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NOx

In atmospheric chemistry, NOx is shorthand for nitric oxide (NO) and nitrogen dioxide (NO
2
), the nitrogen oxides that are most relevant for air pollution.[1][2] These gases contribute to the formation of smog and acid rain, as well as affecting tropospheric ozone.

NOx gases are usually produced from the reaction between nitrogen and oxygen during combustion of fuels, such as hydrocarbons, 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 by lightning.

NOx does not include nitrous oxide (N
2
O
),[1] a fairly inert oxide of nitrogen that contributes less severely to air pollution, notwithstanding its involvement in ozone depletion[4] and high global warming potential.

NOy is defined as the sum of NOx plus the NOz compounds produced from the oxidation of NOx which include nitric acid, nitrous acid (HONO), dinitrogen pentoxide (N2O5), peroxyacetyl nitrate (PAN), alkyl nitrates (RONO2), peroxyalkyl nitrates (ROONO2), the nitrate radical (NO3), and peroxynitric acid (HNO4).[5][6]: 30 

Formation and reactions edit

Because of energy limitations, 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 NOx refers to the total concentration of NO and NO2 since the conversion between these two species is rapid in the stratosphere and troposphere.[6] During daylight hours, these concentrations together with that of ozone are in steady state, also known as photostationary state (PSS); the ratio of NO to NO2 is determined by the intensity of sunshine (which converts NO2 to NO) and the concentration of ozone (which reacts with NO to again form NO2).

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 reaction 2. Equation 4 relates the concentrations of NOx and ozone, and is known as the Leighton relationship.

The time   that is needed to reach a steady state among NOx and ozone is dominated by reaction (3), which reverses reactions (1)+(2):

 

 

 

 

 

(5)

for mixing ratio of NO, [NO] = 10 part per billion (ppb), the time constant is 40 minutes; for [NO] = 1 ppb, 4 minutes.[8]: 211 

Formation of smog edit

When NOx and volatile organic compounds (VOCs) react in the presence of sunlight, they form photochemical smog, 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 with NOx quickly and increase the concentration of ozone and peroxide compounds, especially peroxyacetyl nitrate (PAN).[9]

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.[10]

Formation of nitric acid and acid rain edit

NO2 is further oxidized in the gas phase during daytime by reaction with OH

NO2 + OH (+M) → HNO3 (+M),

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

NO2 + O3 → NO3 + O2.

During the daytime, NO3 is quickly photolyzed back to NO2, but at night it can react with a second NO2 to form dinitrogen pentoxide.

NO2 + NO3 (+M) → N2O5 (+M).

N2O5 reacts rapidly with liquid water (in aerosol particles or cloud drops, but not in the gas phase) to form HNO3,

N2O5 + H2O(liq) → 2 HNO3(aq)

These are thought to be the principal pathways for formation of nitric acid in the atmosphere.[8]: 224–225  This nitric acid contributes to acid rain or may deposit to soil, where it makes nitrate, which is of use to growing plants. The aqueous phase reaction

NO2 + H2O → HNO2 + HNO3

is too slow to be of any significance in the atmosphere.[8]: 336 

Sources edit

Natural sources edit

Nitric oxide is produced during thunderstorms due to the extreme heating and cooling within a lightning strike. This causes stable molecules such as N2 and O2 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 produce nitric acid (HNO3), this can be precipitated out as acid rain or deposited onto particles in the air. Elevated production of NOx 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 reactive NOx. With 1.4 billion lightning flashes per year, multiplied by 7 kilograms per lightning strike, they estimated the total amount of NOx 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 atmospheric NOx.[3] Atmospheric constituents such as nitrogen oxides can be stratified vertically in the atmosphere. Ott noted that the lightning-produced NOx 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]

