air pollution-part notes
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Air PollutionTRANSCRIPT
Environmental Auditing-Day 2
AIR POLLUTION
ATMOSPHERIC FACTORS
In order to understand topics related to the effects and control of air pollution, it is first
necessary to know something about the composition and physical behaviour of the
atmosphere itself. What does the “pure” atmosphere consist, of, and how do
meteorological or weather conditions affect the mixing and dispersion of pollutants?
COMPOSITION OF THE ATMOSPHEREThe atmosphere comprises of a mixture of many different gases, but mostly it consists of
molecular nitrogen and oxygen. About 78 percent of dry air is nitrogen, and bout 21
percent is oxygen. This is expressed on a column basis. In other words, a container
holding 1000 L of air (at standard pressure) would include about 780 L of nitrogen and
210 L of oxygen.
The nitrogen and oxygen add up to only 990/1000 or 99 percent of the total volume. The
remaining 10 L, or 1 percent of the “pure” atmosphere, normally includes several other
gases. Most of that 1 percent (roughly 0.9 percent) is the inert gas Argon. The rest
includes carbon dioxide, methane, hydrogen, helium, neon, ozone, and other gases in
trace amounts. Figure illustrates the relative amounts of atmospheric gases in graphic
form.
The relative amounts or concentrations of gases in air can be expressed in terms of parts
per million (ppm), as well as in terms of percentage. For example, since 10 000 ppm = 1
percent the oxygen concentration of 21 percent in air can also be expressed as 21 000
ppm. Obviously, it is more convenient to simply express that concentration in percent. On
the other hand, the average global concentration of carbon dioxide, 0.0340 percent, may
be more conveniently expressed as 340 ppm. Natural ozone concentrations can be as low
as 0.02 ppm.
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Figure Molecular nitrogen and oxygen are the main constituents of the atmosphere, but
“clean” air also contains argon, carbon dioxide, and farce amounts of several other gases.
Source: Basic Environmental Technology by JERRY A. NAHANSON. COM
(For private circulation only)
Water vapor is also a normal component of the atmosphere, but the amount may vary
significantly over time and location. Local climate is a major factor that affects the
amount of atmospheric moisture. In humid regions, the water or moisture content of air
may be as high as 5 percent.
Atmospheric Layers
The full atmosphere extends upward roughly 160 km (100 miles) above the surface of the
earth. But the relative composition of gases just outlined pertains only to the troposphere,
which is the lowermost layer of the atmosphere. The troposphere is only about 12 km (8
miles) thick. It is in this relatively thin layer of air that oxygen-dependent life is
sustained, clouds are formed, weather patterns develop, and most of our air pollution
problems occur.
The density of air decreases significantly with an increase in altitude or distance above
the earth’s surface. Consequently, most of the total air mass of the atmosphere is
contained within the lower layer or troposphere. The “skin of the apple” mentioned
previously refers to this life-supporting layer. Above the troposphere, there is not enough
oxygen to support life.
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The layer of air above the troposphere, called the stratosphere, is a stable layer that
extends upward to an altitude of about 30 km (20 miles). Even though it is deeper than
the troposphere, the stratosphere contains only a small fraction of the total air mass,
because of the lower air density. It does, however, contain much more ozone, 03, than the
troposphere.
The ozone in the stratosphere plays an important role in protecting live organisms on the
earth from the sun’s harmful ultraviolet (UV) radiation. The UV rays are absorbed by
ozone molecules and are then converted into heat energy. The ozone in effect, acts as a
protective filter. It is conceivable that an accumulation of certain pollutants (e.g., freon
from aerosol cans) in the stratosphere could react with ozone, diminishing its UV filtering
capacity. There is concern that this may lead to an increase of skin cancer and other
health problems in humans.
Layers of the atmosphere above the stratosphere include the mesosphere, the ionosphere,
and the thermosphere. These portions of the atmosphere are essentially unaffected by air
pollution.
THE EFFECT OF WEATHER
Air pollutants are mixed, dispersed, and diluted in the atmosphere by movement of air
masses, both horizontally and vertically. This air movement, and therefore air quality, is
very dependent upon local meteorological or weather conditions.
