Urban Ozone: The Photochemical Smog Process
Urbanization undergoes episodes of air pollution where high levels of ground-level ozone in appreciable
concentrations at low altitudes in the air are produced as a result of the light-induced chemical reaction
of pollutants. This phenomenon is called photochemical smog.
The chief original reactants in an episode of photochemical smog are molecules of nitric oxide, NO, and
of unburned and partially oxidized hydrocarbons that are emitted into air from internal combustion of
engines. Nitric oxide, on the other hand, is released from electric power plants. The concentrations of
these chemicals are orders of magnitude greater than found in clean air.
Also, Volatile Organic Compounds (VOCs) including hydrocarbons and their derivatives that readily
vaporize into the air react in photochemical smog.
Gaseous hydrocarbons and partially oxidized hydrocarbons are VOCs that are present in urban air as a
result of the evaporation of solvents, liquid fuels, and other organic compounds.
Another vital ingredient in producing photochemical smog is sunshine, which serves indirectly to
increase the concentration of free radicals that participate in the chemical processes of smog formation.
Substances that are emitted directly into air are called primary pollutants; in the smog reaction, these
are NO and VOCs, most of which are relatively innocuous with regard to human health.
An approximate overall chemical equation for the smog reaction is
Substances into which the primary ones are transformed are called secondary pollutants; in smog they
include ozone, nitric acid, HNO3.