air pollutants - nptel

Environmental Chemistry and Analysis Prof. M.S.Subramanian

Indian Institute of Technology Madras

MODULE 3.2

Air Pollutants Carbon Oxides 1

Carbon monoxide 1

Sources of CO pollution 1 Industrial processes 1

CO emission from vehicle exhaust 2

Natural processes 2

Sinks 3

Toxicity of CO 4

Control of CO emissions 4

Carbon Dioxide And Global Warming 5 Sulphur Dioxide Sources And Removal 9

Sulphur dioxide reactions in the atmosphere 9

Effects of atmospheric sulphur dioxide 10

Nitrogen Oxides In The Atmosphere 11 Harmful effects of nitrogen oxides 13

Control of NOx emissions 13

Acid Rain 15 Particles In The Atmosphere 16

Particle formation 18

Radioactive particles 19



MODULE 3.2

Air Pollutants

A wide range of chemicals can pollute the air, but only those pollutants

which are generally viewed as needing control measures are discussed in this

chapter and in the next chapter. Air pollutants can be arbitrarily classified

according to chemical composition as (1) inorganic air pollutants and (ii) organic

air pollutants

• The following inorganic air pollutants are discussed in this chapter.

• Oxides of carbon (eg., CO and CO2) even though carbondioxide is

a natural and essential constituent of atmosphere, it may turn out to be

a deadly air pollutant because of it's potential as a green house gas.

• sulphur dioxide (eg., SO2) and

• Oxides of nitrogen(eg., NOx)

• particulates comprising of finely divided solids or liquids and often

exist as colloidal states as aerosols.

Carbon Oxides:

Carbon monoxide:

Carbon monoxide is a colourless, odourless tasteless gas, that is by far

the most abundant of the criteria pollutants.

Sources of CO pollution

Industrial processes:

• Carbon monoxide is formed during the incomplete combustion of carbon

containing compounds.

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+ →22C O 2CO ....................................................................................(1)

• It is also produced in large amounts during the reaction between carbon

containing materials at high temperatures as in blast furnaces.

+ →2CO C 2CO ....................................................................................(2)

• Carbon monoxide is also produced during the dissociation of CO2 at high

temperature.

+ →2CO C 2CO .....................................................................................(3)

CO emission from vehicle exhaust:

Most of the CO in the ambient air comes from vehicle exhaust. Internal

combustion engines do not burn completely to CO2 and water; some unburnt fuel

will always be exhausted, with CO as a component.

CO in vehicle exhaust can be reduced by using partially oxidised fuels like

alcohol and by a variety of after burner devices. It tends to accumulate in areas

of concentrated vehicle traffic.

Natural processes:

Volcanic action, natural gas emission, electrical discharge during storms,

seed germination, marsh-gas production etc, are the natural processes that

contribute to a small measure for the presence of CO in the atmosphere. Forest

fires contribute to 7.2% of CO emissions and agricultural burning contribute 8.3%

of emissions. The atmospheric back ground concentration of CO is 0.1 ppm.

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Sinks:

In soil, the major CO sink is by soil microorganisms. The major sink

process in the atmosphere is however is the conversion to CO2 by reaction with

hydroxyl radical. This process is however rather slow and the reduction in CO

level away from the source area is almost entirely a function of atmospheric

dilution processes.

The residence time of CO in the atmosphere is of the order of 4 months

and it is removed from the atmosphere by reaction with hydroxyl radical, HO•:

+ → +i 2CO HO CO H ..................................................................(4)

The reaction of atomic hydrogen with atmospheric oxygen produces

hydroperoxyl radical, as a product:

+ + → +i2O H M HOO M ..........................................................(5)

(M is an energy absorbing third body, usually a molecule of O2 or N2)

HO• is regenarated from HOO• by the following reactions:

+ → +i i 2HOO NO HO NO .........................................................(6)

+ → +i i 2 2 2HOO HOO H O O .......................................................(7)

The hydrogen peroxide formed undergoes photochemical dissociation to

regenerate HO• :

+ υ→ i2 2H O h 2HO ....................................................................(8)

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Toxicity of CO:

At levels of CO that occur in urban air, there are apparently no detrimental

effects on materials or plants, but those levels can adversely affect human

health. After entering the blood stream through the lungs, carbon monoxide

reacts with haemoglobin (Hb) to convert oxyhaemoglobin (O2Hb) to carboxy

haemoglobin (COHb).

