Mining and quarrying operations send mineral dusts into the air. For example, asbestos mines (together with asbestos processing and manufacturing plants) send into the air countless threadlike mineral particles some of which are so small that they can be seen only with the electron microscope. These particles travel widely and are inhaled by humans, lodging permanently in the lung tissue. Nuclear test explosions inject into the atmosphere a wide range of particles, including many radioactive substances capable of traveling thousands of miles in the atmospheric circulation. Certain of these pollutants belong to a very special and dangerous class, for they are sources of ionizing radiation.
Man-induced forest and grass fifes add greatly to smoke palls in certain seasons of the year. Ploughing, grazing, and vehicular traffic raise large amounts of mineral dusts from surfaces of deserts and steppes. Bacteria and viruses, which we have not as yet mentioned, are borne aloft in air turbulence when winds blow over contaminated surfaces, such as farmlands, grazing lands, city streets, and waste disposal sites.
The proportions will, of course, change somewhat from one year to the next. Much of the carbon monoxide, half of the hydrocarbons, and about a third of the oxides of nitrogen comes from exhausts of gasoline and diesel engines in vehicular traffic. Generation of electricity and various industrial processes contribute most of the sulphur oxides, because the coal and lower-grade fuel oil used for these purposes are comparatively rich in sulphur. These same sources also supply most of the particulate matter. Fly ash is the term applied to the coarser grades of soot particles emitted from smokestacks of generating plants. These particles settle out quite quickly within close range of the source.
Combustion used for heating of buildings is a comparatively minor contributor to pollution, because the higher grades of fuel oil are low in sulphur and are usually efficiently burned. Very finely divided carbon comprises much of the smoke of combustion and is capable of remaining in suspension almost indefinitely because of its collodial size. Forest fires comprise a secondary contributor of particles. Burning of refuse is a minor contributor in all categories of pollutants.
In the smog of cities there are, in addition to the ingredients mentioned above, certain chemical elements contained in particles contributed by exhausts of automobiles and trucks. Included are particles that contain lead, chlorine, and bromine. About half of the particles in automobile exhaust contain lead.
Primary pollutants are conducted upward from the emission sources by rising air currents that are a part of the normal convective process. The large particles settle under gravity and return to the surface as fallout. Particles too small to settle out are later swept down to earth by precipitation, a process called washout. By a combination of fallout and washout the atmosphere tends to be cleansed of pollutants. In the long run a balance is achieved between input and output of pollutants, but there are large fluctuations in the quantities stored in the air at a given time. Pollutants are also eliminated from the air over their source areas by winds that disperse the particles into large volumes of cleaner air in the downwind direction. Strong winds can quickly sweep away most pollutants from an urban area, but during periods when a stagnant anticyclone is
present, the concentrations rise to high values.
In polluted air, certain chemical reactions take place among the components injected into the atmosphere, generating a secondary group of pollutants. For example, sulphur dioxide (SO2) may combine with oxygen to produce sulphur trioxide (SO3), which in turn reacts with water of suspended droplets to yield sulphuric acid (H2SO4). This acid is both irritating to organic tissues and corrosive to many inorganic materials. In another typical reaction, the action of sunlight upon nitrogen oxides and organic compounds produces ozone (03), a toxic and destructive gas. Reactions brought about by the presence of sunlight are described as photochemical. One toxic product of photochemical action is ethylene, produced from hydrocarbon compounds.
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