<strong>Characteristics:</strong> Nitrogen (N<sub>2</sub>) is an
inert (non-reactive) gas that makes up about 80 percent of the
atmosphere. Nitrogen gas is an essential element to plant
metabolism and is harmless to humans. However, at high
temperatures (e.g., gas, oil and coal combustion) and under
certain conditions, nitrogen combines with oxygen
(O<sub>2</sub>), forming various oxides of nitrogen
(NO<sub>x</sub>). Nitrogen oxides are an important precursor to
both ozone and acid rain, and may affect both terrestrial and
aquatic ecosystems. Among all nitrogen oxides, nitrogen dioxide
(NO<sub>2</sub>), the criteria pollutant, and nitric oxide (NO)
are the most serious air pollutants.
<p>
Nitrogen dioxide is a colored gas, yellowish-orange at low
temperatures and reddish-brown at relatively high
concentrations.
The relatively toxic NO<sub>2</sub> has a pungent, irritating
odor. Because of its high oxidation rate, it is extremely
corrosive.
<p>
<strong>Sources:</strong> Nitrogen oxides form in
high-temperature fuel combustion. Only 10 percent of nitrogen
oxides are emitted in the form of NO<sub>2</sub>, with the rest
emitted as gaseous NO. The major mechanism for the formation of
NO<sub>2</sub> in the atmosphere is the oxidation of NO.
Because
NO and NO<sub>2</sub> are photochemically interchangable, they
are usually collectively designated as NO<sub>x</sub>. The
primary sources of NO<sub>x</sub> are transportation and
stationary fueld combustion sources, such as electric utilities
and industrial boilers.
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<strong>Concentration:</strong> Background concentration of NO
and NO<sub>2</sub> are approximately 0.5 and 1 parts per billion
(ppb), respectively. In urban areas, one-hour average
concentrations of NO may reach 1-2 parts per million (ppm), with
maximum NO<sub>2</sub> levels of about 0.5 ppm. As pollutant
urban air spreads to rural areas, NO<sub>x</sub> concentrations
drop to near background levels.
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Atmospheric levels of NO and NO<sub>2</sub> show daily
variations
reltaed to the human transportation/work cycle. Maximum
concentrations of NO are observed in early morning hours (6 a.m.
to 8 a.m.), followed by a second peak later in the day (4 p.m.
to
6 p.m.). High morning concentrations of NO are followed several
hours later by peak levels of NO<sub>2</sub> produced by
oxidation of NO.
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Seasonal trends can also be observed. Emissions of NO increase
in winter months, when there is higher consumption of heating
fuel. The warm and sunny days of summer bring higher
NO<sub>2</sub> levels, due to photochemical oxidation of NO.
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<strong>Health Effects:</strong> Nitric oxide is a
non-irritating gas with little health threat at ambient air
concentrations. However, NO rapidly oxidizes to NO<sub>2</sub>,
which is toxic.
<p>
Nitrogen dioxide can penetrate deep into the lungs and damage
the
delicate structure of lung tissues. Increased respiratory
disease has been detected at exposure levels of 0.5 ppm.
Exposure at higher levels can cause more severe lung damage,
which in laboratory animals resembles emphysema. There is also
evidence indicating that NO<sub>2</sub> can damage respiratory
defense mechanisms, allowing bacteria to proliferate and invade
lung tissues. Thus, NO<sub>2</sub> lowers the resistance of
animals to such diseases as pneumonia and influenza.
<p>
The health effects of exposure to NO<sub>2</sub> are similar to
those of ozone. Short-term exposure to concentrations greater
than 3ppm decreases lung function; levels as low as 0.1 ppm can
irritate lungs; and long-term exposure can destroy lung tissue,
leading to emphysema.
<p>
<strong>Ecological Effects</strong> Nitrogen oxides
(NO<sub>x</sub>) may cause direct and indirect injury to plants
and animals. In vegetation, NO<sub>x</sub> can cause bleaching
of plant tissue (chlorosis), death of plant tissue, loss of
leaves and reduced growth rate. Chronic exposure to relatively
low levels of NO<sub>2</sub> (0.5 ppm for 10-20 days) can
supress
growth in some species.
<p>
NO<sub>x</sub> may react with water to produce acids that are a
component of acid precipitation. Acid rain affects both
terrestrial and aquatic ecosystems. Acidic rain, fog and snow
can harm fish, damage forests and contribute to the
deterioriation of buildings and historical monuments.
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Exposure to acid aerosols and acid-forming gases such as
NO<sub>x</sub> may result in deterioration of fabrics and fading
of fabrid dyes. Nitrate salts formed from NO<sub>x</sub> can
corrode metals.
<p>
NO<sub>x</sub> can also react with other pollutants, such as
hydrocarbons, to produce photochemical oxidants, including ozone
and a group of chemicals called peroxyacetyl nitrates (PAN).
These unstable compounds can damage plants and irritate the eyes
and respiratory system.