In recent years, the level of urban air quality in China has seriously declined, and many big cities can only see a vast expanse of whiteness when they go out. Many people criticize the poor air quality, so many people will post pictures of the city "Wonderland" in Weibo or friends circle. We slowly began to pay attention to the daily weather forecast to see whether the air quality of the next day is suitable for travel; Poor air quality has also taught us some new terms, such as smog, PM2.5 and so on. As far as I know, the value of PM2.5 in Hefei, where I live, has more than doubled compared with five years ago. In severe cases, you can't see it beyond 10 meter, and your breathing is uncomfortable, just like inhaling small particles. So, why has the air around us become so bad?

I think there are three reasons:

First, I think the air quality must be related to the factory. We are still a developing country. In most cities and regions, the development of industry is dominant, and the proliferation of factories is a matter of course, but at the expense of air quality. Some factories directly pollute the air, such as coal smelters, brick kilns and thermal power plants, there will be a chimney-shaped building, from which sulfur, carbon monoxide, sulfur dioxide and other gaseous particles or air pollutants will emerge. If there are more such factories, the air will certainly be polluted to some extent. There is also a class of factories that pollute the air with secondary pollutants, such as oil plants, fertilizer plants and pharmaceutical factories. Most pollutants discharged by these factories are not gaseous, but washed away by rivers. They first cause water pollution, and then pollute the air through some chemical reactions. So poor air quality has a lot to do with many factories.

Secondly, I think part of the reason is that there are too many motor vehicles, which leads to the decline of air quality. In recent years, people's living standards have gradually improved, and cars have become cheaper and cheaper. Almost every family has a car, and some families even have several cars. There are more and more cars on the highway. I stand by the window every day. As long as you go to work, you can see the traffic, and all kinds of cars are dizzying. We should know that most of the gases emitted by automobile exhaust are gases that seriously pollute the air, such as carbon monoxide, hydrocarbons and nitric oxide. Some are even harmful to the human body. If there are more cars in the city, the air will naturally get worse.

Finally, I think the poor air quality is also related to the wanton construction of houses, highways and railways. Building houses and building roads can't avoid cutting down trees and destroying forests. People nowadays want to develop too much. There are high-rise buildings everywhere in the city. The result of using all the space to build high-rise buildings is that there are not so many trees in the city to absorb dust and release fresh oxygen, which leads to the deterioration of air and the increase of PM2.5.