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.

1

+ →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.

2

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)

3

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.

4

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

5

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

6

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.

7

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

8

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.

9

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.

10

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)

11

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

12

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

13

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.

14

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

16

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)

18

+ → +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.

20

Organic Air Pollutants Natural source of hydrocarbons: 1

Hydrocarbons 2

Oxygen-Containing Organic Compounds 4 Aldehydes and ketones 4

Organohalide Compounds 5

Chlorofluoro Carbons And Depletion Of Ozone Layer 7

CFC substitutes 8

Consequences Of Ozone Depletion 11

Photo Chemical Smog 12

Chemical reactions involved in smog formation in the atmosphere

14

Organo Nitrogen Compounds 18 Organic Particles In The Atmosphere 19

Organic Air Pollutants

A variety of organic compounds are emitted into the atmosphere by

natural and human activities. They are so diverse that it is difficult to classify

them neatly. However they can be divided into two categories namely primary

pollutants and secondary pollutants. The primary pollutants are those that are

emitted directly from the sources. Typical example of an organic pollutant in this

category is vinyl chloride which can cause cancer. The secondary pollutants are

those that are formed in the atmosphere by chemical interactions among the

primary pollutants and normal atmospheric constituents. Secondary pollutants

are formed from chemical and photochemical reactions in the atmosphere. An

example of this category in the formation of photochemical smog due to the

interaction between terpene hydrocarbons from conifer trees and nitrogen oxides

from automobiles.

Natural source of hydrocarbons:

Most of the organic compounds in the atmosphere (85%) originate from

the natural source from vegetation. Among them the methane is of concern since

it is the most important greenhouse gas after carbondioxide. Methane is

produced by the bacterial action, when dead organic matter is subjected to an

oxygen-depleted highly reducing aqueous or terrestrial environment as per the

following equation.

2 22{CH O} (bacterial action) CO (g) CH (g)(biomass)

→ + 4 ………………………………….(1)

1

The present tropospheric concentration of methane is about 1.77 ppm and

it is increasing at the rate of 0.5% every year. Methane undergoes photochemical

dissociation in the stratosphere to give water vapour. But in the troposphere it

undergoes photochemical reaction to give CO and O3.

Ethylene, C2H4, is released to the atmosphere by a variety of plants. Most

of the hydrocarbons emitted predominantly by trees are terpenes. Others are

α - pinene, limonene, β - pinene; mycene; ocimene; α - terpinene and isoprene.

These compounds contain olifinic bonds and hence are the most reactive

compounds in the atmosphere. Terpenes react rapidly with hydroxyl radical, HO•

and with other oxidizing agents in the atmosphere, particularly ozone, O3. Such

reactions form aerosols, which cause much of blue haze in the atmosphere

above some heavy growths of vegetation. The compounds emitted by plants

mostly consist of esters such as coniferyl benzoate. But the quantities emitted by

them are small and have little influence in atmospheric chemistry. The principal

sink for methane decomposition is oxidation via hydroxyl radicals in the

troposphere.

4 3CH OH CH H O+ → +i 2 …………………………………….(2)

As discussed earlier this reaction is only the first step of a sequence which

transforms methane ultimately to CO and then CO2. The other sinks for methane

gas are the reaction with soil and loss to the stratosphere.

Hydrocarbons:

The gaseous and volatile liquid hydrocarbons are of particular interest as

air pollutants. Hydrocarbons can be saturated or unsaturated , branched or

straight-chain, or can have a ring structure as in the case of aromatics or other

2

cyclic compounds. In the saturated class, methane is by far the most abundant

hydrocarbon constituting about 40 to 80 percent of total hydrocarbons present in

the urban atmosphere. Hydrocarbons prodominate among the atmospheric

pollutants because of their widespread use in fuels. They enter the atmosphere

either directly from the fuel or as by-products of partial combustion of other

hydrocarbons, which tend to be unsaturated and relatively reactive. Several

alkenes including ethylene, propylene, butadiene and styrene are among the top

50 chemicals produced each year and are released to the atmosphere during

their production and use similarly aromatic hydrocarbons such as benzene,

toluene, ethylbenzene, xylene and cumene are among the top 50 chemicals

produced each year and these are also released into the atmosphere during their

production and use.

