unit v air pollution_6th civil

7/31/2019 UNIT v Air Pollution_6th Civil

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Dr. Ajay Kumar Agrawal, Rungta College of Engineering & Technology, Raipur Page 1

UNIT V

AIR POLLUTION

INTRODUCTION

Air Pollution is basically the presence of foreign substances in air in excessive concentration

which adversely affects the well being of the individual or causes damage to property.

Some Definitions:

1. According to WHO: - Air pollution is substances put into air by the activity of mankindinto concentration sufficient to cause harmful effect to his health, vegetables, property

or to interfere with the enjoyment of his property.2. According to Bureau of Indian Standards (BIS): - Air pollution is the presence in ambient

atmosphere of substances resulting from the activity of man, in sufficient concentration,

present for a sufficient time and under circumstances which interfere significantly with

the conformity, health or welfare of persons or with the full use or enjoyment of

property.

3. According to U. S. Public health service: - Air pollution may be defined as the presencein the outdoor atmosphere of one or more contaminants or combination thereof in such

quantities and of such duration as may be, or may tend to be injurious to human, plant or

animal life, or property, or which unreasonably interfere with the comfortable enjoyment

of life, or property, or the conduct of business.

SOURCES OF AIR POLLUTION

Air pollution may be personal, occupational or community level. The sources of air pollution

are natural and manmade (anthropogenic):-

1. Natural Sources: - It includes pollen grains, marsh gases, dust and smoke (from forestfires and volcanic ash) emitted in to the Atmosphere.

2. Manmade (anthropogenic sources): - Anthropogenic air pollutants enter theatmosphere from stationary and mobile sources. The stationary source include larger

factories, electrical power plants, mineral smelters, petroleum refineries and different


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small scale industries, while mobile sources include all sorts of transport vehicles moving

by road, rail and air.

CLASSIFICATION OF AIR POLLUTANTS

The air pollutants may be classified in different ways as follows:

1. According to Origin:(a) Primary pollutants which are directly emitted into the atmosphere and are found

as such, e.g., CO, NO2, SO2, hydrocarbons, and particulate matter.

(b)Secondary pollutants which are derived from the primary pollutants due tochemical or photochemical reactions in the atmosphere, e.g., Ozone,

Peroxyacetyl nitrate (PAN), photochemical smog etc.

2. According to chemical composition:(a) Organic pollutants: - hydrocarbons, aldehyde, ketones, amines and alcohols.(b) Inorganic pollutants:

Carbon compounds: - CO and Carbonates

Nitrogen compounds: - NOx and NH3

Sulphur compounds: - H2S, SO2, SO3 and H2SO4

Halogen Compounds: - HF, HCl, and metallic fluorides

Oxidising agents: - ozoneInorganic particles: - fly ash, silica, asbestos and dusts from transport, mining,

metallurgical and other industrial activities

3. According to state of matter: -(a) Gaseous pollutants which get mixed with the air and do not normally settle out,

e.g. CO, NOx, and SO2.

(b)Particulate pollutants which comprise of finally divided solids or liquids and oftenexist in colloidal state as aerosols, e.g., smoke, fumes, dust, mist, fog, smog and

sprays.

EFFECTS OF AIR POLLUTANTS: -

1. Damage to materials: - the materials that may be affected by air pollutants includemetals, building materials, rubbers, elastomers, paper, textiles, leather, dyes, glass,


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enamels and surface coatings. The types of possible damage to these materials by air

pollutants include corrosion, abrasion, deposition, direct chemical and indirect chemical

attack. The intensity of damage depends upon factors such as moisture, temperature,

sunlight, air movement and of course the nature and concentration of the pollutant.

2. Damage to vegetation: - Air pollutants, such as SO2, particulate fluorides, smog, oxidantslike ozone, ethylene (from automobiles), NOx, Cl2 and herbicide and weedicide sprays

exert toxic effects on vegetation. The damage usually manifests in the form of visual

injury such as chlorotic marking, banding, silvering, bronzing of the underside of the leaf.

Retardation of plant growth may also occur in some cases. The extent of damage to

plants depends upon the nature and concentration of the pollutant, time of exposure,

soil and plant condition, stages of growth, relative humidity and the extent of sunlight.

3.

Damage to farm animals: - Arsenic, lead and fluorides are the main pollutant which causedamage to livestock. These airborne contaminants accumulate in vegetation and forage

and poison the animals when eaten by them.

Arsenic occurs as an impurity in coal and many ores. It is also used in insecticides.

Livestock near smelting and other industrial operations suffer arsenic poisoning with

symptoms like salivation, thirst, liver necrosis, inflammation and depression of central

nervous system.

Lead is emitted from metallurgical smelters, coke ovens and coal combustion

operations, lead arsenate sprays and automobile exhausts. Lead poisoning occurs in

horses and other animals with symptoms such as depression, lethargy, gastritis, paralysis

and breathing troubles.

Cattle and sheep are particularly susceptible to fluorine toxicity which may

cause fluorosis of teeth and bones.

4. Effect on Aquatic life: - Air pollutants mixing up with rain cause acid rain which adversalyaffects aquatic life. Most planktons, molluses and fish cannot tolerate water having pH

below 5.0. Records of 15 years or more from Scandinavian lakes show an increase in

acidity accompanied by a decrease in fish population. In acidified lakes, phytoplanktons

(algae) do not grow due to nutrient unavailability. Small animals that feed on the algae

thus have little to eat. The fishes which are typically predators of small vertebrate

animals also lack food. The acidic water has other adverse effects on reproduction of

aquatic organisms. The elevated concentration of heavy metals under acidic water clog


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the grills and causes suffocation of fishes. Heavy metals may pose health hazards to

humans, because they may become concentrated in fish and then passed on to birds,

other mammals and people when the fishes are eaten.

5. Effect on human health and human activities: - Air pollution can affect the health ofworkers within the industrial premises, causing absenteeism, sickness and drop in

production. Industrial hygiene measures are being taken by many industrial management

to combat these occupational disease. However, apart from the effects on industrial

workers, air pollution also affects larger segments of general population. The notorious

London smog of 1952, which lasted for 5 days causing 4000 deaths, is an example.

