NO_x

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 NO_x 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.

${\displaystyle {\ce {NO2 + h\nu -> NO + O}},\qquad \lambda <398~{\ce {nm}}}$[7]

(1)

${\displaystyle {\ce {O + O2 + M -> O3 + M}}}$

(2)

${\displaystyle {\ce {O3 + NO -> NO2 + O2}}}$

(3)

${\displaystyle {\frac {{\ce {[NO2]}}}{{\ce {[NO]}}}}={\frac {k_{3}[{\ce {O3}}]}{j_{{\ce {NO2}}}}}}$

(4)

The symbol ${\displaystyle {\ce {M}}}$ 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 NO_x and ozone, and is known as the Leighton relationship.

The time ${\displaystyle \tau }$ that is needed to reach a steady state among NO_x and ozone is dominated by reaction (3), which reverses reactions (1)+(2):

${\displaystyle \tau ={\frac {1}{k_{3}[{\ce {NO}}]}}}$

(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

There is strong evidence that NO_x 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]

NO_x reacts with ammonia, moisture, and other compounds to form nitric acid vapor and related particles.

NO_x 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]

NO_x 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 NO_x, to ozone has been found that predominantly occurs in coastal areas via formation of nitryl chloride when NO_x comes into contact with salt mist.[32]

The direct effect of the emission of NO_x 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 NO_x exceeds a certain level, atmospheric reactions result in net ozone formation. Since tropospheric ozone can absorb infrared radiation, this indirect effect of NO_x is intensifying global warming.

There are also other indirect effects of NO_x 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 NO_x emissions can counter the effect of greenhouse gases. For instance, ship traffic emits a great amount of NO_x which provides a source of NO_x 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 NO_x emissions from ship travel may lead to a net global cooling.[34] However, NO_x in the atmosphere may undergo dry or wet deposition and return to land in the form of HNO₃/NO−3. 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, NO_x emissions have a negative contribution to global warming.[35]

NO_x 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 NO_x from the atmosphere.[36]

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 NO_x and sometimes increase NO_x emissions, subsequent investigation has shown that blends of up to 20% biodiesel in USEPA-approved diesel fuel have no significant impact on NO_x emissions compared with regular diesel.[38] The state of California uses a special formulation of diesel fuel to produce less NO_x 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 NO_x emissions. The reduction of NO_x 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. NO_x 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 NO_x 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 NO_x emission to less than 0.2 g/km. Low-temperature combustion or LTC technology[2] may help reduce thermal formation of NO_x during combustion, however a tradeoff exists as high temperature combustion produces less PM or soot and results in greater power and fuel efficiency.

Selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) reduce post combustion NO_x 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. NO_x was the main focus of the Volkswagen emissions violations.

Other technologies such as flameless oxidation (FLOX) and staged combustion significantly reduce thermal NO_x in industrial processes. Bowin low NO_x 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 NO_x 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-NO_x technologies are favored today besides the requirement of a more complex system.