The atmosphere is a mixture of nitrogen, oxygen, carbon dioxide, water vapor and solid impurity particles in a certain proportion. As far as clean air is concerned, in terms of volume, in the standard state, nitrogen accounts for 78.08%, oxygen accounts for 20.94%, argon accounts for 0.93%, carbon dioxide accounts for 0.03%, and the volume of other gases can be ignored. Various natural changes often cause changes in atmospheric composition. For example, when a volcano erupts, a large amount of dust, carbon dioxide and other gases are injected into the atmosphere, which causes the volcanic eruption area to be full of smoke and toxic gases; Large-scale forest fires caused by natural causes such as lightning will also increase the content of carbon dioxide and smoke particles. Generally speaking, this natural change is local and short-lived. With the development of modern industry and transportation, more and more substances are continuously discharged into the atmosphere, and the types are becoming more and more complex, causing drastic changes in atmospheric composition. When substances other than normal components in the atmosphere are harmful to human health, animal and plant growth and meteorological climate, we say that the atmosphere is polluted.

Air pollution source is the source of air pollutants, mainly in the following three ways:

(1) Industry: Industry is an important source of air pollution. There are many kinds of pollutants discharged into the atmosphere by industry, including smoke, sulfur oxides, nitrogen oxides, organic compounds, halides, carbon compounds and so on. Some are smoke and some are gas.

(2) Living stoves and heating boilers: A large number of living stoves and heating boilers in cities need to consume a lot of coal, and coal will release a lot of harmful substances such as dust, sulfur dioxide and carbon monoxide to pollute the atmosphere during combustion. Especially when heating in winter, it often makes the polluted area filled with smoke and makes people cough, which is also a pollution source that cannot be ignored.

(3) Transportation: Cars, trains, planes and ships are the main means of transportation at present, and the waste gas produced by burning coal or oil is also an important pollutant. In particular, the number of cars in cities is large and concentrated, and the pollutants discharged can directly hit people's respiratory organs, which has caused serious air pollution in cities and has become one of the main sources of air pollution in big cities. The exhaust gas emitted by automobiles mainly includes carbon monoxide, sulfur dioxide, nitrogen oxides and hydrocarbons, and the first three substances are very harmful.

The hazards of air pollution mainly include the following aspects:

(1) Harm to human health: People need to breathe air to maintain their lives. An adult breathes about 20,000 times a day and inhales 1.5 ~ 20 cubic meters of air. Therefore, polluted air has a direct impact on human health.

(2) Harm to plants: Air pollutants, especially sulfur dioxide and fluoride, are very harmful to plants. When the concentration of pollutants is high, it will cause acute damage to plants, damage to the surface of plant leaves, or directly cause leaves to wither and fall off; When the concentration of pollutants is not high, it will cause chronic damage to plants, make the leaves of plants fade, or have no harmful symptoms on the surface, but the physiological functions of plants are affected, leading to the decline of plant yield and deterioration of quality.

(3) Impact on weather and climate: The impact of air pollutants on weather and climate is very significant, which can be explained from the following aspects:

(1) Reduce the amount of solar radiation reaching the ground: A large number of smoke particles discharged into the atmosphere by factories, power stations, automobiles, household heating equipment, etc. make the air very turbid, which blocks the sunlight and reduces the amount of solar radiation reaching the ground. According to observation and statistics, in the days when smoke and dust are not scattered in large industrial cities, the direct sunlight on the ground is reduced by nearly 40% compared with the days without smoke and dust. In cities with serious air pollution, people, animals and plants will be stunted due to lack of sunshine every day.

(2) Increase atmospheric precipitation: Many particles emitted by large industrial cities have the function of water vapor condensation nuclei. Therefore, when there are other precipitation conditions in the atmosphere, there will be precipitation weather. In the downwind areas of large industrial cities, there is more precipitation.

③ Acid rain: Sometimes, the rain falling from the sky contains sulfuric acid. This kind of acid rain is the oxidation of sulfur dioxide, a pollutant in the atmosphere, to form sulfuric acid, which falls with the precipitation in nature. Sulfuric acid rain can destroy large areas of forests and crops, corrode and destroy paper products, textiles and leather products, degrade metal antirust coatings and reduce their protective effects, and also corrode and pollute buildings.

(4) Raise the atmospheric temperature: Over large industrial cities, the air temperature near the ground is higher than that in the surrounding suburbs due to a large amount of waste heat discharged into the air. This phenomenon is called "heat island effect" in meteorology.

⑤ Impact on global climate: In recent years, people have gradually noticed the impact of air pollution on global climate change. After research, people think that carbon dioxide plays an important role in various air pollutants that may cause climate change. About 50% of the carbon dioxide released into the atmosphere by countless chimneys and other exhaust pipes on the earth remains in the atmosphere. Carbon dioxide can absorb long-wave radiation on the ground and raise the air temperature near the ground, which is the so-called "greenhouse effect". It is roughly estimated that the near-surface temperature will increase by 0.5 ~ 2℃ for every 25% increase of carbon dioxide content in the atmosphere. If 100% is increased, the near-surface temperature can be increased by 1.5 ~ 6℃. Some experts believe that the carbon dioxide content in the atmosphere will increase at the current rate, which will melt the ice in the North and South poles in a few years, leading to global climate anomalies.