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Nitrogen dioxide

Nitrogen dioxide
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
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)

Infobox disclaimer and references

Nitrogen dioxide is the chemical compound NO2. It is one of the several nitrogen oxides. This reddish-brown gas has a characteristic sharp, biting odor. NO2 is one of the most prominent air pollutants and a poison by inhalation.

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]. The most important source of NO2 is internal combustion engines, which emit nitrogen oxides near people. A major industrial source is pulp mills.

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 [2].  

See also

  • Nitryl
  • 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, related to CO but with one additional electron.
  • 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


    This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Nitrogen_dioxide". A list of authors is available in Wikipedia.
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