Polycyclic aromatic hydrocarbons (PAHs) commonly occur in urban

atmospheres up to about 20µg m-3 level. Elevated levels of PAHs are observed

in polluted urban atmospheres, in the vicinity of forest fires and burning of coal.

Terpenes are a particular class of volatile hydrocarbons emitted largely by

natural sources. These are cyclic non-aromatic hydrocarbons found in pine tar

and in other wood sources as mentioned earlier.

The hydrocarbon in air by themselves alone cause no harmful effects.

They are of concern because the hydrocarbons undergo chemical reactions in

the presence of sunlight and nitrogen oxides forming photochemical oxidants of

which the predominant one is ozone.

3

Oxygen-Containing Organic Compounds:

Aldehydes and ketones:

The aldehydes and ketones enter the atmosphere from a large number of

sources and processes. These include direct emission from internal combustion

engine exhausts, incinerator emission, spray painting, polymer manufacture,

printing, printing, and lacquer manufacture. Formaldehyde and acetaldehyde are

produced by microorganisms and acetaldehyde is emitted by some kind of

vegetation. They are also produced from hydrocarbons.by the photochemical

oxidation in the atmosphere. These aldehydes and ketones are somewhat stable

species, but they undergo reactions of their own as shown below.

3 3CH CHO OH CH CO H O+ → +i

i 2

2

……………………………………………….(3)

………………………………………….…(4)

…………………………………..(5)

3 2 3CH CO O M CH C(O)OOacetyl proxy

+ + →i

i

3 2 3CH C(O)OO NO CH C(O)OONOperoxyacetic nitrite

+ →i

There is yet another reaction initiated by hydroxyl radicals followed by the

addition of dioxygen and then reaction with nitrogen dioxide to form a member of

PAN family of compounds. A second possibility also starts with reactions as

explained in 3,4 &5. The acetyl peroxy radical then gives up an oxygen to nitric

oxide followed by cleavage of the C-C bond to form a methyl radical and carbon

dioxide.

4

3 3CH C (O)OO NO CH C(O)O NO+ → +i 2i

2

3

4

…………………………….…….(6)

…………………………………………..............(7) 3 3CH C(O)O CH CO→ +i i

A third important route for decomposition is by photolysis. Aldehydes are

capable of absorbing UV radiation longer than 290 nm and this leads to

photochemical degradation. For acetaldehyde the observed photolytic reactions

are

…………………………….……(8)

…………………………………......(9)

h , 290nm3CH CHO CH HCOυ λ⎯⎯⎯⎯⎯⎯→ +

ii

h , 290nm3CH CHO CH COυ λ⎯⎯⎯⎯⎯⎯→ +

The alkenyl aldehydes because of the presence of both double bonds and

carbonyl groups are especially reactive in the atmosphere. Acrolein is the most

common of these found in the atmosphere which is used as an industrial

chemical. The life time of aldehydes in the atmosphere is in the range of 24

hours.

Organohalide Compounds:

Organohalide compounds contain atleast one atom of F,Cl, Br or I. They

may be saturated or unsaturated and exhibit different physical and chemical

properties. Chlorofluro carbons (CFCs) are used in wide variety of industrial

applications including aerosol propellents, refrigerants, and foam blowing. The

CFC's which have attracted most attention in ozone depletion are CFCl3, CF2Cl2 .

