air

(also Azbine), a plateau in the southern Sahara, in Niger. It is made up of ancient granites and Quaternary lavas. Its average elevation is 800–900 m, with a maximum of about 1,900 m at an extinct volcano. In the valleys of the wadis there is acacia pricklewood; on the slopes, desert savanna; and on the peaks, bare desert. Livestock—zebu and camel—is raised. In the oases, doom palm, millet, wheat, cotton, and maize are cultivated.

a natural mixture of gases, mainly nitrogen and oxygen, which makes up the earth’s atmosphere. The most important geological processes on the earth’s surface, as well as the formation of weather and climate, take place under the action of air and water. Air is the source of the oxygen that is necessary for normal existence of the overwhelming majority of living organisms. Mankind has long obtained the heat necessary for life and productive activity by burning fuel in air. Air is one of the most important sources of chemical raw materials.

Dry air consists of the following gases (percent by volume): nitrogen (N₂), 78.09; oxygen (O₂), 20.95; argon (Ar), 0.93; and carbon dioxide (CO₂), 0.03. Air contains very small quantities of other inert gases, as well as hydrogen (H₂), ozone (0₃), nitrogen oxides, carbon monoxide (CO), ammonia (NH₃), methane (CH₄), and sulfur dioxide (SO₂). Taking into account the molecular weight of each component and its proportion in the composition of air, it is possible to calculate the mean molecular weight of air as 28.966 (approximately 29). The content of nitrogen, oxygen, and inert gases in air is virtually constant, since a constant concentration of oxygen—and in part nitrogen—is maintained by the plants of the earth. The content of carbon dioxide, nitrogen oxides, and sulfur compounds in air fluctuates substantially; in particular, it increases close to large cities and industrial enterprises. The water content of air is not constant; it may range from 0.00002 percent to 3 percent by volume. A large number of small solid dust particles—from a few million per cu m in clean room air to 100-300 million per cu m in large cities—is always present in air. Such particles often serve as centers for the condensation of atmospheric moisture and cause the formation of fog. Air penetrates the soil, making up 10 to 23-28 percent of its volume. Soil air, because of biological processes in the soil, is substantially different from ordinary air in composition: it contains 78-80 percent O₂, 0.1-20.0 percent N₂, and 0.1-15.0 percent CO₂ by volume.

Historical information. Scientists in ancient times considered air to be one of the elements of which all existing things are composed. Anaximenes of Miletus (sixth century B.C.) called air the basic substance, and Empedocles (fifth century) and Aristotle (fourth century) called it one of the four elements—along with fire, water, and earth—in which all inherent properties of matter are contained.

The notion of air as an independent, individual substance prevailed in science until the end of the 18th century. In 1775-77 the French chemist A. Lavoisier showed that the then recently discovered chemical elements nitrogen and oxygen were components of air. In 1894 the English scientists J. Rayleigh and W. Ramsay discovered another element, argon, in the air; later other inert gases were discovered.

The study of the physical properties of air played a large role in the history of science. The Italian scientist Galileo (1632) found that air is 400 times lighter than water. The Italian scientists V. Viviani and E. Torricelli (1643) discovered the existence of atmospheric pressure and invented the barometer to measure it. The French scientist B. Pascal discovered the decrease in atmospheric pressure with altitude. Studying the relationship between pressure and volume of air, R. Boyle and R. Townley (1662) in England and E. Mariotte (1676) in France discovered the law named after them (the Boyle-Mariotte law); subsequently, as science developed, other gas laws were discovered. It was long impossible to liquefy air and its chief components; they were therefore considered “permanent” gases. The failure of attempts to liquefy air was explained only after D. I. Mendeleev established the concept of critical temperature and pressure (1860). In 1877, using air cooled to below the critical temperature (approximately -140°C) under high pressure, L. P. Cailletet (Paris) and R. Pictet (Geneva) succeeded in converting air to liquid. In 1895 the German engineer C. von Linde designed and built the first industrial establishment for liquefying air.