Epidemiological and toxicological studies indicate a link between air pollution and

respiratory conditions like chronic bronchitis, bronchial asthma, pulmonary emphysema

and lung cancer. The vulnerability to air pollution depends upon age, sex, general healthstatus, nutrition, pre-existing diseases, concurrent exposures, concentration and nature

of the pollutants involved, extent of exposure, temperature and humidity at the time of

exposure. People who are very young or very old and infirm, people of poor health,

smokes, people with asthma, bronchitis and coronary heart disease are usually more

vulnerable. Irritation of nose, eyes and throat and bad odours due to air pollutants cause

annoyance, allergy and health hazards.

PRIMARY AIR POLLUTANTS AND THEIR EFFECTS

The most common primary air pollutants are: (1) Carbon monoxide; (2) Oxides of

Nitrogen (NOx); (3) Oxides of sulphur (Sox); (4) Hydrocarbons and (5) Particulate matters

(1) Carbon Monoxide(CO):Sources: - The atmospheric air contains 0.1 0.12 ppm of CO. the natural process which

contribute to CO in the atmosphere are volcanic activity, natural gas and marsh gas

emissions, electrical discharges in the atmosphere during storms, seed germination etc.

however most of the CO in the atmosphere is due to human activities such as (i) automobile

exhausts (which accounts for 60% of CO in the atmosphere); (ii) forest fires and agricultural

burning (i.e., burning of forest debris, crop residues, bushes, weeds and vegetation, which

contribute to about 17% of Co in the atmosphere) and (iii) industrial operations such as


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electric and blast furnaces in iron and steel industry, petroleum refining, paper industry, gas

manufacture and coal mining (which constitutes about 9.6% of CO in the atmosphere.

Characteristics: - Co is colorless, odourless, toxic gas, slightly water soluble.

Harmful Effects: - CO competitively inhibits combination of oxygen and hemoglobin. It

attacks hemoglobin and displaces oxygen to form carboxyhemoglobin, and thus reducing

the oxygen carrying capacity of blood.

Under normal conditions

O2 + Hb ----- O2Hb (oxyhemoglobin)

In Presence of CO

O2Hb + CO ----- COHb (Carboxyhemoglobin) + O2

The immediate response to CO poisoning is loss of judgment, which is responsible

for many automobile accidents. Further exposure to higher levels of CO leads to variousmetabolic disorders such as asphyxiation and causes death. CO poisoning can be cures by

providing fresh oxygen which reverses the above reaction.

(2) Oxides of Nitrogen (NOx): Out of the eight possible oxides of nitrogen, only N2O, NO andNO2 are the important constituents of the atmosphere. The concentration of N2O is more in

the atmosphere, although NO and NO2 are more significant from air pollution point of view

and usually represented together as NOx. NO is a colorless, odorless gas, but NO2 has a

reddish-brown color and pungent suffocating odor. The formation of NO from nitrogen and

oxygen is favored at high temperatures (~1210 1765 oC) which are usually attained in the

combustion process involving air. The oxidation of NO to NO2 is also favored at high

temperatures (~1100 oC):

2

1100

2

17651210

22

22

2

NOONO

NOON

C

C

o

o

NO2 is also formed by photolytic reaction, but the presence of hydrocarbons disrupts this

photolytic cycle and gives rise to photochemical smog, which is discussed in a subsequent

section.

Oxides of nitrogen may also be formed either by the natural or artificial fixation of nitrogen

from the atmosphere or from nitrogen compounds present in organic matter. The annual

global release of NOx from manmade sources is about 5 x 10 7 tonnes, which is only slightly

less than that discharged by natural bacterial activity. Oxides of nitrogen are produced by the


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combustion of coal, oil, natural gas and other organic matter. Thus, NOx is introduced into

the atmosphere from automobile exhausts, incinerators, furnace stacks, coal based power

plants and other similar sources.

Harmful effects: - Oxidizes cellular lipids, forms bonds with hemoglobin and reduces theefficiency of oxygen transport. Disrupts some cellular enzyme systems. Higher levels and

prolonged exposures may cause pulmonary fibrosis, inflammation of lung tissues and may

eventually leads to death. Causes nitric acid mediated effects like pH lowering, impairing

enzymatic functions and destroying various functional molecules. NO can form addition

compounds with hemoglobin, if it enters in blood stream.

(3) Oxides of Sulphur (SOx): - Combustion of any sulphur bearing materials produces SO2 (97 99%) accompanied by a small quantity of SO3 (1 3%). This mixture is usually denoted as SOx.SO2 is colorless, heavy, water soluble gas with pungent and irritating odour, rapidly diffusing

and acid forming oxidizing agent. Nearly 67% of the global SOx pollution is due to volcanic

activity and other natural sources. The remaining 33% SOx emission is because of human

activities such as smelting of sulphide ores and chemical plants e.g., manufacturing of

sulphuric acid.

SO2 is oxidized to SO3 in atmospheric air by photolytic and catalytic process involving ozone,

NOx and hydrocarbons, giving rise to the formation of photochemical smog. Oxidation of

SO2 can take place in presence of catalysts such as NOx, metal oxides, soot and dust. Under

normal humid conditions of the atmosphere SO3 reacts with water vapour to produce

droplets of H2SO4 aerosol which give rise to the so called Acid rain.

nSOHSOHOHSO

OSOOSO

)(424223

2332

Harmful Effects: - Absorbs quickly and irritates the upper respiratory tract. React with

cellular constituent chemical e.g., enzymes. The H2SO4 formed lowers pH, impairs enzymatic

functions and destroy various functional molecules. Leads to bronchial spasms,

breathlessness, impaired pulmonary functions via airway resistance, impaired lung clearance

and increased susceptibility for infection.


