A polluted atmosphere is generally considered an unnatural atmosphere. While it is true there are numerous natural materials in the earth’s atmosphere that, at times, can prove quite objectionable, such as pollen and dust.
However such materials are not usually referred to as air pollutants. To understand what man has added to the atmosphere, it is necessary to know something about the history and composition of the atmosphere as provided by nature.
In addition to the constituents which occur in a more or less constant concentration in the atmosphere, there are a number of decidedly variable constituents of natural origin.
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It decreases in density as the distance from the earth increases. The atmosphere gradually thins out into space. At about 600 km and above, the atoms and molecules describe free elliptical orbits in the earth’s gravitation field.
The proportion of the different components of the atmosphere remains essentially constant from the earth’s surface to a height of about 10 miles (16 km). Above this height, gravitation separation probably begins, although it does not become important until above about 80 miles (128 km).
The earth’s atmosphere has not always had its present composition, although the latter has remained relatively unchanged throughout most of geological time. It is generally agreed that the predecessor of our present atmosphere consisted mainly of carbon dioxide, nitrogen and water.
It may have been similar in composition to the gases emitted by present day volcanoes, which do not contain any oxygen. In some manner, life on earth started and the chlorophyll-containing plants converted some of the carbon dioxide and water into oxygen.
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Whether these phenomena occurred simultaneously or consecutively is probably immaterial at present. What is important is that changes have occurred and the probable nature of these changes helps us understand the chemistry of the atmosphere.
The hydrosphere and biosphere must be considered first in any discussion of the atmosphere. There is a constant shift of substances between these three spheres.
The gases exist in the atmosphere; the hydrosphere contains all the waters of the oceans, the lakes, and the streams; and the biosphere comprises all living matter. Of the three, the biosphere has the least weight. They may be arranged in the following order according to their relative weights:
There is a constant exchange between the earth’s crust and atmosphere.
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The common constituent gases of the atmosphere have been rather intensively studied. Much is known about the reactions in which such gases have taken part since the formation of the earth approximately three million years ago.
Oxygen:
The atmosphere is constantly acquiring oxygen from chlorophyll bearing plants, which take up carbon dioxide and evolve oxygen. Some of the oxygen in the atmosphere dissolves in the hydrosphere.
At the same time, oxygen is being removed from the atmosphere by the oxidation of the -various reducing materials in the earth’s crust such as sulphur, iron oxides, and mangnese salts.
The amount of oxygen lost from the atmosphere by the oxidation of FeO to Fe2O3, MnO to Mn02 and SO2to SO3 has been estimated on the basis of the analysis of the ratio of ferric to ferrous iron in various types of rocks and on the weight of igneous rocks which have been eroded from the earth during geological time.
Based on certain assumptions made in the calculation, the amount of ‘fossil’ oxygen is between about 0.56 and 0.26 kg/sq. cm. of the earth’s surface. The amount of oxygen in the hydrosphere is equivalent to 0.002 kg/sq. cm. and in the atmosphere to 0.230 kg/cm sq.
Thus, the total free and fossil oxygen is between 0.79 kg/sq. cm. and 0.49 kg/sq. cm. In other units, about 2.5 x 1015 metric tonnes of oxygen has been produced by plants during the 3 billion years of the earth’s existence, a large part of which, at present, is combined oxygen in sedimentary rocks.
It may be pointed out that there are, at least, two other hypotheses to explain the occurrence of oxygen in the atmosphere. One is that free oxygen was formed by thermal decomposition of water vapour while the earth and its atmosphere were very hot.
However, it is difficult to refute the objection that the hydrogen and oxygen would later recombine on contact with the still hot surface of the earth.
Another hypothesis is that photochemical decomposition of water vapour occurred, at a later and cooler stage, in the upper atmosphere. However, it is unlikely that there was ever sufficient water vapour at that high altitude to account for all the oxygen which was produced.
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Carbon Dioxide:
All living matter on the earth is involved in the rejection of the carbon dioxide in the atmosphere. Carbon dioxide is rejected into the atmosphere by volcanoes, by decay of organic matter, and by the combustion of fuels.
It is also in equilibrium with the dissolved carbon dioxide in the waters of the earth. In addition, much is stored in limestones and dolomites; carbon which was once atmospheric carbon dioxide is stored in coal and petroleum. Goldschmidt estimated the following values for the weight of carbon dioxide per square cm.
The annual production of carbon dioxide by respiration and decay is approximately 0.040 g/sq. cm year. Over the entire earth’s surface of 5.1 x 1018 sq. cm. total C02 produced in one year is 2 x 1011 metric tonnes.