The lifetime of CFCl3 is about 77 years, while that for CF2Cl2 is 139 years. Hence

any reduction in CFC release will have little effect in the immediate future. The

most abundant organochlorine compounds in the atmosphere are CCl2F2, CCl3F,

CH3Cl, CCl4 and CH3CCl3 . It has been reported that trace amounts of bromine

5

might efficiently catalyse the ozone reduction. The predominant anthroprogenic

source is CH3Br and CF3Br. Sometimes sources such as CF2BrCl and C2H4Br2

are entirely man made. CH3Br is used as a soil fumigant and other compounds of

bromine as fire retardants and fuel additives.

Vinyl chloride which is an unsaturated halide is used primarily in the

production of polyvinylchloride resins. It is known to cause cancer. Methylchloride

is used in the manufacture of silicones.Trichloroethylene is a solvent that was

quite commonly used as a cleaning agent and it is a suspected carcinogen.

1,2-dichloroethane is a metal degreaser and used in the manufacture of number

of products such as varnish removers, fumigants and soap compounds. High

level exposure of this solvent is known to cause injury to the central nervous

system, liver and kidneys.

The organochlorine compounds such as methyl chloride, methyl

chloroform and carbon tetrachloride have tropospheric concentrations ranging

from ten to several tenths of ppm. Methyl chloroform is relatively persistent in the

atmosphere with residence time of several years. Therefore they may pose the

threat to ozone layer as CFCs. Another class of organohalides are

polychlorinated biphenyls (PCBs). They are made by chlorinating the aromatic

compound biphenyl. A complex mixture results with variable numbers of chlorine

atoms substituted at various positions of the rings. They were mainly used as the

coolant in the power transformers and capacitors because they are excellent

insulators, are chemically stable, and have low flammability and vapour pressure.

In later years they were also used as heat transfer fluids in other machinery and

as plasticisers for polyvinylchloride and other polymers; they found additional

uses in carbonless copy paper, as de-linking agents for recycled newsprint and

as weathering agents. As a result of industrial discharges and the disposal of all

6

these products, PCBs were spread widely in the environment. These PCBs can

be transported with atmospheric particles.

Chlorofluoro Carbons And Depletion Of Ozone Layer:

Chlorinated fluorocarbons (CFCs) are a class of compounds initially

developed in 1930s; they have properties that make them particularly useful in a

number of applications. As mentioned in the previous paragraph under organo-

halide compounds, these CFCs have desirable properties such as low viscosity,

low surface tension, low boiling point, and chemical and biological inertness. The

last property accounts for their being non-toxic and non-flammable. Because of

these advantageous properties, these compounds were used as refregerants,

solvents for cleaning electronic and other components, and blowing agents for

polymer foams.

One of the important environmental properties of these CFCs is the

ozone depletion potential (ODP). It is defined as the ratio of the impact on ozone

from a specific chemical to the impact from an equivalent mass of CFC-11

(CFCl3), the standard by which all others are calculated. The ODP values takes

into account the reactivity of the species, its atmospheric lifetime, and its molar

mass. The The amount of chlorine in the chemical species is also important.

Most of the CFCs have ODP values between 0.1 and 1.0, while

hydrofluorocarbons (HCFCs) have ODP values which are about ten times lower

(0.01-0.1). Hydrofluorocarbons, which contain no chlorine, have a zero ODP

value.

Another important property of the CFCs is that they are almost completely

inert both biologically and chemically in the earth's environment, in the

troposphere. Because they do not react, they circulate through the troposphere

7

until they escape into the stratosphere. While unreactive in the troposphere, at

higher altitudes they are able to undergo ultraviolet photolytic decomposition, as

a consequence of being exposed to intense flux of energetic ultraviolet radiation.

3CFCl h ( 290 nm) CFCl Cl+ υ λ < → +i 2 i

2

………………………………….(10)

The released chlorine radical is now able to take part in the catalytic

cycles shown in reactions 11,12 & 13.

3Cl O ClO O+ → +i i ………………...(11)

2ClO O Cl O+ → +i i ……………….(12)

Net reaction 3O O 2O2+ → ……………………(13)

A second and possibly third chlorine radical could also be produced by

further decomposition of the remnant. 2CFCli radical, generating additional

potential for ozone depletion.