Physical properties. The air pressure at 0° C and sea level is 101,325 newtons (N) per sq m, which is equal to 1.01325 bars, 1 atmosphere (atm), or 760 mm of mercury (mm Hg); under these conditions the mass of 1 liter (l) of air is 1.2928 g. For most practical purposes, air may be regarded as an ideal gas; in particular, the partial pressure of each component gas does not depend on the presence of the other components of air. The critical temperature is —140.7° C; the critical pressure is 3.7 MN/m² (37.2 atm). The following properties of air are given at a pressure of 101,325 N/m², or 1.01325 bars (so-called standard pressure): specific heat at constant pressure C_p, 10.045 x 10³ joules/(kg - ° K), or 0.24 cal/(g . ° C) in the range of 0°-100° C, and at a constant volume C_v 8.3710 x 10³ joules/(kg . ° K), or 0.2002 cal/(g . ° C) in the range of 0°-1500° C; coefficient of thermal conductivity, 0.024276 watts/(m . ° K), or 0.000058 cal/(cm . sec . ° C), at 0° C, and 0.030136 watts/(m . ° K), or 0.000072 cal/(cm . sec . ° C), at a temperature of 100° C; coefficient of thermal expansion, 0.003670 (0°-100°C); viscosity, 0.0171 (0° C) and 0.0181 (20° C) MN . sec/m² (centipoise); degree of compressibility z = pV/p₀V₀, 1.00060 (0°C), 1.09218 (25° C), or 1.18376 (50° C); index of refraction, 1.00029; dielectric permeability, 1.000059 (0° C); and solubility in water (in cu cm for 1 / of water), 29.18 (0° C) or 18.68 (20° C). Since the solubility of oxygen in water is somewhat higher than that of nitrogen, the ratio between these gases upon solution in water changes and becomes 35 percent and 65 percent respectively. The speed of sound in air at 0° C is approximately 330 m/sec.

Liquid air. Liquid air is bluish liquid with a density of 0.96 g/cm³ (at - 192 ° C and standard pressure). Freely evaporating liquid air at standard pressure has a temperature of approx. -190° C. Its composition is not constant, since nitrogen and argon volatilize more rapidly than oxygen. Fractional evap-oration of liquid air is used to obtain pure nitrogen and oxygen, argon, and other inert gases. Liquid air is stored and transported in Dewar flasks or specially constructed tanks. Compressed air is stored in steel cylinders at 15 MN/m2 (150 atm); the cylinders are black, with the white label “Compressed Air.”

Physiological and sanitary significance of air. The fluctuations in the nitrogen and oxygen content of atmospheric air are insignificant and do not have a significant effect on the human organism. The percentage composition of air, particularly the partial pressure of oxygen, is important for man’s normal vital activity. The partial pressure of oxygen in the air at sea level is 21,331.5 N/m² (160 mm Hg); the first symptoms of oxygen deficiency, which in healthy persons are readily compensated by increased frequency and depth of respiration, acceleration of blood flow, and an increase in the quantity of erythrocytes, appear when the partial pressure is decreased to 18,665 N/m² (140 mm Hg). With a decrease to 14,665.4 N/m² (110 mm Hg), compensation is insufficient, and symptoms of hypoxia appear; a decrease to 6,666.1-7,999.3 N/m² (50-60 mm Hg) is dangerous to life. An increase in the partial pressure of oxygen all the way to respi-ration of pure oxygen (a partial pressure of 101,325 N/m², or 760 mm Hg) can be tolerated by healthy persons without adverse consequences.

