Ozone and Layer Depletion
Content

The causes of ozone layer depletion are:


 
1. Presence of NO and NO2 in the atmosphere:

NO + O3 --> NO2 + O2
NO2 + O -->  NO + O2
O + O3 -->  2 O2

 Nitrogen oxides (NO and NO2) have short-time persistence in the atmosphere - a few days. This time is too short for their transport from the ground air to the stratosphere. The main source of NO and NO2 in the stratosphere is the photodissociation of nitrous oxide (N2 O).
2. Catalytic cycles of chlorine and fluorine radicals:

2 x (Cl + O3  --> ClO + O2 ) 
Cl0 + ClO + M --> Cl2O2 + M 
Cl2O2 + hn --> Cl + ClO2
ClO2 + M --> Cl + O2 + M 
2 O3 -->  3 O

 Halogenated hydrocarbons (from aerosol sprayers, fire extinguishers, cooling equipment and others) are the active medium for chlorine and fluorine transport to the stratosphere. For example, the mean atmospheric lifetime of the Freon 12 - the mostly used freon - is 102 years. Ozone is chemical stable in the troposphere, slightly diffuse to the stratosphere, where the active chlorine is released by photolysis and catalytically destroys the ozone.
3. Ozone holes above the Antarctic At the extreme low temperature (-80°C) the polar stratospheric cloudiness is created. Heterogeneous reactions in the polar stratospheric cloudiness support creation of the hydrated nitric acid crystals. These crystals settle to the lower layers of stratosphere whereas ozone dissociation precursors (Cl2, HOCl,) are accumulated as gas in the dehydrated and denitrificated layers of the stratosphere during the polar night. During the polar day it comes to photodissociation of these substances and released radicals catalytically dissociate ozone.