What Is Meant By Smog?

The word smog is the combination of smoke and fog. If it contains high contents of sulfur dioxide it is chemically reducing in nature and is known as reducing smog. The main cause of reducing smog is combustion of coal. Photochemical smog consists of higher concentrations of oxidants like ozone and is also termed as oxidizing smog, it is a yellowish brownish grey haze which is formed in the presence of of water droplets and chemical reactions of pollutants in the air. It has unpleasant odour because of its gaseous components. The main reactants of photochemical smog are nitric oxide and unburnt hydrocarbons. Nitric oxide is oxidized to nitrogen oxide within minutes to hours depending upon the concentration of pollutant gas. The yellow color in photochemical smog is due to the presence of nitrogen oxide. The following conditions are required for the formation of smog:
(1) There must be sufficient NO, hydrocarbons and volatile organic compounds (VOC) emitted by the vehicular traffic.
(2) Sunlight, so that some of the chemical reactions may occur at a rapid rate.
(3) The movement of air mass must be little so that the reactions are not disturbed.
The overall result of photochemical smog in afternoon is the built up of oxidizing agents in the air.