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(4)Hydrocarbons: - Hydrocarbons are emitted into the atmosphere by natural biological activityas well as anthropogenic sources such as automobile exhausts; burning of coal, oil, wood and

refuse; and solvent evaporation. Anthropogenic sources accounts for about 15% of total

hydrocarbon emission in to the atmosphere. Methane is the major hydrocarbon emitted into

the atmosphere by natural activities such as anaerobic decomposition of organic matter in

water, soil and sediments by micro-organisms. About twenty other hydrocarbons have been

identified in ambient atmosphere in areas of heavy vehicular traffic, which include ethane, n-

butane, n-pentane, isopentene, isobutene, m-xylene, propane, ethylene, acytylene and

toluene. These hydrocarbons being thermodynamically unstable tend to get oxidized in the

atmosphere by a series of chemical and photochemical reactions. This gives rise to the

formation of various end products such as CO2, solid organic particulates and water soluble

acids and aldehydes, which are washed down by rain. Hydrocarbons are very reactive and animportant ingredient for the formation of ozone.

Harmful Effects: some of these compounds can react with the constituent of the cells.

Carcinogenic hydrocarbons like benzopyrene can react with DNA causing mutation and

cancer.

(5) Particulate matter: - About 2000 million tones of particulate matter per year are releasedfrom natural agencies such as volcanic eruptions, wind and dust storms, salt sprays etc.

manmade activities such as burning of wood, coal, oil and gaseous fuels, inducstrial

processes, smelting and mining operations, fly-ash emissions from power plants, forest fires,

burning of coal refuse and agricultural refuse etc, release about 450 million tones of

particulate per year. The diameter of particulates may range from 0.0002 500 with

varying life times depending upon the size and density of the particles and turbulence of air

which control their settling rate.

Particulates include Fe3O4, V2O5, CaO, PbCl2, PbBr2, fly-ash, aerosols, soot etc. polycyclic

aromatic hydrocarbons (PAH) are important constituent of several organic particulates

which are carcinogenic. Soot is highly condensed product of PAH compounds and can itself

absorb many PAH compounds and toxic trace metals e.g., Be, Cd, V, Cr, Ni & Mn as well as

carcinogenic organics such as benzo pyrenes.


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Characteristics: - Small solid particles and liquid droplets are collectively termed as

particulates. Particulate category includes about 5% of the weight of all pollutants present in

the atmosphere. These are various types of particulates such as:

1.

Dust: - Dispersion aerosols with solid particle are called dusts. They are heterogeneous incomposition and the spectrum of particle size varies considerable.

2. Smoke: - Condensation aerosols with solid dispersed phase or a solid and a liquiddispersed phase are called smokes.

3. Fumes: - There are solid particles of the size ranging from 0.1 1 mm and are normallyreleased from chemical or metallurgical processes.

4. Mist: - It is made-up of liquid droplets generally smaller than 10 mm which are formed bycondensation in the atmosphere or are released from industrial operation.

5. Fog: - It is mist in which the liquid is water and is sufficiently dense to observe vision.6. Aerosol: - Under this category are included all air borne suspensions either solid or liquid,

these are generally smaller than 1 m.

Harmful Effects: - Effects vary with the nature of the particles. Carbon particles and other

particles cause scarring of lungs via complex walling off and fibrogenic reactions leading to a

disease conditions known as Pneumoconiosis. Particles carrying absorbed mutagens lead

to damage of DNA in the lungs and elsewhere.

SECONDARY POLLUTANTS: -

In the influence of electromagnetic radiations primary pollutants produces secondary

pollutants. Secondary pollutants include ozone, acid rain, photochemical smog, peroxyacetyl

nitrate (PAN). We will discuss all these pollutants separately in detail below: -

1. OZONE: - Ozone is a major ingredient of smog and is produced by passing a high voltagethrough dry atmospheric air between two electrodes. It is unstable and changes into

oxygen molecule and nascent oxygen. Ozone is an allotropic form of oxygen and present

in traces in the air. The total ozone content of air is very little, 90% atmospheric ozone

occurs in the stratosphere while remaining 10% in the troposphere.

Harmful effects of Ozone: - (a) Ozone emission from vehicles exhaust causes narrowing

of air path in the animal lungs. (b) Ozone is also associated with lung cancer, visual


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impairment and loss of immune system. (c) Increased concentration of ozone can change

RNA & DNA structure which may cause mutation. (d) Ozone retards the growth of plants

and also affects the food production. (e) Vegetation damage in the plants in the form of

chlorosis and leaf abscission is also due to high concentration of ozone. (f) It is also a

strong irritant.

Control of Ozone: - Concentration of ozone can be reduced by controlling the emission

of the hydrocarbon, NOx, CO etc.

ACID RAIN: -

The term acid rain is used to describe all precipitation or deposition, which is

more acidic than normal, it results, when gaseous emission of SOx & NOx interact with

water vapor and sunlight and are chemically converted to acidic compounds such assulphuric, sulphurous, nitric and nitrous acids.

When these compounds along with other organic and inorganic chemicals are

deposited on the earth as aerosols and particulates the deposition is called dry

deposition and when these are carried to the earth by raindrops, snow, fog or dew , it is

called wet deposition.

Generally clean rain is slightly acidic as it dissolves varying amounts of CO2.

The lowest pH of rain is 5.6, when it is clean, this is the pH level produced by carbonic

acid. Chemical reactions in the atmosphere converts SO2, NOx and volatile organic

compounds (VOC) to acidic compounds associated oxidants.

SunlightVOCNOx

COHOHCO

HNOorHNOOHONOx

SOHorSOHOHOSOx

3222

3222

423222

22OHVOCSunlight

Harmful Effects of Acid Rain: -


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1. Damage to building and structural materials of valuable ancient sculpture, carvedfrom marble, lime stone, sand stone etc, because of pitting and mechanical

weakening due to attack by the acidic components.

2. Acidification of soils with the consequent effects on microbial and soil fauna andfixation of nitrogen. Acidification of soil also reduces forest productivity.

3. Foliar damage to crops and forests, leaching of nutrients from leaves andalteration of seed.

4. Potential effects on aquatic systems such as acidification decreases alkalinity andmobilization of metals like aluminum.

