Nitrogen oxides
The term (NxOy) is applied to several gaseous binary chemical compounds formed by the combination of oxygen and nitrogen. The most common formation process for these inorganic compounds is combustion at high temperatures, a process in which air is usually the oxidizer.
Depending on the atomic valence used by nitrogen, nitrogen oxides have different formulations and different nomenclatures are applied to them:
| Formula | Systematic nomenclature | Stock Nomenclature | Traditional nomenclature |
|---|---|---|---|
| N2O | Dinitrogen monoxide | Nitrogen oxide (I) | Nitrous oxide (hypnitrous ahydride) |
| NO | Nitrogen monoxide | Nitrogen oxide (II) | Nitric oxide |
| N2O3 | Dinitrogen trioxide | Nitrogen oxide (III) | Nitrous anhydrate |
| N2O4 | Dinitrogen tetraoxide | Nitrogen oxide(IV) | Nitrogen tetraoxide |
| NO2 | Nitrogen dioxide | Nitrogen oxide (IV) | Nitrogen dioxide |
| N2O5 | Dinitrogen Pentaoxide | Nitrogen oxide (V) | Nitric anhydride |
Nitrogen monoxide and nitrogen dioxide are two of the most toxicologically important nitrogen oxides; neither is flammable.
Nitrogen monoxide is a gas at room temperature with a pungent sweet odor, easily oxidizable to nitrogen dioxide. While nitrogen dioxide has a strong unpleasant odor. Nitrogen dioxide is a liquid at room temperature, but turns into a reddish-brown gas at temperatures above 21°C.
The gaseous diatomic nitrogen, being formed by a triple bond, is very unreactive, but in combustions carried out at high temperatures, nitrogen manages to react with oxygen (which is very reactive) forming various types of oxides of nitrogen.
Nitrogen oxides are released into the air from motor vehicle exhaust (particularly diesel and lean-burn), from the combustion of coal, oil, or natural gas, and during processes such as arc welding, electroplating, metal engraving and dynamite detonation. They are also produced commercially by reacting nitric acid with metals or with cellulose.
Nitrogen oxides are also generated in nature, the most frequent causes being forest fires, volcanic activity and the bacterial decomposition of certain nitrates.
Nitrogen oxides, once released into the air by combustion (NOx) form, through photochemical reactions, secondary pollutants, for example PAN (peroxyacetyl nitrate), forming photochemical smog or Los Angeles type fog, typical of areas with a high concentration of motor vehicles. The reactions produced in the atmosphere by these compounds are very complex, and radicals OH, O3, NO, and others are involved.
Oxides also indicates various compounds whose combination comes from oxygen mixed with nitrogen. Nitrogen oxides are used in the production of nitric acid, lacquers, dyes, and other chemicals, such as rocket fuel, in the nitrification of organic chemicals, and in the manufacture of explosives. The engines that produce them the most are diesel and lean-burn engines.
Due to its adverse effects on health, its levels have been regulated by environmental and/or sanitary standards. In the European Union, European Directive 50/2008 (which each member state must transpose into its legislation, in Spain RD 102/2011 did) sets the maximum hourly levels, the maximum annual average level, and the maximum number of days in which the maximum hourly levels can be exceeded. In Spain, many urban areas do not meet these levels.
From the Euro 3 standard (year 2000) NOx limits are established for automotive manufacturers