What is smog and its types

The combination of smoke and fog is called smog. It is of two types:
(a) London smog or sulphurous smog or classical smog: London smog formation is initiated by a mixture of SO₂ particulate and high humidity in the atmosphere. The chemicals present in the particulates catalyse the conversion of SO₂ to SO₃ which then combines with H₂O of the humidity forming a fog of sulphuric acid droplets. These then condense on the surface of the particulate.
2SO₂ + O₂  → 2SO₃
SO₃ + H₂O  → H₂SO₄
This type of smog is seen in the early morning hours of winters months.
(b) Photochemical smog or Los Angeles smog: It is formed when the air contains NO₂ and hydrocarbons and the mixture is exposed to sunlight. As the reaction occurs in the presence of sunlight to form the smog, it is called photochemical smog) This type of smog is formed in the months of summer (strong sunlight) during the daytime when NO₂  and hydrocarbons are present in very large amounts due to heavy vehicular traffic.