Nitrogen dioxide

Nitrogen dioxide is a reddish-brown gas with a pungent, acrid odor above {{convert|21.2|C|F K}} and becomes a yellowish-brown liquid below {{convert|21.2|C|F K}}. It forms an equilibrium with its dimer, dinitrogen tetroxide ({{chem2|N2O4}}), and converts almost entirely to {{chem2|N2O4}} below {{convert|-11.2|C|F K}}.

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 ({{chem2|O3}}) the ground electronic state of nitrogen dioxide is a doublet state, since nitrogen has one unpaired electron,{{Greenwood&Earnshaw2nd|page=455-7}} which decreases the alpha effect compared with nitrite and creates a weak bonding interaction with the oxygen lone pairs. The lone electron in {{chem2|NO2}} also means that this compound is a free radical, so the formula for nitrogen dioxide is often written as {{chem2|^{•}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 {{chem2|NO + O}}, atomic oxygen); in the atmosphere the addition of the oxygen atom so formed to {{chem2|O2}} results in ozone.

{{See also|Ostwald process}}

Industrially, nitrogen dioxide is produced and transported as its cryogenic liquid dimer, dinitrogen tetroxide. It is produced industrially by the oxidation of ammonia, the Ostwald Process. This reaction is the first step in the production of nitric acid:

:{{chem2|4 NH3 + 7 O2 -> 4 NO2 + 6 H2O}}

It can also be produced by the oxidation of nitrosyl chloride:

:{{chem2|2 NOCl + O2 -> 2NO2 + Cl2}}

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 {{chem2|NO2}}:{{Greenwood&Earnshaw2nd|page=456}}

:{{chem2|Pb(NO3)2 -> PbO + 2 NO2 +}} {{frac|1|2}} {{chem2|O2}}

Alternatively, dehydration of nitric acid produces nitronium nitrate...

:{{chem2|2 HNO3 -> N2O5 + H2O}}

:{{chem2|6 HNO3 +}} {{frac|1|2}} {{chem2|P4O10 -> 3 N2O5 + 2 H3PO4}}

...which subsequently undergoes thermal decomposition:

:{{chem2|N2O5 -> 2 NO2 +}} {{frac|1|2}} {{chem2|O2}}

{{chem2|NO2}} is generated by the reduction of concentrated nitric acid with a metal (such as copper):

:{{chem2|4 HNO3 + Cu -> Cu(NO3)2 + 2 NO2 + 2 H2O}}

Nitric acid decomposes slowly to nitrogen dioxide by the overall reaction:

:4 {{chem2|HNO3}} → 4 {{chem2|NO2}} + 2 {{chem2|H2O}} + {{chem2|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 {{chem2|NO2}}.

At low temperatures, {{chem2|NO2}} reversibly converts to the colourless gas dinitrogen tetroxide ({{chem2|N2O4}}):

:{{chem2|2 NO2 <-> N2O4}}

The exothermic equilibrium has enthalpy change {{nowrap|1=ΔH = −57.23 kJ/mol}}.Holleman, A. F.; Wiberg, E. (2001) Inorganic Chemistry. Academic Press: San Diego. {{ISBN|0-12-352651-5}}.

At {{convert|150|C|F K}}, {{chem2|NO2}} decomposes with release of oxygen via an endothermic process ({{nowrap|1=ΔH = 14 kJ/mol}}):

:2 NO₂ →2 NO +  {{chem2|O2}}

As suggested by the weakness of the N–O bond, {{chem2|NO2}} is a good oxidizer. Consequently, it will combust, sometimes explosively, in the presence of hydrocarbons.{{cite journal |last=Chan |first=Wai-To |last2=Heck |first2=Simone M. |last3=Pritchard |first3=Huw O. |title=Reaction of nitrogen dioxide with hydrocarbons and its influence on spontaneous ignition. A computational study |journal=Physical Chemistry Chemical Physics |volume=3 |issue=1 |date=2001 |doi=10.1039/b006088g |pages=56–62}}

NO₂ reacts with water to give nitric acid and nitrous acid:

