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International Heat Transfer Conference Digital Library International Centre for Heat and Mass Transfer Digital Library Begell House Journals Annual Review of Heat Transfer

AIR POLLUTION

DOI: 10.1615/AtoZ.a.air_pollution

The air in the environment is never absolutely pure; it always contains traces of pollutants the force of suspended solid particles (dust and smoke), droplets of liquid (mist), vapors, and contaminant gases. The sources of pollution are grouped into those of natural and anthropogenic origin, i.e., the products of human activities. The former include volcanic eruptions, dust storms, wood fires and rotting of plants. They have been known for millions of years, and the atmosphere spontaneously cleans in some finite period of time as a result of sedimentation, chemical oxidation or, conversely, reduction, and by absorption by the ocean and soil. Ultimately an equilibrium concentration of atmospheric pollution of all kinds is established for ongoing natural pollution. Essential disturbances of the equilibrium in the earth's atmosphere were brought about in the past only by large-scale volcanic eruptions such as Krakatau eruption in 1883.

An intensified level of human activity, i.e., a vigorous growth of production, the mining industry, metallurgy, power generation, chemistry, and transport, has given rise to ejection into the atmosphere of vast amounts of dust, gases, and condensed vapors including those containing toxic and radioactive substances. As a rule, these ejections are concentrated over confined geographic regions (viz, industrial centers) and carried over the earth's surface by atmospheric air flows. Therefore, their concentration in various regions is not identical; if it substantially exeeds the natural background, the atmosphere, in the process of self-cleaning, fails to dispose of these substances. This results in the global pollution of the atmosphere.

Figure 1 shows as an example the global cycle of sulfur, which typifies the main sources of pollution, and shows how various substances find the way from the earth's surface to the atmosphere and back.

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Figure 1. Global cycle of sulfur pollution.

At present, the term "pollution of the atmosphere" implies the presence in the surrounding atmosphere of one or more substances and their combinations in such amounts and during such span of time that they can have an injurious effect on the environment and human health.

Until the mid-70s neither water vapor nor CO2 were thought to be atmospheric contaminants, but currently carbon dioxide attracts attention not only as a substance whose high local ejections lead to a reduction of oxygen concentration but also as one producing, on the global scale, a "Greenhouse Effect" due to a reduction of the transmission of thermal radiation from the earth's surface in the infrared region. Water vapor, when ejected in large quantities, produces the same effect.

Concentration of a gaseous contaminants in air is coventionally expressed as the number of parts per million (ppm) and that of condensed contaminator (dust, droplets), in μg/m3. The main contaminants are suspended particles of various chemical composition, gaseous sulfur and nitrogen compounds, vapor of organic substances, as well as halogen compounds and radioactive substances. The degree of natural and anthropogenic pollution of the atmosphere can be judged from the data of Table 1, where the annual discharge of basic contaminants is presented.

Table 1. The main sources of atmospheric pollution

Suspended particles are classified into coarse dust (over 100 μm in size), and so-called aerosols, i.e., fine dust (under 100 μm), mists (from 0.01 to 10 μm), and smokes (from 0.001 to 1 μm). These particles can be in suspension from a few seconds to an infinitely long time. Thus, 0.1 μm and smaller particles are subject to Brownian diffusion and virtually do not settle. The particles from 0.1 to 1 μm have sedimentation rates much lower than even the weak wind velocity, and their sedimentation by gravity is negligible. Slow sedimentation is also observed for particles from 1 to 5 μm, and only particles tens of microns in size and larger actively settle from the atmosphere back to the earth. Atmospheric precipitation such as rain, snow and drizzle play an important role in purifying air from suspended particles, particularly, if account is taken of the fact that particles are the main condensation sites of water vapor. This can be observed in the atmosphere over industrial centers with large dust ejection. Here, rainfall is lower on days when the industry is not operational and when the particle concentration in the atmosphere falls.

The negative impact of suspended particles on life manifests itself not only in reduced visual range and solar radiation due to light absorption and scattering, but also in a direct influence on animal and human health. Solid particles are inhaled and retained in the lungs. They may be toxic owing to their chemical and physical properties; for instance, particles of heavy metals, may hinder clearing of respiratory tracts. Solid particles may be the carriers of toxic substances adsorbed on them. The latter is most dangerous because the statistical analysis of diseases has shown that suspended particles combined with other contaminants (e.g., SO2, NOx) disturb human health more gravely than each contaminant taken separately.

Carbon monoxide, if its concentration in the air is higher than 750 ppm, causes pathological changes in human organs and is ultimately lethal. The reason is that CO, combining with haemoglobin, reduces the capacity of the blood in transporting oxygen.

The injurious effect of sulfur is still more diverse. Sulfur dioxide ejected into the atmosphere is re-oxidized to form SO3 and, in humid air, sulfuric acid forms, which condenses on either suspended particles or, at high supersaturation, on its own nuclei, and generates mist. Production of particles is also attributed to atmospheric photochemical reactions between SO2, NOx, hydrocarbons, and suspended particles. As a result, aerosols of sulfuric acid and its salts constitute up to 20% of the total number of suspended particles in the air of cities. They are the source of the so-called smog that is characteristic of many cities of north-western Europe, where sulfur-bearing coal is used as fuel. Smog is formed, as a rule, in winter in windless weather. Smoke particles that are the sites of condensation of moisture and sulfuric acid become heavy and hang over the city gradually descending on buildings and streets. They are responsible for metal corrosion, fracture of marble, limestone, and other construction materials as well as fabrics primarily, nylon. The destructive effect of sulfuric acid aerosol, though weaker, is observed even in the absence of smog.

