1. AIRBORNE CHEMICALS 



Chemical Contaminants in the Atmosphere 



Atmospheric contamination is dif- 

 ficult to define precisely, since "pure 

 air" itself is a mixture. Water con- 

 tamination is a simple concept, since 

 "pure water" is a single chemical sub- 

 stance. Even with human influences 

 absent, the air has a variable compo- 

 sition in both time and space. There 

 is evidence that there was more oxy- 

 gen in the atmosphere at the peak of 

 the carboniferous era than there is 

 today; and the atmosphere close to 

 an erupting volcano is bound to be 

 different in composition from the air 

 in the midst of a pine forest. 



Definition of Contamination 



To discuss the contamination of a 

 mixture it is necessary to define an 

 arbitrary composition as "pure." It is 

 usual to define dry pure air as con- 



taining roughly 78 percent nitrogen, 

 21 percent oxygen, 0.03 percent car- 

 bon dioxide, and the remaining 0.97 

 percent noble gases. (See Figure X-l) 

 Water vapor is present in pure air in 

 highly variable amounts. 



Under this definition, all air is con- 

 taminated to some degree. Much of 

 the contamination is both natural and 

 beneficial. The development of clouds 

 and precipitation, for example, re- 

 quires the presence of "nucleation 

 centers," usually consisting of dust, 

 sea salt, and particles formed in the 

 air by reactions between gaseous 

 contaminants. 



Some contaminants are intrinsically 

 harmful to things that humans value 

 or harmful in excessive concentra- 

 tions. If these contaminants are pro- 

 duced directly or indirectly by human 



Figure X-1 — COMPOSITION OF CLEAN, DRY AIR 



The table shows the major and some of the trace constituents of clean, dry air 

 near sea level. The concentrations of some of the gases may differ with time and 

 from place to place. Some of the data are open to question, but the values are 

 meant to indicate the order of magnitude. 



activities, they are called air pollut- 

 ants. In a few instances a pollutant 

 is not, paradoxically, a contaminant. 

 For example, excessive industrial 

 steam meets the usual criteria of a 

 pollutant if it obscures visibility on 

 a major highway; yet strictly speak- 

 ing it is no more than a part of the 

 variable fraction of water vapor as- 

 cribed to "pure" air. 



The distinction between a harmful 

 contaminant and a pollutant may be 

 a narrow one. Natural contamina- 

 tions, such as a rain of volcanic ash 

 or a desert sandstorm, are not classed 

 as pollutants. But when human ac- 

 tivity is responsible for their occur- 

 ring — as it was in the great dust 

 storms of the 1930's — these contami- 

 nants are classed as pollutants. We 

 can legitimately consider even a sand- 

 storm in the Sahara as pollution, 

 since the great desert, at least in its 

 present extent, appears to have been 

 caused by overgrazing. 



A final case needs to be distin- 

 guished in the definition of contami- 

 nation. Air may be rendered harmful 

 to life, not by the presence of con- 

 taminants, but by the absence of 

 oxygen. Since combustion within the 

 48 contiguous United States now re- 

 quires twice as much oxygen as all 

 the green plants in the area produce, 

 the hazard of depleted air may be- 

 come acute. 



The Environmental Problem 



The sources of pollutants are gen- 

 erally well characterized and the com- 

 position of most is known. Much less 

 is known about natural contaminants. 

 Many come from enormous areas at 

 miniscule concentrations. For exam- 

 ple, air that blows into Barbados 

 from the tropical Atlantic contains a 



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