:{{chem2|2 NO2 + H2O -> HNO3 + NO}}

This reaction is one of the steps in the Ostwald process for the industrial production of nitric acid from ammonia.{{cite encyclopedia |author=Thiemann, Michael |author2=Scheibler, Erich |author3=Wiegand, Karl Wilhelm |year=2005|encyclopedia=Ullmann's Encyclopedia of Industrial Chemistry|publisher=Wiley-VCH|place=Weinheim|doi=10.1002/14356007.a17_293|chapter=Nitric Acid, Nitrous Acid, and Nitrogen Oxides|isbn=978-3-527-30673-2}} 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.{{Cite journal|last1=Finlayson-Pitts|first1=B. J.|last2=Wingen|first2=L. M.|last3=Sumner|first3=A. L.|last4=Syomin|first4=D.|last5=Ramazan|first5=K. A.|date=2002-12-16|title=The heterogeneous hydrolysis of NO₂ in laboratory systems and in outdoor and indoor atmospheres: An integrated mechanism|journal=Physical Chemistry Chemical Physics|language=en|volume=5|issue=2|pages=223–242|doi=10.1039/B208564J|url=https://escholarship.org/content/qt8wx9v8h9/qt8wx9v8h9.pdf?t=njz4b2}}

{{chem2|NO2}} is used to generate anhydrous metal nitrates from the oxides:

:{{chem2|MO + 3 NO2 -> M(NO3)2 + NO}}

Alkyl and metal iodides give the corresponding nitrates:

:{{chem2|TiI4 + 8 NO2 -> Ti(NO3)4 + 4 NO + 2 I2}}

The reactivity of nitrogen dioxide toward organic compounds has long been known.{{cite journal |doi=10.1021/cr60114a002 |title=The Reactions of Nitrogen Tetroxide with Organic Compounds |date=1945 |last1=Riebsomer |first1=J. L. |journal=Chemical Reviews |volume=36 |issue=2 |pages=157–233 }} For example, it reacts with amides to give N-nitroso derivatives.{{cite journal |doi=10.15227/orgsyn.047.0044 |title=Deamination of Amines. 2-Phenylethyl Benzoate Via the Nitrosoamide Decomposition |journal=Organic Syntheses |date=1967 |volume=47 |page=44|author=Emil White }} It is used for nitrations under anhydrous conditions.{{March6th|page=687}}

{{chem2|NO2}} is used as an intermediate in the manufacturing of nitric acid, as a nitrating agent in the manufacturing of chemical explosives, as a polymerization inhibitor for acrylates, as a flour bleaching agent,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. [http://www.nap.edu/read/11170/chapter/12 Chapter 12: Nitrogen Dioxide] in Emergency and Continuous Exposure Guidance Levels for Selected Submarine Contaminants. National Academies Press, 2007. {{ISBN|978-0-309-09225-8}}{{rp|223}} and as a room temperature sterilization agent.{{cite web |title=Mechanism Overview, June 2012 |url=http://noxilizer.com/pdf/news/WhitePaper-Mechanism_Overview_6_14_12.pdf |archive-url=https://web.archive.org/web/20160412094958/http://noxilizer.com/pdf/news/WhitePaper-Mechanism_Overview_6_14_12.pdf |archive-date=12 April 2016 |access-date=2 July 2013 |work=noxilizer.com |publisher=Noxilizer, Inc.}} It is also used as an oxidizer in rocket fuel, for example in red fuming nitric acid; it was used in the Titan rockets, to launch Project Gemini, in the maneuvering thrusters of the Space Shuttle, and in uncrewed space probes sent to various planets.Cotton, Simon (21 March 2013) [http://www.rsc.org/chemistryworld/2013/04/nitrogen-dioxide-podcast Nitrogen dioxide]. RSC Chemistry World.

File:Aura_OMI_Nitrogen_dioxide_troposphere_column.png column density in 2011.]]

Nitrogen dioxide typically arises via the oxidation of nitric oxide by oxygen in air (e.g. as result of corona discharge):

:2 {{chem2|NO + O2 → 2 NO2}}

{{chem2|NO2}} is introduced into the environment by natural causes, including entry from the stratosphere, bacterial respiration, volcanos, and lightning. These sources make {{chem2|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.WHO Air Quality Guidelines – Second Edition. Chapter 7.1 [http://www.euro.who.int/__data/assets/pdf_file/0017/123083/AQG2ndEd_7_1nitrogendioxide.pdf Nitrogen Dioxide].