5. Other biological effects on aquatic life are decline in productivity of fish andamphibian, skeletal deformities and increased fish mortality.

6.

Corrosive damage to steel, zinc, oil based paints and automobile coatings.7. Possible effects on human lungs, skin hair may be affected. The heavy metals

released by acid rain also cause potential threat to human health.

8. Acidification of drinking water reservoirs and concurrent increase in heavy metalion concentration exceed public health limits and cause injurious effect.

Control of Acid rain:

1. Emission of SO2 & NO2 from industries and power plants should be reduced byusing pollution control equipments.

2. Liming of lakes and soils should be done to correct the adverse effect of acid rain.PHOTOCHEMICAL SMOG: -

When the atmosphere is loaded with large quantities of automobile exhausts

during warm sunny days with gentle winds and low level inversion, then the exhaust

gases are trapped by the inversion layers with stagnant air masses and simultaneously

exposed to intense sunlight. Then, a number of photochemical reactions, involving NO2,

hydrocarbons and other organic compounds and free radicals take place leading to the

formation of ozone, peroxides and other photochemical oxidants in the atmosphere.

This gives rise to the phenomenon of photochemical smog, which is characterized by

the formation of aerosols that reduce visibility, generation of brown hazy fumes that


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irritate the eyes and lungs, and also causes extensive damage to vegetation and rubber

goods.

Photochemical smog was observed in some parts of Los Angeles and Denver in

USA and that is why it is sometimes referred as Los Angeles Smog. Photochemicalsmog is oxidizing smog and it should be clearly distinguished from the usual reducing

smog which forms due to the combination of smog and fog. In India, Mumbai, Kolkata

are ideal candidates for the formation of photochemical Smog.

Photochemical Smog Cycle:

In the photochemical smog cycle given above involves following reactions:


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1. Reactive hydrocarbons from automobile exhausts interact with ozone to form ahydrocarbon free radical .2RCH

.

23Re RCHOnHydrocarboactive

2. .2RCH rapidly reacts with oxygen to form another free radical3. .22ORCH reacts with NO to produce NO2 and the free radical RCH2O.4. This new free radical next interacts with oxygen to yield a stable aldehyde, RCHO and

hydroperoxy radical, .2

HO

5. .2

HO then reacts with another molecule of NO to give NO2 and HO.

6. Hydroxyl radical is extremely reactive and rapidly reacts with a stable hydrocarbonRCH3 to yield water and regenerate the hydrocarbon free radical .2RCH

One complete cycle yields two molecules of NO2, one molecule of RCHO and

regenerates the free hydrocarbon radical .2RCH to start all over again. Very soon there is

a rapid build-up of smog molecules.

7. The aldehyde RCHO interacts with the hydroxyl radical and form acyl radical RC.=OAcyl radical further change into peroxyacetyl radical by the absorption of

oxygen and finally changes into peroxyacetyl nitrate (PAN). PAN is most potent eye

irritant found in smog.

Control of Photochemical Smog: - The emission of primary pollutants viz; hydrocarbons and

NOx should be controlled.

Peroxyacetyl nitrate (PAN): It is formed by the photochemical reaction between VOCs and

NOx and damages plants and forests and irritant to mucous membrane of eyes and lungs.

OZONE LAYER DEPLETION

The stratospheric ozone layer mostly concentrated between the altitudes of 12 -35 km and

protects earth from harmful ultraviolet radiation.

Radiation and importance of the shield


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Solar radiation emits electromagnetic waves with a wide range of energies and wavelength.

UV light is part of this electromagnetic radiation, which on penetrating the atmosphere and

being absorbed by biological tissues, damages protein and DNA molecules at the surface of

all living things (This damage is what occurs when you get a sunburn). If the full amount of

UV radiation falling on the stratosphere will reach earths surface, it is doubtful that any life

could survive. We are spared damaging effects of UV rays because most of UV radiation

(more than 99%) is absorbed by ozone in the stratosphere. For this reason, stratospheric

ozone is commonly referred to as the Ozone shield.

Formation and Breakdown of the Ozone Shield

Ozone is formed in the stratosphere when UV radiation acts on oxygen molecules. The high

energy UV radiation first causes some molecular oxygen to split apart into free oxygenatoms, and these atoms then combine with molecular oxygen to form ozone via the

following reactions:

)(

)(

32

)320280(

2

iiOOO

iOOUVO nmUVA

It is not necessary that all of the oxygen atoms will combine with oxygen molecules

to form ozone, but some of the oxygen atoms may combine with ozone molecule to form

two oxygen molecules as reaction given below:

)(223 iiiOOOO

Finally when ozone absorbs UVB (320 400 nm) radiation, it is converted back to free

oxygen atom and molecular oxygen:

)(23 ivOOUVBO

Thus the amount of ozone in the stratosphere is dynamic. There is equilibrium due to

the continual cycle of reactions of formation and reactions of destruction. Because of

seasonal changes in solar radiation, ozone concentration in the Northern Hemisphere is

highest in summer and lowest in winter. Also, in general, ozone concentrations are highest at

the equator and diminish as latitude increases again, a function of higher overall amounts


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of solar radiation. However, the presence of other chemicals in the stratosphere can upset

the normal ozone equilibrium and promote undesirable reactions there.

Chlorofluoro carbons (CFCs) are a type of halogenated hydrocarbon. CFCs are non-

reactive, non-flammable, non-toxic organic molecules in which both chlorine and fluorineatoms have replaced some hydrogen atoms. At room temperature, CFCs are gases under

normal pressure, but they liquefy under modest pressure, giving off heat in the process and

becoming cold. When they revaporise, they reabsorb the heat and become hot. These

attributes led to the widespread use of CFCs for the following applications: -

1. In refrigerators, air conditioners, and heat pumps as the heat transfer fluid.2. In the production of plastic foams.3.

By the electronics industry for cleaning computer parts, which must be meticulouslypurified.

