Pure air is colourless and odourless. But various pollutants from natural and man-made sources are entering the atmosphere daily and these disturb the dynamic equilibrium in the atmosphere. This leads to air pollution when the normal properties of air are upset and both man and environment suffer.
Natural sources of air pollution are:
• Volcanic activity, vegetation decay, forest fires emitting carbon monoxide, sulphur dioxide and hydrogen sulphide and tiny particles of solids or liquids sprayed from the seas and land by wind.
Man-made sources are:
• Gases, mists, particulates and aerosols emitted by industries and other chemical and biological processes used by man.
There are five primary pollutants which together contribute more than 90 per cent of global air pollution:
- Carbon monoxide, CO
- Nitrogen oxides, NOX
- Hydrocarbons, HC
- Sulphur oxides, SOX and
Transportation accounts for more than 46 per cent of the total pollutants produced per year and hence remains the principal source of air pollution. Carbon monoxide is the major industrial pollutant, with a tonnage matching that of all other pollutants together. However, particulate pollutants, though minor, are the most dangerous among the primary pollutants (100 times more harmful than carbon monoxide).
Table:Primary Air Pollutant sources and Their Quantities (Million Tons per Year).
The above data are taken from those in USA (1990). As a matter of fact, USA and other developed countries contribute most to air pollution.
Carbon Monoxide, CO
It is a colourless, odourless and tasteless gas which is injurious to our health. Each year 350 million tons of CO (275 million tons from human sources and 75 million tons from natural sources) are emitted all over the world in which USA alone shares 100 million tons.
Transportation accounts for 70 per cent of CO emission. That is to say, diesel and petroleum engines in automobiles are primarily responsible for about 70 per cent of CO emissions.The sources of carbon monoxide, CO are the chemical reactions:
(i) incomplete combustion of fuel or carbon containing compounds:
2C + O2 ————–> 2CO
(carbon) (oxygen) (carbon monoxide)
(ii) reaction of carbon dioxide and carbon-containing materials at elevated temperatures in industries, e.g. in blast furnaces:
CO2 + C ———————————> 2CO
(carbon dioxide) (carbon)
(iii) dissociation of carbon dioxide at high temperatures:
CO2 —————– CO + O
Part of Carbon monoxide is lost in the upper atmosphere. The major sink is soil micro-organisms. A potting soil sample weighing 28 kg can completely remove in 3 hours 120 ppm carbon monoxide from ambient air. The same soil sample on sterilization failed to remove carbon monoxide from air.
Control of CO Pollution
The petroleum and diesel-fed automobiles account for major share of carbon monoxide emission. Hence efforts for carbon monoxide pollution control are mainly aimed at automobiles. Use of catalytic converters in the internal combustion engines of automobiles helps in cleaning up the exhaust emissions.
Such converters built into the automobile engines promote oxidation-reduction cycles and ensure complete combustion of carbon monoxide, nitrogen oxides and hydrocarbons. The following figure and flow-sheet illustrate the action of catalytic converters: Use of catalytic converters in two stages helps in
elimination of pollutants from exhaust gases before they are discharged into the atmosphere.
In the first converter nitrogen oxides are reduced to nitrogen (+ ammonia) in presence of finely divided catalyst platinum, and the reducing gases, carbon monoxide and hydrocarbons. The production of ammonia is kept at a minimum under carefully controlled conditions.
In the second converter, air is introduced to provide an oxidizing atmosphere for complete oxidation of carbon monoxide and hydrocarbon into carbon dioxide and water in presence of finely divided platinum catalyst.
Fig.1 Catalytic converters for treating auto emissions.
Thus by means of platinum catalytic converters, auto exhaust emissions are cleaned up through reduction-oxidation reactions. In all developed countries it is mandatory by law for all automobiles to fit their engines with catalytic converters. In India some automobile companies have plans to fit their automobile engines with catalytic converters.
Nitrogen Oxides, NOX
It consists of mixed oxides, nitric oxide and nitrogen dioxide (NO and NO2 respectively)—the former is a colourless and odourless gas but the latter (NO2) has a reddish brown colour and pungent smell.
The formation of NO and NO2 is based on the chemical reactions:
These reactions occur inside the automobile engines so that the exhaust gases consist of NOX. The latter concentration in rural air is much less than in urban air. In air NOX is converted into nitric acid, HNO3 by natural processes:
This nitric acid is one of the constituents of acid rain discussed in a subsequent section. From auto exhaust emissions NOX is removed as discussed above by means of catalytic converters.
Hydrocarbons and Photochemical Smog
Natural processes, particularly trees, emit large quantities of hydrocarbons in air. Methane, CH4 is a major hydrocarbon. It is generated in large quantities by bacteria formed by anaerobic decomposition of organic matter in water, sediments and soil.
