletion of ${O_3}$ layer, U.V. radiations fall on the earth surface
UV radiations damage the cornea and lens of the eyes.
The UV radiations affect plants proteins and thus reduce the chlorophyll.
The UV radiations disturb the heat balance of earth.
The influx of UV radiation reaching the surface of earth would increase which would increase the risk of skin cancer due to exposure of UV radiation, UV radiation also tends to damage the immune system.

Note – Whenever we come up with this type of question, one must know that the concept stratosphere is taken from environmental chemistry. Environmental chemistry is the study of sources, reactions, transport, effects and fates of chemical species in the air, soil and water environments; and the effect of human activities on these.