Photochemical oxidant is a secondary pollution produced in the air when the nitrogen
oxides (NOx ) and hydrocarbon (H• C) emitted largely by automobiles react to the ultraviolet
rays of the sunlight. An oxidant is the general term of the substances which have the
potential of oxidizing those which are unoxidizible in the atmosphere. Ozone, aldehyde
and PANs are the main ingredients of the oxidant, and then are colorless and non-smog.