+ → +2O Hb CO COHb O2 .......................................................(9)

Carbon monoxide, infact, has a much greater affinity for haemoglobin than

does oxygen, so that even small amounts of CO can seriously reduce the

amount of oxygen conveyed throughout the body. With this blood stream carrying

less oxygen, brain function is affected and heart rate increases in an attempt to

offset the oxygen deficit.

Control of CO emissions:

As mentioned earlier since major contribution to CO pollution is from

transportation sources and gasoline fed internal combustions are primarily

accountable for it, control measures have been concentrated on the automobiles.

Carbon monoxide emissions may be lowered by using a relatively low air-fuel

mixture, that is one in which the weight ratio of air to fuel is relatively high. At air

fuel ratios (weight : weight) ratios exceeding approximately 16 : 1, an internal

combustion engine emits virtually no carbon monoxide.

Modern automobiles use catalytic exhaust reactors to cut down on carbon

monoxide emissions. Excess air is pumped into the exhaust gas and the mixture

is passed through a catalytic converter in the exhaust systems, resulting in

oxidation of CO to CO2.

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The greatest problem with catalytic reactors at present is lack of

sufficiently durable (50,000 driven miles) catalytic material. The catalysts now in

use are subjected to poisoning (deactivation) by the adsorption of materials on

their surfaces. One of the most effective catalytic poisons is lead and this is one

reason for the development of lead free gasoline.

Carbon Dioxide And Global Warming:

It may be recalled that our atmosphere is made of almost nitrogen, oxygen

and other gases and particles. If we focus our attention on other gases, Carbon

dioxide is a relatively insignificant non - pollutant species (present level 356ppm)

in the atmosphere. However its increasing concentration in the atmosphere is of

serious environmental concern.Among the constituents of the atmosphere

methane, chloro fluorohydrocarbons, nitrous oxide, water vapour and carbon

dioxide contribute to global warming. The relative contribution of radiatively active

gases is shown in the following Table 1.

Table.1 Relative contribution of radiatively active gases to

temperature rise

Active gas % Contribution to temperature rise

CO2 50

CH4 19

CFC 17

O3 8

N2O 4

H2O 2

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The figure shows that the main influence is by CO2 but the contribution of

other green house gases, specially by CH4 cannot be ignored.

Fig. 1 Influence of H2O and CO2 on the absorption of IR radiation

emitted from the earth’s surface

IR radiation emitted from earth's surface

Fig.1 shows that most of the long-wavelength energy radiated by

the earth is affected by a combination of radiatively active gases most importantly

H2O and CO2. The watervapour strongly absorbs thermal radiation with

wavelengths less than 8 µm amd greater than 18 µm. CO2 shows a strong

4 8 12 16 20 24Wavelength (µm)

H2O absorption

CO2 absorption

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absorption band centered at 15 µm and extending from 13 to 18 µm, as well as

band centered at 2.7 and 4.3 µm. Between 7 and 12 µm there is relatively clear

sky for outgoing thermal radiation indicated as atmospheric window.

Radiatively active gases that absorb wavelengths longer than 4 mm are

called green house gases. As the fig.2 suggests, CO2 and watervapour trap good

portion of the outgoing thermal radiation attempting to leave the earth's surface.

Thus these green house gases act as thermal blanket around the globe raising

the earth's surface temperature beyond the equivalent temperature.

Although the relative share of the radiatively active gases such as

chlorofluorocarbons, nitrousoxide, CO2 and methane has been significantly

increasing each year, the largest effect is still due to CO2. Anthropogenic

production of CO2 from burning fossil fuels exceeds that of the other green house

gases. Although the natural flux of CO2 to the atmosphere due to the constant

respiration of the biosphere is much greater, this flux is in balance with

photosynthesis. Every year the CO2 declines to a minimum concentration in

summer when photosynthesis in forests of northern hemisphere converts CO2 to

biomass and it rises to a maximum in winter when the dead vegetation days,

releasing its stored carbon as CO2 . The oscillatory pattern is regular from year to

year but it is super imposed on a rising background average CO2 concentration,

which increased from 314 ppm in 1958 to 365 ppm in 2000, a 17.5 percent

increase in four decades.

