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}} In atmospheric chemistry, is a generic term for the nitrogen oxides that are most relevant for air pollution, namely nitric oxide (NO) and nitrogen dioxide (). These gases contribute to the formation of smog and acid rain, as well as tropospheric ozone. gases are usually produced from the reaction among nitrogen and oxygen during combustion of fuels, such as hydrocarbons, in air; especially at high temperatures, such as occur in car engines. In areas of high motor vehicle traffic, such as in large cities, the nitrogen oxides emitted can be a significant source of air pollution. gases are also produced naturally by lightning. The term is chemistry shorthand for molecules containing one nitrogen and one or more oxygen atom. It is generally not meant to include nitrous oxide (N2O), a fairly inert oxide of nitrogen that has many uses as an oxidizer for rockets and car engines, an anesthetic, and a propellant for aerosol sprays and whipped cream. Nitrous oxide plays hardly any role in air pollution, although it may have a significant impact on the ozone layer.A. R. Ravishankara, J. S. Daniel, R. W. Portmann (2009), "Nitrous Oxide (N2O): The Dominant Ozone-Depleting Substance Emitted in the 21st Century". Science, volume 326, issue 5949, pages 123–125. (reactive, odd nitrogen) is defined as the sum of plus the compounds produced from the oxidation of which include nitric acid.

Formation and reactions

Oxygen and nitrogen do not react at ambient temperatures. But at high temperatures, they undergo an endothermic reaction producing various oxides of nitrogen. Such temperatures arise inside an internal combustion engine or a power station boiler, during the combustion of a mixture of air and fuel, and naturally in a lightning flash. In atmospheric chemistry, the term means the total concentration of NO and . During daylight, these concentrations are in equilibrium; the ratio NO /  is determined by the intensity of sunshine (which converts to NO) and the concentration of ozone (which reacts with NO to again form ). In the presence of excess oxygen (O2), nitric oxide (NO) reacts with the oxygen to form nitrogen dioxide (). The time required depends on the concentration in air as shown below: When and volatile organic compounds (VOCs) react in the presence of sunlight, they form photochemical smog, a significant form of air pollution, especially in the summer. Children, people with lung diseases such as asthma, and people who work or exercise outside are particularly susceptible to adverse effects of smog such as damage to lung tissue and reduction in lung function. |accessdate=2007-12-26 |publisher= United States Environmental Protection Agency }}

Formation of nitric acid and acid rain

Mono-nitrogen oxides eventually form nitric acid when dissolved in atmospheric moisture, forming a component of acid rain. This chemical reaction occurs when nitrogen dioxide reacts with water: 2  + → HNO2 + HNO3 where nitric oxide will oxidize to form nitrogen dioxide that again reacts with water, ultimately forming nitric acid: 4 NO + 3 O2 + 2  → 4 HNO3 Combining these three equations gives a single equation that describes the aerobic conversion of nitrogen dioxide to nitric acid: 4  + 2  + O2 → 4 HNO3 Mono-nitrogen oxides are also involved in tropospheric production of ozone.

Industrial sources (anthropogenic sources)

The three primary sources of in combustion processes:
  • thermal
  • fuel
  • prompt
Thermal formation, which is highly temperature dependent, is recognized as the most relevant source when combusting natural gas. Fuel tends to dominate during the combustion of fuels, such as coal, which have a significant nitrogen content, particularly when burned in combustors designed to minimise thermal . The contribution of prompt is normally considered negligible. A fourth source, called feed is associated with the combustion of nitrogen present in the feed material of cement rotary kilns, at between 300 and 800 °C, where it is also a minor contributor.


