Agriculture Reference
In-Depth Information
engine is a major source of oxides of nitrogen, as are electricity generating sta-
tions, which use boilers to make steam to turn turbines to convert mechanical
energy into electrical energy. Approximately 90 to 95% of the nitrogen oxides
generated in combustion processes are in the form of nitric oxide (NO), but
like the oxides of sulfur, other nitrogen oxides can form, especially nitrogen
dioxide (NO
2
), so air pollution experts refer to NO and NO
2
collectively as
NO
x
. In fact, in the atmosphere the NO emitted is quickly converted photo-
chemically to nitrogen dioxide (NO
2
). Such high-temperature/high-pressure
conditions exist in internal combustion engines, like those in automobiles and
other “mobile sources.” Thus, NO
x
is one of the major mobile source air
pollutants (others include particulate matter, hydrocarbons, carbon monoxide,
and in some countries, the heavy metal lead).
In addition to atmospheric nitrogen, other sources exist, particularly the
nitrogen in fossil fuels. The nitrogen oxides generated from atmospheric ni-
trogen are known as
thermal
NO
x
since they form at high temperatures, such
as near burner flames in combustion chambers. Nitrogen oxides that form
from the fuel or feedstock are called
fuel
NO
x
. Unlike the sulfur compounds,
a significant fraction of the fuel nitrogen remains in the bottom ash or in
unburned aerosols in the gases leaving the combustion chamber (i.e., the fly
ash). Nitrogen oxides can also be released from nitric acid plants and other
types of industrial processes involving the generation and/or use of nitric acid
(HNO
3
).
Nitric oxide is a colorless, odorless gas and is essentially insoluble in water.
Nitrogen dioxide has a pungent acid odor and is somewhat soluble in water.
At low temperatures such as those often present in the ambient atmosphere,
NO
2
can form the molecule NO
2
-O
2
N or simply N
2
O
4
, which consists of
two identical simpler NO
2
molecules. This is known as a
dimer
. The dimer
N
2
O
4
is distinctly reddish brown and contributes to the brown haze that is
often associated with photochemical smog incidents.
Both NO and NO
2
are harmful and toxic to humans, although atmospheric
concentrations of nitrogen oxides are usually well below the concentrations
expected to lead to adverse health effects. The low concentrations are a re-
sult of the moderately rapid reactions that occur when NO and NO
2
are
emitted into the atmosphere. Much of the concern for regulating NO
x
emis-
sions is to suppress the reactions in the atmosphere that generate the highly
reactive molecule ozone (O
3
). Nitrogen oxides play key roles as important
reactants in O
3
formation. Ozone forms photochemically (i.e., the reaction is
caused or accelerated by light energy) in the lowest level of the atmosphere,
the troposphere. Nitrogen dioxide is the principal gas responsible for absorb-
ing sunlight needed for these photochemical reactions. So in the presence
of sunlight, the NO
2
that forms from the NO stimulates the photochemical
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