Horizontal dispersion of air pollutants depends upon wind speed and direction. The
concentration of pollutants decreases with decreases with increasing wind speed, because
as the pollutants are discharged from the source, they are more rapidly separated and
dispersed by the swiftly moving air. Knowledge of prevailing wind speed and direction in
a given locality makes it possible to select sites for new industrial facilities or power
plants so as to minimize local air pollution effects. Locating such sites downwind of
residential areas is preferable, naturally, to upwind location.
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Temperature Inversion
In addition to wind, another important meteorological factor that has a significant effect
on the dispersion of pollutants is atmospheric stability. The atmosphere is said to be
stable when there is little or no vertical movement of air masses. As a consequence there
is little or no mixing or air pollutants in the vertical direction, and pollutants tend to
accumulate near the ground. Under such conditions of stability, air pollution problems
may become severe.
An unstable atmosphere, on the other hand, is one in which air masses move naturally in
a vertical direction, and carry pollutants upward, away from the ground. A condition of
instability then is preferable to conditions of stability in the atmosphere, with regard to air
quality.
Atmospheric stability depends on the relationship between air temperature and altitude
that prevails at a particular time and location. Normally, in the troposphere, air
temperature decreases with increasing altitudes as you go higher, it gets cooler. The
lower-most layer of the atmosphere is warmed by heat energy reradiated from the earth’s
surface. But the relatively warm air near the surface then tends to rise, as it is displaced
by cooler and denser air from above. This may result in an unstable condition with
constant vertical mixing of air masses, if the rate of temperature decrease with altitude is
sufficient to sustain the mixing process. The rate at which temperature actually changes
with increasing altitude at any given time is called the environmental lapse rate, or simply
the lapse rate. The specific lapse rates the represents the separation or boundary between
a stable and unstable atmosphere is called the adiabatic lapse rate. It is equal to 10C/100
m (-5.48F/100 ft.) The negative sign indicates that air temperature decreases as the
altitude increases.
As long as the environmental lapse rate exceeds the adiabatic lapse rate the atmosphere
will be unstable and vertical mixing of air masses will occur. The colder air from above
will descend as the warmed air rises in a manner similar to the “turnover” of a stratifies
lake in the fall. This condition is illustrated in figure.
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Certain weather patterns can cause the environmental lapse rate to be less than, and other
make it greater than, the adiabatic lapse rate. In fact, under some circumstances, it is
possible for the lapse rate to change direction entirely, that is to represent an increase
rather than a decrease in temperature will altitude. Such a condition is called a
temperature inversion, and it is a most undesirable condition with respect to air quality.
Environmental lapse rate
Line A shown the adiabatic lapse rate, and line E shown an environmental or prevailing
lapse rate. When the air temperature decreases faster than the adiabatic rate, as shown
here, air pollutants are dispersed and diluted in the atmosphere.
When local weather conditions temporarily cause air temperatures to increase with
altitude, an inversion has occurred. The atmosphere is stable during an inversion; air
pollutant levels build up because of the lack of mixing and dispersion in these air.
An inversion is illustrated in Figure. The denser, colder air is trapped below the warmer
air, and vertical motion of air masses is restricted. Since vertical motion is restricted there
is essentially no mixing or dispersion of air pollutants in an upward direction.
In an urban area, air quality will decreases rapidly during this period of stability or
stagnation, until the weather conditions change and the normal lapse rate is restored.
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Temperature inversions can be caused by variety of local meteorological conditions, and
they can occur just about anywhere. But there are certain geographical conditions that can
increases the frequency and duration of these inversions. The situation can be particularly
severe, for example, for a community located in a valley, which acts as a holding basin or
sink for cold, dense air masses near the ground. The surrounding hills also tend to block
horizontal air motion, thus adding to the stagnation problems. The city of Los Angeles,
for example, lies in a mountain-rimmed “bowl” that traps air pollutants during frequent
temperature inversion.
Sometimes a temperature inversion will begin at a certain elevation above the ground
surface, leaving a relatively thin lyre of unstable air below. Such a condition is illustrated
in Figure. This type of inversion forms a “lid” in effect, that traps pollutants and prevents
further vertical mixing. The plume are mixed in the thin but unstable layer near the
ground, causing a condition of fumigation for surrounding communities.
Figure: - When a temperature inversion begins above the ground, because of local
weather conditions, it acts as a lid or ceiling that prevents further vertical mixing and
traps pollutants below it.