File:AirQualityLondon1.jpg for air quality monitoring in the City of London.]]

Nitrogen dioxide also forms in most combustion processes. At elevated temperatures nitrogen combines with oxygen to form nitrogen dioxide:

:{{chem2|N2 + 2 O2 -> 2 NO2}}

For the general public, the most prominent sources of {{chem2|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 {{chem2|NO2}}. Nitrogen dioxide accounts for a small fraction (generally well under 0.1) of NOx auto emissions. {{Cite web | url=https://www.sciencedirect.com/science/article/abs/pii/S1352231016308469 | access-date=2025-02-25 | title= On-road measurements of vehicle NO2/NOx emission ratios in Denver, Colorado, USA }}

Outdoors, {{chem2|NO2}} can be a result of traffic from motor vehicles.{{Cite web |url=https://www.who.int/phe/health_topics/outdoorair/outdoorair_aqg/en/ |archive-url=https://web.archive.org/web/20140309212121/http://www.who.int/phe/health_topics/outdoorair/outdoorair_aqg/en/ |archive-date=March 9, 2014 |title=Air quality guidelines – global update 2005 |website=WHO |access-date=2016-10-19}} Indoors, exposure arises from cigarette smoke,US Dept. of Health and Human Services, Public Health Service, Agency for Toxic Substances and Disease Registry, Division of Toxicology. April 2002 [http://www.atsdr.cdc.gov/toxfaqs/tfacts175.pdf ATSDR Nitrous Oxides]. and butane and kerosene heaters and stoves.{{cite web |date=2013-03-21 |title=The Impact of Unvented Gas Heating Appliances on Indoor Nitrogen Dioxide Levels in 'TIGHT' Homes |url=http://www.ahrinet.org/App_Content/ahri/files/Product%20Section/Vent-Free%20NO2%20Modeling%20Study%20Final%20Summary%20March%2021%202013.pdf |work=ahrinet.org |access-date=2018-09-25 |archive-date=2020-08-05 |archive-url=https://web.archive.org/web/20200805094718/http://www.ahrinet.org/App_Content/ahri/files/Product%20Section/Vent-Free%20NO2%20Modeling%20Study%20Final%20Summary%20March%2021%202013.pdf }} Indoor exposure levels of {{chem2|NO2}} are, on average, at least three times higher in homes with gas stoves compared to electric stoves.{{Cite journal |last1=Garrett |first1=Maria H. |last2=Hooper |first2=Martin A. |last3=Hooper |first3=Beverley M. |last4=Abramson |first4=Michael J. |date=1998-09-01 |title=Respiratory Symptoms in Children and Indoor Exposure to Nitrogen Dioxide and Gas Stoves |url=https://research.monash.edu/files/304219582/304218990_oa.pdf |journal=American Journal of Respiratory and Critical Care Medicine |volume=158 |issue=3 |pages=891–895 |doi=10.1164/ajrccm.158.3.9701084 |pmid=9731022}}{{EPA content|url=https://www.epa.gov/no2-pollution|article=Nitrogen Dioxide Basic Information|access-date=February 23, 2016}} File:Nizhniy tagil ntmk main entrance.JPG|left]]Workers in industries where {{chem2|NO2}} is used are also exposed and are at risk for occupational lung diseases, and NIOSH has set exposure limits and safety standards. Workers in high voltage areas especially those with spark or plasma creation are at risk.{{cn|date=December 2022}} Agricultural workers can be exposed to {{chem2|NO2}} arising from grain decomposing in silos; chronic exposure can lead to lung damage in a condition called "silo-filler's disease".{{cite journal|last1=Chan-Yeung |first1=M. |last2=Ashley |first2=M. J. |last3=Grzybowski |first3=S. |title=Grain dust and the lungs |journal=Canadian Medical Association Journal |volume=118 |issue=10 |pages=1271–4 |pmid=348288 |pmc=1818652 |year=1978}}{{cite journal|doi=10.1148/radiographics.11.4.1887117 |title=Agricultural disorders of the lung|journal=Radiographics|volume=11|issue=4|pages=625–34|pmid=1887117|year=1991|last1=Gurney|first1=J. W.|last2=Unger|first2=J. M.|last3=Dorby|first3=C. A.|last4=Mitby|first4=J. K.|last5=von Essen|first5=S. G.|doi-access=free}}