Biogenic sources edit

Agricultural fertilization and the use of nitrogen fixing plants also contribute to atmospheric NOx, by promoting nitrogen fixation by microorganisms.[15][16] The nitrification process transforms ammonia into nitrate. Denitrification is basically the reverse process of nitrification. During denitrification, nitrate is reduced to nitrite, then NO, then N2O 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-wide NOx 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 as nitrate 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 in NOx and in turn, changes in the types of trees which predominate. Due to human activity and climate change, the maples, sassafras, and tulip poplar have been pushing out the beneficial oak, beech, and hickory. 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]

Industrial sources (anthropogenic sources) edit

The three primary sources of NOx in combustion processes:[20][21]

  • thermal NOx
  • fuel NOx
  • prompt NOx

Thermal NOx formation, which is highly temperature dependent, is recognized as the most relevant source when combusting natural gas. Fuel NOx 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 NOx. The contribution of prompt NOx is normally considered negligible. A fourth source, called feed NOx 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.

Thermal edit

Thermal NOx refers to NOx formed through high temperature oxidation of the diatomic nitrogen found in combustion air.[22] The formation rate is primarily a function of temperature and the residence 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 extended Zel'dovich mechanism) producing thermal NOx 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.[23] The last reaction of atomic nitrogen with the hydroxyl radical, HO, was added by Lavoie, Heywood and Keck[24] to the mechanism and makes a significant contribution to the formation of thermal NOx.

Fuel edit

It is estimated that transportation fuels cause 54% of the anthropogenic (i.e. human-caused) NOx. The major source of NOx production from nitrogen-bearing fuels such as certain coals and oil, is the conversion of fuel bound nitrogen to NOx during combustion.[22] 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 NOx 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 than NOx. The second pathway 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 NOx, since much of the NOx that forms during this process is reduced to nitrogen by the char, which is nearly pure carbon.

Prompt edit

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, and CH2 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 prompt NOx is comparatively small and it is generally only of interest for the most exacting emission targets.

Health and environment effects edit

There is strong evidence that NOx 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 with ammonia, moisture, and other compounds to form nitric acid vapor and related particles.

NOx reacts with volatile organic compounds in the presence of sunlight to form 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.[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 form NO2 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 the nitrate radical some of which may cause DNA mutations. Recently another pathway, via NOx, to ozone has been found that predominantly occurs in coastal areas via formation of nitryl chloride when NOx comes into contact with salt mist.[32]

The direct effect of the emission of NOx has positive contribution to the greenhouse effect.[33] Instead of reacting with ozone in Reaction 3, NO can also react with HO2· and organic peroxyradicals (RO2·) and thus increase the concentration of ozone. Once the concentration of NOx exceeds a certain level, atmospheric reactions result in net ozone formation. Since tropospheric ozone can absorb infrared radiation, this indirect effect of NOx is intensifying global warming.

There are also other indirect effects of NOx that can either increase or decrease the greenhouse effect. First of all, through the reaction of NO with HO2 radicals, OH radicals are recycled, which oxidize methane molecules, meaning NOx emissions can counter the effect of greenhouse gases. For instance, ship traffic emits a great amount of NOx which provides a source of NOx over the ocean. Then, photolysis of NO2 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, the NOx 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 of HNO3/NO3. Through this way, the deposition leads to nitrogen fertilization and the subsequent formation of nitrous oxide (N2O) in soil, which is another greenhouse gas. In conclusion, considering several direct and indirect effects, NOx emissions have a negative contribution to global warming.[35]

NOx in the atmosphere is removed through several pathways. During daytime, NO2 reacts with hydroxyl radicals (·OH) and forms nitric acid (HNO3), which can easily be removed by dry and wet deposition. Organic peroxyradicals (RO2·) can also react with NO and NO2 and result in the formation of organic nitrates. These are ultimately broken down to inorganic nitrate, which is a useful nutrient for plants. During nighttime, NO2 and NO can form nitrous acid (HONO) through surface-catalyzed reaction.[36] Although the reaction is relatively slow, it is an important reaction in urban areas.[36] In addition, the nitrate radical (NO3) is formed by the reaction between NO2 and ozone. At night, NO3 further reacts with NO2 and establishes an equilibrium reaction with dinitrogen pentoxide (N2O5).[36] Via heterogeneous reaction, N2O5 reacts with water vapor or liquid water and forms nitric acid (HNO3). As mentioned above, nitric acid can be removed through wet and dry deposition and this results in the removal of NOx from the atmosphere.[36]