4. As the pressurizing agent in aerosol cans.In 1974, chemist Sherword Rowland and Mario Molina published a classic paper

concluding that CFCs could damage the stratospheric ozone layer through the release of

chlorine atoms. Rowland and Molina reasoned that, although CFCs would be stable in the

troposphere, in the stratosphere they would be subjected to intense UV radiation, which

would break them apart, releasing free chlorine atoms via the following reaction:

)(23 vCFClClUVCFCl

Ultimately, all of chlorine of a CFC molecule would be released as a result of further

photochemical breakdown. The free chlorine atoms would then attack stratospheric ozone

to form chlorine monoxide (ClO) and molecular oxygen:

)(33 viOClOOCl

Furthermore, two molecules of chlorine monoxide may react to release more

chlorine and an oxygen molecule:

)(2 2 viiOClClOClO


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Reaction (vi) and (vii) are called the chlorine catalytic cycle, because chlorine is

continuously regenerated as it reacts with ozone. Thus, chlorine acts as a catalyst, a chemical

that promotes a chemical reaction without itself being used up in the reaction. Because every

chlorine atom in the stratosphere can last from 40 100 years, it has potential to break down

100,000 molecules of ozone. Thus CFCs are judged to be damaging because they act as

transport agents that continuously more chlorine atoms in the stratosphere.

THE OZONE HOLE:

In the fall of 1985, British atmospheric scientists working in Antarctica reported a gaping

hole (actually, a serious thinning) in the stratospheric ozone layer over the South Pole.

There, in an area the size of United States, ozone levels were 50% lower than normal.

Scientists had assumed that the loss of ozone, if it occurred, would be slow, gradual, anduniform over the whole planet. The ozone hole came as a surprise, and if it had occurred

anywhere but over the South Pole, the UV damage would have been extensive. As it is, the

limited time and area of ozone depletion there have not apparently brought on ant

catastrophic ecological events so far.

Effects of Ozone Depletion:

1. The most adverse effect of ozone depletion is the increasing rate of UV rays in the earthssurface, temperature of the earth will increase.

2. Effect will be on human beings, increasing rate of skin cancer, psychological growth ofhuman bodies, reducing the body strength to fight bacterial infection on agricultural

crops.

THE GREEN HOUSE EFFECT

The green house effect was first recognized in 1827 by French scientist Jean Batiste Fourier,

who pointed out the similarity in what happens in the atmosphere and in a green house.

The green house effect can be explained by using the example of a car sitting in the sun with

window closed. The interior of car heats up by absorbing the sunlight by the seats and other

interior objects, which is given off in the form of infrared radiation. Unlike sunlight, infrared

radiation is blocked by glass and so cannot leave the car. The trapped heat energy causes the


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interior air temperature to rise. This is the same phenomenon that keeps a green house

warmer than the surrounding environment.

GREEN HOUSAE GASES

On a global scale, water vapor, CO2 and other gases in the atmosphere play a role analogous

to that of the glass in a green house. Therefore, they are called green house gases (GHGs).

Light energy comes through the atmosphere and is absorbed by earth and converted to heat

energy at the planets surface. The infrared heat energy radiates back upward through the

atmosphere and into space. The GHGs that are naturally present in the troposphere absorb

some of the infrared radiation and reradiate it back toward the surface; other gases (N2 &

O2) in the troposphere do not. The GHGs are like a heat blanket, insulating earth and delaying

the loss of infrared energy (heat) to space without this insulation, average surfacetemperatures on earth would be about -19 oC instead of 14 oC and life as we know it would be

impossible.

The green house effect may be defined as the progressive warming up of the earth surface

due to blanketing effect of GHGs in the atmosphere or we can say, green house effect is

the phenomenon due to which the earth retains heat.

Major Sources of Green House Gases:

1. Factories, power stations those use fossil fuels2. A large fleet of automobiles, railways, aircraft etc.3. Burning of fire woods and deforestation

Impact of Green House Effect on Global Climate:

The green house effect will bring about the following important changes in the climate of the

earth:

1. Rise in the temperature results ocean to get warm up and sea level would rise floodinglow lying regions.

2. In temperate regions, the winter will be shorter and warmer and summer will be long andhotter.

3. Plants and animals will be affected resulting in the disruption of the whole ecosystem.


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Control and Remedial Measures of Green House Effect:

It can be controlled by taking the following important measures:

1. Reducing the consumption of fossil fuels (coal and petroleum). This can be achieved bydepending more or non-conventional renewable sources of energy such as wind, solar,

nuclear and biogas.

2. Disposing of green house gases as they are formed, elsewhere than in the atmosphere.3. Recovering green house gases present already in the atmosphere and disposing of them

elsewhere.

4. Greenery and reforestation of the landscape is the immediate solution to control the CO2concentration.

METHOS USED FOR AIR POLLUTION CONTROL:

1. Zoning: - the zoning of the industries is done on the basis of the type of industries, theirfunctions etc. every city has its own zoning rules. Zoning of the industries may be based

on their functions and their performance. The functions include linkage of industries

sidings whereas performance includes industries on the basis of traffic congestion

obnoxious and hazardous emission, industrial nuisance as smoke, dust odor etc. In India

the zoning system varies from city to city, proper zoning results considerable

improvement of health of the atmosphere. Zoning prevents the invasion of undesirable

industries in industrial area and harmful gases are prevented from entering the

atmosphere.

2. Control at the source: - the air pollution can be prevented at the source by the change ofraw materials or by the alteration in the equipment or by the modification of process or

by the separation of pollutants at source. If air pollution occurs due to the utilization of

raw materials a desirable substitute of raw material can be used which is less polluting.

Modified or new process can reduce pollution e.g., the use of exhaust hoods and

ducts over several types of industrial ovens allow the recovery of various solvents. New

type of equipment poses much less air pollution problem e.g., basic oxygen furnaces

which are replacing the open hearth furnaces in steel industry, use of floating roof tanks

etc.


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Removal of pollutants at source is generally more economical to remove pollutants

when they are present in relatively higher concentrations rather that at some point away

from the source.