Domestic animals (cattle, buffaloes, etc.) contribute about 85 million tons of methane to the atmosphere each year. Auto-mobiles are significant sources of hydrocarbons. In presence of ozone,carbon monoxide, nitrogen oxides and hydrocarbon participate in photochemical reactions (in presence of sunlight). A chain reaction proceeds in which the free radical R CH2• is generated in the first step.
Other free radicals which are formed are: R CH2O2• in the second step by reaction with oxygen, R CH2O•; R CH2O• in the third step by reaction with nitric oxide; HO2• in the fourth step by reaction with oxygen—a stable aldehyde R CHO is another product at this stage; HO• is formed in fifth step by reaction with nitric oxide (nitrogen dioxide is another product here); and finally, the starting free radical R CH2• is regenerated by reaction with hydrocarbon, R CH3 thereby sustaining the chain reaction.
The harmful products in the chain reaction are NO2 and aldehyde, R CHO. A side reaction also follows by another route through the aldehyde, R CHO; it gives an injurious end product, peroxy acyl nitrate (PAN) which is a strong eye irritant.
These reactions lead to photochemical smog formation, which is characterized by brown hazy fumes which irritate the eyes and lungs and also cause serious damage to plants. Photochemical smog occurs in coastal cities in winter climate, e.g. in Los Angeles, USA which have the heaviest vehicular traffic.
Sulphur Dioxide, SO2
Sulphur dioxide is a colourless gas with a pungent odour. It is produced from the combustion of any sulphur-bearing material. Sulphur dioxide, SO2 is always associated with a little of sulphur trioxide, SO3.
Man-made sources—coal-fired power stations and other industries contribute about 33 per cent of SOX pollution while natural sources, viz. volcanoes provide about 67 per cent of SOX pollution. Soot particles containing metal oxides, catalyze the oxidation of sulphur dioxide to trioxide.
The first reaction above occurs in presence of ozone and water vapour. The product, sulphuric acid is formed on aerosol (fine particle suspended in air as in smoke, fog, mist, etc.) droplet. Sulphuric acid is one of the constituents of acid rain. In winter sulphur oxides from thermal power plants along
with other gases leads to smog formation, e.g. London smog.
This is known as reducing smog in contrast with photochemical smog which is known as oxidising smog (consisting of hydrocarbons, nitrogen oxides and ozone). London smog (1952) is well-known for its disastrous effect. Heavy smog (SO2) conditions prevailed in London for five days which killed about
4,000 people. The causes of death were bronchitis, pneumonia, and other respiratory troubles particularly among aged people.
Control of SOX Pollution
SOX (sulphur oxides) from flue gases of industrial plants can be removed by means of chemical scrubbers. The flue stack gases are led through a bed of (slurry) of limestone, CaCO3 (calcium carbonate) which absorbs sulphur dioxide quite efficiently.The method is economical but the disposal of solid waste,
calcium sulphate is a problem.
Alternatively, sulphur oxide in aqueous solution is treated with citric acid salt and the resulting solution is exposed to a stream of hydrogen sulphide gas whereby sulphur is deposited. This sulphur can then be recovered and utilised.
Thermal power plants, major sources of man-made SOX pollution, are normally constructed with tall chimneys to disperse the emissions over a wide area. This reduces the local problem but creates problems for far away areas through acid rains (see below).
It has been described above that much of nitrogen oxides, NOX and sulphur oxides, SOX entering the atmosphere are transformed into nitric acid (HNO3) and sulphuric acid (H2SO4) respectively. These combine with hydrogen chloride, HCl from HCl emissions (both by man-made and natural sources) and
generate acidic precipitation, known as acid rain.
Acid rain is a major environmental issue as it badly damages the environment. It damages buildings and structural materials of marble, limestones, slate and mortar. These materials become structurally weak as calcium carbonate reacts with sulphuric acid to form soluble sulphate, which is leached out by rain water:
Fig. 6.6 Acid rain in Greece and Italy
In Greece and Italy invaluable stones and statues have been partly dissolved by acid rain. Besides these, acid rain damaged forests in Germany and lakes in Sweden and Canada. Acid rain originated from U.K. but far away in Sweden, it damaged some 8,000 lakes of which 4,000 are dead. Taj mahal stones are affected by acid rain.
Control of Acid Rain
Acid rain can be checked if its constituents, sulphur oxide and nitrogen oxide are controlled as discussed above.
Small solid particles and liquid droplets are collectively termed particulates. They originate both from natural and man-made sources. Every year natural sources discharge 800–2,000 million tons and man-made sources 200–500 million tons of particulates. Among man-made sources, fly ash from thermal power plants deserve mention. Table 6.3 gives a list of annual production of particulate matter from the two sources.
Table 2. World-wide Addition of Particulate Matter to the Atmosphere (in Million Tons)
Particulates range in size from 0.0002 µ (about the size of a molecule) to 500 µ (l µ = 10–6 metre). The number of particles in the atmosphere vary from several hundred per cm3 in clean air to more than 100,000 per cm3 in highly polluted air (urban/ industrial area).