Biodiesel and NOx edit

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.[37] 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.[38] 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 NOx 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 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[2] may help reduce thermal formation of NOx 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 edit

Selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) reduce post combustion NOx by reacting the exhaust with urea or ammonia to produce nitrogen and water. SCR is now being used in ships,[39] 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. NOx was the main focus of the Volkswagen emissions violations.

Other technologies such as flameless oxidation (FLOX) and staged combustion significantly reduce thermal NOx in industrial processes. Bowin low NOx 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.[40] Water Injection technology, whereby water is introduced into the combustion chamber, is also becoming an important means of NOx 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 besides the requirement of a more complex system.

References edit

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this, article, about, nitrogen, oxides, produced, during, combustion, more, extensive, list, nitrogen, oxides, nitrogen, oxide, other, meanings, atmospheric, chemistry, shorthand, nitric, oxide, nitrogen, dioxide, nitrogen, oxides, that, most, relevant, pollut. This article is about nitrogen oxides produced during combustion For a more extensive list of nitrogen oxides see nitrogen oxide For other meanings of Nox see Nox In atmospheric chemistry NOx is shorthand for nitric oxide NO and nitrogen dioxide NO2 the nitrogen oxides that are most relevant for air pollution 1 2 These gases contribute to the formation of smog and acid rain as well as affecting tropospheric ozone NOx gases are usually produced from the reaction between nitrogen and oxygen during combustion of fuels such as hydrocarbons 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 by lightning NOx does not include nitrous oxide N2 O 1 a fairly inert oxide of nitrogen that contributes less severely to air pollution notwithstanding its involvement in ozone depletion 4 and high global warming potential NOy is defined as the sum of NOx plus the NOz compounds produced from the oxidation of NOx which include nitric acid nitrous acid HONO dinitrogen pentoxide N2O5 peroxyacetyl nitrate PAN alkyl nitrates RONO2 peroxyalkyl nitrates ROONO2 the nitrate radical NO3 and peroxynitric acid HNO4 5 6 30 Contents 1 Formation and reactions 1 1 Formation of smog 1 2 Formation of nitric acid and acid rain 2 Sources 2 1 Natural sources 2 2 Biogenic sources 2 3 Industrial sources anthropogenic sources 2 3 1 Thermal 2 3 2 Fuel 2 3 3 Prompt 3 Health and environment effects 4 Biodiesel and NOx 5 Regulation and emission control technologies 6 ReferencesFormation and reactions editBecause of energy limitations 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 NOx refers to the total concentration of NO and NO2 since the conversion between these two species is rapid in the stratosphere and troposphere 6 During daylight hours these concentrations together with that of ozone are in steady state also known as photostationary state PSS the ratio of NO to NO2 is determined by the intensity of sunshine which converts NO2 to NO and the concentration of ozone which reacts with NO to again form NO2 In other words the concentration of ozone in the atmosphere is determined by the ratio of these two species NO 2 h n NO O l lt 398 nm displaystyle ce NO2 h nu gt NO O qquad lambda lt 398 ce nm nbsp 7 1 O O 2 M O 3 M displaystyle ce O O2 M gt O3 M nbsp 2 O 3 NO NO 2 O 2 displaystyle ce O3 NO gt NO2 O2 nbsp 3 NO 2 NO k 3 O 3 j NO 2 displaystyle frac ce NO2 ce NO frac k 3 ce O3 j ce NO2 nbsp 4 The symbol M displaystyle ce M nbsp represents a third body a molecular species that is required to carry away energy from the exothermic reaction 2 Equation 4 relates the concentrations