3. Control by Different Devices: - use of control devices depends upon the collectionproperty, capacity of the device and nature of the processes used by the particular

industry. The various collecting devices and techniques employed for the particulate and

gaseous air pollutants are summarized below;

Control Devices for Particulate Matter

Internal

Separator

Gravity SettlingChamber

CycloneCollector

Fabric Filters

Wet Collection

Devices

Spray TowerCyclonicScrubber

VenturyScrubber

Packed Bed

Scrubbers

Electrostatic

Precipitators

Control Devices for Gaseous Emissions

Absorption Adsorption

Physisorption Chemisorption

Combustion

Direct

Combustion

Thermal

Incineration

Catalytic

Combustion

Condensation

(ColdTrapping)


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(A)DEVICES USED FOR PARTICULATE MATTER1. INTERNAL SEPARATORS: - These separates dust particles from the gas and are of following

types:

(a) Gravity Settling Chamber: - It consists of a chamber in which dust is separated from gas bythe reduction in the velocity of passing gas. Due to this dust particles settle down in the

chamber and coarse particles are removed. By this method particle having size 25 30

can be removed.

FIGURE: Gravitational Settling Chamber

Advantages: Cheap, Low initial cost, low maintenance, and continuous disposal of solid

particulate matter.

Disadvantages: - poor efficiency for small size particles and needs large space.

(b) Cyclone Collector: - The working principle of cyclone collector is when a centrifuged forcerotation is provided to incoming gas, it throws the heavy particles of the gas to the outer

periphery of the cyclone then these heavy particles slide down into the collector.

In the cyclone collector the gas is first allowed to flow through a light circular

spiral which produces centrifugal force on the suspended particles, which in turn are

forced to move upwards at the central portion of the cyclone. Due to inertia the dust

particles tend to settle on the surface of the cyclones wall, from where they are collected

in receivers. The cyclone collector has an efficiency of 95% for the removal of particles

having size of 10 40 .


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Figure: Cyclone Collector

Advantages: - Easy in operation, low initial cost, low maintenance.

Disadvantages: - Low efficiency for particles below 10 .

(c) Fabric Bag Filters: - In the fabric filters a dusty gas is allowed to pass through a fabricon which dust is attached. If the gas is flowing at low velocity and contains

particulates. These settle down as a result of sedimentation. Due to electrostatic

charges, fine particles are also attached to the fabric.

FIGURE: Fabric Bag Filter


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In bag filters gas enters from the bottom of the hopper. Bags are 1 m x 7 m in

size and its collection efficiency is about 99%. Due to gravity heavy particles settle

down in the hopper and fine particles are on the fabric bag. The bags are cleaned by

blowing compressed air in the reverse direction.

Advantages: - High collection efficiency for particles less than 10 size. Simple in

operation and construction, low electricity consumption

Disadvantages: - High maintenance and fabric replacement cost. Bigger in size, limit

in operating temperature.

2. WET COLLECTION DEVICES: In wet collection devices mixed phases of gases and liquidsare used. In this device particles are washed out of the gas flow by a water spray. The aim

of wet collection device is to transfer particulate matter in the gas to the scrubbing liquidwhich can be readily removed. This leaves the gas clean to pass onwards to the process

for which it is being used. Wet collection devices are generally of four types:

(a) Spray Tower: - It is the simplest wet scrubber in which water is introduced by spraynozzles. The polluted gas flows upward and the particle collection results because of

inertial impaction and interception on the droplets. It can remove particulate matter of

less than 0.2 in diameter and also remove gaseous contaminants.

FIGURE: Spray Tower (a) Countercurrent-flow spray tower (b) Crosscurrent-flow spray

tower


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(b)Cyclonic Scrubber: - In cyclonic scrubber the aerosol is introduced in a centrifugalmanner. Water is sprayed at the entrance of the gas and plates are provided to remove

the moisture from the gas after the removal of the dust. This is followed by control

equipment such as gravity settling chamber or cyclones. Cyclonic scrubbers have an

efficiency of 90% and it can remove dust particles of 5 size. It is capable of cleaning

about 2000 liters of gas per minute.

Figure: Cyclonic Scrubber

(c) Ventury Scrubber: - It consists of a ventury throat through which dirty gas is passed at arate of about 3400 12600 m3 per minute. Water is added in the direction of flow so thatthe water enters at the throat. The efficiency of ventury scrubber is about 99% and it is

capable of cleaning even very fine particles. A cyclonic separator has to be used after the

ventury scrubber, if moisture is to be removed from the gas.

Figure: Typical Ventury Scrubber Configuration


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(d)Packed Bed Scrubber: - It is used to scrub particulate matter from polluted gas. In thepacked bed scrubber dirty gas moves upward and comes in contact with the scrubbing

liquid which is moving downward over the packing in a film. The scrubber can be packed

by coke, broken stone etc. packed towers can be chosen for scrubbing particles that are

soluble in scrubbing liquid.

Figure: Packed Bed Scrubber

Advantages of Wet Collection Devices:

1. It needs moderate amount of space2. In this simultaneous removal of gases and particulate occur3. It shows effective performance over a wide loading range4. It reduces hazards of explosive dust air mixture5. By this corrosive gases may be neutralized using proper scrubbing liquid

Disadvantages of Wet Collection Devices:

1. In these devices corrosion is a problem2. In these disposal of wet slide is also a problem3. These consume high energy


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(3) Electrostatic Precipitator (ESPs): - Particles with diameter as small as 0.0001 cm can be

removed by passing the dusty gas through an electrostatic precipitator. It works on the

principle that when the particulates move through a region of high electric potential, they

become charged and then they are attracted to an oppositively charged area where they are

collected and removed. The electrostatic precipitator consists of a series of plates, which are

charged to high voltages, alternatively +ive and ive, particles approaching given plate tend

to acquire its charge. They are then attracted to the surface of the next plate, from which

they fall into the hopper below. Thus particles pickup charge as they pass through plates and

are precipitated on plates of opposite charge. The potential across the plates is around

50000 V. The high voltage in the wire produces billions of electrons and bombards the gas

molecules, which become positively and negatively charged. The positive ions returns to the

negative end electrode and gain electrons while the negative ions combine with the dust

particles and make them negatively charged. The negatively charged dust particles collect at

the positively charged plates. When the thickness of the dust layer becomes more then 6

mm, the electrical attraction becomes week. A sharp potential gap is given which causes the

dust layer to separate and fall down into the hopper. The efficiency of electrostatic

precipitator is 99.9% and it is capable of cleaning 150000 liters of gas per minute at a

temperature of 600 oC.

