The primary offenders in the photochemical smog that blankets the Los Angeles basin and covers an increasing number of other cities on bad days are unburned hydrocarbons and oxides of nitrogen. Even with stringent controls on stationary sources of pollution, as in Los Angeles, the automobile remains a serious polluter.

Smog from oxides of nitrogen begins with nitric oxide, NO, which is formed at high temperatures from and in the combustion chambers of automotive engines:

+ 2NO

NO is emitted in the exhaust and is oxidized by atmospheric to brown nitrogen dioxide:

NO + 1/2

is brown because its unpaired electron leads to closely spaced electronic energy levels that permit absorption of light in almost the entire visible spectrum.