Some of the commonest sources of air pollution are the following:
(1) Carbon monoxide, mostly released from motor vehicles, heating and industry. Major portions of atmospheric CO result from engines powered with petroleum derivatives and used for transportation purposes.
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(2) Sulphur oxides, mostly released from power-generating plants and industrial concerns.
(3) Nitrogen oxides, mostly released by motor vehicles, power plants and industries.
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(4) Hydrocarbons, mostly discharged by motor vehicles and industries.
(5) Particulate matter, mostly from industries, power plants and refuse disposal. Most of this particulate matter has not yet been adequately identified.
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(6) Naturally produced air pollutants, e.g., pollen, volcanic gases, marsh gas, etc.
(7) Metals such as nickel, beryllium, arsenic, tin, vanadium, titanium, lead, cadmium, etc., which have been detected in the atmosphere in the form of solid particles or liquid droplets or gases. Mostly they come from fumes and dust from metallurgical processes; also from seaspray and volcanic ash, etc.
Fortunately for us, the release of reasonably low levels of any of the above pollutants often does not lead to any serious effects. This is because the global atmosphere is endowed with considerable absorptive capacity, and the natural regulatory systems can cope with the reasonable amounts of these pollutants.
However, when any of these pollutants reaches dangerously high or otherwise intolerable concentrations over a limited area, then the tolerance of the natural regulatory cycle is exceeded and dangerous consequences arise. Secondly, the emission of industrial smoke in some advanced countries has been greatly reduced through improved design of industrial burners and combustion equipment. This has been achieved through appropriate legislative measures.
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Increasing petroleum scarcity or unavailability may call for increased use of coal in the future. Coal is a complex heterogeneous material containing both organic (C, N, H, O) and inorganic elements. The mineral matter (impurities) may be broadly divided into two categories, those forming ash and those that contribute sulphur. Some elements of environmental concern present in significant quantities in coal include S, As, Be, Cd, Cu, Pb, Hg, Mn, N, Se and Zn.
Coal preparation (cleaning) processes generate million tonnes of wastes. Interaction of air and water in pyrite-rich coal wastes converts the pyritic sulphur to dilute H2SO4 leachate which may also contain high concentrations of trace elements and other dangerous pollutants. Some impurities present in coal are chemically bound and cannot be removed; others can be removed to varying extents mechanically.
Future projections indicate that coal will be utilized to produce synthetic fuels by gasification and liquefaction. These processes in turn may pose new toxicological problems. Researchers are now underway to produce synthetic fuels from such non-petroleum sources of combustible hydrocarbons as oil shale and tar sands.
The aqueous condensates derived from coal gasification and liquefaction processes constitute primary effluents. The major organic components in these condensates are phenols, cresols and some low molecular weight carboxylic acids. Even when highly diluted, these condensates are toxic to Daphnia.
Coal combustion produces large amounts of SOx, NOx and particulates all of them which are hazardous to agricultural crops and human health. But modern pollution control devices can sharply reduce the emission of pollutants into the atmosphere. The technology to limit emissions of pollutants from coal burning includes such processes as coal washing, and use of electrostatic precipitators and stack gas scrubbers.
About 75 per cent or more of the sulphur oxides emitted by power plants can be removed by means of commercially available “scrubbers.”
Coal cleaning involves crushing the coal and using the differences in density between coal and its contaminants to separate the two. Such cleaning can at least remove a fraction of the sulphur content of the coal and thus minimize the SOx emissions.
High efficiency electrostatic precipitators can remove much of the particulate matter arising from coal combustion, but the finer particles (smaller than 3 microns) which pose serious health hazard cannot be precipitated, Fabric filters are now being used to control particulate emissions from utility boilers burning low sulphur coal.
Nitrogen oxides probably pose the most serious air pollution control challenge. These oxides come both from automobiles and from certain stationary combustion sources. Fuel combustion in a wide variety of equipment is estimated to contribute about 99 per cent of technology associated NOx emissions. About 95 per cent of the NOx is emitted as NO and the remaining 5 per cent as NOx In the atmosphere, NOx react photochemically with hydrocarbons and SO2 to form undesirable secondary compounds, with a shift of residual NO to NO2.
The main method of NO control is combustion modification, which uses changes in the combustion conditions within the boiler plant to minimize high-temperature fixation of atmospheric nitrogen with oxygen. The combustion modification programme seems to develop technology capable of controlling emissions from the two chief stationary sources of NOx viz., thermal NOx and fuel NOx. Thermal NOx may be controlled by lowering the combustion temperature whereas fuel NOx may likewise be controlled by lowering oxygen concentration in combustion zone.
