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  3. A useful definition of an air pollutant is a compound added directly or indi

A useful definition of an air pollutant is a compound added directly or indi

游客2024-03-07  61
问题     A useful definition of an air pollutant is a compound added directly or indirectly by humans to the atmosphere in such quantities as to affect humans, animals, vegetation, or materials adversely. Air pollution requires a very flexible definition that permits continuous change. When the first air pollution laws were established in England in the fourteenth century, air pollutants were limited to compounds that could be seen or smelled—a far cry from the extensive list of harmful substances known today. As technology has developed and knowledge of the health aspects of various chemicals has increased, the list of air pollutants has lengthened. In the future, even water vapor might be considered an air pollutant under certain conditions.
    Many of the more important air pollutants, such as sulfur oxides (硫氧化物), carbon monoxide, and nitrogen oxides, are found in nature. As the earth developed, the concentrations of these pollutants were altered by various chemical reactions; they became components in biogeochemical cycles. These serve as an air purification scheme by allowing the compounds to move from the air to the water or soil. On a global basis, nature’s output of these compounds dwarfs that resulting from human activities. However, human production usually occurs in a localized area, such as a city.
    In this localized region, human output may be dominant and may temporarily overload the natural purification scheme of the cycles. The result is an increased concentration of noxious chemicals in the air. The concentrations at which the adverse effect appears will be greater than the concentrations that the pollutants would have in the absence of human activities. The actual concentration need not be large for a substance to be a pollutant; in fact the numerical value tells us little until we know how much of an increase this represents over the concentration that would occur naturally in the area. For example, sulfur dioxide has detectable health effects at 0. 08 parts per million (ppm), which is about 400 times its natural level. Carbon monoxide, however, has natural level of 0. 1 ppm and is not usually a pollutant until its level reaches about 15 ppm. [br] The numerical value of the concentration level of a substance is useful only if________.
选项 A、it is in a localized area
B、it can be calculated quickly
C、the naturally occurring level is also known
D、the other substances in the area are known
答案 C
解析 由题干关键词the numerical value定位至第三段第四句。细节辨认题。由定位句可知，一种物质要成为污染物，其浓度并不需要很大；实际上，数值并不能告诉我们什么，除非我们知道这个数值代表的增加量超过了该地区自然浓度多少。由此可知，C“还知道其自然浓度的水平”为正确答案。
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