TYPES AND SOURCES OF AIR POLLUTANTS
Air pollution may be simply defined as the presence of “foreign” substances in the
atmosphere in high enough concentrations, and for long enough durations, to cause
undesirable effects. What are these so-called foreign substances, where do they come
from, and what are the undesirable effects? In this section, the nature and sources of
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common air pollutants will be discussed, and in the following section, some of the most
undesirable effects will be considered.
First though, we should make a distinction between so-called natural air pollution, and
pollution caused by industry, transportation and other human activities. Not all air
pollution is caused by human activity. In fact, at certain times the pollution from natural
sources can be far more severe and long lasting than pollution from human activity.
Perhaps the most dramatic and recent example of natural air pollution in the United States
was caused by the 1980 eruption of Mount St. Helens, in the state of Washington. Vast
quantities of gases and dust were spewed into the atmosphere in a relatively short period
of time. Local communities, including the city of Portland, Oregon, were blanketed with
volcanic ash for quite a while, In addition to discharges such as those from Mount St.
Helens and other active volcanoes around the world, natural air pollutants include smoke
and gases from forest fires, windblown dust from deserts, salt seaspray pollen grains, and
other naturally occurring substances.
Those substances that are generally recognized to be of major concern as air pollutants
from human activity include the following.
1. Particulates
2. Sulfur dioxide
3. Nitrogen dioxide
4. Carbon monoxide
5. Hydro carbons
6. Ozone
7. Lead
The principal sources of these air pollutants are considered to be either mobile (e.g.,
automobiles.) or stationary (e.g. coal fired electric power generating stations.) The
distinction between mobile and stationary sources of air pollutants is important because
of the different dispersion patterns and pollution control technology applied to each type.
Chemical manufacturing, fuel combustion for heat, and solid waste incineration are
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additional stationery sources, but electric power generation is the most significant with
respect to total emissions.
EFFECTS OF AIR POLLUTION
For discussion, we classify or group the effects of air pollution into five general
categories, according to its effects on;
1. Human health
2. Materials
3. Vegetation, agricultural crops, and livestock
4. Atmospheric conditions
5. Aquatic and terrestrial ecosystems
HUMAN HEALTH
Of primary concern are the adverse effects air pollution has on human health. Generally,
air pollution’s is most harmful to the very old and the very young. Many elderly people
already suffer from some form of lung or heart disease, and their weakened condition
makes them very susceptible to additional harm from pollution. The sensitive respiratory
systems of newborn infants are also susceptible to harm from dirty air. But it is not just
the elderly or the very young who suffer; healthy people of all ages can be adversely
affected by high concentrations of air pollution. In general, major health effects include.
1. Acute (short-term but severe) illness, or death
2. Chronic (long-term) respiratory illness, including bronchitis, emphysema, asthma,
and possibly lung cancer.
3. Temporary eye and throat irritation, coughing, chest pain and malaise or general
discomfort
The intermittent occurrence of exceptionally high air pollutant concentrations in a
community and the acute public health problems that manifest themselves during the
same period is called an air pollution episode. One of the most severe episode of record
occurred in London, is 1952. During a one-week period of very high sulfur dioxide and
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particulate levels, about 4000 “excess deaths” (more than ordinarily would be expected in
that time period) were noted.
In the United States, the first major air pollution episode on record occurred in Donora,
Pennsylvania, in 1984. In only a few days during October of that year, 20 excess death
and about 600 illnesses were attributed to air pollution from local industry. Because of
the relatively small population of 14000 people in Donora, the “per capital death rate”
was actually the highest ever recorded during an air pollution episode.
Many other pollution episodes have occurred in the recent past in many different
countries, and we cannot yet rule out the possibility of their recurrence. In general, a
typical episode lasts about two to seven days and is characterized primarily by stagnant
air and unusually high concentration of SO2 and particulates. The stagnant air results
from temporary weather conditions, including a temperature inversion and negligible
winds speeds. Illness and excess deaths occur in all age groups, but mostly the very old,
the very young, and previously ill persons are affected.
It is difficult for public health experts to match up any specific air pollutant with a
specific disease or health effect, with absolute certainty. But some general conclusions
can drawn from available data. Usually sulfur dioxide, nitrogen dioxide, or ozone cause
eye and throat irritation, coughing, and chest pain. These pungent gases can harm lung
tissue when inhaled into the respiratory tract, and are associated with bronchitis,
emphysema, and other lung diseases.
