Nitrogen dioxide

Nitrogen dioxide is a reddish-brown gas with a pungent, acrid odor above and becomes a yellowish-brown liquid below . It forms an equilibrium with its dimer, dinitrogen tetroxide (), and converts almost entirely to below . ==Preparation==

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: :   Alternatively, dehydration of nitric acid produces nitronium nitrate... : :   ...which subsequently undergoes thermal decomposition: :   is generated by the reduction of concentrated nitric acid with a metal (such as copper): : Nitric acid decomposes slowly to nitrogen dioxide by the overall reaction: :4 → 4 + 2 + 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 . ==Selected reactions==

Nitrogen dioxide interconverts to other N-O compounds. At low temperatures, reversibly converts to the colourless gas dinitrogen tetroxide (): : The exothermic equilibrium has enthalpy change . At , decomposes with release of oxygen via an endothermic process (): :2 NO2 →2 NO +   Nitric dioxide combines with nitric oxide to reversibly give dinitrogen trioxide: : This reaction is a step in the lead chamber process for producing sulfuric acid from sulfur doxide. Absorption of light at wavelengths shorter than about 400 nm results in photolysis to form (atomic oxygen). In the atmosphere the addition of the oxygen atom so formed to results in ozone. Hydrolysis NO2 reacts with water to give nitric acid and nitrous acid: 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. Conversion to nitrates is used to generate anhydrous metal nitrates from the oxides: For example, it reacts with amides to give N-nitroso derivatives. It is used for nitrations under anhydrous conditions. ==Uses==

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, and as a room temperature sterilization agent. 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. ==Environmental presence==

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): Anthropogenic sources 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: : For the general public, the most prominent sources of are internal combustion engines, as combustion temperatures are high enough to thermally combine some of the nitrogen and oxygen in the air to form . Outdoors, can be a result of traffic from motor vehicles. Indoors, exposure arises from cigarette smoke, and butane and kerosene heaters and stoves. Indoor exposure levels of are, on average, at least three times higher in homes with gas stoves compared to electric stoves. |leftWorkers in industries where is used are also exposed and are at risk for occupational lung diseases, and NIOSH has set exposure limits and safety standards. ==Toxicity==

, 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. Acute exposure Acute harm due to 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. 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. 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. Long-term Exposure to low levels of over time can cause changes in lung function. 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. 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. == Environmental effects ==

Interaction of 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. Elevated levels of can also harm vegetation, decreasing growth, and reduce crop yields. ==See also==