CFC substitutes:

Current research is centered on modifying the relative amounts of

fluorine, chlorine and hydrogen in new compounds. A common approach has

been to incorporate hydrogen in the structure which is then called as

hydroflurocarbons (HCFCs) or replace chlorine altogether and produce what are

known as hydrofluorocarbons (HFCs). Increasing the hydrogen content reduces

the inertness, in this way making the tropospheric lifetime shorter. The presence

of hydrogen increases the reactivity and flammability of the compound and higher

reactivity is a disadvantage in some applications. Increasing the proportion of

fluorine at the expense of chlorine tends to to produce a highly stable compound.

8

However such compounds are excellent and persistent green house gases. For

compounds containing no chlorine, obviously no chlorine atoms can be released,

so that there is no potential to deplete ozone.

From the combined industrial-environmental perspective, one of the

more promising new compounds is HFC-134a (CF3CH2F). This HFC has

intermediate stability; it is oxidised by the hydroxyl radical in the troposphere, and

has a residence time of 18Y. It is not very flammable. Since it does not contain

any chlorine, it will have no ozone depletion potential. Unfortunately it is

expensive to manufacture. Another CFC replacement is HCFC-123 (CF3CHCl2) ,

which contains chlorine but has only about one-tenth the ozone depletion

potential of CFC-11, largely because of its relatively short tropospheric lifetime.

It is important to mention here that there are other processes that occur

in the stratosphere which are in competition with the catalytic cycles. Their

relative importance further complicates our ability to make predictions about the

extent of ozone destruction that may occur under various conditions. Null and

holding cycles are two other types of reaction sequences that prevent the

species from taking part in the catalytic processes.

Null (do nothing) cycles interconvert the species X and XO while effecting

no net odd oxygen removal. Null cycles involving nitrogen oxides are shown

below.

3 2NO O NO O+ → + 2

h NO O+ υ → +

O O+ υ → +

……………….(14)

NO …………………(15) 2

Net reaction O h …………………(16) 3 2

9

This sequence competes with catalytic cycle and is important only during

daytime as it requires radiation in the near-ultraviolet region. While the net effect

is ozone photolysis, ozone is rapidly and stoichiometrically synthesised by

reaction (17).

2 3O O M O M+ + → + ……………………(17)

Another example of a null cycle is the reaction involving NO2 which results

in the production of NO3 and the establishement of a cycle.

2 3 3NO O NO O+ → + 2

2

2 O

…………..(18)

……………(19) 3NO h NO O+ υ→ +

Net reaction 3O h NO+ υ→ + ……………..(20)

Some of the NO3 reacts in a three-body process to produce N2O5.

3 2 2 5NO NO M N O M⎯⎯→+ + +←⎯⎯ ……………....(21)

The N2O5 is a relatively stable species and therefore it behaves as an

unreactive reservoir of NOx. N2O5 is not permanently stable as it ultimately

decomposes back to NO2 and NO3. Reaction (21) therefore acts as a holding

cycle, temporarily preventing the catalytic decompostition of ozone.

Nitric acid and hydrochloric acid are formed in the stratosphere and these

reactions serve as reservoirs for ozone depleting nitric oxide and chlorine

species.

2NO OH M HNO M+ + → +i i 3

3i

………………………(22)

……………………………..(23) 4Cl CH HCl CH+ → +i

10

Almost 50% of NOx is stored in nitric acid reservoir and 70% of chlorine is

stored in hydrochloric acid reservoir.

The nitric acid undergoes photolysis in the day light , producing nitrogen

dioxide in the reverse reaction of (22) and the hydrochloric acid releases chlorine

and water after reaction with hydroxyl radical.

In addition several other species have been identified as reservoirs for

NOx and Cl in the stratosphere as indicated by the following equations.