File:No2toxpathwaysEPA.png, 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.{{rp|4–62}}|660x660px]]

{{main|Nitrogen dioxide poisoning}}

{{chem2|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 {{chem2|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.U.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. [https://www.federalregister.gov/articles/2016/01/28/2016-01548/integrated-science-assessment-for-oxides-of-nitrogen-health-criteria Federal Register Notice Jan 28, 2016] Free download available at [https://cfpub.epa.gov/ncea/isa/recordisplay.cfm?deid=310879 Report page at EPA website].

Acute harm due to {{chem2|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.Toxnet [https://web.archive.org/web/20180612163553/https://toxnet.nlm.nih.gov/cgi-bin/sis/search2/r?dbs+hsdb:@term+@rn+10102-44-0 Nitrogen dioxide: Human Health Effects] Page accessed March 28, 2016.CDC NIOSH [https://www.cdc.gov/niosh/ipcsneng/neng0930.html International Chemical Safety Cards (ICSC): Nitrogen Dioxide] Page last reviewed: July 22, 2015; Page last updated: July 1, 2014.

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.Agency for Toxic Substances and Disease Registry via the CDC [https://web.archive.org/web/20100528070306/http://www.atsdr.cdc.gov/MMG/MMG.asp?id=394&tid=69 Medical Management Guidelines for Nitrogen Oxides] Page last reviewed: October 21, 2014; Page last updated: October 21, 2014University of Kansas Hospital, Poison Control Center [http://www.kumed.com/~/media/Imported/kumed/documents/kdhe-20nitrogen-20dioxide.ashx?la=en Poison Facts: Medium Chemicals: Nitrogen Dioxide] {{Webarchive|url=https://web.archive.org/web/20160411001233/http://www.kumed.com/~/media/Imported/kumed/documents/kdhe-20nitrogen-20dioxide.ashx?la=en |date=2016-04-11 }} page accessed March 28, 2016

It is classified as an extremely hazardous substance in the United States as defined in Section 302 of the U.S. Emergency Planning and Community Right-to-Know Act (42 U.S.C. 11002), and it is subject to strict reporting requirements by facilities which produce, store, or use it in significant quantities.{{Cite web | publisher = Government Printing Office | title = 40 C.F.R.: Appendix A to Part 355—The List of Extremely Hazardous Substances and Their Threshold Planning Quantities | url = http://edocket.access.gpo.gov/cfr_2008/julqtr/pdf/40cfr355AppA.pdf | edition = July 1, 2008 | access-date = October 29, 2011 | archive-url = https://web.archive.org/web/20120225051612/http://edocket.access.gpo.gov/cfr_2008/julqtr/pdf/40cfr355AppA.pdf | archive-date = February 25, 2012 }}

Exposure to low levels of {{chem2|NO2}} over time can cause changes in lung function.{{cite journal

| last = Int Panis

| first = L

| title = Short-term air pollution exposure decreases lung function: a repeated measures study in healthy adults

| journal = Environmental Health

| volume = 16

| issue = 1

| page = 60

| year = 2017

| doi = 10.1186/s12940-017-0271-z

| pmid =28615020

| pmc = 5471732

| doi-access = free

| bibcode = 2017EnvHe..16...60I

}} Cooking with a gas stove is associated with poorer indoor air quality. Combustion of gas can lead to increased concentrations of nitrogen dioxide throughout the home environment which is linked to respiratory issues and diseases. Children exposed to {{chem2|NO2}} are more likely to be admitted to hospital with asthma.{{Cite journal |last1=Wang |first1=Weiyi |last2=Gulliver |first2=John |last3=Beevers |first3=Sean |last4=Freni Sterrantino |first4=Anna |last5=Davies |first5=Bethan |last6=Atkinson |first6=Richard W. |last7=Fecht |first7=Daniela |date=2024 |title=Short-Term Nitrogen Dioxide Exposure and Emergency Hospital Admissions for Asthma in Children: A Case-Crossover Analysis in England |journal=Journal of Asthma and Allergy |volume=17 |pages=349–359 |doi=10.2147/JAA.S448600 |doi-access=free |issn=1178-6965 |pmid=38623450|pmc=11016460 }}