Inhalation of particulates also affects the breathing process adversely. Although particles
larger that about 1um tend to be captured by the protective mucus lining and cilia (very
small hairs) in the nose and throat, smaller particles can penetrate deep into the lungs.
Certain particulate are especially dangerous because of their toxic or carcinogenic
properties; lead fumes in automobile exhausts and asbestos fibers are only two such
examples.
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Carbon monoxide is a colorless and odorless gas that is virtually unnoticeable to our
senses. But this makes it all the more dangerous because it can be inhaled without
causing irritation or immediate discomfort. It is extremely toxic because it readily
combines with hemoglobin in the blood, and takes up the place ordinarily occupied by
oxygen, which the body needs continuously. The inhaled CO reduces the ability of the
blood to transfer oxygen to body cells, leading to asphyxiation or suffocation.
A CO concentration of about 1000 ppm can cause unconsciousness in a healthy person,
in one hour of exposure; death by asphyxiation will occur in about four hours at that
concentration. Even much lower concentrations can cause illness or reduced mental
awareness; a maximum allowable eight-hour exposure limit for workers in the United
States has been set at 50 ppm. Under certain circumstances, particularly in the immediate
vicinity of heavily congested highways, atmospheric CO levels may reach one-hour
peaks as high as 400 ppm.
MATERIALS
Damage to materials, due to air pollution, occurs continuously in urban areas. It includes
the soiling and deterioration of building surfaces, public statues and other outdoor works
of art, the corrosion of metals, and the weakening and deterioration of textiles and
leather, as well as rubber, nylon, and other synthetic products.
Deposition or settling of particulates on materials is the cause of soiling; the frequent
cleaning of-soiled surfaces and clothing leads to more rapid deterioration. Abrasion,
caused by particulates carried in the wind at high speeds, eventually erodes and wears
away solid surfaces. Examples of direct and irreversible chemical attack include the
cracking of rubber that is exposed to ozone, and the severe discoloration of leaded house
paint that is exposed to hydrogen sulfide gas. Leather becomes brittle when exposed to
sulfur dioxide; the SO2 is absorbed into the leather material; and is converted to sulfuric
acid in the presence of moisture.
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The damage of material by air pollution is not merely an aesthetic problem, but also an
economic problem of major proportions. Although this is not immediately apparent to the
casual observer, it should be noted that the total cost of cleaning and repairing damage
caused by air pollution is estimated to exceed 1 billion per year in the United States.
PLANTS, ANIMALS AND THE ATMOSPHERE
Air pollutants can damage fruits, vegetables, trees and flowers in various ways. Some
pollutants cause collapse of the leaf tissue; others bleach or discolor the leaves. The total
cost of air pollution damage to agricultural hundred million dollars per year in the United
States. Certain air pollutants also cause harm to cattle and other livestock, but this is
usually a localized problem on farms near specific industrial plants that cause the
pollution.
To the general public, the most noticeable effect of air pollution is on the atmosphere
itself. Specifically, it is the haze and reduction of visibility due to the scattering of light
by suspended particles. Particulates can also affect weather conditions by increasing the
frequency of fog formation and rainfall.
GREENHOUSE EFFECT
At the present time, it seems that any increase in the earths reflectively is being
counterbalanced by a phenomenon called the greenhouse effect. The greenhouse effect is
caused by carbon dioxide, CO2 which is not ordinarily considered to be an air pollutant.
In fact, it is a normal although minor component of the atmosphere, with an average
concentration of about 0.034 percent or 340 ppm. And it does not cause any adverse
effects on human health.
But carbon dioxide is released into the atmosphere in vast quantities as a by-product of
fossil fuel combustion (foal, oil, gas), which is used in industrial activity and power
generation. It is estimated that the average worldwide concentration of carbon dioxide is
increasing at a rate of almost 1 ppm per year. This does not cause a public health hazard,
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nor does it cause damage to plants, animals, or materials. What, then, is the problem with
atmospheric carbon dioxide, and why is it called the greenhouse effect?