Thermal refers to formed through high temperature oxidation of the diatomic nitrogen found in combustion air. The formation rate is primarily a function of temperature and the residence time of nitrogen at that temperature. At high temperatures, usually above 1600 °C (2900 °F), molecular nitrogen (N2) and oxygen (O2) in the combustion air disassociate into their atomic states and participate in a series of reactions. The three principal reactions (the extended Zel'dovich mechanism) producing thermal are: N2 + O → NO + N N + O2 → NO + O N + OH → NO + H All three reactions are reversible. Zeldovich was the first to suggest the importance of the first two reactions. The last reaction of atomic nitrogen with the hydroxyl radical, •HO, was added by Lavoie, Heywood and Keck to the mechanism and makes a significant contribution to the formation of thermal .


It is estimated that transportation fuels cause 54% of the anthropogenic (i.e. human-caused) . The major source of production from nitrogen-bearing fuels such as certain coals and oil, is the conversion of fuel bound nitrogen to during combustion. During combustion, the nitrogen bound in the fuel is released as a free radical and ultimately forms free N2, or NO. Fuel can contribute as much as 50% of total emissions when combusting oil and as much as 80% when combusting coal. Although the complete mechanism is not fully understood, there are two primary paths of formation. The first involves the oxidation of volatile nitrogen species during the initial stages of combustion. During the release and before the oxidation of the volatiles, nitrogen reacts to form several intermediaries which are then oxidized into NO. If the volatiles evolve into a reducing atmosphere, the nitrogen evolved can readily be made to form nitrogen gas, rather than . The second path involves the combustion of nitrogen contained in the char matrix during the combustion of the char portion of the fuels. This reaction occurs much more slowly than the volatile phase. Only around 20% of the char nitrogen is ultimately emitted as , since much of the that forms during this process is reduced to nitrogen by the char, which is nearly pure carbon.


This third source is attributed to the reaction of atmospheric nitrogen, N2, with radicals such as C, CH, and CH2 fragments derived from fuel, where this cannot be explained by either the aforementioned thermal or fuel processes. Occurring in the earliest stage of combustion, this results in the formation of fixed species of nitrogen such as NH ( nitrogen monohydride), HCN ( hydrogen cyanide), H2CN ( dihydrogen cyanide) and •CN ( cyano radical) which can oxidize to NO. In fuels that contain nitrogen, the incidence of prompt is especially minimal and it is generally only of interest for the most exacting emission targets.

Health and environment effects

reacts with ammonia, moisture, and other compounds to form nitric acid vapor and related particles. Small particles can penetrate deeply into sensitive lung tissue and damage it, causing premature death in extreme cases. Inhalation of such particles may cause or worsen respiratory diseases, such as emphysema or bronchitis, or may also aggravate existing heart disease. reacts with volatile organic compounds in the presence of sunlight to form and to destroy ozone. Ozone can cause adverse effects such as damage to lung tissue and reduction in lung function mostly in susceptible populations (children, elderly, asthmatics). Ozone can be transported by wind currents and cause health impacts far from the original sources. The American Lung Association estimates that nearly 50 percent of United States inhabitants live in counties that are not in ozone compliance. Ozone, Environmental Protection Agency. In South East England, ground level ozone pollution tends to be highest in the countryside and in suburbs, while in central London and on major roads NO emissions are able to "mop up" ozone to form and oxygen. London Air - What is Ozone?, King's College London, Environmental Research Group also readily reacts with common organic chemicals, and even ozone, to form a wide variety of toxic products: nitroarenes, nitrosamines and also the nitrate radical some of which may cause DNA mutations. Recently another pathway, via , to ozone has been found that predominantly occurs in coastal areas via formation of nitryl chloride when comes into contact with salt mist. emissions also cause global cooling through the formation of •OH radicals that destroy methane molecules, countering the effect of greenhouse gases. The effect can be significant. For instance, according to the OECD "the large emissions from ship traffic lead to significant increases in hydroxyl (OH), which is the major oxidant in the lower atmosphere. Since reaction with OH is a major way of removing methane from the atmosphere, ship emissions decrease methane concentrations. (Reductions in methane lifetimes due to shipping-based emissions vary between 1.5% and 5% in different calculations)." "In summary, most studies so far indicate that ship emissions actually lead to a net global cooling. However, it should be stressed that the uncertainties with this conclusion are large, in particular for indirect effects, and global temperature is only a first measure of the extent of climate change in any event."http://www.oecd.org/greengrowth/greening-transport/45095528.pdf The ultimate destination of much is to end up in the soil as nitrite or nitrate, which are useful to growing plants.

