294 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1956 



idling and deceleration effects (largely due to hydrocarbon losses) . It 

 seems, however, that many of the more recent motor-vehicle engines, 

 with an increased compression ratio, produce relatively larger propor- 

 tions of nitrogen dioxide. The relative importance of nitrogen di- 

 oxide effects as compared to hydrocarbon effects may, therefore, be 

 quite different in the future. 



This discussion is presented here to indicate a method for an anal- 

 ysis and does not give final results, which require the use of much more 

 extensive data than it was possible to use in our extremely simplified 

 model of atmospheric pollution. 



AIR ZONING FOR NEW POLLUTION SOURCES 



All these studies are closely connected with air zoning problems for 

 an existing urban area. Their purpose is to reduce the pollution of 

 both the over-all urban area and the local neighborhood. Such a re- 

 duction may be needed not only when there is danger of reaching the 

 allowable threshold concentrations at any location of the urban area 

 but also to make possible the installation of new industries or other 

 sources of pollution that accompany a normal expansion of a city. 

 A study of the influence of a new source of pollution on the mean con- 

 centration patterns may therefore be an important air zoning problem. 

 It should be emphasized that, as far as air zoning is concerned, there 

 is no need to impose restrictions on the installation of an industry at 

 any location of an urban area. Limitations should refer only to the 

 amount of pollution that such a new industrial source of pollution 

 would contribute to the atmospheric pollution of its neighborhood and 

 of the over-all urban area. 



Conventional land zoning has as its main purpose promoting art 

 and amenity in urban building. The urban area is divided into resi- 

 dential, commercial, industrial, and other zones with specifications 

 describing the character and the use of the authorized buildings and 

 enterprises. To accomplish its purposes in each zone, the means used 

 by land zoning are to apply certain restrictions to the buildings in 

 the same zone. Air zoning proposes to limit the contamination of 

 the urban atmosphere, and would specify the allowable levels of 

 concentration for the atmosphere of each zone. However, building 

 restrictions for each zone must take into account the contributions 

 of the whole urban area to the contamination of its atmosphere. The 

 necessary restrictions will therefore depend on the meteorological 

 conditions of the urban area and result in different requirements for 

 the same classes of zones, according to their location in the area. 

 When building restrictions are considered, one must of course de- 

 termine some specifications that would apply for all weather condi- 

 tions. Such restrictions could be based on an analysis of the me- 



