Dr. Asaduddin M. Laskar, Assistant Professor, Department of Chemistry, Jazan University, Saudi Arabia

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   Smog refers to a smoky fog with opacity and odor. The air quality is measured by a relative scale called PSI (Pollutant Standard Index). PSI is calculated by measuring the average concentration of a pollutant. Clean air means air in which the concentrations of pollutants, namely CO, O₃, NO₂, SO₂, lead, hydrogen sulfide and vinyl chloride, fall below the standards (PSI value). Good air refers to air having PSI value of 0-50. Fog is dispersed water drops, while smoke refers to a mixture of sulfur dioxide, soot and other gases and particulates. Smog refers to a smoky fog with opacity and odor. Common constituents of smog are nitrogen oxides, sulfur oxides, sulfur, ozone, smoke or particulates among others (namely carbon monoxide, CFCs and radioactive sources). Major anthropogenic activities which release these constituents are coal emissions, vehicular emissions, industrial emissions, forest and agricultural fires and photochemical reactions of these emissions. There are two types of smog:
Classical (also called London-type) smog is formed in the regions where emission of sulfur-bearing fossil fuels (Example: coal) occurs.
Photochemical (also called Los-Angeles-type) smog is formed in the regions where high emissions from automobiles, which contain reactive hydrocarbons, occur. The fuel gasoline is a mixture of over hundred different alkanes, alkenes, and aromatic hydrocarbons.

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