The presence of smoke in fog is called smog. Classical smog occurs in cool and humid climate when smoke and SO2 are suspended in air. Due to the presence of SO2 it is found to be reducing in nature. On the other hand photochemical smog occurs in warm, dry and sunny climate. Photochemical smog is produced by the action of light on unsaturated hydrocarbons and NOx in air. It is oxdising in nature.