Why does smog happen?

Smog is a result of a combination of photochemical reactions which involve:

1. Ground-Level Ozone (made from VOCs)

2. Nitrogen oxides.

3. Peroxyacytyl Nitrate (a dangerous gas).

4. Dirt and matter (stuff that "clogs" the air: dust and burnt matter (smoke))

Smog is linked to heavy traffic, high temperatures, and little wind, NOT CO2, a common misconception.