Produced due to incomplete combustion of carbon and due to metallurgical operations. Mainly released in air by
Photochemical smog formation -
Emission of nitrogen oxides and hydrocarbons from automobiles exhausts and solar energy by chain of reactions. NO3 and O3 react with unburnt hydrocarbons to produce formaldehyde, aerolein and proxy-acyl nitrates (PAN)
and hydrocarbon are all produced by automobile exhaust.
Amonth them is the main product.