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An Air Pollutant
Keywords: an air pollutant, Air pollution, water vapor, actual concentration, sulfur dioxide, Carbon monoxide
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, 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 effects appear 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 a natural level of 0.1 ppm and is not usually a pollutant until its level reaches about 15 ppm.
1. What does the passage mainly discuss?
(A) The economic impact of air pollution
(B) What constitutes an air pollutant
(C) How much harm air pollutants can cause
(D) The effects of compounds added to the atmosphere
2. The word "adversely" in line 3 is closest in meaning to
(A) negatively
(B) quickly
(C) admittedly
(D) considerably
3. It can be inferred from the first paragraph that
(A) water vapor is an air pollutant in localized areas
(B) most air pollutants today can be seen or smelled
(C) the definition of air pollution will continue to change.
(D) a substance becomes an air pollutant only in cities
4. The word "altered" in line 12 is closest in meaning to
(A) eliminated
(B) caused
(C) slowed
(D) changed
5. Natural pollutants can play an important role in controlling air pollution for which of the following reasons?
(A) They function as part of a purification process.
(B) They occur in greater quantities than other pollutants.
(C) They are less harmful to living beings than are other pollutants.
(D) They have existed since the Earth developed.
6. According to the passage, which of the following is true about human-generated air pollution in localized regions?
(A) It can be dwarfed by nature's output of pollutants in the localized region.
(B) It can overwhelm the natural system that removes pollutants.
(C) It will damage areas outside of the localized regions.
(D) It will react harmfully with naturally occurring pollutants.
7. The word "noxious" in line 19 is closest in meaning to
(A) harmful
(B) noticeable
(C) extensive
(D) weak
8. According to the passage, the numerical value of the concentration level of a substance is only useful if
(A) the other substances in the area are known
(B) it is in a localized area
(C) the naturally occurring level is also known
(D) It can be calculated quickly
9. The word "detectable" in line 24 is closest in meaning to
(A) beneficial
(B) special
(C) measurable
(D) separable
10. Which of the following is best supported by the passage?
(A) To effectively control pollution local government should regularly review their air pollution laws.
(B) One of the most important steps in preserving natural lands is to better enforce air pollution laws.
(C) Scientists should be consulted in order to establish uniform limits for all air pollutants.
(D) Human activities have been effective in reducing air pollution.
参考答案：DACDD BACCA
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