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CO2

Fig. 2 Sources of sinks of carbon dioxide

The major important sink for CO2 is ocean (Fig.2). Because sea water is

alkaline and CO2 is acidic, the oceans are vast reservoir of CO2. However only

the surface layer of ocean, the top 75 meters, is in equilibrium with the

atmosphere and its capacity to absorb CO2 is limited. Exchange of surface layer

with deep oceans takes hundreds of years. The location of the remaining carbon

dioxide has been a subject of considerable debate, but it is now widely accepted

that vegetation absorbs much of the CO2. But even with this natural sink sources,

the CO2 level is continuously increasing resulting in global warming. Global

warming can shift the climate zones and the existing forests may not be able to

adapt, especially if the shift is rapid and again the result may be loss of biomass.

Global warming may also lead to increased evaporation of water thereby

reducing water available for agricultural, municipal, and industrial use.

CO2 HCO3-

CO32-organiccarbon

(Bio-mass)

CO2CO2

CO2

IndustriesPlants

AutoOcean

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Sulphur Dioxide Sources And Removal :

The two natural sources of SO2 are volcanic eruptions and sulphur-

containing geothermal sources like geysers and hot springs.

Some important industrial sources of SO2 are (1) nonferrous smelters (2)

oil refining and (3) paper and pulp manufacture.

Nonferrous smelters: With the exception of iron and aluminium, metal ores are

sulphur compounds. When the ore is reduced to the pure metal, its sulphur is

ultimately oxidised to SO2. Thus when Cus ore is reduced to copper, its sulphur

is oxidised to SO2 .

Oil refining: Sulphur and hydrogen sulphide are constituents of crude oil and

H2S is released as a gas during catalytic cracking. Since H2S is considerably

more toxic than SO2 it is burned to produce SO2 before release to the ambient

air.

Pulp and paper manufacture: The sulphite process for wood pulping uses hot

H2SO3 and thus emits SO2 in air. The kraft pulping process produces H2S, which

is then burned to produce SO2.

Sulphur dioxide reactions in the atmosphere:

Sulphur dioxide once released can convert to SO3, in a series of reaction

which, once again, involve a free radical such as OH•

2SO OH HOSO+ →i 2 i

2 i

…………………………………….(10)

2 2 3HOSO O SO HO+ → +i …………………………....(11)

Sulphur trioxide react quickly with H2O to form sulphuric acid, which is the

principal cause of acid rain.

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3 2 2SO H O H SO+ → 4 …………………………………………………..(12)

Sulphuric acid molecules rapidly become particles by either condensing

on existing particles in the air or by merging with water vapour to form

H2O - H2SO4 droplets. Often significant fraction of particulate matter in the

atmosphere consist of such sulphate aerosols.

The formation is promoted by the presence of hydrocarbons and nitrogen

oxides, which are key components of photochemical smog.

(b) In relatively humid atmospheres, SO2 is probably oxidized by reactions

occurring inside water aerosol droplets, which proceed faster in the presence of

ammonia and catalysts such as manganese (II), iron (II), nickel (II), copper (II),

etc.

+ −+ + → +3 2 2 4NH SO H O NH HSO3

−3

.............................................(13)

− ++ → + 23 3 4NH HSO NH SO ...................................................(14)

Effects of atmospheric sulphur dioxide:

When sulphur is entrained in an aerosol, it is possible for sulphur oxides to

reach far deeper into the lungs. The combination of particulate matter and

sulphur oxides can then act synergistically, with the effects of both together being

much more detrimental than either of them separately. Sulphur dioxide is one of

the serious air pollutants which is responsible for smog formation, which has

resulted in several incidents of loss of human lives.

Atmospheric sulphur dioxide is harmful to plants and leaf tissue is killed

with exposure to high levels of gas.

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Sulphurous pollutants can discolour paint, corrode metals, and cause

organic fibres to weaken. Airborne sulphates significantly reduce visibility and

discolour the atmosphere.

