In 1967, the concept of the “airshed” as a unit for planning and management of air (resource) quality—hitherto a notion espoused by a scattering of scholars—received official blessing. In his January Message to the Congress, “Preserving the Quality of Our Natural Environment,” President Johnson stated, “regional air quality commissions should be established to enforce pollution control measures in ‘regional airsheds’ which cut across state and local boundaries … I recommend that the Air Quality Act of 1967 authorize the Secretary of Health, Education, and Welfare to:
—designate those interstate areas where effective regional airshed pollution programs are needed, that do not exist;
—establish in consultation with the states and local communities affected a regional air quality commission in each such area.”
In November, both houses passed such a bill (Public Law 90-148). The Air Quality Act of 1967 provides for expansion and improvement of research and development programs for air pollution control, and for planning and control programs on a regional basis.
Like the Water Quality Act of 1965, on which the present act was largely modeled, the Air Quality Act requires standard setting and enforcement by the states and permits strong federal action if the states do not act. Ambient air quality standards would be established for the regions to reflect the geographical variations in factors influencing the degree of air pollution. These would include concentration of waste generators and of receiving populations, emission characteristics, meteorology, and topography.
The act also establishes a Presidential Air Quality Advisory Board, and calls for registration of fuel additives, a study of national emissions standards and of emissions from jet aircraft, and comprehensive cost studies.
The concept that air quality should be managed on a regional (airshed) basis is, of course, a descendant of the concept of the watershed as the areal unit for water resource management. The watershed has proved to be an appropriate area in which to consider and implement water quality control programs. Water units usually (but not always) make sense both hydrologically and economically. For example, if upstream reservoir storage is provided to regulate downstream flows, it may benefit power plants all along the stream by providing more even streamflow for power generation. Because of this, benefits occurring in the entire watershed must be considered when the decision is made whether to build upstream storage and to what capacity. Similarly—and more comparable to problems of air resources—in deciding how much water quality control is desirable, all affected downstream uses of water must be considered. The various uses of the water in the watershed are physically and often economically interdependent.
Air currents dilute and transport emissions to the atmosphere in much the same way that water currents dilute and transport pollutants. In deciding what the best levels and measures of control are, these processes must be understood and forecast. The impacts on all uses within the affected airshed must be evaluated, and the effects on these uses of alternative levels and types of emissions control must be estimated. From this point of view, the airshed is quite as sensible a concept as the watershed as a resource management unit.
Clearly, however, the airshed is considerably more difficult to define geographically. Streams run in one direction only, and their surface drainage areas have rather clearly defined physical boundaries. Even in watersheds, though, the highly variable nature of streamflow (within a single year and from year to year), the amount of which may vary by a factor of 100 or more, creates difficulties in planning, designing, and operating water management works. The airshed adds more dimensions to this problem. It may be likened to a stream which varies its course rapidly (within defined boundaries), changes specific gravity, and from time to time decides to flow uphill or not to “flow” at all.
The problem, however, is not so intractable as it may at first appear. With the aid of contemporary computers and modern meteorological models, it is possible to define airsheds, even though their boundaries may be fuzzy and their sizes may differ in terms of the composition of gaseous wastes. The latter is the case because some materials degrade rapidly in the air or settle to the earth, while other gases may for long persist in the atmosphere. A dense smoke, for example, may be harmful in the immediate area of emission, but the particulates forming it will normally settle to the earth fairly quickly. At the extreme, atomic radiation and gases such as carbon dioxide (which is emitted in massive quantities due to the burning of fossil fuels) may become mixed throughout the world’s atmosphere.
However, for those emissions of greatest concern at present—hydrocarbons, sulfur dioxide, particulates, carbon monoxide, and the like—it should be possible, using contemporary tools, to define reasonably acceptable airshed areas.
The Environmental Sciences Services Administration of the Department of Commerce; the Travelers Research Center; the National Center for Air Pollution of the Department of Health, Education, and Welfare; and others have been working on atmospheric diffusion models that lend themselves quite well to forecasting the boundaries of airsheds. They could be established directly if air quality measurements were abundant; but this is not now the case, and it is tremendously expensive to measure continually the concentration of a variety of gaseous discharges over large areas for a number of years.
At this point, the meteorological model approach appears much more feasible. Atmospheric diffusion models utilize information on the rate of emissions of wastes from different sources, and on such meteorological factors as wind speed and direction at various elevations and temperature at various heights above the ground. Using these data and mathematical models programmed for computers, it is possible to forecast the concentration of gaseous wastes in the ambient air at different distances from their sources. Calculations of this kind, plus a certain amount of judgment, should be capable of yielding fairly acceptable boundaries for airsheds, some of which may be quite large. It would not be surprising, for example, if one airshed were found to stretch from Boston to Washington.
A version of this article appeared in print in the January 1968 issue of Resources magazine.