of NOx and ozone and is known as the Leighton relationship The time t displaystyle tau nbsp that is needed to reach a steady state among NOx and ozone is dominated by reaction 3 which reverses reactions 1 2 t 1 k 3 NO displaystyle tau frac 1 k 3 ce NO nbsp 5 for mixing ratio of NO NO 10 part per billion ppb the time constant is 40 minutes for NO 1 ppb 4 minutes 8 211 Formation of smog edit When NOx and volatile organic compounds VOCs react in the presence of sunlight they form photochemical smog 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 with NOx quickly and increase the concentration of ozone and peroxide compounds especially peroxyacetyl nitrate PAN 9 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 10 Formation of nitric acid and acid rain edit NO2 is further oxidized in the gas phase during daytime by reaction with OH NO2 OH M HNO3 M 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 NO2 O3 NO3 O2 During the daytime NO3 is quickly photolyzed back to NO2 but at night it can react with a second NO2 to form dinitrogen pentoxide NO2 NO3 M N2O5 M N2O5 reacts rapidly with liquid water in aerosol particles or cloud drops but not in the gas phase to form HNO3 N2O5 H2O liq 2 HNO3 aq These are thought to be the principal pathways for formation of nitric acid in the atmosphere 8 224 225 This nitric acid contributes to acid rain or may deposit to soil where it makes nitrate which is of use to growing plants The aqueous phase reaction 2 NO2 H2O HNO2 HNO3is too slow to be of any significance in the atmosphere 8 336 Sources editNatural sources edit Nitric oxide is produced during thunderstorms due to the extreme heating and cooling within a lightning strike This causes stable molecules such as N2 and O2 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 produce nitric acid HNO3 this can be precipitated out as acid rain or deposited onto particles in the air Elevated production of NOx 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 reactive NOx With 1 4 billion lightning flashes per year multiplied by 7 kilograms per lightning strike they estimated the total amount of NOx 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 atmospheric NOx 3 Atmospheric constituents such as nitrogen oxides can be stratified vertically in the atmosphere Ott noted that the lightning produced NOx 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 Biogenic sources edit Agricultural fertilization and the use of nitrogen fixing plants also contribute to atmospheric NOx by promoting nitrogen fixation by microorganisms 15 16 The nitrification process transforms ammonia into nitrate Denitrification is basically the reverse process of nitrification During denitrification nitrate is reduced to nitrite then NO then N2O 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 wide NOx 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 as nitrate 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 in NOx and in turn changes in the types of trees which predominate Due to human activity and climate change the maples sassafras and tulip poplar have been pushing out the beneficial oak beech and hickory 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 Industrial sources anthropogenic sources edit The three primary sources of NOx in combustion processes 20 21 thermal NOx fuel NOx prompt NOxThermal NOx formation which is highly temperature dependent is recognized as the most relevant source when combusting natural gas Fuel NOx 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 NOx The contribution of prompt NOx is normally considered negligible A fourth source called feed NOx 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 Thermal edit Thermal NOx refers to NOx formed through high temperature oxidation of the diatomic nitrogen found in combustion air 22 The formation rate is primarily a function of temperature and the residence 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 extended Zel dovich mechanism producing thermal NOx are N2 O NO N N O2 NO O N OH displaystyle ce nbsp NO H displaystyle ce nbsp 