Prolonged exposure to sulphates causes serious damage to buildings

made of marble, limestone and mortar, as the carbonates of these materials are

replaced by sulphates,

3 2 4 4 2 2CaCO H SO CaSO CO H O+ → + + …………………………………(15)

which are water soluble.

Nitrogen Oxides In The Atmosphere:

Although there are seven oxides of nitrogen known to occur, the only two

that are important in the study of air pollution are nitric oxide (NO) and nitrogen

dioxide (NO2). The most abundant oxide is nitrous oxide. This is however

chemically rather unreactive and is formed from the natural biological processes

in the soil. Nitrous oxide first undergoes photochemical reaction. The formed

atomic oxygen reacts with another molecule of N2O to give NO. The formed nitric

oxide reacts with ozone, thereby causing ozone depletion. They can be

represented by the following equations.

+ υ→ +2N O h N O2

2

.................................................................................(16)

+ → +2N O O NO NO ..............................................................................(17)

3 2NO O NO O+ → + ...............................................................................(18)

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Nitric oxide is formed by the combustion of nitrogen-containing compounds

(including fossil fuels) by the thermal fixation of atmospheric nitrogen.

1210 17652 2N O 2NO

−⎯⎯⎯⎯⎯→+ ←⎯⎯⎯⎯⎯ ……………………………………………..….(19)

Thus all high temperature processes produce NO, which is then oxidised to NO2

in the ambient air.

22NO O 2NO⎯⎯→+ ←⎯⎯ 2 …………………………………………….………(20)

Nitrogen dioxide is very reactive and significant species in the atmosphere. At

wavelengths below 398nm it undergoes photodissociation to oxygen atoms,

2NO h NO O+ υ→ + ……………………………………………………(21)

giving rise to significant inorganic reactions, in addition to host of atmospheric

reactions involving organic species.

The principal reactions among NO,NO2, and HNO3 are indicated below:

Reactions of NO:

2

2

3 2

HOO NO NO HOROO NO NO ROO NO NO O

+ → +

+ → +

+ → +

i ii i

2

Reactions of NO2:

2 3

2

NO HO HNONO h NO O

+ →

+ ν → +

i

Removal of HNO3

3

3 2

HNO (Pr ecipitation)HNO h NO HO+ ν → + i

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Nitric oxide and nitrogen dioxide are important constituents of polluted air.

These oxides collectively designated as NOx, enter the atmosphere mainly from

combustion of fossil fuels in both stationary and mobile sources.

Harmful effects of nitrogen oxides:

NO, the atmospheric precursor of NO2, is not an irritant gas; infact it is

often used as an anaesthetic. High concentrations of NO2 can produce

pulmonary edema-an abnormally high accumulation of fluid in lung tissue. For

exposures ranging from several minutes to one hour, a level of 50 – 100 ppm

NO2 causes inflammation of lung tissue for a period of 6 – 8 weeks, after which

time the subject normally recovers. Exposure of the subject to 150 – 200 ppm of

NO2 causes bronchititis fibrosa obliterans , a conditions fatal within 3 – 5 weeks

after exposure. Death generally results within 2 – 10 days after exposure to 500

ppm or more of NO2.

NO2 also causes extensive damage to plants through its secondary

products such as peroxy acyl nitrate formed in smog. Exposure of plants to

several parts per million of NO2 in the laboratory causes leaf spotting and break

down of plant tissue. It also causes fading of dyes and inks used in some

textiles. Much of the damage to materials caused by NOx, such as stress –

corrosion cracking of electrical apparatus, comes from secondary nitrates and

nitric acid.

Control of NOx emissions:

NOx emissions are difficult to control because efficiency energy

conversion requires high combustion temperatures, whether in cars or

powerplants. Moreover there is a trade_off between NOx and unburned gases as

the ratio of air to fuel in the combustion chamber is varied. The NO production

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rate is maximum near the stoichiometric ratio (just enough O2 to completely

oxidise the fuel), where the highest temperature is reached. If less air is admitted

to the combustion zone ("fuel-rich"), the NO production rate falls along with the

temperature, but the emission of CO and unburned hydrocarbon (HC) increases.

