Chemistry Of Air Pollution

Type of physical science: Chemistry

Field of study: Environmental chemistry

Air pollution is the presence in the atmosphere of a substance or substances, put there directly or indirectly by a human act, in such an amount as to be detrimental to the well-being of living organisms. It has, therefore, enormous impact on the health, safety, and economic life of human society.

Overview

Air pollution is any substance present in the atmosphere that is detrimental to the health of humans or lower life-forms. Offensive or objectionable to humans either internally or externally, it either indirectly or directly adversely affects the welfare of humans.

The natural composition of air is a mixture of 78 percent nitrogen, 20 percent oxygen, less than 0.05 percent carbon dioxide, and traces of other inert gases. None of these components tends to interact with the others, leaving the oxygen always available for respiration. Natural air is still considered pure even when it has some tiny amount of fog, salt spray from the oceans, pollen from plants, dust from the soil, or smoke from forest fires. This composition has been relatively constant for the last billion years.

The most important air pollutants include compounds containing the elements carbon (C), nitrogen (N), and sulfur (S). While lead (Pb) compounds are particularly dangerous, they are not, on the whole, as widespread. Air pollutants exist as gases, vapors, and particulate matter. A vapor is a substance that, though present in the gaseous phase, generally exists as a liquid or solid at room temperature and the pressure of air normal at sea level. These liquid and solid substances become vaporized by the greatly raised temperatures or lowered pressures created by industrial processes or motor vehicles.

Particulate matter is what most people think of when they hear the phrase "air pollution" because it includes the plumes from smokestacks and the visible haze hanging over cities. It includes both settleable matter, which will soon fall to the ground, and suspended matter, which may stay suspended for great distances and long periods of time and be much more harmful.

The sources of pollution in general include the emissions from fuel consumption in motor vehicles and electricity-generating facilities, refuse burning, and industrial activities such as roasting and processing of various chemicals.

The number of carbon compounds alone is vast. There are thousands already, and modern technology is creating more every year. It is possible here to introduce only a few categories from this vast array. One such group is called the aromatic hydocarbons and includes benzene, toluene, and xylene. These compounds occur in solvents, dyes, paints, and fumigants, and pollute the air by evaporation. The aliphatic hydrocarbons of the paraffin series include methane, light and heavy gasolines, and lubricating oils. Gasoline vapors in engines typically enter the air when they get past pistons without being burnt. Aldehydes such as formaldehyde and acetaldehyde also enter the air from incomplete, inefficient combustion.

Another very dangerous category includes the chlorofluorocarbons--for example, the freons. These compounds have been used as aerosol propellants, refrigerants, and the main ingredients in Styrofoam. They enter the air by leakage or emerge from burning Styrofoam objects.

One of the simplest carbon compounds chemically is perhaps the most prevalent in modern motorized society: the carbon monoxide that pervades vehicle emissions. A concentration hazardous to most humans is often reached or exceeded in heavy motor-vehicle traffic.

Interestingly, the definition of air pollution includes the concept of amount, for carbon dioxide has been a normal component of air for eons. It has for all that time been used by plants to produce food by photosynthesis. When those plants die and decompose, it is slowly released again into the atmosphere. Alternatively, those plants that were buried and became fossil fuels over hundreds of millions of years have had their carbon released as huge amounts of carbon dioxide relatively suddenly over the last thousand years. This carbon dioxide is a pollutant because plant life cannot use such amounts and it remains in the air.

Nitrogen is another element found among the primary pollutants in the form of nitrogen oxides. Both nitric oxide and nitrogen dioxide also occur naturally, formed by lightning and the decomposition of organic compounds from dead animal matter. The vast amount of these substances produced by combustion in motor vehicles and power plants, however, is filling our air. They are produced quite simply by the heat of combustion combining the free nitrogen and oxygen of the air. Once formed by inadequately controlled internal combustion engines, they can form the basis of the secondary pollutants.

The compounds of sulfur are more often produced by factories and power plants than by motor vehicles, since sulfur is a frequent contaminant of the coal and oil that they burn.

Hydrogen sulfide, sulfur dioxide, sulfur trioxide, and the salts that they form are, perhaps, the air pollutant vapors of greatest concern to the general public because of their great capacity to become secondary pollutants. They now exist in huge quantities, far beyond the natural amounts given off by decaying organic matter and occasional volcanic eruptions.

