276 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1956 



must therefore face the problem of reducing the degree of contamina- 

 tion without at the same time seriously affecting many of those human 

 activities that are responsible for the contamination and which, as 

 much as the clean air, are necessary to the population. 



Zoning of an urban area for the purpose of eliminating or limiting 

 atmospheric pollution has been considered in several instances. The 

 usual purpose of zoning in urban planning is the regulation of resi- 

 dential building. However, the principles and methods used in such 

 zoning cannot be applied directly to atmospheric pollution zoning. 

 In the regulation of residential building, one is concerned with zoning 

 the use of the land; in the regulation of atmospheric pollution, one 

 should be concerned with zoning the use of the atmosphere above the 

 land. In the present article, I shall refer to such air zoning and I 

 shall examine how it can be achieved. 



An urban area usually includes residential and industrial buildings. 

 Both the industrial and residential parts of the area contain sources 

 of pollution. In a residential area, those sources may include house 

 chimneys, motorcar exhausts, household incinerators, and others. In 

 the industrial area, we may have smokestacks, oil refineries, steel mills, 

 and other sources of industrial pollution. We should differentiate 

 between two general areas of contact of pollutants with the ground. 

 The first is the area near the source of pollution itself, and the second 

 is the area at a considerable distance from the source. Most of the 

 industrial sources emit very large amounts of pollutants but often 

 discharge them through tall stacks and avoid the high concentrations 

 of pollution at ground level. Therefore a large part of the industrial 

 pollutants comes in contact with the ground at considerable distances 

 from the industry that produces them. In residential buildings, auto- 

 mobiles, and house incinerators, the most important area of contact 

 of pollutants with the ground is near the source of pollution itself. 



The problem of air zoning is to limit the amount of atmospheric 

 pollution tolerable at various locations in the area. In a populated 

 community such limitation may be based on health requirements, 

 nuisance level, visibility, or the pollution deposits on buildings, cars, 

 and other objects. In rural areas it is necessary that atmospheric 

 contamination be reduced to such an extent that it will not poison 

 the soil, damage crops and vegetation, or injure livestock. 



ATMOSPHERIC DISPERSION OF POLLUTANTS 



The most obvious dangers of contamination are limited in some 

 areas to the periods of time when the meteorological conditions are 

 very unfavorable to the dispersion of the pollutants. Since not much 

 can as yet be done about the weather, consideration is often given to 

 shutting down sources of pollution when such unfavorable conditions 