In windy weather a substantial portion of sulfur-containing aerosols is carried at long distances. Their precipitation on the earth is facilitated by rains, which are of a clearly pronounced acid nature and, therefore, are known as acid rains. Their effect on soils with pH > 7 is reduced to a considerable extent by the soil alkalinity, while for soils with pH < 7 precipitation of acid rains is extremely unfavorable since an increase in soil acidity reduces fertility and an increased acidity of water basins leads to the mortality of fish, primarily of the most valuable varieties. Precipitation of acid rains is injurious to plant leaves.

Sulfure dioxide and mixtures of sulfur-containing components, in particular, heavily irritate the rispiratory tract in humans, stimulating diseases of respiratory tract and other diseases, and lead to a higher mortality. Among nitrogen oxides the most harmful for men is nitrogen dioxide that, together with atmospheric moisture, forms a strong nitric acid the effect of which on men is similar to that of sulfuric acid.

Methane, halogen compounds, and NO2 are regarded currently to be destructive of the earth's ozone protective layer. The main sources of atmospheric air pollution are power generation, metallurgy, chemical industry, and transport. The former three deliver to the atmosphere, first and foremost, dust, sulfur and nitrogen oxides, whereas vehicles chiefly deliver CO.

Monitoring of the steadily increasing pollution of the atmosphere required the establishment of permissible limiting values. In addition to a local pollution (maximum allowable concentration determined in the near-earth layer or the "breathing layer"), a limit also has to be established for the total discharge.

Scattering of polluting substances in the atmosphere to a great extent depends on convective and turbulent mixing. The height of the air layer in which active mixing occurs depends on the season, the weather, the time of day, and the topography of the ground. The greater the height on which the pollutant is lifted, the greater the space into which the pollution is diluted.

The jet configuration of stacks under different weather conditions is demonstrated in Figure 2. Figure 2a depicts the most characteristic, "ordinary" conditions, when dispersion in the horizontal and vertical directions is approximately the same and the jet cross section approaches a circle. Under inversion when a stack mouth rises above the inversion layer (IL) "hanging" over the earth (Figure 2b) the plume may extend only horizontally and upward, therefore, its section is of a triangular shape. Figure 2c illustrates the plume shape when the pollutant is emitted below the inversion layer.

Effect of emission position relative to inversion layers.

Figure 2. Effect of emission position relative to inversion layers.

The mathematical description of dispersion is often represented in a simplified form, e.g., the problem is considered one of the propagation of an impurity from a point source in a final direction with the wind blowing with a constant velocity and direction (Figure 3). The jet is round in cross section and distribution of impurities along the radius is assumed to be Gaussian. Since the gases discharged from the stack mouth have some velocity, of the order of 10 m/s, and a temperature higher than the ambient air temperature, the jet rises upward by Δh, and this is the distance by which the effective point source for the emission (A in Fig. 3) is raised over the stack. An equation allowing to calculate the impurity concentration at any point of the jet C(x, y, z) has the form

where x, y, z are the distances along the appropriate axes (m); Sy and Sz, the diffusion in the y and z directions (m); , the intensity of the source (g/s); C, the concentration at a given point (g/m3); u, the wind velocity (m/s); and H, the effective height of discharge (m). H is equal to the sum of the stack height h and an additional jet rise height Δh as discussed above.

Parameter governing spread of pollution from a point source.

Figure 3. Parameter governing spread of pollution from a point source.

Of particular interest is the calculation of impurity concentration in the near-earth layer when the jet touches the earth's surface. For this parameter the following empirical formulas, based on a vast body of experimental evidence, are used:

where A is a coefficient allowing for vertical and horizontal dispersion conditions; Fm, a coefficient relating to physical state of a given harmful substance m; , the maximum overall discharge of harmful impurity from all the stacks (g/s); h, the geometric height of the chimney (m); N, the number of stacks of identical height; , the overall volumetric flow of the flue gases (m3/s); and ΔT, the difference between the temperature of discharged gases and that of the ambient air (K).

High buildings not far from the pollution source may essentially alter the aerodynamic situation, and the jet may descend to the earth.

Unfavorable environmental conditions in a city may result from a large accumulation of heat by bulky buildings in the daytime. In this way a stable self-maintaining system of polluted air recirculation, a so called "heat island", upset only by a strong wind, is established in and over the city.

For monitoring atmospheric pollution in the near-earth layer and for collecting and analyzing data there is a large network of observation stations equipped with analytical automatic instruments regularly sending information to a central processing system.

REFERENCES

Wark, K. and Warner, C. F. (1976) Air Pollution: Its Origin and Control, IEP-Dun-Donnelley, New York, London.

Brimblecombe, P. (1986) Air Composition and Chemistry, Carbridge Unit Press, London, New York.

Strauss, W. and Mainwaring, S. J. (1984) Air Pollution. Edward Arnold Ltd., London.

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