Lead occurs in both element and compound form in polluted air, since it is among the most widely used of metals. It is emitted into the atmosphere from smelters that recover lead from ores and scrap metal, from the combustion of coal, and especially from gasoline. Both tetramethyl and tetraethyl lead are fuel additives placed in gasoline for antiknock purposes, thus raising the octane rating of the fuel.

The various pollutants discussed above exist along with an increasing amount of ozone.

This substance is formed from ordinary oxygen when the very active, or volatile, hydrocarbons and nitrogen oxides react with that oxygen in the presence of sunlight.

Polluted air, then, is a volatile mixture of various hydrocarbons, carbon monoxide, inordinate amounts of carbon dioxide, nitrogen and sulfur oxides, lead, and ozone. All these substances coexist with dust from excessive soil erosion, the carbon particles impregnated with tar that is called soot, ash from burning, and all types of aerosol compounds. Together, the mixture known as polluted air has great potential to damage living things and their environment.

Applications

The practical effects of air pollution are varied but can be grouped in certain general categories. Among the primary pollutants, particulate matter is responsible for many odors, reduced visibility, and an enormous amount of soiling of skin, hair, clothing, and households.

The volatile hydrocarbons, such as benzene, can damage bone marrow and the liver if exposure is repeated. In addition, many hydrocarbons are suspected, and in many cases proven, causes of mutations in unborn children and of innumerable cancers. Most of the hydrocarbons are irritants of delicate eye tissue.

The chlorofluorocarbons, which tend to escape to the upper atmosphere, are very destructive to the ozone layer that has formed there over the ages. That ozone layer acts as a shield for all on earth from the harmful ultraviolet light rays in sunlight. Increasing destruction of the ozone is permitting increasing amounts of ultraviolet light to cause increasing numbers of cases of skin cancers, cataracts, and premature aging of the skin.

Carbon monoxide does most of its damage by reducing the oxygen-carrying capacity of the blood. The hemoglobin in red blood cells, which carries oxygen to all parts of the body, has two hundred times greater affinity for carbon monoxide than it has for oxygen. In addition, the carbon monoxide becomes permanently attached to the hemoglobin, making it unavailable to carry oxygen for the body. That accidental deaths and suicides can be caused by carbon monoxide in enclosed places is well known. By comparison, the high levels of carbon monoxide in polluted air cause dizziness, fatigue, nausea, and varying amounts of damage to the nervous system.

The increased level of carbon dioxide does not greatly affect animals or humans directly. Instead, it is dramatically changing the entire environment by what is called the greenhouse effect. A very easy way to understand this physical phenomenon is to compare it to an automobile in direct sunlight. Like the atmosphere, a car's windows are transparent to the sun's visible rays, which warm the metal and upholstery within. The materials, like the earth, re-radiate some of this absorbed energy as infrared light. The window glass, like the carbon dioxide in the atmosphere, absorbs and thus traps the heat-producing infrared light, warming the car's interior. Although the heat trapped in the car can be fairly rapidly dissipated by opening the windows for a while, the enormous amounts of heat being trapped by the atmosphere in the greenhouse effect is causing a significant amount of global warming. It is estimated that only a doubling of the normal carbon dioxide content can cause a 2-to-5-degree-Celsius increase in average temperature worldwide. This would be ten times as rapid an increase as in the last few thousand years. A chain reaction of events will follow, since warming alters global patterns of winds, rainfall, and oceanic currents. These climatic changes, in addition to the increase in sea level because of polar ice cap melting, will have pronounced effects on farming and life-styles.

Ultimately, the greenhouse effect can cause a jarring redistribution in wealth and, most likely, political power.

Although the post-industrial level of carbon dioxide is only approximately 25 percent greater than before the industrial revolution, scientists drilling the Greenland ice cap have discovered that lead levels in air we breathe had increased two thousand times since the start of the industrial revolution. Lead enters the body through inhaling. Other lead precipitates out into the soil and is taken up by plants. When we eat those plants or the animals that have fed on those plants, we are prone to headaches and increased anxiety levels, since lead acts as a neurotoxin.

Both liver and kidneys become damaged as these organs attempt to rid the body of the lead before it can do permanent brain damage.

