Nitrogen dioxide is a chemical compound with the formula NO2. One of several nitrogen oxides, nitrogen dioxide is a reddish-brown gas. It is a paramagnetic, bent molecule with C2vpoint group symmetry. Industrially, NO2 is an intermediate in the synthesis of nitric acid, millions of tons of which are produced each year, primarily for the production of fertilizers.
Nitrogen dioxide is poisonous and can be fatal if inhaled in large quantities.[8] The LC50 (median lethal dose) for humans has been estimated to be 174 ppm for a 1-hour exposure.[9] It is also included in the NOx family of atmospheric pollutants.
Properties
Nitrogen dioxide is a reddish-brown gas with a pungent, acrid odor above 21.2 °C (70.2 °F; 294.3 K) and becomes a yellowish-brown liquid below 21.2 °C (70.2 °F; 294.3 K). It forms an equilibrium with its dimer, dinitrogen tetroxide (N2O4), and converts almost entirely to N2O4 below −11.2 °C (11.8 °F; 261.9 K).[6]
The bond length between the nitrogen atom and the oxygen atom is 119.7 pm. This bond length is consistent with a bond order between one and two.
Unlike ozone (O3) the groundelectronic state of nitrogen dioxide is a doublet state, since nitrogen has one unpaired electron,[10] which decreases the alpha effect compared with nitrite and creates a weak bonding interaction with the oxygen lone pairs. The lone electron in NO2 also means that this compound is a free radical, so the formula for nitrogen dioxide is often written as •NO2.
The reddish-brown color is a consequence of preferential absorption of light in the blue region of the spectrum (400–500 nm), although the absorption extends throughout the visible (at shorter wavelengths) and into the infrared (at longer wavelengths). Absorption of light at wavelengths shorter than about 400 nm results in photolysis (to form NO + O, atomic oxygen); in the atmosphere the addition of the oxygen atom so formed to O2 results in ozone.
Industrially, nitrogen dioxide is produced and transported as its cryogenic liquid dimer, dinitrogen tetroxide. Direct production of the monomer usually occurs on a laboratory scale.
Nitric acid decomposes slowly to nitrogen dioxide by the overall reaction:
4 HNO3 → 4 NO2 + 2 H2O + O2
The nitrogen dioxide so formed confers the characteristic yellow color often exhibited by this acid. However, the reaction is too slow to be a practical source of NO2.
Instead, most laboratory syntheses stabilize and then heat the nitric acid to accelerate the decomposition. For example, the thermal decomposition of some metal nitrates generates NO2:
At 150 °C (302 °F; 423 K), NO2 decomposes with release of oxygen via an endothermic process (ΔH = 14 kJ/mol):
2 NO2 →2 NO + O2
As an oxidizer
As suggested by the weakness of the N–O bond, NO2 is a good oxidizer. Consequently, it will combust, sometimes explosively, in the presence of hydrocarbons.[12]
This reaction is one of the steps in the Ostwald process for the industrial production of nitric acid from ammonia.[13] This reaction is negligibly slow at low concentrations of NO2 characteristic of the ambient atmosphere, although it does proceed upon NO2 uptake to surfaces. Such surface reaction is thought to produce gaseous HNO2 (often written as HONO) in outdoor and indoor environments.[14]
Conversion to nitrates
NO2 is used to generate anhydrous metal nitrates from the oxides:[11]
MO + 3 NO2 → M(NO3)2 + NO
Alkyl and metal iodides give the corresponding nitrates:[10]
NO2 is introduced into the environment by natural causes, including entry from the stratosphere, bacterial respiration, volcanos, and lightning. These sources make NO2 a trace gas in the atmosphere of Earth, where it plays a role in absorbing sunlight and regulating the chemistry of the troposphere, especially in determining ozone concentrations.[18]
Nitrogen dioxide also forms in most combustion processes. At elevated temperatures nitrogen combines with oxygen to form nitrogen dioxide:
N2 + 2 O2 → 2 NO2
For the general public, the most prominent sources of NO2 are internal combustion engines, as combustion temperatures are high enough to thermally combine some of the nitrogen and oxygen in the air to form NO2.[8]
