What is the composition of London smog?

Photochemical smog is composed of many different compounds, but the three major ones are ozone, PAN and VOCs (Volatile Organic Compounds).

Ozone is written as O3 and is the product of the following reaction:

O+O2--->O3.

The singular O is formed when NO2 decomposes because of sunlight

PAN(peroxyacetylnitrate)is the product of VOCs oxygen and nitrogen oxide.

VOCs are uncombusted fuels, often the product of engines.