The sulfur compound called hydrogen sulfide also causes headaches in small concentrations; large amounts of it can cause paralysis of those nerve centers that enable our heart and lungs to function. Like the hydrocarbons, carbon monoxide, and lead discussed above, it does its damage to the body directly and therefore is termed a primary pollutant.

The other principal sulfur compounds that are pollutants, the sulfur oxides, tend to react with other substances in the air, producing new and more dangerous secondary pollutants. The sulfur oxides combine with smoke, carrying its varied particulate burden, to form smog. Sulfur dioxide reacts with the oxygen of the air to form sulfur trioxide.

In a given place, the emission level of all pollutants present may be below the amount able to do damage, called the established maximum threshold limits. Their cumulative effect, however, or their interactions because of meteorological conditions can present grave health hazards such as serious respiratory conditions. We tend to think of a bright, sunny day as very pleasant, but the ultraviolet portion of the sunlight combines the water vapor in the air with sulfur trioxide to form sulfuric acid. This strong acid then does damage to building materials, whether stone or metal, and painted surfaces like autos and works of art, and it destroys the strength of clothing fibers as different as cotton, hemp, rayon, and nylon. Sulfuric acid also directly affects the vegetation upon which it descends and the ability of soil to sustain plant life.

The same ultraviolet light that is forming sulfuric acid is also creating an abundance of other secondary pollutants. These substances are thus called photochemical oxidants. The hydrocarbons from incomplete combustion in vehicle engines and the nitrogen oxides formed by the heat of those engines create another type of smog. This photochemical smog is filled with nitric acid, ozone, and peroxyacetyl nitrate (PAN). All three are destructive to crops and forests, and dangerous to lungs and eyes.

The presence in air of both sulfuric and nitric acids gives it a much lower pH than unpolluted air. Instead of a pH or acid content of approximately pH 6, which is only mildly acidic, polluted air usually has a pH 4 rating, which is almost one hundred times as acidic. The nitric and sulfuric acids are carried upward by water vapor, which condenses as clouds and returns to earth as acid precipitation. Air masses move in characteristic patterns around the globe; hence, air made acidic in a particular urban area will release its acid precipitation in a highly predictable fashion. It will rain or snow down on another, less polluting city, making it as polluted, or on a country area that is then the recipient of the burden of this pollution.

Since the base of a cloud tends to hold most of the acids, the forests high up in the mountains, which are so often wreathed in clouds, are more completely and rapidly damaged.

Also, the acid snow that falls upon them remains on their surrounding soil for many months, enabling the acids to enter that soil and change its composition.

Snow and rain bring these same acids down into lakes. Since most organisms die at levels lower than pH 4, whole lakes or rivers can quite simply "die" when all their diverse living residents die. The pollution thus moved by weather patterns is termed transboundary pollution.

Other pollution stays closer to its source because of the physical phenomenon called inversion. Ordinarily, warm air rises, carrying pollutants aloft, where they are diluted by spreading out or carried by clouds to distant areas. In an inversion, cooler air moves over a city and finds itself trapped below the warm air it had displaced and pushed upward. The next day's pollutants are then held down by the warm air, engulfing the city and its inhabitants. Inversions are particularly dangerous if the city is surrounded by mountains, which hold the polluted air mass in place. The longer the inversion lasts, the greater the build-up of pollutants and, therefore, the greater the potential for disaster.

Context

Pollutants have a fairly unique place in the context of chemistry as a whole. The entire earth, including its atmosphere, consists entirely of chemicals. Since its inception, chemistry has studied those chemicals and their actual as well as possible interactions with each other. That branch of chemistry called environmental chemistry deduced the many geochemical cycles by which naturally occurring chemicals are changed from one form to another. Living organisms, also built entirely of chemicals, have had important roles in these chemical interrelationships for millions of years. Modern technology has placed a great burden on our atmosphere, which must function as a pool and a chemical reaction vessel for a host of new substances, and for naturally occurring substances in enormously increased amounts. Air pollution, thus, is a large-scale human interference in previously well-balanced biogeochemical cycles.

