Unit 5 Type and Sources of Air Pollutants 空气污染物的类型和来源
What is air pollution? Air pollution is normally defined as air that contains one or more chemicals in high enough concentration to harm humans, other animals, vegetation or materials. 什么是空气污染？空气污染通常是指那种包含一种或多种化学物质富集到高浓度并足以伤害人类、其他动物、植物或材料的空气。
There are two major types of air pollution.主要有两种类型的空气污染。
A primary air pollution is a chemical added directly to the air that occurs in a harmful concentration.初级空气污染是指有害的浓度直接进入到空气中的化学物质。
It can be a natural air component, such as carbon dioxide, that rise above its normal concentration, or something not usually found in the air, such as a lead compound emitted by cars burning leaded gasoline. 这种化学物质可以是一个自然的空气成分，如超过了正常浓度的二氧化碳，或通常不会在空气中发现的，如汽车燃烧含铅汽油所排放出来的主要尾气。
A secondary air pollution is a harmful chemical formed in the atmosphere through a chemical reaction among air components.次生空气污染是指在大气中由气体成分之间发生化学反应的有害化学物质。
Serious air pollution usually results over a city or other area that is emitting high levels of pollutants during a period of air stagnation.严重的空气污染通常是因为一个城市或其他地区在空气流动停滞期间排放了高浓度污染物。
The geographic location of some heavily populated cities, such as Los Angeles and Mexico City, makes them particularly susceptible to frequent air stagnation and pollution buildup.一些人口稠密的城市的地理位置，例如洛杉矶和墨西哥城，使他们特别容易受到频繁的空气停滞和空气污染累积的影响。
We must be careful about depending solely on concentration values in determining the severity air pollutants.在仅从浓度来决定严重空气污染物方面，我们必须十分小心。
By themselves , measured concentrations tell us nothing about the danger caused by pollutants , because threshold levels , synergy, and biological magnification are also determining factors.通过测出的浓度自身并不能告知我们有关污染物所带来的危险的信息，因为临界浓度、协同作用还有生物放大效应都是决定因素。
In addition, we run into the issue of conflicting views of what constitutes harm. 除此之外，我们碰到关于什么才构成危害的问题的什么才构成危害的问题的争论。
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