(b) unburnt carbon particles or hydrocarbons suspended in air
The combustion of fossil fuels increases the number of suspended particles in the air. These suspended particles could be unburnt carbon particles or substances called hydrocarbons. Presence of high levels of all these pollutants causes visibility to be lowered, especially in cold weather when water also condenses out of the air. This is known as smog and is a visible indication of air pollution.