Ecology: Menace in the Skies

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Colorless Contamination. The most obvious component of polluted air is the smoke that pours from millions of home chimneys, power-plant and factory smokestacks, incinerators and garbage dumps. It consists of tiny pieces of carbon, ash, oil, grease, and microscopic particles of metal and metal oxides. Some of these particles are so large that they settle rapidly to earth, but many are small enough to remain suspended in the atmosphere until they are removed by rain or wind. Though the participates, as they are called, are highly visible and often the first target of antipollution officials, they constitute only about 10% of the pollution in the air over the U.S.

Cities such as Pittsburgh and St. Louis, which after World War II enforced vigorous and successful campaigns to clear smoke from their skies, have now discovered that their drives against pollution have only just begun. A full 90% of U.S. air pollution consists of largely invisible but potentially deadly gases. More than half of the contamination in the air over the U.S., for example, consists of colorless, odorless carbon monoxide, most of it issuing from the exhaust pipes of automobiles, trucks and buses.

The second most plentiful gas pollutant is composed of oxides of sulphur, produced by home, power-plant and factory combustion of coal and oil containing large percentages of sulphur. More than a tenth of air pollution consists of hydrocarbons, most of them emanating as unburned or only partially burned gaseous compounds from automobile fuel systems. Combustion also produces large quantities of carbon dioxide, nitrogen oxides and other gases.

As if these products of combustion were not unpleasant or dangerous enough by themselves, some also undergo complicated chemical changes in the atmosphere that make them even less attractive. In the presence of sunlight, the hydrocarbons and nitrogen oxides emitted largely by automobile exhausts react to produce the sort of brownish and irritating photochemical smog that blankets Los Angeles for most of the year. "Los Angeles smog" is a highly complex soup containing, among other things, nitrogen dioxide, hydrocarbons, ozone (a highly active and poisonous form of oxygen) and peroxyacyl nitrate (commonly called PAN). "London smog," on the other hand, usually contains high quantities of sulphur oxides that react with moisture to produce a dilute but corrosive sulphuric-acid mist. Though air conditioners can effectively filter pollutant particles out of the air, the troublesome gaseous contaminants pass through unhindered. Thus city dwellers who feel that they have found sanctuary from the smog in sealed and air conditioned offices and apartments are actually in an atmosphere that may be little better than the foul air of the streets.