Here's why:

* Photochemical Smog: The type of smog most commonly associated with urban areas is called photochemical smog. This smog is formed through a complex series of chemical reactions triggered by ultraviolet (UV) radiation from the sun.

* Nitrogen Oxides (NOx): Smog formation begins with the presence of pollutants like nitrogen oxides (NOx), released primarily from vehicle exhaust and industrial processes.

* Volatile Organic Compounds (VOCs): Another key ingredient is volatile organic compounds (VOCs), emitted from sources like gasoline, paints, and industrial processes.

* Sunlight's Role: Sunlight's UV radiation provides the energy to break down NOx molecules. This initiates a chain reaction where NOx reacts with VOCs to form ozone (O3), a major component of smog.

In summary:

Sunlight acts as a catalyst by providing the energy necessary to start the chain reaction that leads to the formation of ozone and other pollutants in smog.