What is smog? How is classical smog different from photochemical smogs ? 

The word smog is derived from smoke and fog. This is the most common example of air pollution that occurs in many cities throughout the world. There are two types of smog:

$(i)$ Classical smog: This type of smog occurs in cool humid climate. It is a mixture of smoke, fog and sulphur dioxide. It is also called as reducing smog because it is chemically reducing mixture.

$(ii)$ Photochemical smog: This type of smog occurs in warm, dry and sunny climate. It is produce due to the action sunlight on unsaturated hydrocarbons and nitrogen dioxide produced by automobiles and factories. It is also called as oxidising smog because it has high concentration of oxidising agent.