Aerosols

systems consisting of solid or liquid particles suspended in a gaseous medium.

Aerosols are categorized according to their nature as dispersive and condensative. Dispersive aerosols are formed by spraying liquids. For example, water mists are formed by waterfalls, ocean surf, fountains, and so forth. Mucous aerosols are formed by coughing and sneezing, and insectici-dal aerosols are derived from the use of aerosol cylinders. Dispersive aerosols are also formed by the pulverization of solids and stirring up of powder, such as dust raised from the ground by the wind, by automobile tires, by the sweeping or pouring of fine powders (flour, chalk, and others), by stone crushers destroying stone walls, by the crushing of coal, and so forth. Condensative aerosols such as natural clouds and ground fogs similar to clouds result from the condensation of vapors. Natural clouds and ground fogs are formed from water droplets or ice crystals generated by the condensation of water vapor in the atmosphere. In the process of smelting, the vapors of the metals burn, and the products of their combustion condense to form smoke consisting of solid particles of metallic oxides. For example, smoke is formed in this way by the combustion of fuel (in stoves and automobile motors), tobacco, gunpowder and various organic substances. However, in the case of the combustion of organic substances, the smoke contains droplets of resinous substances, in addition to solid particles of carbon black.

The properties of aerosols are determined by the nature of the substance of which the particle is composed and the nature of the gaseous medium, as well as by the mass concentration of the aerosol (that is, the total mass of the particles that have combined to form a volumetric unit of an aerosol) and the calculated concentration (the number of particles per unit volume), diameter, shape, and charge of the particles. All of these variables may have the most varied values. Thus, on a windless, clear day in the countryside, one liter of air may contain several tens of thousands of particles with a total mass of less than 10^-4 mg, but in a mine near a coal combine, one liter of air may contain a million particles with a total mass of up to several dozen milligrams. The atmosphere absorbs 10¹⁰ tons of salt per year in the form of aerosols from the surface of the seas and oceans alone. The diameters of particles in aerosols vary from approximately 1 nanometer (nm) to fractions of a millimeter; for example, cloud droplets have diameters of 5–50 microns (μm), particles of tobacco smoke measure tenths of a micron, and dusts usually contain particles with very different diameters. Particles of dispersive aerosols have rather high electrical charges, both positive and negative. In condensative aerosols formed at low temperatures particles are uncharged, but they gradually acquire low charges by adsorbing the light ions which are always present in gases.

The most important processes occurring in aerosols are sedimentation, the Brownian motion, coagulation, and vaporization of particles. The rate of sedimentation (settling due to the effect of gravity) is approximately proportional to the square of the size of the particles; this amounts to several dozen cm/sec for particles with a diameter of 100 μm and several mm/sec for particles with a diameter of 10 μm. For particles smaller than 1 μm, the rate of sedimentation is extremely low. The smaller the particles, the more rapid is the Brownian motion, which is noticeable only in the case of particles smaller than 1 μm. Under the effect of the Brownian motion, particles are deposited on any surfaces with which an aerosol comes into contact. Under the effect of sedimentation, particles are deposited only on surfaces facing upward; therefore, there are always more particles on such surfaces than on the vertical surfaces. Coagulation of aerosols occurs as a result of the collision of particles under the effect of the Brownian motion and the different rates of sedimentation of particles with different diameters, which are overtaking one another owing to electrical and other forces. Solid particles adhere when they collide, but liquid particles combine, and the number of free particles decreases. The rate of coagulation of particles—that is, the decrease in the number of particles in a given time—is proportional to the square of their concentration. Therefore, at a concentration of 10¹⁰ per cm³, the rate of coagulation is halved in 0.7 sec, but at a concentration of 10⁶ per cm³, the rate is halved in 12 minutes. Vaporization of particles is observed in aerosols generated by volatile substances—for example, in the melting of clouds. All of these processes lead to the destruction of aerosols; however, the formation of new particles usually occurs simultaneously in the above-mentioned ways.

The most important optical properties of aerosols are the scattering and absorption of light. Passage of a light beam through an aerosol (for example, the passage of the rays of a searchlight through the atmosphere at night or of solar rays through a crack into a dark room) is observed as a luminous Tyndall cone. The higher the concentration and the larger the diameters of the particles, the brighter the Tyndall cone is. The individual particles scattering the light can be observed with an ultramicroscope. However, the scattering of light rapidly decreases as the diameters of the particles decrease, so that only particles larger than 0.1 μm can be seen. Fine aerosols scatter primarily short light waves; therefore, they appear to be blue. For example, smoke coming from the lighted end of a cigarette appears to be blue.

Aerosols play a major positive role in the life of man. Clouds are the most important links in the natural water cycle. By absorbing solar rays and heat rays from the earth, clouds moderate the extremes of hot and cold. The pollination of many plants, including grasses, is done by another aerosol, pollen. All liquid and almost all solid fuel is now ignited in aerosol form. The battle against pests and diseases of cultivated plants and forests is carried on with aerosols containing toxic chemicals. Many important industrial materials, such as carbon black, are obtained in the form of aerosols. Aerosol therapy and aerosol immunization of people and domestic animals are also very important. Aerosols have been used successfully against hailstorms.

Aerosols also cause a great amount of damage. Radioactive aerosols formed by atomic blasts and in the mining and processing of fissionable materials are very dangerous. Dust containing silica causes a serious lung disease called silicosis; beryllium, lead, and chromium dusts are no less dangerous. Therefore, the struggle against industrial dust is one of the most important tasks of industrial sanitation. Bacterial aerosols containing pathogenic microorganisms and produced by coughing and choking can be sources of infectious diseases such as influenza. Natural fog prevents airplanes from landing. Dust storms are real disasters for hot, dry, unforested areas. The struggle against aerosol pollution of the atmosphere in industrial centers is one of our most serious problems. Aerosols formed in the atmosphere are usually called atmospheric aerosols.