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 the hydroxyl radical HO was added by Lavoie Heywood and Keck 24 to the mechanism and makes a significant contribution to the formation of thermal NOx Fuel edit It is estimated that transportation fuels cause 54 of the anthropogenic i e human caused NOx The major source of NOx production from nitrogen bearing fuels such as certain coals and oil is the conversion of fuel bound nitrogen to NOx during combustion 22 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 NOx 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 than NOx The second pathway 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 NOx since much of the NOx that forms during this process is reduced to nitrogen by the char which is nearly pure carbon Prompt edit 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 and CH2 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 prompt NOx is comparatively small and it is generally only of interest for the most exacting emission targets Health and environment effects editThere is strong evidence that NOx 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 with ammonia moisture and other compounds to form nitric acid vapor and related particles NOx reacts with volatile organic compounds in the presence of sunlight to form 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 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 form NO2 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 the nitrate radical some of which may cause DNA mutations Recently another pathway via NOx to ozone has been found that predominantly occurs in coastal areas via formation of nitryl chloride when NOx comes into contact with salt mist 32 The direct effect of the emission of NOx has positive contribution to the greenhouse effect 33 Instead of reacting with ozone in Reaction 3 NO can also react with HO2 and organic peroxyradicals RO2 and thus increase the concentration of ozone Once the concentration of NOx exceeds a certain level atmospheric reactions result in net ozone formation Since tropospheric ozone can absorb infrared radiation this indirect effect of NOx is intensifying global warming There are also other indirect effects of NOx that can either increase or decrease the greenhouse effect First of all through the reaction of NO with HO2 radicals OH radicals are recycled which oxidize methane molecules meaning NOx emissions can counter the effect of greenhouse gases For instance ship traffic emits a great amount of NOx which provides a source of NOx over the ocean Then photolysis of NO2 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 the NOx 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 of HNO3 NO3 Through this way the deposition leads to nitrogen fertilization and the subsequent formation of nitrous oxide N2O in soil which is another greenhouse gas In conclusion considering several direct and indirect effects NOx emissions have a negative contribution to global warming 35 NOx in the atmosphere is removed through several pathways During daytime NO2 reacts with hydroxyl radicals OH and forms nitric acid HNO3 which can easily be removed by dry and wet deposition Organic peroxyradicals RO2 can also react with NO and NO2 and result in the formation of organic nitrates These are ultimately broken down to inorganic nitrate which is a useful nutrient for plants During nighttime NO2 and NO can form nitrous acid HONO through surface catalyzed reaction 36 Although the reaction is relatively slow it is an important reaction in urban areas 36 In addition the nitrate radical NO3 is formed by the reaction between NO2 and ozone At night NO3 further reacts with NO2 and establishes an equilibrium reaction with dinitrogen pentoxide N2O5 36 Via heterogeneous reaction N2O5 reacts with water vapor or liquid water and forms nitric acid HNO3 As mentioned above nitric acid can be removed through wet and dry deposition and this results in the removal of NOx from the atmosphere 36 Biodiesel and NOx editBiodiesel 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 37 