The phenomenon of ‘photochemical smog’ is characterized by the formation of aerosols that reduce visibility and generation of brown hazy fumes that irritate the eyes and lungs. These fumes also affect vegetation and rubber goods.

High concentration of hydrocarbons and oxides of nitrogen in the atmosphere, stagnating of the air and maximum sun light are responsible for the formation of photochemical smog.

Photochemical reactions between oxides of nitrogen and hydrocarbons result in the production of secondary pollutants such as ozone, aldehydyes, ketones and PANs. The reaction mechanism are highly complex and are not fully understood.

CO and SO2 also contributes for photochemical smog formation.

The ‘photochemical smog’ formation is a dynamic process. In morning the NO and the hydrocarbon levels rises which is followed quickly by increase in NO2. NO2 is photo excited by sunlight which results in various cyclic chain reactions. As a result, formation of ozone and other oxidants takes place. The concentration of Ozone increases until it reaches a maximum in the afternoon. Then gradually ozone concentration decreases. NO2 concentration decreases from a maximum as ozone concentration builds up. In late afternoon NO2 concentration is very low.

‘Photochemical smog’ is different from the usual reducing smog which is a mixture of smoke and fog.

Reduction in the release of NO2 and hydrocarbons will decrease the formation of photochemical smog.

 


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