London has always been known for its "black fogs". In the winter of 1952, a milky white fog rolled into the city. As the smoke poured into the air, it was so hard to see that people had to walk in front of the buses to guide them. In this way, the most serious air pollution disaster in history began. When it was over, more than 4,000 people had been killed by the thick black smog.
New York City has had several London-type smogs since 1950. Each time, 100 to 400 deaths were caused by the smog. Although these smogs were not as deadly as London's, New York City has the worst air pollution problem in the United States.
In all the killer smogs, factories and homes poured smoke and fumes into the air from the furnaces. The chemical fumes combined with the water droplets in the fog to form harmful substances. These substances caused the illness of those who breathed the polluted air.
Usually, such harmful fumes rise into the upper air and are blown away by the wind. But sometimes there is an unusual weather condition called a temperature inversion. A layer of cold air remains near the ground as smoke and fumes pour into it. This is covered by an upper layer of warm air that acts like a lid. It prevents the polluted cooler air from rising. The harmful fumes pile up and make people ill. The smog may be so thick that airports are closed and chains of collisions occur on the highways.
Another type of smog occurs in Los Angeles. Here the weather may be clear and sunny. But stinging eyes and dry coughs show that harmful chemicals fill the air. The smog is due to invisible gases, mostly from automobile exhaust. Because these chemicals are changed by the sun high up in the air, it is called photochemical smog. It contains automobile exhaust fumes and nitrogen oxides changed by the sun's rays. Added to these are sulfur dioxide and other fumes from factories and oil refineries. Photochemical smog is found in many large cities all over the world.
Killer smogs don't happen very often, fortunately. But in Beijing, a combination of automobile exhaust fumes, home furnace smoke, and factory waste gases pours into the air. This may also happen in the suburbs, or out in the country, where large factories have been built. A number of harmful substances have been found in the air there. When these substances are breathed in day after day, the health of the population is affected.
小题1:Why do people call this phenomenon "black fog"?
A．It is black and dirty.
B．It can affect the health.
C．It can kill people.
D．It is too thick for people to see something.小题2:What can reduce the air pollution according to the passage'?
A．Water.
B．Wind.
C．The sun.
D．Cold air.小题3:When photochemical smog happens in the city, .
A．many people will be killed
B．black smoke may pour into the air
C．the weather may be fine
D．people have to walk to work小题4:From the passage we know that the most serious black smog disaster took place in .
A．London
B．New York
C．Los Angeles
D．Beijing
小题1:D
小题2:B
小题3:C
小题4:A
题目分析：文章大意：伦敦、纽约、洛杉矶和北京四个大城市曾经经历和正在经历的空气污染虽然有些许不同，但是都对人类的生活产生了极大的危害。
小题1:D细节理解题。根据第一段第三句中的“it was so hard to see that people had to walk in front of the buses to guide them”可知答案为D。故D正确。
小题2:B细节理解题。根据第四段的第一句“Usually, such harmful fumes rise into the upper air and are blown away by the wind.”可知答案。故B正确。
小题3:C细节理解题。根据第五段的第二句“Here the weather may be clear and sunny.”可知答案为C。故C正确。
小题4:A推理判断题。根据第一段中的“the most serious air pollution disaster in history began”以及第二段第三句中的“these smogs were not as deadly as London’s”可推知，最严重的浓黑的烟雾发生在伦敦。故A正确。