Air pollution has accompanied human society from its beginnings, in the form of smoke from wood fires. Historically, the air has renewed itself by slow interactions with vegetation and the oceans. From the Middle Ages on through the industrial revolution, coal was used increasingly, until, by the early twentieth century, most cities were enclosed in black shrouds. In London and in Pittsburgh, for example, headlights were needed at midday for vehicles to find their way.

Such high levels of pollutants can most easily permeate the body of humans and their domestic animals, since respiratory organs are so efficient in exchanging gases and fine particles between the air and the blood. It has been discovered that comparable damage can be done by long-term exposure to low levels or short-term exposure to high levels of pollutants. The former is often harder to observe, while the latter can be seen from the disastrous effects of periods of extreme pollution. A few of the most striking examples include the sixty people killed and the six thousand made ill in 1930 in the Meuse Valley of Belgium; the twenty killed and six thousand made ill in Donora, Pennsylvania, in 1948; most especially, the 1952 tragedy in which four thousand Londoners died and tens of thousands were made ill. Each of these events lasted five days and involved temperature inversions in addition to massive emissions of pollutants. In these cases, as is usual with pollution, those at greatest risk were infants, children, the elderly, and those with previous respiratory problems.

Interestingly, studies to establish so-called limits of tolerance or safe limits of pollutants are usually conducted on healthy adults under limited-duration, controlled conditions, using only one or a small number of pollutants. In the real world, a large segment of the population suffers from health deficiencies that make them more susceptible to air pollution damage.

Obviously, human society is not about to return to its preindustrial life-style. If air pollution is not to bring about its destruction, the new focus of technology must be how to cleanse the air of those pollutants already put into it, and the soil and water of their indirect acquisitions. Even more important, technology must find ways to keep more pollutants from entering our atmosphere in the future.

Principal terms

ACID PRECIPITATION: rain, fog, dew, or dry particles with a pH rating of 4.0 or below; a pH of 4.0 is one hundred times more acidic than ordinary, unpolluted precipitation

COMBUSTION: burning; the chemical reaction of a substance with oxygen, usually from air

INVERSION: a condition of the vertical temperature profile in which temperature increases, rather than decreases, with height

PRIMARY POLLUTANT: a pollutant emitted into the air directly from a source

SECONDARY POLLUTANT: a pollutant formed in the air from other substances, as opposed to being emitted directly from a source

Bibliography

Bassow, Herbert. AIR POLLUTION CHEMISTRY. Rochelle Park, N.J.: Hayden, 1976. Designed for beginning chemistry students at high school and freshman college level. Large sections of each chapter will be very useful to the lay reader, including the suggested readings.

Bellini, James. HIGH TECH HOLOCAUST. San Francisco: Sierra Club Books, 1986. Well-written, nontechnical explanation of the wide-reaching effects of pollution in general. Chapter 2 discusses lead, and chapter 4 concerns acid rain, caused by sulfur and nitrogen compounds.

Corson, Walter H. THE GLOBAL ECOLOGY HANDBOOK. Boston: Beacon Press, 1990. A very practical supplement to the Public Broadcasting System's series RACE TO SAVE THE PLANET. Its 414 pages on all aspects of pollution contain many graphs, charts, drawings, and lists of further information.

Eckholm, Erik. DOWN TO EARTH. New York: W. W. Norton, 1982. Very interesting and highly readable treatment of all types of pollution. Chapters 6 and 7 focus on air pollution. Contains extensive bibliography.

Lynn, David. AIR POLLUTION: THREAT AND RESPONSE. Reading, Mass.: Addison-Wesley, 1976. A relatively nontechnical introduction to the study of air pollution and its control. Although a textbook, it is easy to read. Contains a fifteen-page bibliography and a glossary.

Mohnen, Volker A. "The Challenge of Acid Rain." SCIENTIFIC AMERICAN 259 (August, 1988): 30-38. An excellent summary of the acid rain problem: its causes, effects, and some possible remedies. Very well illustrated and highly readable.

Parker, Homer. AIR POLLUTION. Englewood Cliffs, N.J.: Prentice-Hall, 1977. Chapters 1 and 2 are appropriate for those not trained in science. The remainder is an excellent, well-illustrated source of information on every type of control device. These chapters require a math and physics background. Includes extensive lists of references.

Air pollutants and methods of detection

Federal ambient air quality standards

Acids and Bases

Carbon and Carbon Group Compounds