2ClO NO M ClO NO Mchlorine nitrate

+ + → +i i 2

2

…………………..(24)

……………………(25)

……………………(26)

2 2 2HOO NO M HO NO Mpernitricacid

+ + → +i i

2ClO NO M ClO NO Mchlorine nitrate

+ + → +i i

These compounds act as reservoirs for Cl and NOx until they are released

as active catalysts into the troposphere.

The appearence of Antartic and Arctic 'Ozone holes' is the consequence

of the release of these as active catalysts.

Consequences Of Ozone Depletion:

Ozone depletion has a number of consequences for human health and

agriculture. These include increased rate of skin cancer and eye cataracts,

weakening of immune systems, damage to crops, reductions in primary produces

(plankton) in the ocean and increasing air pollution.

11

Photo Chemical Smog:

Photo chemical smog was first observed in Los Angeles, U.S.A in the mid

1940s and since then this phenomenon has been detected in most major

metropolitan cities of the world. The conditions for the formation of photochemical

smog are air stagnation, abundant sunlight, and high concentrations of

hydrocarbon and nitrogen oxides in the atmosphere. Smog arises from the

photochemical reactions in the lower atmosphere by the interaction of

hydrocarbons and nitrogen oxide released by exhausts of automobiles and some

stationary sources. This interaction results in a series of complex reactions

producing secondary pollutants such as ozone, aldehydes, ketones, and

peroxyacyl nitrates. The nature of the dynamic photochemical smog is illustrated

in Fig.1.

The fig.1 illustrates the characteristic variation with time of the day in

levels of NO, NO2, hydrocarbons,aldehydes and oxidants under smoggy

atmospheric conditions in a city with heavy vehicular traffic. From the figure it can

be seen that shortly after sun rise, the level of NO in the atmosphere decreases

markedly followed quickly by increase in NO2. NO2 reacts with sunlight leading to

various chain reactions and ultimately to the production of ozone and other

oxidants. During the midday when the concentration of NO has fallen to a very

low level, the levels of aldehydes and oxidants become relatively high. The

concentration of total hydrocarbon in the atmosphere reaches it maximum in the

morning and then decreases during the remaining daylight hours. The typical

smog episode occurs in hot, sunny weather under low humidity conditions. The

characteristic symptoms are the brown haze in the atmosphere, reduced

visibility, eye irritation, respiratory distress and plant damage.

12

0

0.1

0.2

0.3

0.4

0.5

M 4 A.M 8 A.M N 4 P.M 8 P.M M

non methanehydrocarbons

aldehydes

OxidantNO

NO2

Fig. 1 Dynamic behaviour of photochemical smog

(Redrawn by permission of Lewis Publishers, Chelsea, Michigan 48118, USA from

Fundamentals of Environmental Chemistry, S.E.Manahan, p.630,1993)

The control of photochemical smog may require substantial reduction in

NOx produced in urban areas. At the same time it is necessary to control the

release of hydrocarbons from numerous mobile and stationary sources. Catalytic

converters are now used to destroy the pollutants in exhaust gases. A reduction

catalyst is employed to reduce NO in the exhaust gas and after injection of air an

oxidation catalyst is used to oxidise the hydrocarbon and CO.

13

Chemical reactions involved in smog formation in the atmosphere:

The starting mechanism is the absorption of uv light from the sun by NO2 .

This causes the nitrogen dioxide to decompose into nitric oxide.

2NO h NO O+ υ→ + ............................................................................(26)

and reactive atomic oxygen (26). The atomic oxygen initiates oxidising processes

or quickly combines with molecular oxygen to form ozone, which itself is reactive

and acts as oxidant.

