Aerosols
Aerosols
Aerosols are collections of tiny particles of solid and/or liquid suspended in a gas. The size of these particles can range from about 0.001 to about 100 microns. While a number of naturally occurring aerosols exist, the most familiar form of an aerosol is the pressurized spray can. Aerosols are produced by a number of natural processes and are now manufactured in large quantities for a variety of commercial uses. They are also involved in a number of environmental problems, including air pollution and destruction of ozone in the atmosphere.
Classification
Aerosols are commonly classified into various subgroups based on the nature and size of the particles of which they are composed and, to some extent, the manner in which the aerosol is formed. Although relatively strict scientific definitions are available for each subgroup, these distinctions may become blurred in actual practical applications. The most important of these subgroups are the following:
Fumes
Fumes consist of solid particles ranging in size from 0.001 to 1 micron. Some typical fumes are those produced by the dispersion of carbon black, rosin, petroleum solids, and tobacco solids in air. Probably the most familiar form of a fume is smoke. Smoke is formed from the incomplete combustion of fuels such as coal, oil, or natural gas. Its particles are smaller than 10 microns in size.
Dusts
Dusts also contain solid particles suspended in a gas, usually air, but the particles are larger in size than those in a fume. They range from about 1 to 100 microns (and even larger) in size. Dust is formed by the release of materials such as soil and sand, fertilizers, coal dust, cement dust, pollen, and fly ash into the atmosphere. Because of their larger particle size, dusts tend to be more unstable and settle out more rapidly than do fumes, which do not settle out at all.
Mists
Mists are dispersions in a gas of liquid particles less than about 10 microns in size. The most common type of mist is that formed by tiny water droplets suspended in the air, as on a cool summer morning. If the concentration of liquid particles becomes high enough to affect visibility, it is then called a fog. A particular form of fog that has become significant in the last half century is smog, which forms when natural moisture in the air interacts with man-made components, such as smoke and other combustion products, to form chemically active materials.
Sprays
Sprays form when relatively large (10+ micron) droplets of a liquid are suspended in a gas. Sprays can be formed naturally, as along an ocean beach, but are also produced as the result of some human invention such as aerosol can dispensers of paints, deodorants, and other household products.
Sources
About three-quarters of all aerosols found in Earth’s atmosphere come from natural sources. The most important components are sea salt, soil and rock debris, products of volcanic emissions, smoke from forest fires, and solid and liquid particles formed by chemical reactions in the atmosphere. As an example of the last category, gaseous organic compounds released by plants are converted by solar energy in the atmosphere to liquid and solid compounds that may then become components of an aerosol. A number of nitrogen and sulfur compounds released into the atmosphere as the result of living and nonliving changes undergo similar transformations.
Volcanic eruptions are major, if highly irregular, sources of atmospheric aerosols. The eruptions of Mount Hudson in Chile in August 1991 and Mount Pinatubo in the Philippines in June 1991 produced huge volumes of aerosols that had measurable effects on Earth’s atmosphere.
Remaining atmospheric aerosols result from human actions. Some, such as the aerosols released from spray cans, go directly to form aerosols in the atmosphere. Others undergo chemical changes similar to those associated with natural products. For example, oxides of nitrogen and sulfur produced during the combustion of fossil fuels may be converted to liquid or solid nitrates and sulfates, which are then incorporated into atmospheric aerosols.
Physical properties
The physical and chemical properties of an aerosol depend to a large extent on the size of the particles that comprise it. When those particles are very large, they tend to have the same properties as a macroscopic (large size) sample of the same material. The smaller the particles, however, the more likely they are to take on new characteristics different from those of the same material in bulk. Aerosols tend to coagulate, or collide and combine with each other, to form larger bodies. A cloud, for example, consists of tiny droplets of water and tiny ice crystals. These particles move about randomly within the cloud, colliding with each other from time to time. As a result of a collision, two water particles may adhere (stick) to each other and form a larger, heavier particle. This process results in the formation of droplets of water or crystals of ice heavy enough to fall to Earth as rain, snow, or some other form of precipitation.
Synthetic production
The synthetic production of aerosols for various commercial purposes has become such a large industry that the term aerosol itself has taken on a new meaning. Average citizens who know little or nothing about the scientific aspects of aerosols recognize the term as referring to devices for dispensing a wide variety of products.
