In atmospheric chemistry, NOx is a generic term for the nitrogen oxides that are most relevant for air pollution, namely nitric oxide (NO) and nitrogen dioxide (NO2). 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 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. NOx gases are also produced naturally by lightning.
The term NOx is chemistry shorthand for molecules containing one nitrogen and one or more oxygen atom. It is generally meant to include nitrous oxide (N2O), although nitrous oxide is 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, and is a significant greenhouse gas.
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 NOx denotes the total concentration of NO and NO2. During daylight, these concentrations together with that of ozone are in steady state; 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). The time constant to establish the steady state is 1/ where k is the rate coefficient of the reaction
- NO + O3 → NO2 + O2
When NOx 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.
Formation of nitric acid and acid rain
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 day 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 nitric acid HNO3,
- N2O5 + H2O(liq) → 2 HNO3(aq)
These are thought to be the principal pathways for formation of nitric acid in the atmosphere.: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 + HNO3
Nitric oxide is produced during thunderstorms due to the extreme heat of lightning, and is caused by the splitting of nitrogen molecules. This can result in the production of acid rain, if nitric oxide forms compounds with the water molecules in precipitation.
Scientists Ott et al. 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.
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. It should also be noted that 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).
Industrial sources (anthropogenic sources)
The three primary sources of NOx in combustion processes:
- 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 and 800 °C, where it is also a minor contributor.
Thermal NOx refers to NOx 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 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. 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 NOx.
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. During combustion, the nitrogen bound in the fuel is released as a free radical and ultimately forms free N2, or NO. Fuel NOx 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 NOx. 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 NOx, since much of the NOx 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 NOx is especially minimal and it is generally only of interest for the most exacting emission targets.
Health and environment effects
NOx 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.
NOx 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. 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.
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.
NOx 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 NOx 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 NOx 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."
The ultimate destination of much NOx is to end up in the soil as nitrite or nitrate, which are useful to growing plants.
Biodiesel and NOx
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. 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. 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. 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
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, 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 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. 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). Inline emulsified fuel/water mixtures show NOx reductions between 4 and 83%.
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