Factsheet containing information on nitrogen oxides. Including information on why the substance is included in the Pollution Inventory and it's effects on the environment and human health.
What type of substance is it?
The term 'nitrogen oxides' (NOx) is usually used to includes 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. Nitrogen oxides contribute to acid rain, depletion of the ozone layer and have detrimental effects on health. They are also greenhouse gases. Nitrogen dioxide is one of the eight substances for which the government has established an air quality standard as part of its national Air Quality Strategy.
How is it released?
Major man-made releases of nitrogen oxides are primarily from fuel combustion (including vehicles), biomass burning and some production processes. There are also minor natural sources such as lightning, natural fires and biological processes in soils and waters.
Detailed information
Scientific name:
Oxides of nitrogen (NOx), which is generally a description for the grouping of nitric oxide (NO) and nitrogen dioxide (NO2)
Other names:
NOx, oxides of nitrogen; nitrogen dioxide (nitrogen (IV) oxide) and nitric oxide (nitrogen (II) oxide)
CAS Number:
10102-44-0 (nitrogen dioxide); 10102-43-9 (nitric oxide)
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 is a dimer in equilibrium with NO2 and N2O5 is very unstable, usually only present in significant quantities at night (it is broken down 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.
Nitric oxide (NO) is a colourless, odourless gas. Nitrogen dioxide (NO2) is a reddish-brown gas (although the colour is only seen at high concentration of the gas) with a pungent odour, soluble in water. Nitric oxide reacts with oxygen or ozone in the air to form nitrogen dioxide. Inhalation of the pure gases is rapidly fatal.
Where is it released?
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 dintrogen oxide (or nitrous oxide - N2O) and nitrogen (N2).
Local environmental effects
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 amongst other effects. When sulphur dioxide is present with nitrogen dioxide this has a synergistic effect increasing the damage done to plant life - the combination is greater than the sum of 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.
Global environmental effects
Nitrogen dioxide can react with organic peroxy radicals (formed from the breakdown of Volatile Organic Compounds in the air) to form PANs (peroxyacetyl nitrates), which can serve as a temporary reservoir for reactive nitrogen and may be transported long distances.
Possible health concerns
Excessive exposure to nitrogen oxides may affect the blood, liver, lung and spleen. The Environment Agency aims to ensure that environmental exposures are too low to harm human health.
Legislation
Why was this substance selected for the Pollution Inventory?
Included in : European Pollutant Emission Register (EPER) reporting requirements; UK National Air Quality Strategy (NAQS) substances; United Nations Economic Commission for Europe (UNECE) Convention on Long-Range Transboundary (LRTAP) reporting requirements.
Standard risk phrases for the pure substance
The standard risk phrases provided here are generally those used by suppliers of chemicals to describe substances - for example on packaging materials. The most important source of these phrases are the CHIP Regulations - Chemicals (Hazard Information and Packaging for Supply) - provided by the Health and Safety Executive. Some substances do not have CHIP risk phrases and in these circumstances we have used other risk phrases, the sources of which are indicated.
No risk phrases available for this substance
Controlling legislation and international agreements
Nitrogen dioxide is one of the eight main air pollutants in the UK's Air Quality Strategy. The air quality standard - which should not be exceeded as a one hour mean - has been set at 150 parts per billion (ppb) or 287 microgrammes per cubic metre (ug / m3). The EC Integrated Pollution Prevention and Control (IPPC) Directive and the UK Pollution Prevention and Control (PPC) legislation control emissions of VOCs and nitrogen dioxide from large plants. Releases are also controlled under EC Directive 96/62/EC 'Ambient Air Quality Assessment and Management' Internationally nitrogen oxides are controlled under the UNECE Convention on Long-Range Transboundary Air Pollution and the Gothenburg UN/ECE 'multi-effect, multi-pollutant protocol on nitrogen oxides and related substances'
These factsheets have been compiled to provide users with information on the Pollution Inventory substances and represent our best efforts to summarise a large number of disparate and sometimes conflicting data sources. We emphasise that this information describes potential hazards rather than actual effects and that the Environment Agency seeks to regulate releases to minimise emissions and hence any risk of detrimental effects occurring.