2 3O O M O M+ + → + ………………………………………………….(27)

…………………………………………………..(28) 3 2O NO NO O+ → + 2

In equation (27) an energy absorbing molecule or particle (M) is required

to stabilise O3 or else it will rapidly decompose. Under normal conditions, the

ozone formed will be quickly removed by reaction with NO to provide NO2 and O2

according to equation (28);however when the hydrocarbons are present in the

atmosphere, this mechanism is partially eliminated as NO reacts with the

hydrocarbon radical peroxyacyl (RCO3•) according to equation (32) RCO3• also

reacts with O2 to give O3 (equation 33) and as a result ozone concentration

builds up to dangerous levels.

Hydrocarbons, indicated by symbol HC compete for free oxygen released

by NO2 decomposition to form oxygen bearing free radicals such as acyl radical.

HC O RCO (acyl radical)+ → i ………………………………………(29)

14

This radical takes part in a series of reactions involving the formation of

still more reactive species , which in turn react with O2 , hydrocarbons and nitric

oxide.

………………………(30) 2 3RCO O RCO (peroxyacyl radical)+ →i i

3

2

RCO HC RCHO(aldehydes)R CO (Ketones)

+ →i………………………………(31)

3 2RCO NO RCO NO+ → +i i 2

3i

2

…………………………………….(32)

3 2 2RCO O RCO O+ → +i …………………………………….(33)

Reactions represented by equation(31) are termination reactions forming

aldehydes and ketones; however, in equations (32) and (33) the peroxyacyl

radical reacts with NO and O2 to produce another oxidised hydrocarbon radical

(RCO2• ) as well as more NO2 and O3. Further the acylate radical (RCO2• ) can

react with NO to generate even more NO2.

2RCO NO RCO NO+ → +i i …………………………………(34)

The NO level in the atmosphere eventually drops off with the accumulation

of NO2 and O3. When the reactions such as these increase the NO2 level

sufficiently, another reaction begins to compete for the peroxyacyl radical.

3 2 3 2RCO NO RCO NO (PANS)+ →i ……………………….(35)

The end products are known as peroxyacyl nitrates or PANS. Numerous

PANS could be formed, corresponding to different possible R groups.

15

Three of the common members of PAN family are:

H-C-OO NO2 :

O

CH3-COO NO2

O

: Peroxy acyl nitrate (PAN)

Peroxy formyl nitrate (PFN)

C6H5-C-OO NO2 : Peroxy benzoyl nitrate (PBN)O

The ozone formed according to equations (27) and (33) reacts with

hydrocarbons to generate more aldehydes and ketones.

3 2 2HC O RCO RCHO, R CO+ → +i …………………………(36)

The above equation represent in a broad sense the nature of the overall

photochemical reactions leading to the formation of smog.

It has been observed that CO and SO2 also play a significant part in the

process of formation of smog by strongly interacting with many species present

in the smog and accelerate the oxidation process.

For example carbon monoxide does this through a series of reactions

whose net effect is to convert CO, NO and O2 into CO2 and NO2 thus

accelerating the oxidation of NO. First CO is oxidised to CO2 by OH• radical

(equation 38).

n the smogy atmosphere the OH• radical may be produced when

aldehydes are attacked by atomic oxygen.

3 3O CH CHO CH CO OH+ → + i…………………………..(37)

16

2CO OH CO H+ → +i i

i

2

2

3

…………………………………….(38)

2 2H O M HO M+ + → +i …………………………………(39)

The H• radical react with O2 to form hydro peroxyl radical HO2• which is the

principal agent for rapid conversion of NO into NO2.

2HO NO OH NO+ → +i i ……………………………….(40)

The overall reaction is

2 2CO O NO CO NO+ + → + …………………. …….(41)

This sequence of reaction provide another route for the oxidation of NO without

the participation of O3 .

Similarly, the reaction of SO2 with HO2• radical may be an important step in the

mechanism of the oxidation of SO2 to SO3 .

2 2HO SO OH SO+ → +i i ………………………..(42)

In addition, the hydrocarbon radicals may give off an oxygen atom to SO2

to form SO3, which in turn is converted to H2SO4 droplets resulting in the

formation of haze.