Carbon dioxide molecules in the air absorb the heat energy reradiated from the earth’s
surface. The energy coming from the sun is able to penetrate the atmosphere. But when
the warmed surface of the earth radiated some of the energy back into space, it is trapped
by the carbon dioxide in the troposphere, as if it were a blanket of insulation, or the glass
enclosure of a greenhouse. This is illustrated in Figure. As the CO2 concentration
increases, less heat will escape through the troposphere, and average global temperatures
will increase.
The greenhouse effect should not be dismissed as an example of scientific speculation or
environmentalists’ “ doomsday” propaganda. Two independent federal studies published
in 1983, one by the Environmental Protection Agency and the other by the National
Academy of Sciences, concluded that the warming trend is both imminent and inevitable.
It is expected that global temperatures will increase by about 2 C (3.6 F) within the next
50 years, and by as much as 15 C (27 F) by the year 2100.
Figure. Energy from the sun can penetrate the atmosphere to warm the earth. But the
type of heat energy radiated back from the earth is absorbed by carbon dioxide from
combustion will lead to an increase in atmospheric temperatures, called the greenhouse
effect.
Both studies also concluded that even if the use of fossil fuel was banned as of today, the
greenhouse effect would not be halted or reversed, there is no known strategy that will
mitigate the problem. The only alternative is to plan for ways to cope effectively with the
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changes in climate that are expected to accompany the warming of the atmosphere. Some
of these changes may be beneficial; for example, agricultural production may be
improved in certain regions because of a longer growing season and more efficient
photosynthesis. On the other hand, the melting of the Arctic ice packs is expected to raise
the sea level by about 1 m (3 ft)’ this will cause extensive economic and social hardship
in coastal areas all over the world.
ACID RAIN
A current environmental issue of major public concern is the problem knows as acid rain.
The description “acid rain” refers to the fact that the average pH of rainfall has been
decreasing significantly below its normal value, in recent years. The strength of an acidic
solution is measured by its pH value. Briefly, values of pH range between 0 and 14. With
a pH of 7 representing a neutral condition. Values less than 7 indicate acidic conditions;
the lower the pH is, the stronger the acid is.
“Pure” rain, in rural areas far removed from human activity, has some natural acidity,
with a pH of bout 5.5. This is primarily from the formation of carbonic acid, H2CO3, by
the reaction of moisture and carbon dioxide in the atmosphere. But recent scientific
studies show that in urban and industrial areas of the United States, and in other
countries, the average pH of rain is less than 4.5. (A pH of 2.2 as acidic as vinegar, was
recorded during a rainfall in Scotland in 1974.) On the logarithmic pH scale, a drop of
one pH unit represents an increase in acidity by a factor of 10. What is the relationship
between acid rain and air quality, and what are the adverse effects of acid rain?
The fact that sulfur dioxide reacts with water vapor to form a mist of sulfuric acid was
already discussed. Nitrogen dioxide also reacts with atmospheric moisture to form nitric
acid. Oxides of sulfur and nitrogen are among the major air pollutants and their primary
source is power-generating stations. The atmospheric mists of sulfuric and nitric acid
eventually reach the surface of the earth in the form of rainfall, dew. or snow. There are
several environmental problems attributed to this excessively acidic precipitation,
including contamination and damage of;
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1. Fresh water lakes
2. Forests
3. Agricultural crops
4. Drinking water
5. Materials
Many species of fish, trees and agricultural crops are very sensitive to pH values and do
not thrive under acidic conditions Hundreds of lakes in certain regions of the United
States as well as in many other countries, no longer support fish life, most scientists agree
that the death” of these once productive lakes is directly attributable to acid rainfall Acid
rain also accelerates the rate at which minerals leach out of the soil. This reduces soil
fertility, diminishing the growth and productivity of forests and agricultural crops.
Leaching of certain metals from the soil into the groundwater may also contaminate some
drinking water supplies. Finally, acid rainfall undoubtedly speeds up the physical
deterioration of concrete, metal, and other exposed material.
A factor that complicates the acid rain problem is that most of the sulfur and nitrogen
dioxide is emitted from the tall smokestacks or chimneys at power generating plants. The
purpose of the tall stacks is to increase the dispersion and dilution of the stack gases and
to protect the surrounding community from high levels of air pollution. But discharge
from these tall chimneys allows the pollutants to be carried long distances in the
atmosphere. The pollution is, in effect, transferred by “air mail” to other regions of the
country. For example, most of the acid rain falling in the north-eastern region of the
United States is believed to be the result of fossil fuel combustion by industries and
power plants located in the Midwestern section of the nation. About 16 million tons of
sulfur emissions each year come from the Midwest. Also, acid rain the Norway is
believed to come from industrial areas in England and in continental Europe.
