Encyclopedia of Environmental Science and Engineering, Volume I and II

746

NITROGEN OXIDES REDUCTION

INTRODUCTION

Nitrogen oxides are one of the most persistent categories of

globally emitted air pollutants because they are combustion

products of both stationary and mobile sources. By far the

highest concentration of the oxides formed during combus-

tion are NO and NO 2. Given time, and in the presence of O 2

either in the flue gases or in the atmosphere most of the NO

is converted to NO 2.

There are six commonly encountered oxides of nitrogen:

(1) nitric oxide (NO); (2) nitrous oxide (N 2 O); (3) nitrogen

dioxide (NO 2 ); (4) nitrogen trioxide (N 2 O 3 ); (5) nitrogen

tetroxide (N 2 O 4 ); (6) nitrogen pentoxide (N 2 O 5 ). Typically,

nitrogen oxides, or NO x , refers to a collective name for

nitric oxide, NO, and nitrogen dioxide NO 2 , both of which

are formed as a by-product during combustion. Nitric oxide

is a colorless, odorless, toxic, nonflammable gas which is

slightly soluble in water. Nitrogen dioxide, however, is a

reddish-brown gas that is toxic and highly corrosive with

a pungent odor. This gas can contribute to highly visible

plumes. Emissions of nitrogen oxides are of concern due to

their potential role in ozone formation, acid rain deposition,

health effects, and formation of toxic air pollutants.

EFFECTS OF NO X

Human and Animal Health Effects

Of the two compounds, nitrogen dioxide is the most toxic and

dangerous to humans. A variety of studies has been performed

to observe the effects of NO 2 on humans and animals. Most of

these studies have been performed using pure NO 2. Effects of

acute NO 2 exposure have been reported as nose and eye irri-

tation, obliterative bronchiolitis, pulmonary congestion and

edema, pneumonitis, and death. Most of these reactions, such

as pulmonary edema and obliterative bronchiolitis, can occur

at extremely high concentrations (150–500 ppm) for short

periods of time minutes to an hour. It appears, however, that

mixtures of oxides tend to lessen the discomfort and the poten-

tial to contract severe disorders. It has also been shown that

chronic, intermittent exposure to NO 2 (10–40 ppm) can result

in chronic pulmonary fibrosis and emphysema.^1 In animal

studies, continuous exposure to NO 2 for 90 days at 5 ppm has

resulted in the deaths of 18% of the rats, 13% of the mice and

66% of the rabbits.^1 On the other hand, intermittent exposure

for 18 months at the same level (5 ppm) did not result in any

deaths. In addition to the chronic and acute effects, it appears

that daily exposure to NO 2 concentrations of 5 ppm can lead to

slightly accelerated lung tumor formation, but not at a level of

any statistical significance.

On the other hand, exposure strictly to nitric oxide has

not been reported to result in human poisoning probably

due to its relatively low toxicity and its conversion to NO 2.

The relative toxicity can be seen in the exposure standards

set by NIOSH (National Institute for Occupational Safety

and Health) and OSHA (Occupational Safety and Health

Administration). For nitric oxide, the threshold limit value

(TLV) for an 8-hour time weighted average (TWA) expo-

sure is 25 ppm.^2 For nitrogen dioxide, the TLV is 1 ppm for

a short-term exposure limit (STEL). The short-term expo-

sure limit is a 15-minute TWA exposure that should not be

exceeded at any time during a workday. Likewise, the imme-

diately dangerous to life or health (IDLH) concentration for

nitric oxide is 100 ppm, whereas for nitrogen dioxide, the

concentration is 50 ppm.^2 Toxicologists have reported that

nitric oxide can be a mild nose, eye and throat irritant. At

high concentrations, nitric oxide can lead to a progressive

depression of the central nervous system.^1

Environmental Effects

Not only does NO x affect the human population, but it also

has an adverse effect on the environment, in particular vege-

tation. Gaseous pollutants damage plants by entering through

the stomata during the respiration cycle.^3 The pollutants can

disrupt the photosynthesis process and can destroy plant

chlorophyll. Experiments have shown that concentrations of

NO 2 as low as 0.5 ppm can result in reduced plant growth by

as much as 35%. In particular, it appears that plants are more

susceptible to nitrogen dioxide effects at night than during

the day. Scientists feel that NO x has played a significant role

in the deforestation of central Europe.

Not only does direct exposure to nitrogen oxides result

in plant deterioration, but nitrogen oxides combine with

certain hydrocarbons to form ozone and peroxyacyl nitrates

(PAN’s), two compounds that have been found be more

toxic towards plants than nitrogen oxides alone. Exposure to

these compounds has been shown to result in plant growth

suppression, bleaching, glazing and silvering on the lower

surface of the leaves.^3

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