Inatmospheric chemistry,NOx is shorthand fornitric oxide (NO) andnitrogen dioxide (NO2), thenitrogen oxides that are most relevant forair pollution.[1][2] These gases contribute to the formation ofsmog andacid rain, as well as affectingtropospheric ozone.
NOx gases are usually produced from the reaction betweennitrogen andoxygen duringcombustion of fuels, such ashydrocarbons, in air; especially at high temperatures, such as in car engines.[1][2][3] In areas of high motor vehicle traffic, such as in large cities, the nitrogen oxides emitted can be a significant source of air pollution.NOx gases are also produced naturally bylightning.
NOx does not includenitrous oxide (N2O),[1] a fairly inert oxide of nitrogen that contributes less severely to air pollution, notwithstanding its involvement inozone depletion[4] and highglobal warming potential.
NOy is the class of compounds comprisingNOx and theNOz compounds produced from the oxidation ofNOx which includenitric acid (HNO3),nitrous acid (HONO),dinitrogen pentoxide (N2O5),peroxyacetyl nitrate (PAN),alkyl nitrates (RONO2), peroxyalkyl nitrates (ROONO2), the nitrate radical (NO3), andperoxynitric acid (HNO4).[5][6]: 30
Because of energy limitations, oxygen and nitrogen do not react at ambient temperatures. But at high temperatures, they undergo anendothermic reaction producing variousoxides of nitrogen. Such temperatures arise inside aninternal combustion engine or a power stationboiler, during the combustion of a mixture of air and fuel, and naturally in alightning flash.
Inatmospheric chemistry, the termNOx refers to the total concentration of NO andNO2 since the conversion between these two species is rapid in the stratosphere and troposphere.[6] During daylight hours, these concentrations together with that ofozone are insteady state, also known asphotostationary state (PSS); the ratio of NO toNO2 is determined by the intensity ofsunshine (which convertsNO2 to NO) and the concentration ofozone (which reacts with NO to again formNO2).
In other words, the concentration of ozone in the atmosphere is determined by the ratio of these two species.
| [7] | 1 |
| 2 |
| 3 |
| 4 |
The symbol represents a "third body", a molecular species that is required to carry away energy from the exothermic reaction2. Equation4 relates the concentrations ofNOx and ozone, and is known as theLeighton relationship.
The time that is needed to reach a steady state amongNOx and ozone is dominated by reaction (3), which reverses reactions (1)+(2):
| 5 |
formixing ratio of NO, [NO] = 10 part per billion (ppb), the time constant is 40 minutes; for [NO] = 1 ppb, 4 minutes.[8]: 211
WhenNOx andvolatile organic compounds (VOCs) react in the presence of sunlight, they form photochemicalsmog, a significant form of air pollution. The presence of photochemical smog increases during the summer when the incident solar radiation is higher. The emitted hydrocarbons from industrial activities and transportation react withNOx quickly and increase the concentration of ozone and peroxide compounds, especiallyperoxyacetyl nitrate (PAN).[9]
Children, people with lung diseases such asasthma, 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.[10]
NO2 is further oxidized in the gas phase during daytime by reaction with OH
where M denotes a third molecule required to stabilize the addition product.Nitric acid (HNO3) is highly soluble in liquid water in aerosol particles or cloud drops.
NO2 also reacts with ozone to form nitrate radical
During the daytime,NO3 is quicklyphotolyzed back toNO2, but at night it can react with a secondNO2 to formdinitrogen pentoxide.
N2O5 reacts rapidly with liquid water (in aerosol particles or cloud drops, but not in the gas phase) to formHNO3,
These are thought to be the principal pathways for formation of nitric acid in the atmosphere.[8]: 224–225 This nitric acid contributes toacid rain or may deposit to soil, where it makesnitrate, which is of use to growing plants. The aqueous phase reaction
is too slow to be of any significance in the atmosphere.[8]: 336
Nitric oxide is produced duringthunderstorms due to the extreme heating and cooling within alightning strike. This causes stable molecules such asN2 andO2 to convert into significant amounts of NO similar to the process that occurs during high temperature fuel combustion.[11]NOx from lightning can become oxidized to producenitric acid (HNO3), this can be precipitated out as acid rain or deposited onto particles in the air. Elevated production ofNOx from lightning depends on the season and geographic location. The occurrence of lightning is more common over land near the equator in the inter-tropical convergence zone (ITCZ) during summer months.[12] This area migrates slightly as seasons change.NOx production from lightning can be observed through satellite observations.