Aerosol technology is relatively simple. A spray can is filled with a product to be delivered (such as paint), a propellant, and, sometimes, a carrier to help disperse the product. Pressing a button on the can releases a mixture of these components in the form of an aerosol.
The simplicity of this concept, however, masks some difficult technological problems involved in their manufacture. An aerosol pesticide, for example, must be formulated in such a way that a precise amount of poison is released, enough to kill pests, but not so much as to produce an environmental hazard. Similarly, a therapeutic spray such as a throat spray must deliver a carefully measured quantity of medication. In such cases, efforts must be taken to determine the optimal particle size and concentration in the aerosol by monitoring chlorofluorocarbon (CFC) propellants, which destroy the ozone layer.
The production of commercial aerosols fell slightly in the late 1980s because of concerns about the ozone and other environmental effects. By 1992, however, their manufacture had rebounded. In that year 990 million container units (bottles and cans) of personal aerosol products and 695 million container units of household products were manufactured. In the early 1990s many states passed legislation limiting the volatile organic compound (VOC) content in aerosols such as hairspray and spray paint. These limitations have forced the aerosol industry to seek alternate propellants and solvents. In many cases this substitution has resulted in inferior products from the standpoint of drying time and spray characteristics. The industry continued to struggle with these issues after the year 2000.
Combustion aerosols
Aerosol technology has made possible vastly improved combustion systems, such as those used in fossil-fueled power generator plants and in rocket
KEY TERMS
Acid rain— A form of precipitation that is significantly more acidic than neutral water, often produced as the result of industrial processes.
Chlorofluorocarbons (CFCs)— A group of organic compounds once used widely as propellants in commercial sprays, but outlawed in the United States in 1978 because of their harmful environmental effects.
Dust— An aerosol consisting of solid particles in the range of 1 to 100 microns suspended in a gas.
Electrostatic precipitator— A device for removing pollutants from a smokestack.
Fume— A type of aerosol consisting of solid particles in the range 0.001 to 1 micron suspended in a gas.
Mist— A type of aerosol consisting of droplets of liquid less than 10 microns in size suspended in a gas.
Ozone layer— A region of the upper atmosphere in which the concentration of ozone is significantly higher than in other parts of the atmosphere.
Smog— An aerosol form of air pollution produced when moisture in the air combines and reacts with the products of fossil fuel combustion.
Smoke— A form of smoke formed by the incomplete combustion of fossil fuels such as coal, oil, and natural gas.
Spray— A type of aerosol consisting of droplets of liquid greater than 10 microns in size suspended in a gas.
Stack gases— Gases released through a smokestack as the result of some power-generating or manufacturing process.
engines. The fundamental principle involved is that any solid or liquid fuel burns only at its surface. The combustion of a lump of coal proceeds relatively slowly because inner parts of the coal can not begin to burn until the outer layers are burned off first.
The combustion rate can be increased by dividing a lump of coal or a barrel of fuel oil into very small particles—the smaller the better. Power-generating plants today often run on coal that has been pulverized to a dust, or oil that has been converted to a mist. The dust or mist is then mixed with an oxidizing agent, such as air or pure oxygen, and fed into the combustion chamber. The combustion rate of such aerosols is many times greater than would be the case for coal or oil in bulk.
Environmental factors
A number of environmental problems are associated with aerosols, the vast majority of them associated with aerosols produced by human activities. For example, smoke released during the incomplete combustion of fossil fuels results in the formation of at least two major types of aerosols that may be harmful to plant and animal life. One type consists of finely divided carbon released from unburned fuel. This soot can damage plants by coating their leaves and reducing their ability to carry out photosynthesis. It can also clog the alveoli (air sacs) in lungs, and interfere with respiration.
A second type of harmful aerosol is formed when stack gases, such as sulfur dioxide and nitrogen oxides, react with oxygen and water vapor in the air to form sulfuric and nitric acids, respectively. Mists containing these acids can be carried hundreds of miles from their source before conglomeration occurs and the acids fall to Earth as acid rain. Considerable disagreement exists about the precise nature and extent of the damage it causes, but there seems to be little doubt that in some locations it has caused severe harm to plant and aquatic life.
Ozone depletion
A particularly serious environmental effect of aerosol technology has been damage to Earth’s ozone layer. This damage appears to be caused by a group of compounds known as chlorofluorocarbons (CFCs) which, for more than a half century, were by far the most popular of all propellants used in aerosol cans.