It is possible to lower both NO and HC by carrying out the combustion in

two stages, the first of which is rich in fuel and the second of which is rich in air.

In this way the fuel is burned completely, but the temperature is never as high as

it would be for a stoichiometric mixture. This two-stage approach is being

incorporated in power plants; it has been tried in cars but with less success.

The other approach to reducing emissions is to remove the pollutant from

the exhaust gases.

In automobiles, this is accomplished with a threeway catalytic converter

(i.e it reduces emissions of HC, CO and NO).

In order to deal with both NO and unburned gases the converter has two

chambers in succession. In the reduction chamber, NO is reduced to N2 by

hydrogen, which is generated at the surface of a rhodium catalyst by the action of

water on unburned fuel molecules.

2 2hydrocarbons H O H CO+ → + ……………………………….(22)

………………………………………..(23) 2 2 22NO 2H N 2H O+ → +

In the oxidation chamber, air added, and the CO and unburned hydrocarbons are

oxidised to CO2 and H2O at the surface of platinum/palladium catalyst.

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22CO O 2CO+ → 2

2

3

………………………………………………(24)

………………………….(25) 2 2hydrocarbons 2O CO 2H O+ → +

The catalytic converter is quite effective in reducing automotive emissions.

Acid Rain:

We have seen that in polluted regions the main causes for acid rain are

sulphur dioxide and nitrogen oxides in the atmosphere. Acid rain results when

these gases are oxidised in the atmosphere and return to the ground dissolved in

rain drops. SO2 falls as H2SO3 and H2SO4 while NOx falls as HNO3. A night time

route

2 2 2SO H O H SO+ → …………………………………………………………….(26)

2 2 2 2(soot particle metal oxide)1SO O H O H SO2

+ + ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 4 …………………………..(27)

2 2 211NO H O HNO HNO2 2

+ → + 3

3

…………………………………………..…(28)

……………………………………………..(29) X H2 3 3NO O NO HNO−+ → ⎯⎯⎯→

to nitric acid in hydrogen abstraction from some suitable donor X-H by

nitrate free radical. In water droplets ions such as Mn(II), Fe(II), Ni(II) and Cu(II)

catalyse the oxidation reaction. Soot particles are also known to be strongly

involved in catalysing the oxidation of SO2. HNO3 and H2SO4 combine with HCl

emission (both by natural and anthropogenic sources) to generate acidic

precipitation which is known as ACID RAIN.

Acid rain is classified as regional air pollution problem compared to a local

air pollution problem for smog and a global one for ozone-destroying chlorofluoro

15



hydrocarbons and green house gases. Acid rain causes major damages to our

environment. They are as follows:

• Direct photo toxicity to plants from excessive acid concentrations

• Phytotoxicity from acid-forming gases, particularly SO2 and NO2 which

accompany acid rain.

• Indirect phytotoxicity such as from Al3+ liberated from acidified soil

• Acid rain causes destruction of sensitive forests.

• It affects the respiratory systems in human and other animals.

• It acidifies the lake water with toxic effects especially to fish fingerlings.

• It corrodes the exposed structures, electrical relays, equipment and

ornamental materials. The hydrogen ions from the acid rain dissolve the

lime stone (CaCO3) and thus cause damage to marble structures.

+ ++ → + +23 22H CaCO (s) Ca CO (g) H O2

4

...................................(30)

• It causes reduction of visibility by sulphate aerosols and the influence of

sulphates aerosols on physical and optical properties of clouds.

...............................................(31) + → +2 2 32CaF 3SiO 2CaSiO SiF

Particles In The Atmosphere:

Particles are important constituents of the atmosphere, particularly in

the troposphere and have a diameter of 0.001µm to 10µm. Aerosol particles from

natural sources have a diameter of less than 100µm. These particles originate

from sea sprays, smokes, dusts and the evaporation of organic materials from

vegetation. Other typical particles of natural origin in the atmosphere are

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bacteria, fog, pollen grains, and volcanic ash. Thus particulate matter may be

both organic or inorganic and both types are very important atmospheric

contaminants.

They are important because of the following reasons.