Acid rain is one of the most controversial environmental issues in recent times. In 1984,
several northeastern states petitioned the Environmental Protection Agency to order the
reduction of emissions from coal-burning power plants in the Midwest. The request was
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denied by the EPA, on the basis that the existing requirements of the Clean Air Act were
not being violated.
The political atmosphere of the early 1980 was one that generally supported further
research rather than immediate (and possibly expensive) control of the problem. It was
thought by some that acidity in lakes is not simply caused by acid rain. In some cases,
though, corrective action was initiated on a statewide basis. For example, in 1984 New
York State was the first to require a 30 percent reduction of sulfur emissions by industry
and power utilizes, specifically to help mitigate the acid rain problem. Finally, in 1986, a
5 billion program to develop cleaner coal-burning technology was endorsed by the
federal government; this was part of a joint Canadian-United States effort to control acid
rain.
AIR SAMPLING AND MEASURMENT
In order to evaluate air quality and to design appropriate air pollution control systems, it
is necessary to measures the amount or concentration of the various pollutants. First, of
course, an appropriate sample must be collected. There are basically two different
approaches for sampling and measuring air pollutants One involves the sampling and
analysis of surrounding “outdoor” or ambient air quality. The other involves the sampling
and analysis of specific emissions at their point of generation, and may be referred to as
source sampling or emissions analysis.
Ambient Air Quality
Ambient samples are collected from the open atmosphere, after pollutants form various
sources have been dispersed and mixed together under natural meteorological conditions.
Ambient, or atmospheric sampling, as it is sometimes called, serves several purposes. It
provides “ background” air quality data in urban or rural areas and a basis for dev eloping
and updating ambient air quality standards.
Monitoring ambient air quality also provides data of determine if established standards
are being met or exceeded. Impending air pollution episodes or emergencies can be
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predicted in advance, by examining ambient air quality along with meteorological data;
this provides time for health officials to warn the public.
Even though samples are taken from the “open air” it is most important that the sampling
duration and location be representative of the particular study area and type of pollutant
being examined.
Source Sampling
Source or emissions sampling is performed right at the point or pollutant discharge such
as at a vehicle tailpipe or a smokestack. In fact, it is often called stack sampling at power
plants where discharge is from a chimney. A basic purpose of source sampling is to
evaluate the pollution discharged from a specific generator and to use the results to
determine if the so-called emission standards are being met or compiled with. Other
purposes of emissions sampling are to provide data for designing and operating air
cleaning equipment and to measure the working efficiency of that equipment.
For accurate and meaningful results, stack samples must be isokinetic; that is, collected
by a probe at the same rate at which the gas leaves the stack. The equipment used for this
purpose is called a sampling train, and it includes several interconnected devices. The
basic components are a pitotube probe, a vacuum pump to pull the sample out of the
stack, a flow meter, and a meter to measure the weight or mass of a specific pollutant in
the sample. The temperature of the gas must also be determined. A typical sampling train
is illustrated in Figure.
PARTICULATES
Measurement of particulate air pollutants may be accomplished by several methods,
including a gravity technique, and an inertial technique.
The gravity technique is the simplest method, but it can only measure the amount of
settable particulates (dust and fly ash) in the air. A simple device called a dust fall bucket
has been used for this purpose. The open bucket, containing water to trap and hold the
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particles, is left exposed in a suitable location, often on a building rooftop. After a
collection period of 30 days the water is evaporated and the dust is weighed.
The measurement results for settable particulates may be expressed in terms of grams per
square mile per month (g/m2 month), or more typically, as tons per square mile per month
(tonsmile2/month), based upon the open top area of the collecting bucket. The total
amount of dust that will settle out of the atmosphere in an urban area can be quite high; as
much as 50 tons/mile 2/month of dustfall have been observed in some cities.
Suspended particles that are too small to settle out of the air by gravity can be collected
using the filtration technique. A common filtration apparatus, called the high-volume
sampler, is shown in Figure. It acts basically as a vacuum cleaner, except that the air
stream first passes through a special leak-proof, glass-fiber filter before it reaches the fan.
