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 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.
NOx is chemistry shorthand for molecules containing one
nitrogen and one or more oxygen atom. It is generally 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, and is a significant greenhouse gas.
NOy (reactive, free radical) is defined as the sum of
NOx plus the NOz
compounds produced from the oxidation of
NOx which include nitric
1 Formation and reactions
1.1 Formation of nitric acid and acid rain
2.1 Natural sources
2.2 Biogenic sources
2.3 Industrial sources (anthropogenic sources)
3 Health and environment effects
Biodiesel and NOx
5 Regulation and emission control technologies
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/k[NO] where k is
the rate coefficient of the reaction
NO + O3 → NO2 + O2
for mixing ratio of NO, [NO], = 1 part per billion (ppb), the time
constant is 40 minutes; for [NO] = 10 ppb, 4 minutes.:211
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
NO2 + OH (+M) → HNO3 (+M),
where M denotes a third molecule required to stabilize the addition
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
is too slow to be of any significance in the atmosphere.:336
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
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).
Agricultural fertilization and the use of nitrogen fixing plants also
contribute to atmospheric NOx, by promoting nitrogen fixation by
Industrial sources (anthropogenic sources)
The three primary sources of
NOx in combustion processes:
NOx formation, which is highly temperature dependent, is
recognized as the most relevant source when combusting natural gas.
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.
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)
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
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
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
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
Recently another pathway, via NOx, to ozone has been found that
predominantly occurs in coastal areas via formation of nitryl chloride
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 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
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
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.
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
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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