Outdoors, NO2 can be a result of traffic from motor vehicles.[19] Indoors, exposure arises from cigarette smoke,[20] and butane and kerosene heaters and stoves.[21] Indoor exposure levels of NO2 are, on average, at least three times higher in homes with gas stoves compared to electric stove.[22][23]
Workers in industries where NO2 is used are also exposed and are at risk for occupational lung diseases, and NIOSH has set exposure limits and safety standards.[6] Workers in high voltage areas especially those with spark or plasma creation are at risk.[citation needed]Agricultural workers can be exposed to NO2 arising from grain decomposing in silos; chronic exposure can lead to lung damage in a condition called "silo-filler's disease".[24][25]
NO2 diffuses into the epithelial lining fluid (ELF) of the respiratory epithelium and dissolves. There, it chemically reacts with antioxidant and lipid molecules in the ELF. The health effects of NO2 are caused by the reaction products or their metabolites, which are reactive nitrogen species and reactive oxygen species that can drive bronchoconstriction, inflammation, reduced immune response, and may have effects on the heart.[26]
Acute exposure
Acute harm due to NO2 exposure is rare. 100–200 ppm can cause mild irritation of the nose and throat, 250–500 ppm can cause edema, leading to bronchitis or pneumonia, and levels above 1000 ppm can cause death due to asphyxiation from fluid in the lungs. There are often no symptoms at the time of exposure other than transient cough, fatigue or nausea, but over hours inflammation in the lungs causes edema.[27][28]
For skin or eye exposure, the affected area is flushed with saline. For inhalation, oxygen is administered, bronchodilators may be administered, and if there are signs of methemoglobinemia, a condition that arises when nitrogen-based compounds affect the hemoglobin in red blood cells, methylene blue may be administered.[29][30]
Possible pathways implicated in long-term nitrogen dioxide exposure. Dotted lines indicate findings only supported by animal studies, while solid lines indicate findings from controlled human exposure studies. Dashed lines indicate speculative links to asthma exacerbation and respiratory tract infections. ELF = epithelial lining fluid.[26]: 4–62
Exposure to low levels of NO2 over time can cause changes in lung function.[32] Chronic exposure to NO2 can cause respiratory effects including airway inflammation in healthy people and increased respiratory symptoms in people with asthma.[citation needed] A small number of correlational studies have suggested that the presence of a gas stove inside a home may increase asthma risk, a process believed to be mediated by indoor NO2 concentration.[22][33]
Environmental effects
Interaction of NO2 and other NOx with water, oxygen and other chemicals in the atmosphere can form acid rain which harms sensitive ecosystems such as lakes and forests.[34] Elevated levels of NO 2 can also harm vegetation, decreasing growth, and reduce crop yields.[35]
^"nitrogen dioxide (CHEBI:33101)". Chemical Entities of Biological Interest (ChEBI). UK: European Bioinformatics Institute. 13 January 2008. Main. Archived from the original on 4 March 2016. Retrieved 4 October 2011.
^Mendiara, S. N.; Sagedahl, A.; Perissinotti, L. J. (2001). "An electron paramagnetic resonance study of nitrogen dioxide dissolved in water, carbon tetrachloride and some organic compounds". Applied Magnetic Resonance. 20 (1–2): 275–287. doi:10.1007/BF03162326. S2CID97875925.
^Subcommittee on Emergency and Continuous Exposure Guidance Levels for Selected Submarine Contaminants; Committee on Toxicology; Board on Environmental Studies and Toxicology; Division on Earth and Life Studies; National Research Council. Chapter 12: Nitrogen Dioxide in Emergency and Continuous Exposure Guidance Levels for Selected Submarine Contaminants. National Academies Press, 2007. ISBN978-0-309-09225-8
^US Dept. of Health and Human Services, Public Health Service, Agency for Toxic Substances and Disease Registry, Division of Toxicology. April 2002 ATSDR Nitrous Oxides.
^ abU.S. EPA. Integrated Science Assessment for Oxides of Nitrogen – Health Criteria (2016 Final Report). U.S. Environmental Protection Agency, Washington, DC, EPA/600/R-15/068, 2016. Federal Register Notice Jan 28, 2016 Free download available at Report page at EPA website.
