88 AIR POLLUTION SOURCES
TABLE 9 (continued)
Total National Emissions of Nitrogen Oxides, 1940 through 1994 (thousand short tons)Source Category 1940 1940 1940 1940 1940 1940 1940 1940
industrial NA NA 40 75 99 125 131 136
farm 33 29 50 166 280 230 256 265
airport service NA NA NA 78 113 144 152 159
Aircraft 0 2 4 72 106 139 147 153
Marine Vessels 109 108 108 40 110 173 183 188
Railroads 657 992 772 495 731 929 945 947
MISCELLANEOUS 990 665 441 330 248 373 219 374
TOTAL ALL SOURCES 7,374 10,093 14,140 20,625 23,281 23,038 23,276 23,615
Note(s): Categories displayed below Tier 1 do not sum to Tier 1 totals because they are intended to show major contributors.
1994 emission estimates are preliminary and will be updated in the next report.
Tier 1 source categories and emissions are shaded.Part 1. Pollutant Emissions (continued)Pollutant types Sources and abundance Abatement and controlc. Nitrogen dioxide: Nitrogen dioxide (NO 2 ) is a colored gas which is a
light yellowish orange at low concentrations and
reddish brown at high concentrations. It has a
pungent, irritating odor. It is relatively toxic and
has a rapid oxidation rate which makes it highly
corrosive as well. The oxidation of NO to NO 2
follows the reaction:
2NO O 2 → 2NO 2 (7)
This reaction is slow at low atmospheric levels
and accounts for about 25% of all NO
conversion. The major NO conversion
processes are photochemical, involving
hydrocarbons, ozone, aldehydes, carbon
monoxide, and other compounds.
Background concentrations of NO 2 are
approximately 0.5 ppb with one hour average
concentrations in urban areas of 0.5 ppm. Peak
morning concentrations of NO are followed
several hours later by peak levels of NO 2
produced by the chemical and photochemical
oxidation of the NO. Since the conversion of
NO to NO 2 is related to solar intensity, more
NO 2 is produced on warm, sunny days.In the atmosphere, NO 2 can be
photochemically oxidized to nitrates
which are subsequently removed by
precipitation, dry deposition and
surface absorption.
In motor vehicles, current methods for
controlling NOx emissions include
retardation of spark timing, increasing
the air/fuel ratio (i.e., less fuel to air),
injecting water into the cylinders,
decreasing the compression ratio, and
recirculating exhaust gas. All these
methods reduce the combustion
chamber temperature (which reduces
NOx emissions) without greatly
increasing the emissions of
hydrocarbons and CO. Catalytic
convertors which reduce NO to
elemental nitrogen (N 2 ) can also be
used. The use of alternative fuels, such
as methyl and ethyl alcohol, which
combust at a lower temperature than
gasoline can also be used to lower NOx
emissions.
For stationary sources, one abatement
method is to use a lower NOx producing
fuel; emissions are highest from coal,
intermediate with oil and lowest with
natural gas. For the numerous methods
of control see the article “Nitrogen
Oxides” in this Encyclopedia.- Photochemical Oxidants: Photochemical oxidants
are secondary pollutants which result from a
complex series of atmospheric actions involving
organic pollutants, NOx, O 2 and sunlight. The main
photo-chemical oxidants are ozone, NO 2 (covered in
the section on nitrogen compounds) and, to a lesser
extent, peroxyacetylnitrate.
Ozone (O 3 ) is the most important and widely
reported of the photochemical oxidants. It is a
bluish gas that is 1.6 times heavier than oxygen
and is normally found at elevated levels in the
stratosphere where it functions to absorb harmful
ultraviolet radiation. Ground level ozone is one
of the major constituents of photochemical
“smog” which is a widespread, urban
phenomenon. It is formed when nitrogen dioxide
absorbs ultraviolet light energy and dissociates
into nitric oxide and an oxygen atom:
NO 2 hv → O NO (8)Abatement is achieved through the control
of hydrocarbons and nitrogen oxides as
discussed in other sections of this
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