Scientists Ott et al.[13] estimated that each flash of lightning on average in the several mid-latitude and subtropical thunderstorms studied turned 7 kg (15 lb) of nitrogen into chemically reactiveNOx. With 1.4 billion lightning flashes per year, multiplied by 7 kilograms per lightning strike, they estimated the total amount ofNOx produced by lightning per year is 8.6 million tonnes. However,NOx emissions resulting from fossil fuel combustion are estimated at 28.5 million tonnes.[14]
A recent discovery indicated that cosmic ray and solar flares can significantly influence the number of lightning strikes occurring on Earth. Therefore, space weather can be a major driving force of lightning-produced atmosphericNOx.[3] Atmospheric constituents such as nitrogen oxides can be stratified vertically in the atmosphere. Ott noted that the lightning-producedNOx is typically found at altitudes greater than 5 km, while combustion and biogenic (soil)NOx are typically found near the sources at near surface elevation (where it can cause the most significant health effects).[13]
Agriculturalfertilization and the use ofnitrogen fixing plants also contribute to atmosphericNOx, by promotingnitrogen fixation by microorganisms.[15][16] The nitrification process transforms ammonia into nitrate. Denitrification is basically the reverse process of nitrification. Duringdenitrification, nitrate is reduced to nitrite, then NO, thenN2O and finally nitrogen. Through these processes,NOx is emitted to the atmosphere.[17]
A recent study conducted by the University of California Davis found that adding nitrogen fertilizer to soil in California is contributing 25 percent or more to state-wideNOx pollution levels.[18] When nitrogen fertilizer is added to the soil, excess ammonium and nitrate not used by plants can be converted to NO by microorganisms in the soil, which escapes into the air.NOx is a precursor for smog formation which is already a known issue for the state of California. In addition to contributing to smog, when nitrogen fertilizer is added to the soil and the excess is released in the form of NO, or leached asnitrate this can be a costly process for the farming industry.
A 2018 study by the Indiana University determined that forests in the eastern United States can expect to see increases inNOx and in turn, changes in the types of trees which predominate. Due to human activity andclimate change, themaples,sassafras, andtulip poplar have been pushing out the beneficialoak,beech, andhickory. The team determined that the first three tree species, maples, sassafras, and tulip poplar, are associated with ammonia-oxidizing bacteria known to "emit reactive nitrogen from soil." By contrast, the second three tree species, oak, beech and hickory, are associated with microbes that "absorb reactive nitrogen oxides," and thus can have a positive impact on the nitrogen oxide component of air quality. Nitrogen oxide release from forest soils is expected to be highest in Indiana, Illinois, Michigan, Kentucky and Ohio.[19]
The three primary sources ofNOx incombustion processes:[20][21]
ThermalNOx formation, which is highly temperature dependent, is recognized as the most relevant source when combusting natural gas. FuelNOx 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 thermalNOx. The contribution of promptNOx is normally considered negligible. A fourth source, calledfeedNOx is associated with the combustion of nitrogen present in the feed material of cement rotary kilns, at between 300 °C and 800 °C, where it is considered a minor contributor.
ThermalNOx refers toNOx formed through high temperature oxidation of the diatomic nitrogen found in combustion air.[22] The formation rate is primarily a function of temperature and theresidence time of nitrogen at that temperature. At high temperatures, usually above 1300 °C (2600 °F), molecular nitrogen (N2) and oxygen (O2) in the combustion air dissociate into their atomic states and participate in a series of reactions.
The three principal reactions (the extendedZel'dovich mechanism) producing thermalNOx are:
All three reactions are reversible.Zeldovich was the first to suggest the importance of the first two reactions.[23] The last reaction of atomic nitrogen with thehydroxyl radical,•HO, was added by Lavoie, Heywood and Keck[24] to the mechanism and makes a significant contribution to the formation of thermalNOx.
It is estimated that transportation fuels cause 54% of the anthropogenic (i.e. human-caused)NOx. The major source ofNOx production from nitrogen-bearing fuels such as certain coals and oil, is the conversion of fuel bound nitrogen toNOx during combustion.[22] During combustion, the nitrogen bound in the fuel is released as afree radical and ultimately forms freeN2, or NO. Fuel can contribute as much as 50% of totalNOx emissions through the combusting oil and as much as 80% through the combusting of coal[25]
Although the complete mechanism is not fully understood, there are two primary pathways 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 thanNOx. The second pathway involves the combustion of nitrogen contained in the char matrix during the combustion of thechar portion of the fuels. This reaction occurs much more slowly than the volatile phase. Only around 20% of the char nitrogen is ultimately emitted asNOx, since much of theNOx that forms during this process is reduced to nitrogen by the char, which is nearly pure carbon.
Nitrogen oxides are released during manufacturing of nitrogen fertilizers. Though nitrous oxide is emitted during its application, it is then reacted in atmosphere to form nitrogen oxides. This third source is attributed to the reaction of atmospheric nitrogen,N2, with radicals such as C, CH, andCH2 fragments derived from fuel,[26] rather than 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), NCN (diradical cyanonitrene),[27] HCN (hydrogen cyanide),•H2CN (dihydrogen cyanide) and•CN (cyano radical) which can oxidize to NO.[28] In fuels that contain nitrogen, the incidence of promptNOx is comparatively small and it is generally only of interest for the most exacting emission targets.
