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.

2. Main pollution sources and pollutants in the atmosphere

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, carbon 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. Especially, the number of cars in cities is large and concentrated, and the pollutants discharged can directly hit 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.

3. Hazards of air pollution

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.

The harm of air pollutants to human body is various, mainly manifested in respiratory diseases and physiological dysfunction, as well as the irritation of mucosal tissues such as eyes and nose.

For example,1February 5-8, 952, the soot incident in London, England, killed 4000 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, and the smoke and dust generated by factory chimneys and residents' heating permeated the urban area of London for a long time. The highest concentration of smoke and dust reaches 4.46 mg/m3, and the daily average concentration of sulfur dioxide reaches 3.83 ml/m3. Sulfuric acid droplets produced by chemical reaction of sulfur dioxide attach to smoke or condense on fog droplets, and enter organs with breathing, which makes people sick or accelerates the death of patients with chronic diseases.

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

(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.

4. Air pollution prevention and control

There are many measures to prevent and control air pollution, but the most fundamental thing is to reduce pollution sources. The following measures are usually taken:

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

(2) District heating and central heating: stoves scattered in thousands of households and short chimneys as dense as trees in cities are the main pollution sources of soot dust pollution. It is an important measure to eliminate soot by central heating and district heating, that is, to replace the stoves of thousands of households in several large thermal power plants with efficient dust removal equipment set in the suburbs.

(3) Reduce the pollution of traffic exhaust: The key to reduce the pollution of automobile exhaust is to improve the combustion design of the engine, improve the combustion quality of gasoline, and make the engine oil burn fully, thus reducing harmful exhaust.

(4) Changing fuel composition: changing coal into gas, and stepping 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 can be greatly reduced.

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

The harm of air pollution

According to the definition made by the International Organization for Standardization, air pollution "usually means that a substance enters the atmosphere due to human activities and natural processes, and shows sufficient concentration in enough time, thus endangering human health, comfort and the environment".

Generally speaking, the atmosphere from the surface to the altitude of 1000 km is composed of dry and clean air, water vapor and various impurities. The composition of dry and clean air is basically unchanged, and the water vapor content changes with time and place, and various impurities (such as dust, smoke, harmful gases, etc.). Great changes have taken place due to natural factors or human activities, regardless of its type or content, and even lead to air pollution.

Natural factors leading to air pollution include volcanic activity, forest fire, tsunami, weathering of soil and rocks and air flow in the atmosphere. Generally speaking, due to the self-purification function of the natural environment, various natural phenomena can automatically coordinate the dynamic balance of the ecosystem.

Human activities include life activities and production activities, and the main object of preventing and controlling air pollution is production activities first. Air pollution caused by human activities mainly comes from three aspects: 1. Combustion process of fuel; 2. Industrial production process; 3. Transportation, etc. Among them, the air pollutants produced by fuel combustion (including coal, gasoline, diesel oil, natural gas, etc.). ) accounts for about 70% of all pollutants; Air pollutants produced by industrial production account for about 20%; The air pollutants produced by motor vehicles account for about 10%.

These air pollutants can be divided into aerosol pollutants and gaseous pollutants according to their existing state. Aerosol refers to solid particles, liquid particles or suspension of liquid particles in air medium with negligible settling speed, such as dust, smoke, fly ash, droplets, light fog, black smoke and fog. There are five aspects of gaseous pollutants: sulfur compounds mainly containing sulfur dioxide; Nitrogen compounds mainly containing nitrogen oxides and nitrogen dioxide; Oxides of carbon; Hydrocarbons and halogen compounds, etc.

These air pollutants mainly invade the human body through respiratory inhalation, digestive tract swallowing and skin contact. Among them, respiratory inhalation is the most dangerous.

The harm of air pollutants to human health is mostly manifested as respiratory diseases. Under the sudden action of high concentration pollutants, it can cause acute poisoning and even death in a short time. For example, the "Smoke" incident that shocked the world in London, England in 1952 killed more than 4,000 people in four days. Long-term exposure to low-concentration pollutants can cause chronic bronchitis, bronchial asthma, emphysema and even lung cancer. In addition, air pollutants will also have harmful effects on plants, animals, crops and utensils (buildings, appliances, etc.). ) and climate.

The harm of air pollution to human body is mainly manifested in respiratory diseases; It can inhibit the physiological mechanism of plants, leading to poor growth, weakened disease and insect resistance, and even death; Air pollution will also have adverse effects on the climate, such as reducing visibility and solar radiation (data show that the intensity of solar radiation and ultraviolet radiation in cities is lower than that in rural areas 10 ~ 30% and 10 ~ 25% respectively), leading to an increase in the incidence of rickets in cities; Air pollutants will corrode articles and affect product quality; In recent ten years, acid rain has appeared in many countries, and the acidity in rain and snow has increased, which has made rivers and lakes, soil acidification, fish reduction and even extinction, affecting forest development, which is closely related to air pollution.

Effects of various air pollutants on human body.

Smoke from burning coal

Cause bronchitis, etc. If soot is accompanied by various industrial dusts (such as metal particles), it can cause pneumoconiosis and other corresponding diseases.

Sulfuric acid smoke

It has a strong stimulating and damaging effect on skin, conjunctiva, nasal mucosa and throat. Severe cases such as gastric perforation, vocal cord edema, stenosis, heart failure or stomach irritation are life-threatening.

lead to

When it is slightly higher than the allowable depth of air pollution, it can cause chronic poisoning symptoms such as red blood cells, and high concentration can cause strong acute poisoning symptoms.

sulphur dioxide

When the concentration is 1-5ppm, it can smell, and long-term inhalation of 5ppm can cause cardiopulmonary diseases such as palpitation and dyspnea. In severe cases, it can cause reflex vocal cord spasm, laryngeal edema and even suffocation.

nitric oxide

Mainly refers to nitric oxide and nitrogen dioxide, the characteristic of poisoning is that it acts on the deep respiratory tract, and in severe cases it can reach pulmonary gangrene; It is harmful to mucosa, nervous system and hematopoietic system, and suffocation may occur when inhaled with high concentration of nitric oxide.

carbon monoxide

The affinity for hemoglobin in blood is 2 10 times greater than that of oxygen, which can cause severe hypoxia symptoms, that is, gas poisoning. About 100ppm can make people feel headache and fatigue.

ozone

Its influence is more complicated, ranging from low vital capacity to bronchitis.

hydrogen sulfide

Inhalation of 100ppm for 2- 15 minutes can cause olfactory fatigue, and high concentration can cause systemic injury and death.

cyanide

Mild poisoning has symptoms of mucosal irritation, which can make people faint gradually in severe cases. Although they have straight spasms, their blood pressure drops, and they will soon have breathing disorders and die. The sequelae of cyanide poisoning include headache, aphasia and epilepsy. Cyanide vapor can cause acute conjunctival congestion and asthma.

fluoride

It can invade the human body from respiratory tract, gastrointestinal tract or skin, mainly damaging bones, hematopoiesis, nervous system, teeth and skin mucosa. Severe cases may die of respiratory paralysis and collapse.

chlorine

The toxic effect on human body is mainly through respiratory tract and skin mucosa. When the concentration of chlorine in the air reaches 0.04 ~ 0.06 mg/L, it can cause serious poisoning in 30 ~ 60 minutes. If the chlorine concentration in the air reaches 3 mg/L, it will cause chemical burns to the lungs and die quickly.