The word 'smog' consists of two words 'smoke' and 'fog'. This name was given because Fog in the atmosphere Condense on the carbon particles of smoke to form smog. There are two types of smog.
London smog
The smog formed sulphur oxides (SOx) particulates (soot, ammonium sulphate) and humidity is known as London smog. It is formed by following reactions.
2SO2 + O2 
2SO3
OR
SO3 + H2O H2SO4
In the atmosphere SO2 is oxidized into SO3 by many chemicals (NO, soot particle. SO3 combines with water and forms sulphuric acid droplets. These droplets condense on the carbon particles of soot. It is formed in the early morning hours of winter months. After sunrise it increases due to oxidation of SO2 to SO3 followed by dissolution in moisture.
It causes bronchial irritation, acid rain poor visibility.
los angles smog (photo chemical smog)
It is oxidizing smog formed by action of sunlight on hydrocarbons and the nitrogen oxides. Hydrocarbons + NOx + sunlight photochemical smog.
Photochemical smog is mixture of substances such as O3, PAN, NOx, aldehyde, ketone etc. it is formed by following reactions.
In I.C. engines
N2 (g) + O3 (g) 2NO (g)
Or furnaces
2NO (g) + O2 (g) 2NO2 (g)
U.V light
NO2 (g) NO (g) + (O)
O2 (g) + (O) O3
Hydro carbons + O3 + O2 + O + NO + NO2
Oxidized hydrocarbons (peroxides, peroxyacetyl nitrate (PAN), formaldehyde, aldehyde, acrolin etc.) these oxidized hydrocarbons and ozone in presence of humidity causes photo chemical smog. The photochemical smog dissipates at night. It causes irritation to eyes, nose and lungs, and may also damage the plants.