There is strong evidence thatNOx respiratory exposure can trigger and exacerbate existing asthma symptoms, and may even lead to the development of asthma over longer periods of time. It has also been associated with heart disease, diabetes, birth outcomes, and all-cause mortality, but these nonrespiratory effects are less well-established.[29]
NOx reacts withammonia, moisture, and other compounds to formnitric acid vapor and related particles.
NOx reacts withvolatile organic compounds in the presence of sunlight to formozone. 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.[30] 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 formNO2 and oxygen.[31]
NOx also readily reacts with common organic chemicals, and even ozone, to form a wide variety of toxic products:nitroarenes,nitrosamines and also thenitrate radical some of which may causeDNAmutations. Recently another pathway, viaNOx, to ozone has been found that predominantly occurs in coastal areas via formation ofnitryl chloride whenNOx comes into contact with salt mist.[32]
The direct effect of the emission ofNOx has positive contribution to the greenhouse effect.[33] Instead of reacting with ozone in Reaction 3, NO can also react withHO2· and organic peroxyradicals (RO2·) and thus increase the concentration of ozone. Once the concentration ofNOx exceeds a certain level, atmospheric reactions result in net ozone formation. Since tropospheric ozone can absorb infrared radiation, this indirect effect ofNOx is intensifying global warming.
There are also other indirect effects ofNOx that can either increase or decrease the greenhouse effect. First of all, through the reaction of NO withHO2· radicals,•OH radicals are recycled, which oxidize methane molecules, meaningNOx emissions can counter the effect of greenhouse gases. For instance, ship traffic emits a great amount ofNOx which provides a source ofNOx over the ocean. Then,photolysis ofNO2 leads to the formation of ozone and the further formation of hydroxyl radicals (·OH) through ozone photolysis. Since the major sink of methane in the atmosphere is by reaction with•OH radicals, theNOx emissions from ship travel may lead to a net global cooling.[34] However,NOx in the atmosphere may undergo dry or wet deposition and return to land in the form ofHNO3/NO−3. Through this way, the deposition leads to nitrogen fertilization and the subsequent formation ofnitrous oxide (N2O) in soil, which is another greenhouse gas.
NOx in the atmosphere is removed through several pathways. During daytime,NO2 reacts with hydroxyl radicals (·OH) and formsnitric acid (HNO3), which can easily be removed by dry and wet deposition. Organic peroxyradicals (RO2·) can also react with NO andNO2 and result in the formation oforganic nitrates. These are ultimately broken down to inorganic nitrate, which is a useful nutrient for plants. During nighttime,NO2 and NO can formnitrous acid (HONO) through surface-catalyzed reaction.[35] Although the reaction is relatively slow, it is an important reaction in urban areas.[35] In addition, the nitrate radical (NO3) is formed by the reaction betweenNO2 and ozone. At night,NO3 further reacts withNO2 and establishes an equilibrium reaction with dinitrogen pentoxide (N2O5).[35] Via heterogeneous reaction,N2O5 reacts with water vapor or liquid water and formsnitric acid (HNO3). As mentioned above, nitric acid can be removed through wet and dry deposition and this results in the removal ofNOx from the atmosphere.[35]
Biodiesel and its blends in general are known to reduce harmful tailpipe emissions such as:carbon monoxide; particulate matter (PM), otherwise known assoot; and unburnedhydrocarbon emissions.[36] While earlier studies suggested biodiesel could sometimes decreaseNOx and sometimes increaseNOx emissions, subsequent investigation has shown that blends of up to 20% biodiesel in USEPA-approved diesel fuel have no significant impact onNOx emissions compared with regulardiesel.[37] The state of California uses a special formulation of diesel fuel to produce lessNOx relative to diesel fuel used in the other 49 states. This has been deemed necessary by theCalifornia 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 increaseNOx emissions. The reduction ofNOx 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.NOx 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 onNOx 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 limitNOx emission to less than 0.2 g/km. Low-temperature combustion or LTC technology[2] may help reduce thermal formation ofNOx during combustion, however a tradeoff exists as high temperature combustion produces less PM or soot and results in greater power andfuel efficiency.
Selective catalytic reduction (SCR) andselective non-catalytic reduction (SNCR) reduce post combustionNOx by reacting the exhaust withurea or ammonia to produce nitrogen and water. SCR is now being used in ships,[38] diesel trucks and in some diesel cars. The use ofexhaust gas recirculation andcatalytic converters in motor vehicle engines have significantlyreduced vehicular emissions.NOx was the main focus of theVolkswagen emissions violations.
Other technologies such asflameless oxidation (FLOX) andstaged combustion significantly reduce thermalNOx in industrial processes.Bowin lowNOx technology is a hybrid of staged-premixed-radiant combustion technology with major surface combustion preceded by 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.[39]Water Injection technology, whereby water is introduced into the combustion chamber, is also becoming an important means ofNOx 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). Excessive water addition facilitates hot corrosion, which is the primary reason why dry low-NOx technologies are favored today despite their greater complexity.