Nitrogen dioxide

Nitrogen dioxide
Diossido di azoto.jpg
CAS number [10102-44-0]
Molecular formula NO2
Molar mass 46.0055
Appearance brown gas
Density 1443 kg/m³, liquid
3.4 kg/m³, gas at 294.25 K
Melting point

-11.2°C (261.95 K)

Boiling point

21.1°C (293.25 K)

EU classification Highly toxic (T+)
NFPA 704

NFPA 704.svg

R-phrases R26, R34
S-phrases S1/2, S9, S26, S28,S36/37/39, S45
Except where noted otherwise, data are given for
materials in their standard state
(at 25 °C, 100 kPa)

Nitrogen dioxide is a chemical compound with the formula NO2. It is one of the several nitrogen oxides. At ordinary temperatures and atmospheric pressure, it is a reddish-brown gas with a characteristic sharp, biting odor. It is one of the most prominent air pollutants and a poison by inhalation. This gas is present in small quantities in smog and automobile exhaust fumes. However, chemists find it useful as a catalyst, nitrating agent, and oxidizing agent.



Nitrogen dioxide (NO2) gas converts to the colorless gas dinitrogen tetroxide (N2O4) at low temperatures, and converts back to NO2 at higher temperatures. Both bottles in this photograph contain equal amounts of gas at different temperatures.

Nitrogen dioxide (NO2) is prepared by simple reaction of nitric acid (HNO3) over copper metal. The reaction is the following:

4HNO3(aq) + Cu(s) → Cu(NO3)2(aq) + 2NO2(g) + 2H2O(L)

Safety and pollution considerations

Nitrogen dioxide is toxic by inhalation. Symptoms of poisoning (lung edema) tend to appear several hours after one has inhaled a low but potentially fatal dose. Also, low concentrations (4 ppm) will anesthetize the nose, thus creating a potential for overexposure.

Long-term exposure to NO2 at concentrations above 40–100 µg/m³ causes adverse health effects[1].

Nitrogen dioxide is formed in most combustion processes using air as the oxidant. At elevated temperatures nitrogen combines with oxygen to form nitrogen dioxide:

2O2 + N2 → 2 NO2

The most important sources of NO2 are internal combustion engines [2], thermal power stations and, to a lesser extent, pulp mills.[3]

The map shown below, depicting results of satellite measurements over Europe, illustrates nitrogen dioxide as large scale pollutant, with rural background ground level concentrations in some areas around 30 µg/m³, not far below unhealthful levels. Nitrogen dioxide plays a role in atmospheric chemistry, including the formation of tropospheric ozone. A recent study by researchers at the University of California, San Diego, suggests a link between NO2 levels and Sudden Infant Death Syndrome. [4]

NO2 pollution levels in Europe, from January 2003 to June 2004.

Oxides of nitrogen

  • Nitrous oxide or N2O, "laughing gas," a linear molecule, isoelectronic with CO2 but with a nonsymmetric arrangement of atoms (NNO)
  • Nitric oxide or NO, a problematic pollutant that is short lived because it converts to NO2 in the presence of free oxygen.
  • NOx = all of the above in unspecified proportions but tending toward NO2.

More esoteric nitrogen oxides include N2O5 and the blue species N2O3.

Oxidized (cationic) and reduced (anionic) derivatives of many of these oxides exist: nitrite (NO2), nitrate (NO3), nitronium or NO2+, and nitrosonium or NO+. NO2 is intermediate between nitrite and nitronium:

NO2+ + e → NO2
NO2 + e → NO2

See also


  1. Health Aspects of Air Pollution with Particulate Matter,Ozone and Nitrogen Dioxide. Retrieved May 25, 2008..
  2. Son, Busoon and Wonho Yang, Patrick Breysse, Taewoong Chung and Youngshin Lee (March 2004). Estimation of occupational and nonoccupational nitrogen dioxide exposure for Korean taxi drivers using a microenvironmental model. Environmental Research 94 (3): 291-296.
  3. Air emissions. Botnia. Retrieved May 25, 2008.
  4. Sids Linked to Nitrogen Dioxide Pollution. Retrieved May 25, 2008.


  • Chang, Raymond. 2006. Chemistry, 9th ed. New York: McGraw-Hill Science/Engineering/Math. ISBN 0073221031.
  • Cotton, F. Albert, Geoffrey Wilkinson, Carlos A. Murillo, and Manfred Bochmann. 1999. Advanced Inorganic Chemistry 6th ed. New York: Wiley. ISBN 0471199575.
  • Sloss, Leslie L. 1992. Nitrogen Oxides Control Technology Fact Book. Park Ridge, NJ: Noyes Data Corp. ISBN 0815512945.

External links

All links retrieved January 15, 2015.


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