Why is smog a mixture?

In the case of photochemical smog, which is the one found in cities like Los Angeles, it is formed by a combination of sunlight, nitrogen dioxide and hydrocarbons. If the meteorological conditions are correct, there is time for these 3 things to mix before being dispersed. The nitrogen dioxide and hydrocarbons mainly come from automobile emissions. When the hydrocarbons are released into the atmosphere, they can combine with atomic oxygen or ozone to form complex molecules referred to as PANs or peroxyacyl nitrates. These PAN molecules are strongly oxidizing and are responsible for many of the harmful properties of smog.