In 2019, the Court of Justice of the EU, found that France did not comply with the limit values of the EU air quality standards applicable to the concentrations of nitrogen dioxide (NO₂) in 12 air quality zones.{{Cite web |date=2020-10-30 |title=Air quality: Commission decides to refer France to the Court over failure to meet its obligation to protect citizens against poor air quality |url=https://ec.europa.eu/commission/presscorner/detail/en/ip_20_1880 |access-date=2025-02-11 |website=European Commission}}

Interaction of {{chem2|NO2}} and other {{chem2|NO_{x}|}} with water, oxygen and other chemicals in the atmosphere can form acid rain which harms sensitive ecosystems such as lakes and forests.{{Cite web|last=US EPA|first=OAR|date=2016-07-06|title=Basic Information about NO2|url=https://www.epa.gov/no2-pollution/basic-information-about-no2|access-date=2020-07-03|website=US EPA|language=en}} Elevated levels of {{chem|NO|2}} can also harm vegetation, decreasing growth, and reduce crop yields.{{Cite web |title=Nitrogen oxides |url=https://www.qld.gov.au/environment/pollution/monitoring/air/air-pollution/pollutants/nitrogen-oxides|access-date=2020-07-03|website=Queensland Government|language=en}}

• Dinitrogen tetroxide ({{chem2|N2O4}})
• Nitric oxide ({{chem2|NO}}) – pollutant that is short lived because it converts to {{chem2|NO2}} in the presence of ozone
• Nitrite ({{chem2|NO2-}})
• Nitrous oxide ({{chem2|N2O}}) – "laughing gas", a linear molecule, isoelectronic with {{chem2|CO2}} but with a nonsymmetric arrangement of atoms ({{chem2|NNO}})
• Nitryl

{{reflist|30em}}

• {{cite book | editor= Haynes, William M. | year = 2011 | title = CRC Handbook of Chemistry and Physics | edition = 92nd | publisher = CRC Press | isbn = 978-1-4398-5511-9| title-link = CRC Handbook of Chemistry and Physics }}

{{Commons category|Nitrogen dioxide}}

• [http://www.inchem.org/documents/icsc/icsc/eics0930.htm International Chemical Safety Card 0930]
• [http://www.npi.gov.au/resource/oxides-nitrogen-0 National Pollutant Inventory – Oxides of nitrogen fact sheet]
• [https://www.cdc.gov/niosh/npg/npgd0454.html NIOSH Pocket Guide to Chemical Hazards]
• WHO-Europe reports: [http://www.euro.who.int/document/e79097.pdf Health Aspects of Air Pollution (2003)] (PDF) and "[https://web.archive.org/web/20050909162153/http://www.euro.who.int/document/E82790.pdf Answer to follow-up questions from CAFE (2004)] (PDF)
• [http://www.greenfacts.org/air-pollution/nitrogen-dioxide-no2/index.htm Nitrogen Dioxide Air Pollution]
• [https://earth.nullschool.net/#current/chem/surface/level/overlay=no2/winkel3 Current global map of nitrogen dioxide distribution]
• [https://web.archive.org/web/20160415110253/http://www.iom-world.org/pubs/IOM_TM0403.pdf A review of the acute and long term impacts of exposure to nitrogen dioxide in the United Kingdom] IOM Research Report TM/04/03
• [https://pubs.rsc.org/en/content/articlelanding/2001/cp/b006088g Reaction of nitrogen dioxide with hydrocarbons and its influence on spontaneous ignition]

{{nitrogen compounds}}

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{{DEFAULTSORT:Nitrogen Dioxide}}

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