The simultaneous production of overoxidised pollutants,nitrogen oxide, and underoxidised pollutants, hydrocarbons and carbon monoxide, makes it difficult to clean up the exhaust with a single chemical treatment. The production of each major pollutant depends on the air/fuel ratio. A rich mixture(with less air than the ideal ratio) leads to the formation of carbon monoxide and hydrocarbons. A lean mixture (with excess of air) leads to the formation of nitrogen oxide. Altering the air/fuel ratio merely trades one set of pollutants for another.
This oxide is a pollutant which attacks the ozone layer. It also contributes to the photochemical smog (i will explain this later) and to the formation of nitric acid in acid rain.
This is a poisonous gas. Hydrocarbons by themselves cause little damage, but in the presence of sunlight react with oxygen, ozone and oxides of nitrogen to form photochemical smog. One solution to the problem of pollution by vehicle exhausts is the catalytic converter.