Discuss the various chemical reactions occurring in the atmosphere ?

(i) Reactions occurring in the troposphere: All biological activities occur in the troposphere which is the lowest region and lies just above the surface of the earth (0-11 km). The most important reactions taking place in this region are those involving CO₂and H₂O vapours. When sunlight enters troposphere, CO₂ molecules present in this region absorb a large part of the energy and get excited.
   
other molecules and the excess energy present in them is converted into heat and temperature of the atmosphere may rise. Similar behaviour is also shown by the water vapours in this region.
(ii) Reactions occurring in the stratosphere: The most important reaction taking place in this region (11-50 km) the formation and dissociation of ozone. It involves two steps process:
(i) First step: The U.V. radiations coming from the sun have the efficient energy to split dioxygen (O₂) into two oxygen atoms.

(ii) Second step:  These oxygen atoms react with more of dioxygen to form ozone.

The ozone thus formed on absorbing U.V. radiations dissociates into dioxygen and an oxygen atom. Heat is given off which warms up the stratosphere. 

Thus ozone cycle is completed in the stratosphere. 
Also in the stratosphere, freons undergo photochemical decomposition, giving chlorine atoms. 

These reactive chlorine atoms then destroy the ozone through the following steps of reactions:

(iii) Reactions occurring in the mesosphere and thermosphere (collectively called ionosphere): In the ionosphere, free ions and electrons are formed due to photochemical reactions. For example,
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These free ions and electrons are not present as such below the mesosphere but immediately collide with other ions, atoms or molecules to form neutral species. But in the upper atmosphere, they do not react with other species.