Soot particles originate from fuel combustion and consist of highly condensed product of polycyclic aromatic hydrocarbon (PAH)—roughly 100 condensed aromatic rings. The hydrogen content of soot is 1–3 per cent and oxygen content 5–10 percent due to partial surface oxidation.
Due to large surface area, soot acts as a carrier for toxic organics, e.g. benzo-α -pyrene and toxic trace metals, e.g. beryllium, cadmium, chromium, manganese, nickel, vanadium, etc.A soot particle has an average size 0.1–20 µ . The finer particles (< 3 µ ) are the worst causes of lungs damage due to their ability to penetrate deep in our respiratory tract and thence in lungs where they remain for years and cause all sorts of diseases such an cough, bronchitis, asthma, and finally cancer.
Particulates cause increased corrosion of metals which assume serious dimensions in industrial and urban areas. They are responsible for damage to buildings, sculptures, paintings, etc.
Fig. 3. A soot particle
Particulates play key roles in the atmosphere. They reduce visibility by scattering and absorption of solar radiation. They influence the climate through the formation of cloud, rain and snow by acting as nuclei upon which water can condense into raindrops. Atmospheric particulate levels can be correlated with the extent of precipitation over cities and suburbs.
Control of Particulate Emissions
The removal of particulate matter from gas streams is an essential step for air pollution control. The best equipment is the Electrostatic Precipitator. Electrostatic precipitator is based on the principle that aero-sol particles acquire charge when subjected to an electrostatic field.
Air Pollution and Biosphere
Air pollutants are present largely in the troposphere and lower stratosphere. The ground air, l–100 metres high, is very much polluted in urban and industrial areas. Some pollutants are absorbed on vegetation, buildings and water surfaces. The primary pollutants discharged into the atmosphere, undergo chemical changes in presence of water vapour, oxygen and solar ultra-violet radiation and produce secondary pollutants.
These pollutants (secondary) have harmful effects on soil, vegetation, crops, animals, men and materials.
Plants are affected both by gaseous pollutants and by particulates deposited on soil. Acid rain over a period of time tends to reduce the soil pH (= log H+ i.e. negative logarithm of hydrogen ion concentration which is an index of acidity, alkalinity or neutrality) and renders it acidic and less fertile.
Moreover, deposition of toxic metals on soil in industrial areas makes the soil unsuitable for growth of plants. Some plants are very sensitive to traces of toxic metals as the latter inhibit the action of some plant enzymes. Particulates such as dust and soot are deposited on plant leaves and block the stomata (opening in the epidermis of plants).
This restricts the absorption of carbon dioxide and hence reduce the rate of photosynthesis as well as rate of transpiration. The overall result is retarded growth of plants and decreased yield of crops. In California, USA the presence of sulphur dioxide in air and metallic pollutants in soil killed vegetation in an area of 300 km2 and affected growth on a further 350 km2 of land.
In Leeds, U.K. there was drastic decrease in growth of lettuce and radish in heavily polluted industrial areas compared to less polluted areas of the city. Ozone and peroxyacylnitrate (PAN) (in photochemical
smog, see previous section) are oxidising agents which attack plants by oxidising their sulphydril (–SH) groups of proteins into disulphides.
This leads to inhibition of individual enzyme activity. They also affect photosynthesis by plants. Cattle are affected by air pollution, particularly under smog conditions. They develop breathing troubles, loss of appetite and show low milk yield while many of them die. Man has become the victim of air pollution. Thousands of chemicals pose the problems of health hazards during manufacture and handling.
A typical list of 24 extremely hazardous substances in the atmosphere has been submitted by the United
States Environmental Protection Agency (1973):
- Acrylonitrile, Arsenic, Asbestos, Benzene, Beryllium, Cadmium, Chlorinated solvents, Chlorofluorocarbons, Chromate, Coke oven emissions, Ethylene oxide, Lead, Mercury, Ozone, Sulphur dioxide, Vinyl chloride, Toxic waste disposal emissions and leachates (washings), etc.
Meteorology and Air Pollution
Air pollution, one of the man-made activities, has some impact on meteorology, i.e. the science of atmospheric phenomenon. Meteorology is based on the physical parameters such as temperature, wind, moisture, and movement of air masses in the atmosphere. It is also affected by the chemical properties
of the atmosphere and the chemical reactions going on in the atmosphere.
The air pollutants get dispersed in the atmosphere depending on the patterns of air circulation. In this context temperature inversion plays an important role. It occurs when a warm air mass moves above a cold air mass resulting in air stagnation of the latter (cold air) in which air pollutants get trapped.
The air above the ground becomes polluted. This happens when warm air blows over a mountain range and over cool air on the other side of the range. Such a phenomenon is observed in Denver, USA on the east of the Rocky mountains.
Human activities are partly responsible for changing the meteorology of the earth.