At normal partial pressure, nitrogen is inert. An increase in its partial pressure to 0.8-1.2 MN/m² (8-12 atm) leads to the appearance of narcosis. A significant increase in the content of nitrogen in the air (up to 93 percent and more) as a result of a decrease in the partial pressure of oxygen may lead to anoxemia and even death. The content of carbon dioxide, the physiological stimulant of the breathing center in atmospheric air, is usually 0.03-0.04 percent by volume. A certain increase in its concentration in the air of industrial centers is insignificant to the body. At high carbon-dioxide concentrations and with a decrease in the partial pressure of oxygen, asphyxia may occur. At a carbon-dioxide content of 14-15 percent in the air, death may occur from paralysis of the breathing center. An increase in the concentration of CO₂ in the air in buildings occurs principally because of the respiration and vital activity of people. (An adult at rest at 18°-20° C emits about 20/ of CO₂ per hour.) Therefore, the CO₂ content of air on the one hand, and of organic compounds, microorganisms, and dust on the other, increase simultaneously; the concentration of CO₂ in air in buildings is a sanitary indicator of the cleanliness of the air. The CO₂ content of air in residential premises should not exceed 0.1 percent. The inert gases found in insignificant amounts in atmospheric air—argon, helium, neon, krypton, and xenon—have no effect on the organism at standard pressure. The radioactive gas radon and its isotopes actinon and thoron, which have short half-lives and are observed in atmospheric air in minute concentrations, do not have an unfavorable effect on man.

Various microorganisms (bacteria, fungi, and so on) are ordinarily found in atmospheric air. However, pathogenic microorganisms in the air are extremely rare, as a result of which the transmission of infectious diseases through atmospheric air occurs only in exceptional cases—for example, with the use of bacteriological weapons and in closed areas where there are sick persons who are emitting pathogenic microorganisms into the air with minute droplets of saliva when coughing, sneezing, or speaking. Depending on the resistance of the microorganisms, they may be transmitted through the air by means of droplets, as well as by dust particles—the most resistant microorganisms are the causative agents of tuberculosis and diphtheria.

The temperature, humidity, and movement of the air have great significance for the vital activity of man. The optimal air temperature for an ordinarily dressed person performing light work is 18°-20° C. Heavier work requires a lower air temperature. Thanks to advanced mechanisms of thermal regulation, humans readily tolerate changes in temperature and can adapt to various climatic conditions. The optimal relative humidity of air for humans is 40-60 percent. Dry air is tolerated well under all conditions. High humidity has an unfavorable effect; at high temperatures it facilitates overheating of the body, and at low temperatures it facilitates chilling. The movement of air causes an increase in the body’s heat emission. Therefore, at high temperatures (up to 37° C), wind provides protection of humans from overheating, and at low temperatures it promotes chilling. The combination of wind with low temperatures and high humidity is especially adverse for humans. The significance of ionization of the air is well known. Light ions with negative charges exert a positive effect on the body. A number of devices have been proposed for ionization of air.

Air pollution. The growth of the scale of economic activity increases air pollution. The development of industry, power engineering, and transportation is leading to an increase in the content of carbon dioxide (by 0.2 percent of the amount in the air annually) and a number of other harmful gases in the air. Metallurgical and chemical enterprises and also thermal electric power plants pollute the air with sulfur dioxide, nitrogen oxides, hydrogen sulfide, and halogens and their compounds. Another serious source of air pollution is automobile transportation. According to some calculations, 1,000 automobiles discharge with their exhaust gases 3.2 tons of carbon monoxide, 200-400 kg of other products of incomplete fuel combustions, and 50-150 kg of nitrogen compounds into the air in one day. The pollution of air by solid particles is very great. In Pittsburgh (USA), 610 tons of dust is precipitated annually per square mile (259 hectares). Industrial enterprises, thermal electric power plants, automobile transportation, and forest fires and dust storms occuring as a result of the improper use of land increase the concentration of solid particles (dust and smoke) in the air to such an extent that this substantially (by 20-40 percent) decreases the solar radiation reaching the earth’s surface in the area of large cities. The scale of such processes may be judged by the fact that dust storms from 1930 to 1934 in the USA carried away up to 25 cm of topsoil and transported about 200 million tons of dust for distances of up to 1,000 km.

