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 78.18%, oxygen accounts for 21.94%, argon accounts for 1.93%, carbon dioxide accounts for 1.13%, and the volume of other gases is negligible. 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, causing the volcanic eruption area to be filled with smoke and poisonous gas; 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, which causes drastic changes in atmospheric composition. When substances other than the normal components of the atmosphere are harmful to human health, animal and plant growth and meteorological climate, we say that the atmosphere is polluted.

[ Edit this paragraph ]2. Major pollution sources and pollutants in the atmosphere

Air pollution sources are the following three sources:

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

(2) domestic stoves and heating boilers: a large number of domestic 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 areas filled with smog and makes people cough, which is also a pollution source that can not be ignored.

(3) Transportation: Cars, trains, planes and ships are the main means of transportation at present, and the exhaust gas generated by burning coal or oil is also an important pollutant. Especially, cars in cities are large and concentrated, and the pollutants discharged can directly attack people's respiratory organs, causing serious air pollution in cities and becoming one of the main air pollution sources 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.

[ Edit this paragraph ]3. Hazards of air pollution

Hazards of air pollution mainly include the following aspects:

(1) Hazards to human health: people need to breathe air to maintain their lives. An adult breathes about 21,111 times a day, and inhales 1.5 ~ 21 cubic meters of air. Therefore, polluted air has a direct impact on human health.

The harm of air pollutants to human body is various, mainly manifested in respiratory diseases and physiological dysfunction, and mucosal tissues such as eyes and nose are stimulated to get sick.

for example, on February 5-8, 1952, a coal smoke incident occurred in London, England, killing 4,111 people. People call the smoke of this disaster "the smoke of killing". According to the analysis, this is because there was no wind and fog in London in those days, and the smoke and dust from factory chimneys and residents' heating permeated the urban area of London for a long time. The highest concentration of smoke and dust reached 4.46 mg/m3, and the daily average concentration of sulfur dioxide reached 3.83 ml/m3. After a chemical reaction, sulfur dioxide produces sulfuric acid droplets attached to smoke or condensed on fog droplets, which enter organs with breathing, making people ill or accelerating the death of patients with chronic diseases. This is also called photochemical pollution.

As can be seen from the above example, when the concentration of pollutants in the air is very high, it will cause acute pollution poisoning, or worsen the symptoms, and even kill thousands of people in a few days. In fact, even if the concentration of pollutants in the atmosphere is not high, breathing this polluted air for years will cause diseases such as chronic bronchitis, bronchial asthma, emphysema and lung cancer.

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

(3) Impact on weather and climate: The impact of atmospheric 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 soot particles discharged into the atmosphere from factories, power stations, automobiles and household heating equipment 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 the smoke is not scattered in large industrial cities, the amount of direct sunlight hitting the ground is reduced by nearly 41% compared with the days without smoke. In cities with serious air pollution, it will lead to poor growth and development of people, animals and plants due to lack of sunlight every day.

② increasing atmospheric precipitation: many particles discharged from large industrial cities have the function of condensation nuclei of water vapor. Therefore, when there are other precipitation conditions in the atmosphere, there will be precipitation weather. In the downwind areas of big 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 formed by 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 break paper products, textiles and leather products, deteriorate metal antirust coatings and reduce their protection, and also corrode and pollute buildings.

④ increase the atmospheric temperature: over the big industrial cities, the temperature of the air near the ground is higher than that in the surrounding suburbs because a large amount of waste heat is 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 all kinds of air pollutants that may cause climate change. About 51% of the large amount of carbon dioxide discharged into the atmosphere from countless chimneys and other waste gas pipelines on the earth remains in the atmosphere. Carbon dioxide can absorb long-wave radiation from the ground and increase the air temperature near the ground, which is called "greenhouse effect". It is roughly estimated that if the carbon dioxide content in the atmosphere increases by 25%, the near-surface temperature can increase by 1.5 ~ 2℃. If it is increased by 111%, 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.

[ Edit this paragraph ]4. Prevention and control of air pollution

There are many measures to prevent and control air pollution, but the most fundamental one is to reduce pollution sources. Generally, the following measures are adopted:

(1) Rational industrial layout: this is an important measure to solve air pollution. Factories should not be too concentrated to reduce the emission of pollutants in an area. In addition, chemical plants with raw material supply relations should be put together to reduce exhaust emissions through comprehensive utilization of exhaust gas.

(2) district heating and central heating: stoves scattered in thousands of households and short chimneys densely as trees in the city are the main pollution sources of soot dust pollution. It is an important measure to eliminate soot by adopting the method of district heating and central heating, that is, replacing the stoves of thousands of households with several large thermal power plants with high-efficiency dust removal equipment set up in the suburbs.

(3) Reduce the pollution of traffic exhaust: The key to reduce the pollution of automobile exhaust lies in improving the combustion design of the engine and improving the combustion quality of gasoline, so that the oil can be fully burned, thus reducing harmful exhaust.

(4) change the fuel composition: implement the conversion from coal to gas, and at the same time step up research and development of other new energy sources, such as solar energy, hydrogen fuel and geothermal energy. In this way, the pollution of smoke and dust can be greatly reduced.

(5) Afforestation: Dense forest can reduce the wind speed and the large dust carried in the air. The surface of leaves is rough, some have fluff, some can secrete mucus and oil, so they can absorb a lot of floating dust. Dust-covered leaves can continue to absorb dust after being washed by rain. Such reciprocating blocking and adsorption of dust can purify the air.

