NITROGEN OXIDES REDUCTION 747
In addition to vegetative kill, nitrogen dioxide is an
extremely corrosive gas that can have deleterious effects on
a wide range of materials such as plastics, fabrics, rubber
and metals. Studies on Nylon-6 and Kevlar have revealed
that the ultimate tensile strengths of these materials are
reduced when exposed to a NO x environment in the range of
0.5–0.8% volume concentration.Global Warming/Greenhouse EffectThe greenhouse effect occurs due to the buildup of gases
which can absorb heat. The earth maintains a constant aver-
age temperature by radiating heat to space; thus, greenhouse
gases absorb a portion of this heat and radiate it to the lower
atmosphere. Scientists believe that the heat is trapped in the
lower atmosphere, resulting in global warming, rising sea
level and global climatological alterations. There is consid-
erable debate over the global warming issue when viewed
from the geologic time scale. In any case, there has been
considerable research done in this field and it will continue
to be a topic of concern for years to come.
Principally, carbon dioxide is the most important green-
house gas, contributing to roughly half of the global warming
that has been reported. The other half of the global warming
has been attributed to approximately 20 other gases, most
notably methane, chlorofluorocarbons (CFC-11 and CFC-12),
ozone, nitrous oxide, and nitrogen dioxides. Some scientists
have estimated that a 50% increase in the current concentra-
tion of N 2 O will result in a mean global temperature increase
of 0.2–0.5C.^4 Of the nitrous oxide emissions, 25% has been
attributed to fossil fuel combustion. If these predictions are
accurate, then it becomes even more important to control
the emissions of nitrogen oxides from combustion related
processes.Acid Rain DepositionAcid rain forms when sulfur dioxide and nitrogen oxides
mix with water vapor to form sulfuric and nitric acid. In par-
ticular, nitrogen oxides are transformed into nitric acid by
the following reactions:^5(^)
2NO O 2NO
NO O NO O
2NO 2H O O 4HNO
22
322
22 2 3
+→
+→ +
++→.
On an annual basis, nitric acid is responsible for approximately
30% of the acidity of rainfall. This percentage increases to
around 50% during the winter. Acid rain is one of the most
damaging effects of NO x emissions. It leads to the destruc-
tion of ecosystems in lakes, deforestation, and the stripping
of organic material in soils, creating erosion and potentially
metal laden soils. In addition, NO x emissions contribute
substantially to the acid pulse in snowmelt, which in turn
severely impacts the freshwater ecosystem. Therefore, acid
rain is yet another reason for controlling and limiting NO x
emissions to the atmosphere.
Ozone Formation
In addition to being a cause of acid rain, nitrogen oxides are
also considered one of two precursors to the formation of pri-
mary ozone, O 3. Ozone is thought to be formed from the com-
plex reaction of certain hydrocarbons and nitrogen oxides.
The role of nitrogen oxides in ozone formation is
significant because of the health effects associated with ele-
vated levels of ozone. Ozone exposure can lead to coughing
and chest discomfort, headaches, upper respiratory illness,
reduced pulmonary function, eye irritation, and increased
asthma attacks. For ozone, the NIOSH ceiling exposure limit
is 0.1 ppm; the OSHA threshold limit value (TLV) for an
8-hour time weighted average (TWA) exposure is 0.1 ppm.
The OSHA short-term exposure limit is 0.3 ppm.^2 The
immediately dangerous to life or health (IDLH) concentra-
tion for ozone is 10 ppm, which is five times less than the
IDLH for nitrogen dioxide and nitric oxide, respectively. In
addition to human health effects, studies have shown that
ozone damages agricultural crops and forest ecosystems.
NO x REGULATIONS
Stationary Source Regulations
Because of the harmful effects associated with nitrogen
oxides, governments around the world have established
increasingly stricter regulations over the past few decades
to curb and reduce the amount of NO x emissions. In par-
ticular, the United States federal government enacted the
Clean Air Act (CAA) to protect the nation’s air quality. The
CAA was first passed into legislation in 1970, with substan-
tial amendments being added in 1977 and most recently in
- The amendments in 1990 greatly increased the scope
of the existing CAA. The CAA is divided into seven primary
Titles, I–VII. Some of he new major points in these Titles
are as follows: 6, 7
- Title I —A new system was established to deter-
mine if an area is classified as either an ozone
attainment or an ozone nonattainment area. The
CAA effectively divides the United States into
attainment and nonattainment areas, based on the
level of criteria pollutants in the area’s ambient
air. Because of the formation of ozone from vola-
tile organic compounds (VOC’s) and NO x , Title I
requires more stringent requirements for these
two classes for compounds. - Title II —Clean fuel requirements and increased
restrictions on motor vehicle emissions were
introduced in Title II. Under Title II, new vehicle
tailpipe emissions of NOx were to be reduced by
60%, starting with 40% of all new vehicles in the
1994 model year and increasing to 100% by the
1998 model year. - Title III —Emission limitations for 189 hazard-
ous air pollutants (HAP’s) are to be set by the
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