Question

what is the difference between photochemical and classical smog?

Answer 1

Classical smog is also called London-type smog which occurs in the regions where emission of the sulphur containing compounds is high (e.g., due to burning of coal) and air contains high liquid water contents (e.g., fogs). The main components of classical fog are smoke, fog, sulphur dioxide.

Photochemical smog is also called Los-Angeles-type smog which occurs in the regions where there is high emissions of automobiles, large concentrations of reactive hydrocarbons (RH) (from automobile) and plenty of sunlight (high level of UV radiation).
The main components of Photochemical fog are PAN, acrylonirile, formaldehyde, nitric oxide.