Scientists originally felt little concern about the use of CFCs in aerosol products because they are highly stable compounds at conditions encountered on the Earth’s surface. They have since learned, however, that CFCs behave very differently when they diffuse into the upper atmosphere and are exposed to the intense solar radiation present there.
Under those circumstances, CFCs decompose and release chlorineatoms that, in turn, react with ozone in the stratosphere. The result of this sequence of events is that the concentration of ozone in portions of the atmosphere has been decreasing over at least the past decade, and probably for much longer. This change is not a purely academic concern, since the Earth’s ozone layer absorbs ultraviolet radiation from the sun and protects life on Earth from the harmful effects of that radiation. For these reasons, CFCs have been banned from consumer product aerosols since the late 1970s. They are still employed for certain medical applications, but by and large they have been eliminated from aerosol use. The aerosol industry has replaced CFCs with other propellants such as hydrocarbon gases (e.g., butane and propane), compressed gases (e.g., nitrogen and carbon dioxide), and hydrochlorofluorocarbons (which are much less damaging to the ozone layer.)
Technological solutions
Methods for reducing the harmful environmental effects of aerosols such as those described above have received the serious attention of scientists for many years. As a result, a number of techniques have been invented to reduce the aerosol components of things like stack gases. One device, the electrostatic precipitator, is based on the principle that the particles of which an aerosol consists (such as unburned carbon in stack gases) carry small electrical charges. By lining a smokestack with charged metal grids, the charged aerosol particles can be attracted to the grids and precipitated out of the emitted smoke.
Aerosol sniffing
Another risk associated with commercial aerosols is their use as recreational drugs. Deliberately inhaling of some consumer aerosol preparations may produce a wide variety of effects, including euphoria, excitement, delusions, and hallucinations. Repeated sniffing of aerosols can cause addiction intoxication, damaged vision, slurred speech, and diminished mental capacity.
See also Emission; Ozone layer depletion.
Resources
BOOKS
Baron, Paul A., and Klaus Willeke. Aerosol Measurement: Principles, Techniques, and Applications. 2nd ed. Hoboken, NJ: Wiley-Interscience, 2001.
Hinds, William C. Aerosol Technology: Properties, Behavior, and Measurement of Airborne Particles. 2nd ed. Hoboken, NJ: Wiley-Interscience, 1999.
Smoke, Dust, and Haze: Fundamentals of Aerosol Dynamics (Topics in Friedlander, Sheldon K. Chemical Engineering. 2nd ed. Oxford: Oxford University Press, 2000.)
PERIODICALS
Browell, Edward V., et al. “Ozone and Aerosol Changes during the 1991-1992 Airborne Arctic Stratospheric Expedition.” Science (1993): 1155-158.
Charlson, R. J., et al. “Climate Forcing by Anthropogenic Aerosols.” Science (1992): 423-30.
Charlson, Robert J., and Tom M. L. Wigley. “Sulfate Aerosol and Climatic Change.” Scientific American (1994): 48-55.
Haggin, Joseph. “Pressure to Market CFC Substitutes Challenges Chemical Industry.” Chemical & Engineering News 69 (1991): 27-8.
Miller, Norman S., and Mark S. Gold. “Organic Solvent and Aerosol Abuse.” American Family Physician 44 (1991): 183-89.
Osborne, Elizabeth G. “Administering Aerosol Therapy.” Nursing 23 (1993): 24C-24E.
Penner, J. E., et al. “Unraveling the Role of Aerosols in Climate Change.” Environmental Science & Technology 35, no. 15 (2001): 332a-340a.
Ramanathan, V. “Aerosols, Climate, and the Hydrological Cycle.” Science 249, no. 5549 (2001): 2119-2114.
“The Role of Atmospheric Aerosols in the Origin Of Life.” Surveys in Geophysics 23, no.5-5 (2002): 379-409.
“War Spurs Aerosol Research.” Geotimes 37 (1992): 10-11.
OTHER
NASA Earth Observatory. “Aerosols and Climate Change” <http://earthobservatory.nasa.gov/Library/Aerosols> (accessed October 11, 2006).
NASA Facts Online. “Atmospheric Aerosols:What Are They, and Why Are They So Important?” <http://oea.larc.nasa.gov/PAIS/Aerosols.html> (accessed October 11, 2006).
David E. Newton