In ancient times, air was once considered as a simple substance. 1669, You Mei inferred that the composition of air was very complicated according to the experiment of candle burning. About 1700, Starr of Germany put forward a general chemical theory, namely "phlogiston theory". He believes that there is an invisible so-called phlogiston, which exists in combustible substances. For example, when a candle burns, phlogiston escapes, and the candle shrinks and collapses into ashes. He believes that burning loses phlogiston, that is, candle-phlogiston = ashes. However, phlogiston theory cannot explain some phenomena in natural changes after all, and there are serious contradictions. The first is that no one has ever seen phlogiston; Incredibly, the mass of the second metal increases after combustion, so "phlogiston" must be negative. 1774, French chemist lavoisier put forward the theory of combustion oxidation, which denied the phlogiston theory. When lavoisier was doing the combustion experiments of metals such as lead and mercury, he found that some metals turned into colored powder, and the volume of air in the bell jar decreased by 65,438+0.5 of the original volume. The remaining air could not support the combustion, and animals would suffocate in it. He called the remaining 4/5 gas nitrogen (meaning it doesn't support life). After he proved that the gas produced by priestley and Scheler from the decomposition of mercuric oxide was oxygen, the composition of air was determined as nitrogen and oxygen.

The composition of air is mainly nitrogen and oxygen, which is caused by various changes in nature for a long time. Before the appearance of primitive green plants, the primitive atmosphere was dominated by carbon monoxide, carbon dioxide, methane and ammonia. After the emergence of green plants, the free oxygen released by plants during photosynthesis oxidizes carbon monoxide in the original atmosphere into carbon dioxide, methane into water vapor and carbon dioxide, and ammonia into water vapor and nitrogen. Later, due to the continuous photosynthesis of plants, most of the carbon dioxide in the air was absorbed in the process of plant photosynthesis, which made more and more oxygen in the air, and finally formed a modern air dominated by nitrogen and oxygen.

Air is a mixture and its composition is very complicated. The constant components in the air are nitrogen, oxygen and rare gases, which are almost constant, mainly because of the mutual compensation of various changes in nature. The variable components of air are carbon dioxide and water vapor. The uncertain composition of air varies from region to region. For example, the air near the factory will contain ammonia, acid vapor and so on due to different production projects. In addition, the air also contains trace amounts of hydrogen, ozone, nitrogen oxides, methane and other gases. Dust is more or less an impurity suspended in the air. Generally speaking, the composition of air is relatively fixed.

Due to the strong gravity of the earth, 80% of the air is concentrated in the average range of 15 km from the ground. This air layer has a great influence on human life and production activities. What people usually say about air pollution refers to the air pollution within this range. The development of industry has discharged harmful substances into the air, polluted the air and increased harmful components in the air. When harmful substances in the air reach a certain concentration, it will seriously damage human health and the growth of crops, destroy some substances, reduce people's visibility and affect traffic safety. Therefore, efforts must be made to prevent air pollution.

Harmful substances discharged into the air can be divided into the following categories: dust (such as carbon particles, etc. ), metal dust (such as iron, aluminum, etc. ), wet fog (such as oil fog, acid fog, etc. ), and harmful gases such as carbon monoxide, hydrogen sulfide and nitrogen oxides. ). From a global perspective, sulfur dioxide and carbon monoxide are harmful gases with large emissions and great harm. Sulfur dioxide is produced when coal and oil are burned. Carbon monoxide is mainly emitted when the car is driving. Globally, it is estimated that carbon monoxide emissions exceed sulfur dioxide.