The entire atmosphere contains approximately 2 x 10″ metric tonnes CO2. In other words, decay and respiration are adding carbon dioxide to the atmosphere at the rate of approximately 10 per cent per year.
The combustion of fuels is also adding to the amount of carbon dioxide, but in a relatively insignificant manner. In 1951, the entire world had an energy consumption equivalent to 2.5 x 109 metric tonnes of coal.
Approximately 20 per cent of this energy was hydroelectric power in Europe and, somewhat, less in the United States If it is assumed that 15 percent of the total world’s energy is hydroelectric power and that coal contains 90 percent carbon, then the combustion of coal to carbon dioxide would add only 7 x105 metric tonnes of carbon dioxide per year or one-thirtieth of the amount produced by decay and respiration.
Geological evidence suggests that relatively little change has occurred in the concentration of carbon dioxide from Archaeozoic times to the present. However, there must have been ceaseless minor fluctuations. Such data as exist show that the carbon dioxide content of the atmosphere has increased about 30 ppm in the last 50 years.
The fact that it has not increased more is probably because of its consumption by plants and because the waters of the earth act as a huge reservoir which contain 50 times as much dissolved C02 as is mixed in our gaseous atmosphere.
Nitrogen:
The amount of nitrogen that undergoes changes is small compared to that of oxygen or carbon dioxide. Nitrogen is removed from air by both organic and inorganic processes. Organic processes involve nitrogen fixing micro-organisms and some blue-green algae.
The inorganic processes produce oxides of nitrogen by electrical discharges and photochemical reactions in the upper atmosphere.
The amount of organically fixed nitrogen far exceeds that fixed by inorganic processes. It has been estimated that biological fixation adds 0.07 mg of nitrogen per square centimeter of the earth’s surface per year and non-biological fixation adds not more than 0.005 mg/sq. cm.
Much of this nitrogen is eventually returned to the atmosphere by the decay of organic matter. Adel, who suggested the presence of nitrous oxide in the atmosphere, concludes that it is supplied to the atmosphere as part of the earth’s nitrogen cycle.
The presence of nitrous oxide has been confirmed by infrared spectroscopy and a mass spectrometer. It has been estimated that the average content of the combined nitrogen in sediments is 510 g/tone. Multiplying this by Goldschmidt’s figure of 170 kg/sq. cm. for the total amount of sediments formed during geologic time, gives 86.7 g/sq. cm. of fossil nitrogen.
Nitrogen is also a constituent of all igneous rocks, which contain an average of 0.04 cu cm of nitrogen per gram of approximately 0.005 percent by weight. It is evidently present in a combined form, since it may be released from the rocks as ammonia by means of fusion with soda ash. Ammonia is also a constituent of volcanic gases.
Ozone:
A large gap exists in the present understanding of atmospheric ozone. The concentration at ground level is highly variable both at one locality and from place to place.
Little is known about the effect of weather conditions on its concentration, particularly with reference to its vertical distribution. In most areas of the world where it has been measured, its concentration at ground level ranges between 0 to 5 ppm.
However, there is considerable evidence to prove that the concentration near the ground is often considerably greater. Concentrations of ozone greater than 20 ppm in Alaska have also been reported. The concentrations of ozone, in Southern California, have been found to be as high as 20 to 50 ppm in the Los Angeles area, on Mt. Wilson, and in the desert.
Aerosols:
In our everyday life, visual perception plays an important role and dominates the other senses with which we are equipped. It is, therefore, not surprising that the average person associates the term atmospheric pollution with clouds of dust and palls of smoke. The word ‘particle’ is frequently used to describe the particles in aerosols.
However, gas is also particulate; so aerosols really mean, at least, groups of molecules. Aerosols are set apart from gases such as nitrogen, oxygen, argon, carbon dioxide, and ozone as being present in larger groups than individual molecules of gases.
Dust particles are larger than fine or smoke particles, and although they may be suspended in the atmosphere, they settle according to Stokes’ law. The smaller fume or smoke particles on the other hand, may be either liquid or solid and exhibit Brownian motion.
When a liquid or solid is dispersed to form a cloud or a smoke, many of its properties are greatly altered. For example, the surface per unit weight (specific surface) is greatly increased and the nature of the surface may greatly influence the behaviour of the material. It is assumed that the particles are spherical in shape and that their specific gravities are not changed by dispersion.
These assumptions are not strictly valid, nor are the typical diameters of the particles truly representative in all cases. Nonetheless, the figures provide a general indication of the size of such particles.
As a result of the increase in surface area, the properties that depend upon the activity of the surface molecules are greatly enhanced.