I. Overview of air pollution

Even when the sky is clear, the atmosphere around us is not as clear as it seems. The air is full of invisible solids, liquids and gases, such as pollen, bacteria, smoke and dust, moisture and so on. The so-called air pollution means that the air contains one or more pollutants, the amount, nature and time of which will harm the lives of human beings, plants and animals, damage property, or interfere with the comfortable living environment, such as the existence of odor. In other words, as long as a substance exists in sufficient quantity, nature and time to have an impact on human beings or other creatures and property, we can call it an air pollutant; And the phenomenon caused by its existence is air pollution. We need to know the composition of clean air before we know what substances will cause pollution when they enter the air. The composition of clean air is shown in Table 1: what we usually call "air pollutants" such as nitrogen dioxide and ozone. The content of sulfur dioxide, carbon monoxide and other substances in clean air is very small; But in the case of pollution, some kinds of these specific substances will increase greatly. In other words, the abnormal increment of some substances in the air will produce air pollution.

ii. types of air pollutants

there are many types of air pollutants, and their types may be solid particles, dripped or gas, or a mixture of these types. What kinds of air pollutants are currently defined in China's laws and regulations? According to the definition of air pollution prevention law and relevant regulations, air pollutants can be divided into four major items, namely gaseous pollutants (including sulfur oxides, carbon monoxide, nitrogen oxides, hydrocarbons, chlorine gas, gasified hydrogen, fluoride, chlorinated hydrocarbons, etc.), granular pollutants (including suspended particles, gold bureau soot, black smoke, acid fog, falling dust, etc.), and secondary pollutants (referring to the pollution caused by photochemical reaction of pollutants in the air) Common air pollutants include suspended particles, carbon monoxide, sulfur oxides, nitrogen oxides and hydrocarbons, which are mostly caused by human factors. In China's laws and regulations, most of the air pollutants produced by human factors (such as chimney emissions, vehicle emissions, etc.) have "emission standards" to regulate their emissions.

iii. Air pollution index

Pollutant Standard Index (PSI) refers to the index decided by the US Environmental Protection Agency and other institutions, and represents the degree of air pollution with a value of 1 to 511. The relationship between this indicator value and health impact is divided into the following five grades:

indicator value

1 ~ 51 Health impact: good

51 ~ 111 (good) medium

111 ~ 199 (moderate): bad

211 ~ 299 (unhealthy). Extremely bad

311 ~ 351 (very unhealthy): harmful

(Hazardous) :

Air pollutants include smoke, steam, CharredPaper, Dust, Soot, Grime and CarbonFumes. Odor (Order), ParticulateMatter, RadioactiveMaterials, NoxiousChemicals, or other inclusions in the outdoor atmosphere.

[ Edit this paragraph ]5. Prevention and control of air pollution

Prevention and control of air pollution is a huge systematic project, which requires the concerted efforts of individuals, collectives, countries and even countries around the world. The following measures can be considered:

1. Reduce pollutant emissions. Reforming the energy structure, using pollution-free energy (such as solar energy, wind energy, hydroelectric power generation) and low-pollution energy (such as natural gas), pretreating fuel (such as desulfurization before burning coal) and improving combustion technology can reduce the pollution discharge. In addition, before the pollutants enter the atmosphere, some pollutants in the exhaust gas can be eliminated by using dust removal and smoke elimination technology, condensation technology, liquid absorption technology and recycling technology, which can reduce the amount of pollutants entering the atmosphere.

2. Control emissions and make full use of the self-purification ability of the atmosphere. Different meteorological conditions have different atmospheric capacity for pollutants, and the same amount of pollutants discharged will result in different pollutant concentrations. For areas and time periods with strong wind, good ventilation, strong turbulence and strong convection, the atmospheric diffusion and dilution ability is strong, and it can accept more activities of factories and mines. In areas and time periods with inversion temperature, the diffusion and dilution ability of the atmosphere is weak, so it can't accept more pollutants, otherwise it will cause serious air pollution. Therefore, effective emission control should be carried out in different regions and different time periods.

3. The site selection, chimney design, urban and industrial planning should be reasonable, so as to avoid excessive concentration of large emitters and repeated pollution, resulting in serious local pollution incidents.

4. Afforestation will enable more plants to absorb pollutants and reduce the degree of air pollution.

5. Little things around you. Even small things like throwing confetti can't be ignored. After all, a perfect city will not tolerate this.

[ Edit this paragraph ]6. Measurement of pollutants

Generally, μg/m3 (μ g/m3) is used as the unit; PPM(parts per million (ppm) is also used as the unit.

measurement of common single pollutant:

respirable suspended particles

carbon monoxide

carbon dioxide

all kinds of pollutants in the air have their own pollution index, which is called fractional index. At present, the monitoring items of air quality in key cities in China are uniformly defined as sulfur dioxide (SO2), nitrogen dioxide (NO2) and total suspended particulate matter (TSP), and the values of air pollution index are expressed by numbers between 1 and 511.

the value range of air pollution index is 1 ~ 511, in which 1 ~ 51, 51 ~ 111, 111 ~ 211, 211 ~ 311 and greater than 311 respectively correspond to the pollutant concentration limit values of the daily average in China air quality standards. The air quality grade and its impact on human health are as follows: < P > Grade I, the air quality assessment is excellent, and it has no impact on human health.