One very promising approach to high NOx removal is the recent technique of NOx fuel gas treatment currently being adopted in Japan. Two basic approaches, involving dry and wet processes, have been developed. The dry processes involve a chemical reduction reaction 2NO+2NH3+ 1/2 O2 -> 2N2+3H2O, whereas the wet processes can involve either oxidative and/or reductive chemistry. Some of these wet processes involve oxidation to nitrogen oxide followed by a scrubbing step, others reduce the nitrogen oxides in solution.
Ozone is a typical oxidant for the oxidative/reductive approach. The wet processes seem more advantageous than the dry processes because they can bring out a simultaneous removal of both SOx and NOx. However, the wet processes are somewhat more costly than the dry ones.
Fly-ash leachates from coal combustion have been found to be highly toxic to Daphnia and the toxicity is related to the saline alkaline properties of these leachates.
Contributions to the atmosphere by man are dominated by the mining/ processing industry for Cd, Cu and Zn (around 60%) but by petrol and diesel additives respectively for Pb (60%) and Ni (55%), with smaller contributions from the combustion of wood, coal, and refuse, and from other industrial processes.
Current estimates indicate that the anthropogenic contributions to atmospheric material are calculated to exceed those from natural sources by factors ranging from two fold (Ni) to 20-fold (Pb). Natural sources are apparently dominated by eroded soil materials of ‘normal’ composition (around 70%) except in the case of Cd where occasional volcanic sources tend to dominate (about 60%). Vegetative exudates are important for Zn (20%), Cd and Cu (10- 15%) whilst smaller, localized additions come from forest fires and from the concentration of trace elements in sea spray.
Green plants are more sensitive to acute exposure to SO2 than, are most mammals. Up to 5 ppm of SO2 elicits little effect on mammals but can inflict massive damage to many plants in the form of necrotic lesions on leaves.
Since SO2 reduces crop yield in proportion to leaf area destroyed, the most serious consequence of an increased SO2 content in atmosphere may be the impact on agricultural productivity, especially in areas near coal burning factories.
The mechanism by which SO2 injures plants remains unknown. In cucurbits there seem to be two mechanisms by which resistance to SO2 injury may be achieved. One of these involves genetically determined resistance to SO2 absorption. The other is developmentally controlled, and enables young leaves to absorb SO2 without injury.
A gradual switch over from the presently widespread use of fossil fuels (oil, coal, petroleum products) to the use of natural sources of energy, e.g., sunlight, water, wind and tidal power can greatly minimize air pollution and at the same time conserve our rapidly dwindling reserves of the precious fossil fuels. At the same time, an increasing use of nuclear sources is being made to generate power and energy. Nuclear energy is obtained by fission of atoms which leads to aerial discharge of radioactive gases, e.g., tritium. These constitute yet another source of air pollution.
On a tonnage basis, more CO is emitted to the atmosphere than any other air pollutant, the principal sources of this gas being the exhaust products from motor vehicles.
Some other natural sources of CO include various forms of plant and animal life. The chemical processes associated with the biosynthesis and breakdown of the photosynthetic pigments in algae are also known to release fairly large amounts of CO.
Senescent algal cells produce more CO as compared to young cells. It has been estimated that plants may be the source of more than about 10s tons of CO annually.
The National Environmental Research Center, Cincinnati, U.S.A. has undertaken toxicological assessment of automotive emissions and the effects of certain catalysts in decreasing the harmful effects of environmental pollutants.
Palladium and platinum have been employed in automotive catalytic converters. These catalysts reduce the concentrations of carbon monoxide and hydrocarbons in the exhaust stream by oxidizing them into CO2 and water.
But the use of these catalysts often generates a side problem of potential public health concern viz., the emission of sulphate particles and sulphuric acid aerosols. It is likely that a significant buildup of sulphates may occur in big metropolitan cities by catalyst-equipped automobiles (especially those using high sulphur-content gasoline).
Textile mills emit considerable amounts of cotton dust. Pandit et al., (1972) have analysed the concentration of airborne cotton dust particles (less than 2 mm in size) from coarse and fine cloth mills located in Maharashtra, Vidarbha and Gujarat, and recorded average dust contents of 176,274 and 763 mg/100 m3 for fine mills, coarse mills and ginning presses respectively.
Cotton dust is known to be implicated in the disabling occupational disease known as Byssinosis which is widespread in textile workers in India. Pandit et al., report a definite correlation between cotton dust content of air and Byssinosis incidence and suggest that a safe limit may be 100 mg/100 m3; below this level no Byssinosis occurs whereas at concentrations exceeding 100-150 mg/100 m\ the disease incidence becomes significant. For various mills surveyed in India, the dust content was higher than safety limit and more stringent dust control measures are therefore called for.
In forests the rates of uptake of oxides of sulphur, nitrogen and carbon are determined by turbulent diffusion above and within the canopy, molecular diffusion close to the individual components of the vegetation and within stomata, or rates of solution in intercellular saps, etc.