Photochemical smog is mainly caused by automobile exhaust and industrial emissions. After olefin hydrocarbons and nitrogen dioxide (NO2) in the automobile exhaust are emitted into the air, under the intense ultraviolet irradiation, they will absorb the energy of the sunlight. After the molecules of these materials absorb the energy of the sunlight, they will become unstable, and the existing chemical chain will be destroyed and form into a new substance. This chemical reaction is known as photochemical reaction, and the product is the photochemical smog which is highly toxic.
Nitrogen oxides (NOx) mainly refer to nitric oxide and nitrogen dioxide. Nitric oxide and nitrogen dioxide are both harmful gases to human body. After receiving the intense ultraviolet irradiation in the atmosphere, nitrogen oxides and hydrocarbons (HC) will produce a new secondary pollutant--photochemical smog. During this complex photochemical reaction, it mainly produces photochemical oxidants (mainly ozone) and other complex compounds, which are generally called as "photochemical smog".
As the car plays an increasingly important role in modern life, the pollution caused by vehicle emissions has become increasingly serious, which is a great threat to the physical and mental health of city dwellers.
Through the test and analysis, experts point out, in addition to these main components mentioned above, there are totally 150 to 200 different kinds of compounds in the gases emitted from automobiles, such as nitric acid, aldehyde and ketone, and so on. When these harmful gases spread into the environment, they will cause air pollution. Recently, the study from World Environmental Organization reported that, if the density of the car is more than 100 vehicles per square kilometer, the living environment will be destroyed. While this only refers to the car, the pollution caused by a truck or a bus is equal to that caused by five cars. What's worse, the heavy-duty truck will seriously pollute the air, and damage the vegetation and water beside the road.
In recent years, China has taken a series of comprehensive measures to prevent and control photochemical smog, including the formulation of laws and regulations. For example, in the big cities where the pollution is very serious, our country has formulated strict standard for air quality and for the emission of various types of vehicle exhaust. At the same time, China highly encourages the development of natural gas vehicles and electric vehicles, improve gasoline quality, and promote the use of lead-free gasoline. What's more, the use of natural gas and electric vehicles is also a necessary choice for China to achieve the strategy of sustainable development.
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