Figure: Electrostatic Precipitator

Advantages: High efficiency (99.9%), can be operated at high temperature and pressure.

Disadvantages: High initial cost and large space requirement; safeguard of operating

personnel from high voltage is necessary, collection efficiency can deteriorate gradually.


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(B)DEVICES USED FOR GASEOUS EMISSIONS:Gaseous pollutants can be removed from air stream by the two classes of techniques:

1. By the absorption of pollutants through absorption and adsorption2. By the chemical alteration of the pollutant through combustion or catalytic treatment

(a) Absorption: - Absorption involves the transfer of the pollutant from the gas phase to theliquid phase. In this contaminate gas acts as an absorbate while liquid or gas used in

absorption acts as an absorbent. The amount of gas that dissolved in a liquid absorbent

depends upon the properties of the absorbate and absorbent, temperature, pressure etc.

Different absorbents are used for different absorbate. Selection of a good absorbent is the

important criteria in the absorption. An absorbent must be non-toxic, non-flammable,

chemically stable, non-volatile, non-corrosive, easily available, and less expensive. Absorption

techniques are generally used for controlling of SO2, NO2, H2S, NH3, Hydrocarbons.

To control the gaseous emissions various types of absorbers are used as they are packed

towers, tray or plate towers, spray towers, ventury scrubbers. These equipments designed to

provide intimate contact between the gas and liquid and provide maximum diffusing of gas

into the solution. E.g., NH3, MgO, CaO, CaCO3, P2O5 etc.

(b)Adsorption: - Adsorption is a surface phenomenon in which gas or liquid are adsorbed on theadsorbent. The material upon whose surface the adsorption takes place is called an

adsorbent while the molecular species that get adsorbed are called adsorbate.

Depending upon the nature of force existing the holding the adsorbate molecules and

adsorbent, the adsorption can be classified into two types:

(a) Physical Adsorption: - The force of attraction existing between adsorbate andadsorbent are Vanderwaals force. As force existing between molecules is very week,

therefore this type of adsorption can be easily reversed by heating or by decreasing

the pressure.

(b)Chemical Adsorption: - In this the force of attraction existing between adsorbate andadsorbent is due to chemical interaction. The solid and adsorbed materials are held

to the solid surface by chemical bond and because of very strong chemical bond this

type of adsorption cannot be easily reversed.

Table: Different adsorbents and their uses


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ADSORBENT USES

Activated carbon Removal of odors and trace impurities from gases, purification

of industrial gases and hydrocarbons, solvent recovery

Activated Alumina Dehydration of gases and liquids

Silica Gel Dehydration and purification of gases

Molecular Sieves Selective adsorption of CO2, NH3, C2H2, H2S and SO3

Bone Charcoal For decolorizing sugar solutions

(c) Condensation or Cold trapping: - At a given temperature condensation of acompound will occur if its partial pressure is increased till its vapor pressure.

Condensation is possible if gases temperature reduced to its saturation temperature

and pressure is equal to its vapor pressure. Surface condensers and contact

condensers are two equipments which are used to control air pollution. Thesecondensers are widely used in the field of hydrocarbon emission.

(d)Combustion: - This technique is used when the pollutant contains gases or vapors,which are organic in nature. Combustion converts these pollutants into water vapor

and relatively innocuous products, such as Co2. There are three methods of

combustion:

(i) Direct Combustion: - The equipment used for direct combustion are fumeincinerators, steam injection or ventury flares and after burners. In this

process waste gases are entered directly in the combustor and high

combustible gases are eliminated. Direct combustion or flaring is not a

perfect method when the waste contains pollutants like sulphur, chlorine andfluorine. In these cases waste gas are pretreated. This method is generally

used in petrochemical plants and refineries. Direct combustion can be

economical if the waste gas is used for energy (electricity) production.

(ii) Thermal incineration of Flame Combustion: - This method is used whenwaste gas contains low concentration of combustible gaseous pollutants. In

this process waste gas is preheated and directed into a combustion zone. In

this technique retention time of gases and temperature in combustion zone

decides the efficiency. Thermal incineration requires minimum maintenance

and used for controlling hydrocarbon emission.

(iii) Catalytic Oxidation: - In this method combustion takes place on a catalyst ona catalyst where the temperature is very low in comparison to thermal

incineration. The catalyst used are Pt, Pd, Cu, activated alumina, animal

charcoal etc. A catalytic incinerator consists of a preheating chamber and a

catalytic bed and its efficiency depends upon the concentration of pollutant

gas, temperature of gas, oxygen concentration, contact time and type of

catalyst. It is used for the control of CO2, NOx, Co and hydrocarbons. The


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catalytic oxidation technique is recommended for gases that are free of

particulate matter and certainly free of metallic substances which could

poison the catalyst. The catalyst bed is cleaned periodically by scrub with

water or acid or by heat to high temperature.

(4) Control by Stacks: - Small quantities of air pollutants can be absorbed by theatmosphere but large amount of pollutants cannot be absorbed and cause serious

problem in the atmosphere. If the pollutants are carried away to long distances or

taken to high altitudes, they get reduced in concentration by dilution or by diffusion.

Diffusion of pollutants in air depends mainly on temperature, pressure, speed and

direction of wind. The pollutants are taken to high altitudes by means of stacks. The

height of chimney stack should be raised to such a height through which particulates

get spread evenly over a wide area.

(5) Control by Vegetation and planting trees: - It has been found that the planting of

trees is very helpful in reducing air pollution due to fly ash and coal dust. So treesshould be planted all around. Growing vegetation around the industry also reduces

pollution in the atmosphere.

