air pollution-part notes

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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air pollution-part notes

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