Among the important reactions forming nitric acid are the reaction of N2O5

with water and addition of hydroxyl radical to NO2. The oxidation of NO or NO2 to

nitrate species subsequently may occur after the absorption of the gas by aerosol

droplet. The nitric acid formed interacts with ammonia in the atmosphere to form

salts.

17

Organo Nitrogen Compounds:

The different organonitro nitrogen compounds that may be found as

atmospheric contaminants may be classified as amines, amides, nitriles, nitro

compounds and hetero cyclic nitrogen compounds, The lower-molecular-mass

amines which are volatile, are prominent among the compounds giving rotten fish

odour. A number of amines are widely used as industrial chemicals and solvents.

Decaying organic matter, especially protein wastes, produces amines.

Aromatic amines are of particular concern as atmospheric pollutants,

since they are known , to cause bladder cancer in exposed individuals. Aromatic

amines of potential concern are aniline,benzidine, 3,3' dichlorobenzidine,

naphthylamine, 2-naphthyl-amine, and phenyl-2-naphthyl-amine.They are widely

used as chemical intermediates, antioxidants, and curing agents in the

manufacture of polymers, drugs, pesticides, dyer, pigments, and inks. These

amines also can react with hydroxyl radicals to give rise to harmful products.

Being bases they react with acids in the atmosphere.

Dimethyl formamide is the most encountered among the amides as

atmospheric pollutant. Most amides have relatively low vapour pressures, which

limits their entry into the atmosphere. Acetonitrile and acrylonitrile are the

prominent contaminants under nitriles. Both are used in the synthetic rubber

manufacture. Acrylonitrile, used to make polyacrylonitrile polymer is the

prominant pollutant among the nitrogen containing organic compounds.

The notable pollutants among the nitro compounds include nitromethane

and nitrobenzene. The important nitro compound produced by the photochemical

oxidation of hydrocarbon in urban atmospheres is peroxy acetyl nitrate (PAN).

18

Among atmospheric contaminants nitrosamines are of special concern

since they are known to cause cancer. Two nitrosamines have been detected in

the atmosphere namely N-nitrosodimethylamine and N-nitroso diethylamine.

Organic Particles In The Atmosphere

The combustion processes of automobile engines produce significant

amount of organic particulates. A significant amount of organic particulate matter

is produced by automobile engines in combustion processes. The organic

particulates of greatest concern are polycyclic aromatic hydrocarbons (PAN),

which consist of condensed ring aromatic molecules. The most often cited

example of a PAH compound is benz (a) pyrene, a compound that can be

metabolized in human body to a carcinogenic form.

PAHs can be synthesised from saturated hydrocarbons under oxygen

deficient conditions. Hydrocarbons with very low molecular masses, including

even methane, may act as precursors for the polycyclic aromatic compounds.

The process of PAH formation from low molar mass. hydrocarbons is called

pyrosynthesis. This happens at temperatures exceeding ≈ 500o C at which C-H

and C-C bonds are broken to form free radicals. These radicals undergo

dehydrogenation and combine chemically to form aromatic ring structures. PAHs

also originate from pyrolysis of higher paraffins present in fuels and plant

materials.

Soot is formed as a residue during the combustion of fuel in power plants

and automobiles. It is due to the incomplete combustion of organic products. It is

an impure form of elemental carbon (graphite). Soot particles are roughly

spherical, whereas graphite has a layered structure. Soot forms accretion of

graphite-like precursors. It is known that most PAH compounds are sorbed on

19

soot particles. Soot consists of many condensed aromatic rings containing 1-3%

H, 5-10% O and trace metals such as Be, Cd, Cr, Mn, Ni and vanadium and also

toxic organics such as benzo (a) pyrene. This is illustrated in fig.2.

Soot particle

Adsorbed organics

Adsorbed toxic metals

0.1 - 20 µ

Fig 2. Soot particle from combustion of fossil fuels

20