The conception of air pollution
Air is a mixture of nitrogen, oxygen, carbon dioxide, water vapor, and solid impurities. In standard condition, calculated by volume, nitrogen takes up 78.08%, oxygen 20.94%, argon 0.93%, carbon dioxide 0.03% and the other gases takes up 0.02%. Various natural changes cause the change of air compositions. For example, when a volcano erupts, there is a large amount of dust and carbon dioxide coming into the air. Thus, the volcanic eruption area is pervaded by smoke and toxic gases. Forest fires caused by thunder and lightning also can increase the carbon dioxide and dusts. Generally speaking, those natural changes are partly and temporary. But as modern industries and transportation develop, the air compositions are dramatically changed. When the abnormal compositions achieve to a level that harms the human beings, animals, plants and climates, we say the air is polluted. 

Generally, there are three main source of air pollution.
First, industry: it is a major source of air pollution. Industries usually discharge various and complex pollutants into the air, including dusts, oxides of sulphur, oxides of nitrogen, organic compounds, halides and carbon compounds.

Second, stoves and heating boilers: citizens need to use them to warm themselves and thus, they consume a lot of coal. The burning of coal can release a large amount of dust, sulfur dioxide, carbon dioxide and carbon monoxide. Especially in winter, the polluted area is be permeated with thick fog and smoke.

Third, transportation: cars, trains, airplanes, and steamers are main means of transport. Their power comes from the burning of coal and petroleum. Thus, a lot of pollutants are produced. Especially in cities where cars amass and release many wastes which directly harm people’s respiratory organs. Cars mainly release carbon monoxide, carbon dioxide, nitric oxide and hydrocarbon. The former three waste gases are the most dangerous.

Suggestions for improving the air quality
First, proper distribution of industries: it is a key way to solve the air problems. Industries should not accumulate in an area. Besides, put related industries together so that waste gases can be comprehensively utilized.

Second, installation of regional heating devices: dispersive stoves and chimneys are the main source of dusts. Using some effective large thermal power plants to replace thousands of stoves is a good way to reduce the pollution.

Third, reduce the waste gases released by means of transport. The key is to improve the design of engines and the burning quality of gasoline.

Fourth, change the composition of fuel. Try to explore the new energy as soon as possible such as solar energy, hydrogen fuel and terrestrial heat so as to reduce the pollution.

Fifth, afforestation: dense trees can slow down the wind, and thus decrease the large dusts in the air. As tree leaves are tomentose, they can absorb lots of dusts and thus, the air is much cleaner.