Nitrogen oxides (NOx/NO2), NOX
The term 'nitrogen oxides' (NOx) is usually used to include two gases-nitric oxide (NO), which is a colourless, odourless gas and nitrogen dioxide (NO2), which is a reddish-brown gas with a pungent odour.
Nitric oxide reacts with oxygen or ozone in the air to form nitrogen dioxide. Inhalation of the pure gases is rapidly fatal. Other oxides of nitrogen include NO3 (nitrogen trioxide), N2O (nitrous oxide), N2O4 and N2O5. Nitrous oxide is a potent greenhouse gas and also causes damage to the ozone layer. N2O4, a dimer in equilibrium with NO2 and N2O5, is very unstable, usually only present in significant quantities at night (it is broken by sunlight).
Nitrogen dioxide (NO2) is used as an intermediate in the manufacture of nitric acid (HNO3) and also as a powerful oxidising agent utilised in chemical processes and rocket fuels. Nitric oxide (NO) is used in the manufacture of hydroxylamine, which is based on the reaction of nitric oxide with hydrogen in the presence of a catalyst.
Major man-made releases of nitrogen oxides are primarily from fuel combustion, biomass burning and some production processes. Combustion processes emit (among many other releases) a mixture of nitric oxide (90%) and nitrogen dioxide (10%). The nitric oxide reacts with other chemicals in the air to become nitrogen dioxide. There are also minor natural sources such as lightning and biological processes in soils. The major nitrogen gases emitted from soil micro-organisms are dinitrogen oxide (or nitrous oxide-N2O) and nitrogen N2.
Excessive exposure to nitrogen oxides may cause health effects on the blood, liver, lung and spleen. Nitrogen containing species deposited on plants can act as nutrients, however high levels of NO2 and NO are damaging to plant life – they can hinder growth and stress plant life making it more susceptible to other effects such as disease and frost damage. When sulphur dioxide is present with nitrogen dioxide this may have a synergistic effect increasing the damage more than the sum of the individual effects of nitrogen dioxide and sulphur dioxide alone. Nitrogen dioxide is also one of the gases that contribute to acid rain-causing damage to vegetation, buildings and contributing to the acidification of lakes and streams.
Ground-level ozone is formed by a chemical reaction between oxygen, nitrogen dioxide and volatile organic compounds (VOCs) in the presence of sunlight. High concentrations of ozone can also damage crops and other plant life and damage materials such as rubber. Nitrogen dioxide can react with organic peroxy radicals (formed from the breakdown of volatile organic compounds (VOCs) in the air) to form PANs (peroxyacetyl nitrates), which can serve as a temporary reservoir for reactive nitrogen and may be transported long distances.