• They significantly affect the earth's radiation balance. The effect of

atmospheric particles on the heat flux of the atmosphere depends on

particle size and composition. Large dark particles tend to absorb light,

thus warming earth's atmosphere. In contrast very small particles,

regardless of colour and composition, tend to scatter light, thus increasing

the albedo of the atmosphere.

• Particles in the size 0.1 to 1µm cause serious health hazards. These

particles penetrate the lungs, blocking and irritating air passages and can

have toxic effects. Soot particles pose particular problem because they

can abosrb significant amounts of toxic chemicals on their irregular

surfaces. Coal fires release soot as well as SO2 and in foggy conditions,

the resulting aerosol can combine with soot to produce a toxic smog, with

serious health consequences.

• They provide nucleation bodies for the condensation of atmospheric

water vapour, thereby exerting significant influence upon weather and air

pollution phenomena.

• They are very much involved in several chemical interactions taking place

in the atmosphere such as neutralisation reactions taking place in water

droplets thus providing a surface, and they provide active catalytic surface

upon which heterogeneous chemical reactions can occur.

17



Particle formation:

Particulates originate from a wide variety of sources and processes

ranging from simple grinding of bulk matter to complicated chemical and

biochemical synthesis. For the most part aerosol particles consist of

carbonaceous material, metal oxides and glases, dissolved ionic species, and

ionic solids.

Metal oxides constitute a major class of inorganic particles in the urban

air. These are formed whenever fuels containing metals are burned. For example

particulate iron oxide is formed during combustion of pyrite containing lignite.

+ → +2 2 3 43FeS 8O Fe O 6SO2

2

4

................................................................(32)

Organic vanadium in residual fuel oil is converted to particulate vanadium

oxide. Part of the calcium carbonate in the ash fraction of coal is converted to

calcium oxide and is emitted to the atmosphere through the stack.

+ → +3CaCO heat CaO CO ..................................................................(33)

The SO2 from different sources in the atmosphere is subsequently

oxidised directly to sulphuric acid.

+ + →2 2 2 22SO O 2H O 2H SO ................................................................(34)

The direct reaction of SO2 with O2 is very slow, and the oxidation is carried

out by more reactive species particularly the hydroxyl radical. Some of the

sulphuric acid in the atmosphere is neutralised by ammonia or calcium oxide.

+ →2 4 3 4 2 4H SO (droplet) 2NH (g) (NH ) SO (droplet) ............................(35)

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+ → +2 4 4 2H SO (droplet) CaO(s) CaSO (droplet) H O .........................(36)

When the humidity is low water is lost from these droplets and a solid

aerosol is formed.Atmospheric particulate matter present a wide diversity of

chemical compositions. Organic matter, nitrogen compounds, sulphur

compounds, several metals and radio nuclides are present in polluted urban

atmospheres.

Radioactive particles:

The source for these radioactive particles arise from mining and

processing of ore to produce usable radioactive substances which can be used in

nuclear reactors, in nuclear weapons and in medicinal and industrial applications.

During the processing of uranium large amounts of uranium tailings are produced

which can give rise to radioactive pollution.

The fly ash introduced into the atmosphere during the combustion of fossil

fuels, contain several radionuclides. For example large coal-fired power plants

lacking ash-control equipment, may introduce upto several hundred millicuries of

radionuclides into the atmosphere each year, far more than either an aquivalent

nuclear or oil-fired power plant.

Nuclear weapon testing whether in the air or underground can give rise to

radioactive fall out. For example 90Sr, which is a longlived component of

radioactive fall out is chemically similar to calcium. The 90Sr mixes with calcium in

the soil and is taken by plants, animals and finally by man. By virtue of its

similarity to calcium it enters into bones and cause disorders in blood cell

formation and other related problems.

19



Another natural source of radionuclides in the atmosphere is radon.

Radon being a noble gas readily escapes from soil and porous rock and diffuses

into the lower atmosphere. There the nuclides decay with half-lives of 3.8 days

(222Rn) and 55 s (220Rn) producing a series of shortlived daughter products.

These daughter products attach themselves to the aerosol particles in the

atmosphere which are efficiently deposited in the lungs if inhaled. Their

subsequent α and β emissions can irradiate and damage the lung tissue.

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air pollutants - nptel

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