DISPERSION OF AIR POLLUTANTS:

Pollutants emitted into the atmosphere are mixed thoroughly with the surrounding

air and diluted by atmospheric dispersion. This dispersion is primarily due to turbulent

diffusion and bulk air flow. The dispersion of pollutants in the atmosphere depends upon the

prevailing winds, temperature pressure conditions in the environment.

(i) Wind impact on dispersion of pollutants: - Wind is a type moving air which is causedby the unequal distribution of temperature and pressure over the earths surface and

influenced by the rotation of the earth. The direction of wind is always from high

pressure to low pressure areas. The quicker heating and cooling of the earth as

compared to neighboring sea may also cause the flow of sea breezes and land

breezes. The speed and direction of wind govern the drift and diffusion of polluted

gases emitted near surface. The higher wind speed disperses more rapidly pollutants

from the source at the different concentration.

(ii) Lapse rate: - With an increase in altitude the temperature of ambient air decreases,these change in temperature is called lapse rate or environmental lapse rate (ELR).

Temperature change with altitude has great influence on the motion of air pollutants.

For example, inversion conditions result in only limited vertical mixing. The amount of


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turbulence available to diffuse pollutants is also function of the temperature profile.

The normal or standard lapse rate is 6 oC per km.

(iii) Atmospheric Stability: - Consider a sphere of air which is carried upward over adistance from Z1 to Z2 in the atmosphere. This parcel travels through a region of

decreasing pressure, and consequently expands. The expansion requires work, and

the air sphere temperature drops. Since the process is usually reasonably rapid, it is a

good approximation to assume that this process occurs adiabatically. If the

atmospheric lapse rate were exactly the adiabatic lapse rate, then the parcel of air

would reach its new position, Z2, at the same temperature as the surrounding, and

there would be no buoyant force. We consider this process as having a neutral

stability, a displaced mass of air neither tending to return to its original position nor

tending to continue its displacement.If the sphere of air mass moves upward in adiabatic process but in an atmosphere

with a sub-adiabatic lapse rate, the sphere follows a temperature change given by

the adiabatic slope; but when it arrives at point Z2, it is at a lower temperature than

its surroundings, but at the same pressure. As a result it is heavier than the

surroundings and teds to fall back to its original position. This condition is called

stable. In a stable atmosphere pollutants will only slowly disperse, and turbulence is

suppressed.

In another case, when the air motion results in a temperature rise, the density of air

sphere is less than the surroundings, and the sphere of air continues to rise. Such an

atmosphere is unstable. An unstable condition is favorable for pollutant dispersion.

(iv) Plume and its Types: - The path and extent of released gaseous pollutants in theatmosphere is called plume. Plume behavior decides whether an atmosphere is

stable or not. Various types of plumes are discussed below: -

(a) Looping Plume: - it has a wary character and occurs in super-adiabatic manner.Looping plume produces in highly unstable atmosphere due to rapid mixing. The

dispersion of plume will be rapid if the high degree of turbulence is present. Due

to turbulence high concentration occurs near the ground.

(b)Neutral Plume: - It is the upward vertical rise of plume from the stock. In this ELRis equal to ALR. The upward lifting of plume will continue till it reaches air of

density similar to that of plume itself.


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Figure: Vertical expansion of continuous plumes related to vertical temperature

structure. The dashed lines correspond to the dry adiabatic lapse rate forreference

(c) Coning Plume: - The neutral plume tends to cone as the velocity increases andwhen wind velocity is greater than 32 km/h and when cold cover solar radiation

by day and terrestrial radiation by night. It occurs under sub-adiabatic conditions.


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(d)Fanning Plume: - It occurs under extreme inversion conditions in which theemission will spread horizontally as it cannot lift due to extremely stable

environment.

(e) Lofting Plume: - when there exists a strong super adiabatic lapse rate above asurface Inversion then the plume is said to be lofting. In this the dispersion of

pollutants will be rapid and no concentration will touch the ground.

(f) Fumigating plume: - When an inversion layer occurs at a short distance above thetop of the stock and super adiabatic conditions prevail below the stack, then its

called fumigating plume. The plume is in the form of a well defined cone with

dragged or diffused bottom.

(g)Trapping Plume: - This refers to conditions when the plume is a sandwichedbetween inversion and thus can only diffuse with in a limited vertical height.

AIR QUALITY STANDARDS

Air quality standards indicate the levels of pollutants that cannot be exceeded during

a specified time period in a specified geographic area, with due reference to the method of

measurement, units of measurement, concentration and time of exposure. These are derived

from air quality criteria, which are in turn derived on the basis of effects of ambient air

pollution on human health, vegetation, animals, materials, visibility etc.

Air quality standards represent the ideal requirements which every country should

strive to attain. Table given below summarizes some national ambient air quality standards

for the U. S. proposed in 1971. The primary air quality standards define the levels judged

necessary to protect the public health with an adequate safety margin.

Table: Ambient air quality standards for U. S.

Pollutant Primary air quality standards

SOx 1. 80 g/m3 (0.03 ppm) annual arithmetic mean2. 365 g/m3 (0.14 ppm) maximum 24 h concentration which should not

be exceeded more than once a year

NO2 100 g/m3 (0.05 ppm) annual arithmetic mean


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CO 1. 10000 g/m3 (9 ppm) maximum 24 h concentration2. 40000 g/m3 (35 ppm) maximum 1 h concentration

Particulate 1. 75 g/m3 (0.03 ppm) annual geometrical mean2. 260 g/m3 (0.1 ppm) 24 h maximum concentration which should not be

exceeded more than once a year

Oxidants 160 g/m3 (0.08 ppm) maximum 1 h concentration not to be exceeded more

than once a year

Table: The standards prescribed for air pollution control in INDIA

Sr. No. Area Category Pollutant concentration (g/m3)

SO2 NOx CO Suspended particulate matter

1. Industrial and mixed use areas 120 120 5000 500

2. Residential and rural areas 80 80 2000 200

3. Sensitive areas 30 30 1000 100

unit v air pollution_6th civil

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