Photochemical smog

The chief original reactants in an episode of photochemical smog are:
1. Molecules of nitric oxide, NO, and of unburned.
2. Partially oxidized hydrocarbons that are emitted into the air as pollutants
from internal combustion engines.
3. Nitric oxide is also released from electric power plants.
The concentrations of these chemicals are orders of magnitude greater than
are found in clean air. Gaseous hydrocarbons and partially oxidized
hydrocarbons are also present in urban air as a result of the evaporation of
solvents, liquid fuels, and other organic compounds. Collectively, the
substances, including hydrocarbons and their derivatives, that readily
vaporize into the air are called volatile organic compounds, or VOCs.