7.1 Photochemical Smog
Photochemical smog occurs when sunlight reacts with specific chemicals in the air. One of the most important step in photochemical smog formation is the reaction of nitrogen oxides (NO and NO2 ) in the atmosphere with unburned (unburned) hydrocarbons in the presence of sunlight. This reaction leads to the formation of various secondary pollutants, including peroxyacetyl nitrate (PAN).
The chemical reactions involved in the formation of PAN:
1. Formation of Nitrogen Dioxide (NO2)
Nitric oxide (NO) is emitted from vehicles, industrial processes, and other combustion sources. NO reacts with oxygen (O2) in the atmosphere to form nitrogen dioxide (NO2) :
2. Photolysis of Nitrogen Dioxide (NO2)
NO2 absorbs sunlight (particularly UV light) and undergoes photodissociation to form nitric oxide (NO) and a free oxygen atom (O):
3. Formation of Ozone (O3)
Nascent [O] or atomic oxygen is highly reactive and reacts with atmospheric O2 to form O3 .