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 38 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 NOx 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 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 2 may help reduce thermal formation of NOx 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 editSelective catalytic reduction SCR and selective non catalytic reduction SNCR reduce post combustion NOx by reacting the exhaust with urea or ammonia to produce nitrogen and water SCR is now being used in ships 39 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 NOx was the main focus of the Volkswagen emissions violations Other technologies such as flameless oxidation FLOX and staged combustion significantly reduce thermal NOx in industrial processes Bowin low NOx 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 40 Water Injection technology whereby water is introduced into the combustion chamber is also becoming an important means of NOx 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 besides the requirement of a more complex system References edit a b c Mollenhauer Klaus Tschoke Helmut 2010 Handbook of Diesel Engines Springer pp 445 446 ISBN 978 3540890829 a b c Omidvarborna et al December 2015 NOx emissions from low temperature combustion of biodiesel made of various feedstocks and blends Fuel Processing Technology 140 113 118 doi 10 1016 j fuproc 2015 08 031 a b Annamalai Kalyan Puri Ishwar K 2007 Combustion Science and Engineering CRC Press p 775 ISBN 978 0 8493 2071 2 Ravishankara A R Daniel J S Portmann R W 2009 Nitrous Oxide N2O The Dominant Ozone Depleting Substance Emitted in the 21st Century Science 326 5949 123 125 Bibcode 2009Sci 326 123R doi 10 1126 science 1176985 PMID 19713491 S2CID 2100618 US Environmental Protection Agency Air Monitoring Instrumentation Nitrogen Oxides NOy PDF a b Seinfeld John H Pandis Spyros N 2016 03 29 Atmospheric chemistry and physics from air pollution to climate change Third ed Hoboken New Jersey John Wiley amp Sons ISBN 9781119221166 OCLC 929985467 J B Burkholder S P Sander J Abbatt J R Barker C Cappa J D Crounse T S Dibble R E Huie C E Kolb M J Kurylo V L Orkin C J Percival D M Wilmouth and P H Wine Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies Evaluation No 19 JPL Publication 19 5 Jet Propulsion Laboratory Pasadena 2019 http jpldataeval jpl nasa gov a b c Seinfeld John H Pandis Spyros N 2006 Atmospheric Chemistry and Physics From Air Pollution to Climate Change 2nd rev ed Hoboken New Jersey John Wiley amp Sons ISBN 978 0 471 72018 8 Peter Warneck 2000 Chemistry of the natural atmosphere 2nd ed San Diego Academic Press ISBN 9780127356327 OCLC 162128886 Health and Environmental Impacts of NOx United States Environmental Protection Agency Retrieved 2007 12 26 Murray Lee T 2016 04 25 Lightning NO x and Impacts on Air Quality Current Pollution Reports 2 2 115 133 doi 10 1007 s40726 016 0031 7 ISSN 2198 6592 Hauglustaine Didier Emmons Louisa Newchurch Mike Brasseur Guy Takao Toshinori Matsubara Kouji Johnson James Ridley Brian Stith Jeff March 2001 On the Role of Lightning NOx in the Formation of Tropospheric Ozone Plumes A Global Model Perspective Journal of Atmospheric Chemistry 38 3 277 294 Bibcode 2001JAtC 38 277H doi 10 1023 a 1006452309388 ISSN 0167 7764 S2CID 91569139 a b Ott Lesley E Pickering Kenneth E Stenchikov Georgiy L Allen Dale J DeCaria Alex J Ridley Brian Lin Ruei Fong Lang Stephen amp Tao Wei Kuo 2010 Production of lightning NOx and its vertical distribution calculated from three dimensional cloud scale chemical transport model simulations Journal of Geophysical Research 115 D4 D04301 Bibcode 2010JGRD 115 4301O doi 10 1029 2009JD011880 hdl 10754 552104 Schumann U amp Huntrieser H 2007 The global lightning induced nitrogen oxides source PDF Atmos Chem Phys 7 14 3823 Bibcode 2007ACP 7 3823S doi 10 5194 acp 7 3823 2007 Retrieved 2016 05 31 Galloway J N et al September 2004 Nitrogen cycles past present and future Biogeochemistry 70 2 153 226 doi 10 1007 s10533 004 0370 0 S2CID 98109580 Davidson E A amp Kingerlee W 1997 A