All the suspended particulates in the air stream are trapped on the filter. Which is
weighed before and after the sampling period. The difference represents the weight of the
total suspended particulate (TSP).
The sampling duration is typically 24 hours, in which time about 2000 m3 (70 000 ft3) of
air is pulled through the filter. The air flow rate, which gradually decreases as particulates
accumulate on the filter, is metered and recorded. The measured TSP concentration is
typically expressed in terms of micrograms per cubic meter, ug/m3, although peak values
may reach several hundred ug/m3, out in the “country” TSP levels are generally about 30
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ug/m3. Expressing TSP levels in terms of micrograms can give erroneous impression that
the quantities of material are exceedingly small or negligible. It should be noted that a
TSP value of 200 ug/m3 is roughly equivalent to almost one ton of particles per cubic
mile.
Another filtration-type instrument used to collect and measure suspended particulates is
called the paper tape sampler. Sampling durations with this device are relatively short,
typically two hours. A vacuum pump pulls the air stream through a filter tape, which
moves automatically on a reel is illustrated in Figure.
Figure:- A sheltered high-volume (hi-vol) air sampler, used top analyze suspended
particulate levels. (General Metal Works, inc., A Subsidiary of Andersen Samplers, inc.)
Trapped particulate form a dark spot on the tape, and the amount of particulate correlates
with the darkness of the spot. The relative darkness of the spot is measured by an optical
device called a transmissometer, which gives a reading in terms of the percentage of light
that can pass through the tape. Final results are then expressed in terms of a coefficient o
haze (COH). A value of 1 COH unit is equivalent to an “optical density” of 0.01.
Say, for example, that after a 2-hr sampling period, the amount of light passing through
the clean tape in three times more than the light passing through the spot for trapped
particulates. That ratio is called the “opacity” of the spot. The optical density is equal to
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the logarithm of the opacity. In this example, the logarithm of 3, or 0.477 is the optical
density of the spot.
Since a COH of 1 is equal to an optical density of 0.01, the COH of the air sample in this
case is 48. This can be converted to COH/1000 linear feet, depending on the area of the
spot and the volume of the sample. At a particular location, COH1000 ft values can be
used to monitor hourly fluctuations in particulate air pollution, throughout the day.
However, there is no definite relationship between COH/1000 ft and ug/m3 of
particulates. An advantage of the paper tape sampler is that it is portable and yields
quicker results than the high-volume sampler.
Figure:- A paper tape sampler. Air is pulled through a strip of filter paper that traps
particulates. Particulate levels are measured by a light transmissometer. (RAC Division,
Andersen Samplers, Inc.
The third method of sampling, referred to as the inertial technique, makes use of an
obstacle, placed in the path of the air stream. The air flows around the obstacle, but
because of inertia, the particulates collide with it and become trapped in the device. Once
of the simplest such device is the so-called sticky tape sampler, illustrated in figure. It can
be used to collect and measures TSP, as well as to give an indication of prevailing wind
direction, the particles collide with and stick on the tape as they are carried by the wind.
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Figure: - A sticky tape sampler is a simple inertial device for sampling and measuring
particulates and for obtaining results as a function of wind direction.
Other types of inertial devices are used to collect and analyze specific particulates, such
as pollen grains or bacteria. The cascade impactor, for example, traps particles on a series
of slides that are placed in the air stream. This is illustrated in Figure. The orifice
openings through which the air flows are decreased, thereby increasing the velocity.
Particles of different sizes are captured on each slide, because of their inertia, the sudden
change in direction of flow, and the different flow velocities. The particulate can be
observed on the slides with a microscope.
Smoke Readings
Visual evaluation of smokes plumes that are discharged from a stack or chimney are
made with a so-called Ringlemann Chart, such as the one illustrated in Figure. The
density or darkness of the smoke is compared to the five standard shades of gray on the
chart; Ringlemann smoke readings range from all white (0) to all black (5).
Even though pollutant concentrations are not necessarily correlated exactly with the
shade or darkness of a smoke plume. Ringlemann readings are of value in monitoring air
pollution, and some air quality regulations are still based on smoke density. It should also
be noted that the ability to obtain accurate and consistent readings is not an easy task. At
least one day of special training is required for a technician to be able to use the
Renglemann chart.
