Nitrogen oxides, referred to collectively as NOx, are compounds composed of nitrogen and oxygen. Nitric oxide (NO), whose molecules consist of one atom each of nitrogen and oxygen, is a colorless gas with a sweet odor that is emitted during the combustion of fuels. It is a major contributor to air pollution, especially as a precursor to other pollutants.
Nitrogen dioxide, whose molecules contain one nitrogen atom and two oxygen atoms (NO2), is a reddish-brown gas with a pungent, irritating odor that forms when nitric oxide is oxidized in the atmosphere. It is a major component of air pollution and also an important precursor of other forms of air pollution, including smog and acid rain.
Nitrous oxide (N2O), also known as 'laughing gas,' is a relatively inert nitrogen oxide that is a potent greenhouse gas and harmful to the ozone layer but is not a major component of air pollution. Its main source is agriculture, especially synthetic fertilizers and manure.
Nitrates and nitrites are compounds that contain both nitrogen and oxygen and at least one additional element. The former contain one nitrogen atom bonded to three oxygen atoms. Nitrites contain one nitrogen atom bonded to two oxygen atoms. Both occur naturally in soil, water and some crops and are often added as preservatives and flavoring to processed meat. They can likewise be harmful to the environment, especially to water bodies when present in excess quantities.