global inventory of nitric oxide emissions from soils Nutrient Cycling in Agroecosystems 48 37 50 doi 10 1023 A 1009738715891 S2CID 22290176 Lagzi Istvan Meszaros Robert Gelybo Gyorgyi Leelossy Adam 2013 Atmospheric Chemistry Eotvos Lorand University Houlton Ben New Study Finds Nitrogen Fertilizer a Major Contributor to Smog in California Article EESI www eesi org Retrieved 2018 10 18 Fryling Kevin 2019 01 22 IU study predicts air pollutant increase from U S forest soils News at IU Retrieved 2019 01 27 Nitrogen Oxides What is NOx E Instruments e inst com E Instruments e inst com Retrieved 2018 11 05 Nitrogen Oxides NOx Why and How They Are Controlled PDF Clean Air Technology Center Information Transfer and Program Integration Division Office of Air Quality Planning and Standards U S Environmental Protection Agency November 1999 a b Beychok Milton R March 1973 NOX emission from fuel combustion controlled Oil amp Gas Journal 53 56 Zel dovich Y B 1946 The Oxidation of Nitrogen in Combustion Explosions Acta Physicochimica U S S R 21 577 628 Lavoie G A Heywood J B Keck J C 1970 Experimental and Theoretical Study of Nitric Oxide Formation in Internal Combustion Engines Combustion Science and Technology 1 4 313 326 doi 10 1080 00102206908952211 S2CID 98781153 1 1 3 3 Nitrogen Oxides Compilation of air pollutant emission factors PDF Technical report Vol I Fifth ed US Environmental Protection Agency January 1995 AP 42 Retrieved 2023 12 27 Fenimore C P 1971 Formation of nitric oxide in premixed hydrocarbon flames Symposium International on Combustion 13 1 373 380 doi 10 1016 S0082 0784 71 80040 1 Pfeifle Mark Georgievskii Yuri Jasper Ahren W Klippenstein Stephen J 2017 08 28 Theoretical investigation of intersystem crossing in the cyanonitrene molecule 1NCN 3NCN The Journal of Chemical Physics 147 8 084310 Bibcode 2017JChPh 147h4310P doi 10 1063 1 4999788 ISSN 0021 9606 OSTI 1377972 PMID 28863540 Shrestha Krishna Prasad Seidel Lars Zeuch Thomas Mauss Fabian 2019 05 02 Kinetic Modeling of NOx Formation and Consumption during Methanol and Ethanol Oxidation Combustion Science and Technology 191 9 1628 1660 doi 10 1080 00102202 2019 1606804 ISSN 0010 2202 S2CID 155726862 U S EPA Integrated Science Assessment ISA For Oxides of Nitrogen Health Criteria Final Report 2016 U S Environmental Protection Agency Washington DC EPA 600 R 15 068 2016 Ozone Environmental Protection Agency London Air What is Ozone King s College London Environmental Research Group Potera Carol 2008 Air Pollution Salt Mist Is the Right Seasoning for Ozone Environ Health Perspect 116 7 A288 doi 10 1289 ehp 116 a288 PMC 2453175 PMID 18629329 Lammel Gerhard Grassl Hartmut 1995 Greenhouse effect of NOX Environmental Science and Pollution Research 2 1 40 45 doi 10 1007 bf02987512 ISSN 0944 1344 PMID 24234471 S2CID 42621955 Globalisation Transport and the Environment PDF France Organisation for Economic Co operation and Development Intergovernmental Panel on Climate Change ed 2014 Technical Summary Climate Change 2013 the Physical Science Basis Cambridge University Press pp 31 116 doi 10 1017 cbo9781107415324 005 ISBN 9781107415324 retrieved 2018 11 15 a b c d Finlayson Pitts Barbara J Pitts James N 2000 Chemistry of the upper and lower atmosphere theory experiments and applications San Diego Academic Press ISBN 9780080529073 OCLC 162128929 A Comprehensive Analysis of Biodiesel Impacts on Exhaust Emissions Draft Technical Report PDF US Environmental Protection Agency October 2002 McCormick R L Williams A Ireland J Brimhall M Hayes R R October 2006 Effects of Biodiesel Blends on Vehicle Emissions PDF US National Renewable Energy Laboratory Retrieved 2018 11 17 Wartsila Low NOx Solutions Archived 2015 09 29 at the Wayback Machine Wartsila 2008 Bob Joynt amp Stephen Wu Nitrogen oxides emissions standards for domestic gas appliances background study Combustion Engineering Consultant February 2000 Retrieved from https en wikipedia org w index php title NOx amp oldid 1200984083, wikipedia, wiki, book, books, library,

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