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Figure: - A cascade impactor for collecting and analyzing particulate air pollutants.
(From Vesilind P.A. and J.J. Peirce, Environmental Pollution and Control, with
permission of Ann Arbor Science Publishers.
Figure: - A Ringlemann type smoke chart. (Plibrico Company, Chicago, llinois)
GASEOUS POLLUTANTS
The physical properties and behavior of gases differ markedly from those of particulates.
One important example is the fact that gas molecules are small enough to pass through
the finest filter.
Two techniques for sampling and measuring the amounts of gases in the atmosphere
involve either absorption or adsorption. The process of absorption involves the contact
and trapping of the gas molecules throughout the volume of a liquid, usually by chemical
reaction. The process of adsorption, on the other hand, involves the contact and trapping
of the gas molecules on the surface of a solid substance.
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Absorption of a specific gas from the air may be accomplished with a simple device
called a bubbler, as illustrated in Figure. The air is pumped through a small diffuser and
bubbled up through a liquid, which will either dissolve the gas under study, or react with
it chemically. For example, if a measured volume of air containing sulfur dioxide is
bubbled through hydrogen peroxide, H2 O2 then sulfuric acid is quickly formed, as
described by the following chemical equation.
H2 O2 + SO2 ------ H2SO4
The amount of sulfuric acid that is formed in the reaction can be measured by standard
chemical techniques; from that, the amount and concentration of the sulfur dioxide in the
air sample can be computed.
Figure: - A glass “bubbler” or absorber may be used for sampling specific gaseous
pollutants. For example, hydrogen peroxide will absorb sulfur dioxide from the air,
forming sulfuric acid. The level of sulfur dioxide in the air can be computed after
measuring the amount of sulfuric acid in the bubbler.
An absorption instrument called a twenty-four hour bubbler, shown in Figure, can be
used to test for three different gases at the same time. Separate sampling trains with
suitable collecting liquids in the bubbler are connected in parallel to a vacuum pump. The
rate of airflow can be controlled and measured. A similar device, called a sequential
sampler, can be used to collect up to 12 samples in sequence for fixed periods of time,
typically 2 hours. The sequential sampler allows peak concentrations of a specific
pollutant to be determined on a daily basis.
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In adsorption instruments, the gas molecules are attracted to the surface of a solid and
held there by molecular bonding forces. Activated carbon is usually used as the adsorbent
material; it is a porous solid with a very high surface-area-to-volume ratio. Other
materials, such as silica gels and alumina are also sometimes used as adsorbents. The
adsorbent is percolated with a chemical that reacts with and changes color in proportion
to the amount of gas adsorbed. For example, the adsorbent in a carbon monoxide detector
tube will change from yellow to blue-green, as air containing C O passes through the
tube. The C O concentration can be measured by comparing the tube color to a calibrated
color chart.
Figure: - (a) A three-gas sampler, and (b) the sampler in an all-weather shelter, (RAC
Division, Anderson Samplers Inc.)
Sometimes it is necessary to collect a small sample of air at a particular location for
subsequent analysis in a laboratory. This may have to be done using a minimum of
equipment, by an inexperienced technician. One way of collecting a grab sample, as it is
called, is to utilize an evacuated flack; when the flask is opened at the sampling location,
the air sample is drawn into it by the vacuum.
Another type of grab sampling device that is effective if the gas under study is insoluble
is the liquid-displacement collector, illustrated in Figure. An air sample is drawn in at the
top of the container, displacing the liquid that is drained out at the bottom. In general,
grab samples are present in the air, since the collected volumes are not large enough for
accurate analysis.
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On the other end of the spectrum are the modern and sophisticated continuous monitoring
(CM) instruments. These instruments combine collection and automatic analysis for
many different air pollutants. Electronic detectors, meters, and recording devices are part
of the sampling train of this equipment. Continuous graphs showing the hourly change in
pollutant levels or concentrations can be obtained. Expensive CM equipment is used in
heavily polluted urban areas, as part of an episode warning system.
Figure: - A liquid displacement collector may be used to obtain a grab sample of air for
later analysis in a laboratory analysis in a laboratory. The gaseous pollutant to be
measured should not react or dissolve in the liquid that is used.
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