Air pollution leads to a deterioration of living conditions for humans, animals, and plants. The harmful effect on living organisms under such circumstances is caused not only by the primary components of industrial wastes but also by new toxic substances formed by them—the so-called photoxidants. Air pollution may sometimes attain such a scale that it leads to an increase in morbidity and mortality of the population. Radioactive air pollutants are particularly dangerous; as a result of the constant movement of air masses, they are global in character. Certain air pollutants cause occupational diseases. The influence of air pollutants on living conditions is extremely great. In the USSR laws that provide for the necessity of monitoring air quality and for the responsibility of the heads of industrial enterprises for the thorough cleaning and rendering harmless of industrial gases before their discharge into the atmosphere have been adopted for conservation of nature. As obligatory measures in designing and building cities, villages, and industrial sites, the law provides for the creation of sanitary protection zones (breaks) and the transfer out of residential areas of industrial enterprises that create health hazards.

Analysis of air. The maximum permissible concentrations (usually in mg// or per cu m of air) of harmful and explosive substances in the air environment of a manufacturing plant are regulated by law. Methods of air analysis depend on the aggregate condition of the substance being dealt with. For example, dust and aerosols are usually trapped with cotton wool or paper filters (glass filters are sometimes used for trapping aerosols); fogs and gases are usually absorbed by liquids. The most common methods for determining the content of harmful substances in the air are photometric analysis, nephelometry, and turbidimetry. Automatic gas analyzers are most frequently used for rapid determination of small concentrations of toxic and explosive substances in the air. The determination of radioactive pollutants occupies a special place in air analysis.

Air in technology. Because of the oxygen contained in air, it is used as a chemical agent in various processes, among them the burning of fuel, the smelting of metals from ores (blast-furnace and open-hearth processes), and the commercial manufacture of many chemical compounds (sulfuric and nitric acids, phthalic anhydride, ethylene oxide, acetic acid, acetone, and phenol); the value of air as a chemical agent is substantially increased by increasing its oxygen content.

Air is the most important industrial raw material for obtaining oxygen, nitrogen, and inert gases. The physical properties of air are used in heat and sound insulation materials and in electrical insulation devices; its elastic properties are used in pneumatic tires; and compressed air serves as the operating member for performing mechanical work (pneumatic machines, jet and spraying apparatus, perforators, and so on).

Artificial air. Artificial air, which is more accurately called artificial atmosphere, is a mixture of gases suitable for breathing. It was first used in medicine for diseases accompanied by oxygen deficiency (40-60 percent oxygen mixed with ordinary air, or 95 percent oxygen and 5 percent CO₂). Similar artificial gas mixtures are used in high-altitude aviation and in mine-rescue work. Artificial air is particularly significant in diving. Ordinary air is not suitable for work under pressures substantially higher than normal; under these conditions air has a narcotic effect, and the increase in solubility of nitrogen in the blood and body tissues makes rapid ascent of the diver to the surface dangerous. The discharge of nitrogen bubbles from the blood may cause caisson disease and death. For this reason, nitrogenless gas mixtures containing mainly helium (up to 96.4 percent) and oxygen (2-4 percent) at pressures of 0.7-2 MN/m² (7-20 atm) have been tested in the last 10-15 years for work at great depths (under conditions of high pres-sure). Such mixtures eliminate the danger of caisson disease; however, they produce some discomfort because of the high thermal conductivity of helium; a substantial change in voice timbre is also observed in such an atmosphere.

The problem of artificial air is also being dealt with in the construction of manned spacecraft. The Soviet spacecraft Vostok and Voskhod were equipped with a special system that maintained a close-to-normal air composition: partial pressure of oxygen, 20-40 kN/m²; volume concentration of CO₂, 0.5-1 percent. The American Gemini spacecraft had a pure oxygen atmosphere at a pressure of about 0.3 atm.