Page 209 - Chemistry ICSE Class IX

Atmospheric Pollution 197

What is meant by destruction (or depletion) of ozone layer
The decrease in the concentration of ozone in the upper layer of the
atmosphere is called destruction or depletion of the ozone layer. This dŚĞ ƐƚƌĂƚŽƐƉŚĞƌĞ ;ĂƚŵŽƐƉŚĞƌŝĐ ůĂLJĞƌ
happens when the rate of formation of ozone in the atmosphere decreases ƌŝĐŚ ŝŶ ŽnjŽŶĞͿ ƉƌŽǀŝĚĞƐ ƉƌŽƚĞĐƟŽŶ
and the rate of destruction of ozone increases. In October 1979, a zone of ƚŽ ƚŚĞ ůŝĨĞ ŽŶ ƚŚĞ ĞĂƌƚŚ ĨƌŽŵ ƚŚĞ
ŚĂƌŵĨƵů ĞīĞĐƚƐ ŽĨ ƵůƚƌĂǀŝŽůĞƚ
lowered ozone concentration was detected over Antarctica mainly during ƌĂĚŝĂƟŽŶ͘
September-October period. The concentration of ozone in such areas gets
back to its normal value during November-December period. Such zone of
lower ozone concentration is called ozone hole.
In 1988, a smaller ozone hole appeared over Arctic and also outside
polar regions. dŚŝĐŬŶĞƐƐ ŽĨ ŽnjŽŶĞ ůĂLJĞƌ ŝƐ ŵĞĂƐƵƌĞĚ
ŝŶ ŽďƐŽŶ ƵŶŝƚƐ ; hͿ͘
Why is ozone hole not created at other places in ϭ h с Ϭ͘Ϭϭ ŵŵ
stratosphere KnjŽŶĞ ůĂLJĞƌ ƚŚŝĐŬŶĞƐƐ ĂďŽǀĞ ϮϬϬ h
ŝƐ ĐŽŶƐŝĚĞƌĞĚ ŶŽƌŵĂů͘
Ozone hole is not created at other places in stratosphere due to the following
reactions which occur in the stratosphere:

ClO (g) + NO (g) o ClONO (g)
2
2
nitrogen dioxide

Cl (g) + CH (g) o CH (g) + HCl(g)
4
3

Thus, the chlorine monoxide radical (ClO ) is removed by nitrogen

dioxide and chlorine radical (Cl ) by methane (CH ). Once these radicals
4
are tied up, the chain reaction responsible for ozone destruction cannot
take place. As a result, ozone hole cannot be created. Nitrogen dioxide

(NO ) and methane (CH ) remove the reactive chlorine radical ( Cl) from
4
2
the atmosphere and prevent ozone destruction. Therefore, these are called
scavangers.
What is the cause of ozone destruction

Ozone is mainly destroyed/depleted by chlorine radical (Cl ) and chlorine

monoxide radical ( ClO) released during volcanic activity and also during

reactions of ozone with NO, O and OH.

ƒ Reaction of ozone with chlorine radical (Cl )

O (g) + Cl (g) o ClO (g) + O (g)
3
2

ClO (g) + O(g) o Cl (g) + O (g)
2

The regenerated Cl re-enters the chain reactions to destroy more
ozone molecules.
What chemicals are responsible for the destruction of
ozone layer

Ozone is destroyed by chlorine radical (Cl ), chlorine monoxide radical

(ClO ), nitric oxide (NO), oxygen atom (O) and hydroxyl radical ( OH).
These species are generated in the atmosphere due to certain chemical
reactions or due to the decomposition of certain chemicals released into the
atmosphere during their use on the earth.
The chemical which are the main cause of ozone layer destruction are:
ƒ Chloro uorocarbons, such as CF Cl , CFCl 3 KŶ ĂŶ ĂǀĞƌĂŐĞ͕ ŽŶĞ ĐŚůŽƌŝŶĞ ƌĂĚŝĐĂů
2
ͻ
2
; ů Ϳ ĚĞƐƚƌŽLJƐ ŽŶĞ ƚŚŽƵƐĂŶĚ ;ϭϬϬϬͿ
ƒ Aviation fuel ŽnjŽŶĞ ŵŽůĞĐƵůĞƐ͘