Nitrogen dioxide

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).[7]

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 ground electronic 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.

Nitrogen dioxide typically arises via the oxidation of nitric oxide by oxygen in air (e.g. as result of corona discharge):[11]

NO + 1⁄2 O₂ → NO₂

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

1⁄2 N₂ + O₂ → NO₂

In the laboratory, NO
₂ can be prepared in a two-step procedure where dehydration of nitric acid produces dinitrogen pentoxide:

2 HNO₃ → N₂O₅ + H₂O

6 HNO₃ + 1⁄2 P₄O₁₀ → 3 N₂O₅ + 2 H₃PO₄

which subsequently undergoes thermal decomposition:

N₂O₅ → 2 NO₂ + 1⁄2 O₂

The thermal decomposition of some metal nitrates also generates NO₂:

Pb(NO₃)₂ → PbO + 2 NO₂ + 1⁄2 O₂

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

4 HNO₃ + Cu → Cu(NO₃)₂ + 2 NO₂ + 2 H₂O

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.[14]

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,[15]^: 223 and as a room temperature sterilization agent.[16] 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.[17]

A "fox tail" over Nizhniy Tagil Iron and Steel Works

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.[18]

Indoors, exposure arises from cigarette smoke,[19] and butane and kerosene heaters and stoves.[20]

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.[7] Workers in high voltage areas especially those with spark or plasma creation are at risk. 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".[21][22]

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.[23]

Pathways indicated by a dotted line are those for which evidence is limited to findings from experimental animal studies, while evidence from controlled human exposure studies is available for pathways indicated by a solid line. Dashed lines indicate proposed links to the outcomes of asthma exacerbation and respiratory tract infections. Key events are subclinical effects, endpoints are effects that are generally measured in the clinic, and outcomes are health effects at the organism level. NO₂ = nitrogen dioxide; ELF = epithelial lining fluid.[23]^: 4–62

Nitrogen dioxide diffusion tube for air quality monitoring. Positioned in the City of London.

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.[24][25]

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.[26][27]

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.[28]

Exposure to low levels of NO₂ over time can cause changes in lung function.[29] Chronic exposure to NO₂ can cause respiratory effects including airway inflammation in healthy people and increased respiratory symptoms in people with asthma.

The effects of toxicity on health have been examined using questionnaires and in-person interviews in an effort to understand the relationship between NO₂ and asthma. The influence of indoor air pollutants on health is important because the majority of people in the world spend more than 80% of their time indoors.[30] The amount of time spent indoors depends upon on several factors including geographical region, job activities, and gender among other variables. Additionally, because home insulation is improving, this can result in greater retention of indoor air pollutants, such as NO₂.[30] With respect to geographic region, the prevalence of asthma has ranged from 2 to 20% with no clear indication as to what's driving the difference.[30] This may be a result of the "hygiene hypothesis" or "western lifestyle" that captures the notions of homes that are well insulated and with fewer inhabitants.[30] Another study examined the relationship between nitrogen exposure in the home and respiratory symptoms and found a statistically significant odds ratio of 2.23 (95% CI: 1.06, 4.72) among those with a medical diagnosis of asthma and gas stove exposure.[31]

A major source of indoor exposure to NO₂ is the use of gas stoves for cooking or heating in homes. According to the 2000 census, over half of US households use gas stoves[32] and indoor exposure levels of NO₂ are, on average, at least three times higher in homes with gas stoves compared to electric stoves with the highest levels being in multifamily homes. Exposure to NO₂ is especially harmful for children with asthma. Research has shown that children with asthma who live in homes with gas stoves have greater risk of respiratory symptoms such as wheezing, cough and chest tightness.[31][33] Additionally, gas stove use was associated with reduced lung function in girls with asthma, although this association was not found in boys.[34] Using ventilation when operating gas stoves may reduce the risk of respiratory symptoms in children with asthma.

In a cohort study with inner-city minority African American Baltimore children to determine if there was a relationship between NO₂ and asthma for children aged 2 to 6 years old, with an existing medical diagnosis of asthma, and one asthma related visit, families of lower socioeconomic status were more likely to have gas stoves in their homes. The study concluded that higher levels of NO₂ within a home were linked to a greater level of respiratory symptoms among the study population. This further exemplifies that NO
₂ toxicity is dangerous for children.[35]

Interaction of NO₂ and other NO_x with water, oxygen and other chemicals in the atmosphere can form acid rain which harms sensitive ecosystems such as lakes and forests.[36] Elevated levels of NO
₂ can also harm vegetation, decreasing growth, and reduce crop yields.[37]

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

• Haynes, William M., ed. (2011). CRC Handbook of Chemistry and Physics (92nd ed.). CRC Press. ISBN 978-1-4398-5511-9.

Wikimedia Commons has media related to Nitrogen dioxide.

• International Chemical Safety Card 0930
• National Pollutant Inventory – Oxides of nitrogen fact sheet
• NIOSH Pocket Guide to Chemical Hazards
• WHO-Europe reports: Health Aspects of Air Pollution (2003) (PDF) and "Answer to follow-up questions from CAFE (2004) (PDF)
• Nitrogen Dioxide Air Pollution
• Current global map of nitrogen dioxide distribution
• A review of the acute and long term impacts of exposure to nitrogen dioxide in the United Kingdom IOM Research Report TM/04/03