Biodiesel and

Biodiesel and its blends in general are known to reduce harmful tailpipe emissions such as: carbon monoxide; particulate matter (PM), otherwise known as soot; and unburned hydrocarbon emissions.https://19january2017snapshot.epa.gov/www3/otaq/models/analysis/biodsl/p02001.pdf While earlier studies suggested biodiesel could sometimes decrease NOx and sometimes increase NOx emissions, subsequent investigation has shown that blends of up to 20% biodiesel in USEPA-approved diesel fuel have no significant impact on NOx emissions compared with regular diesel.http://www.nrel.gov/docs/fy07osti/40554.pdf The state of California uses a special formulation of diesel fuel to produce less NOx relative to diesel fuel used in the other 49 states. This has been deemed necessary by the California Air Resources Board (CARB) to offset the combination of vehicle congestion, warm temperatures, extensive sunlight, PM, and topography that all contribute to the formation of ozone and smog. CARB has established a special regulation for Alternative Diesel Fuels to ensure that any new fuels, including biodiesel, coming into the market do not substantially increase NOx emissions. The reduction of emissions is one of the most important challenges for advances in vehicle technology. While diesel vehicles sold in the US since 2010 are dramatically cleaner than previous diesel vehicles, urban areas continue to seek more ways to reduce the formation of smog and ozone. formation during combustion is associated with a number of factors such as combustion temperature. As such, it can be observed that the vehicle drive cycle, or the load on the engine have more significant impact on NOx emissions than the type of fuel used. This may be especially true for modern, clean diesel vehicles that continuously monitor engine operation electronically and actively control engine parameters and exhaust system operations to limit NOx emission to less than 0.2 g/km. Low-temperature combustion or LTC technology. may help reduce thermal formation of during combustion, however a tradeoff exists as high temperature combustion produces less PM or soot and results in greater power and fuel efficiency.

Regulation and emission control technologies

Selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) reduce post combustion by reacting the exhaust with urea or ammonia to produce nitrogen and water. SCR is now being used in ships, Wärtsilä Low Solutions Wärtsilä, 2008 diesel trucks and in some diesel cars. The use of exhaust gas recirculation and catalytic converters in motor vehicle engines have significantly reduced vehicular emissions. was the main focus of the Volkswagen emissions violations. Other technologies such as flameless oxidation ( FLOX) and staged combustion significantly reduce thermal in industrial processes. Bowin low technology is a hybrid of staged-premixed-radiant combustion technology with a major surface combustion preceded by a minor radiant combustion. In the Bowin burner, air and fuel gas are premixed at a ratio greater than or equal to the stoichiometric combustion requirement.Bob Joynt & Stephen Wu, Nitrogen oxides emissions standards for domestic gas appliances background study Combustion Engineering Consultant; February 2000 Water Injection technology, whereby water is introduced into the combustion chamber, is also becoming an important means of reduction through increased efficiency in the overall combustion process. Alternatively, the water (e.g. 10 to 50%) is emulsified into the fuel oil before the injection and combustion. This emulsification can either be made in-line (unstabilized) just before the injection or as a drop-in fuel with chemical additives for long term emulsion stability (stabilized). Inline emulsified fuel/water mixtures show reductions between 4 and 83%.


"green air" © 2007 - Ingo Malchow, Webdesign Neustrelitz
This article based upon the http://en.wikipedia.org/wiki/NOx, the free encyclopaedia Wikipedia and is licensed under the GNU Free Documentation License.
Further informations available on the list of authors and history: http://en.wikipedia.org/w/index.php?title=NOx&action=history
presented by: Ingo Malchow, Mirower Bogen 22, 17235 Neustrelitz, Germany