Air Pollution

Atmosphere is air, which is closely related to people's lives. Smoke and harmful gases pollute the atmosphere and affect human health. Everyone can feel it. However, the pollutants in the atmosphere are affected by meteorological factors, and they will undergo physical and chemical changes, resulting in large-scale and widespread harm, which may not be widely known. For example, haze, smog, acid rain and other phenomena, as air pollution intensifies, we must focus on understanding them, where the pollution source is, how the phenomenon is formed, and what are the harms. It is true that not all air pollutants are emitted by humans, and pollution exists in nature itself. The source of air pollutants in some areas mainly comes from the use of fossil fuels, and fossil fuels are the source of regional air pollution.

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"Haze" is a new term that was only used in Chinese academic circles in 2000. The word "haze" has long existed in the past, and its meaning is the same as haze. Both refer to the turbidity formed by the suspension of large amounts of solid particles in the atmosphere. The reason why it is called "haze" is that this kind of suspended particulate matter is mainly caused by man-made emissions in some areas. Judging from the historical comparison, this view is in line with the facts. The haze phenomenon has already appeared, but the frequency is very low. Many areas with haze phenomenon have only a few days a year, and they have not attracted people's attention. However, in recent years, the phenomenon of haze has frequently appeared, which is called "haze weather", and many cities and regions have reached as many as one to two hundred days a year. Whenever the haze appears, the sky is gray and the eyes seem to be covered with a layer of gauze. The visibility is obviously weakened. Looking at the distance is like looking at flowers in the fog. Traffic accidents increase, which is very annoying. People have to ask, why the increasing number of haze weather can be prevented?

Figure : Haze phenomenon

How does the haze phenomenon form? First of all, we must talk about dust. The dust in the air, or dust, is a variety of solid particles, and the scientific name is "suspended particulate matter" (TSP). Particles are large or small. The large particles are larger than 10 microns in diameter, and they fall to the ground very quickly, which is called "dust fall." Floating for a long time is called "floating dust". Floating dust is easy to be inhaled and attached to the lungs, so it is also called "inhalable particulate matter". A variety of toxic metals and carcinogens attach to the fly dust, and then enter the human body, becoming an important pathogenic factor of human respiratory diseases. Floating dust also weakens the sun's exposure, may reduce the surface temperature, and also makes the sky cloudy, foggy and turbid, reducing visibility, and the appearance of haze is closely related to this.

Haze is very similar to fog, but has a different composition from common water fog. The water mist is formed by tiny water droplets and is dispersed as soon as the sun is exposed to it. The haze can be described as "dry fog", the sun is helpless, the more sunny it is, the more likely it is to appear, and it will not dissipate all day long. Why do people call it "haze weather"? Haze is indeed related to meteorological conditions, not all dust can become haze. The most important meteorological condition is the stability of the regional atmosphere. When the low-altitude atmosphere in the region is highly stable and there is a temperature inversion, the temperature at a high place is higher than the temperature on the ground, and the hot air cannot come down, and the ground is like being covered by a pot cover. At this time, if the pollutants do not diffuse out, the haze phenomenon is prone to appear. With the increase of pollutants emitted by cities and traffic, the concentration of TSP continues to increase. Combined with meteorological conditions that are not conducive to atmospheric diffusion, haze weather occurs frequently. Haze weather often occurs in large cities and urban agglomerations in autumn and winter and early spring, especially the warm winter air in recent years, with little cold air activity and high atmospheric stability, providing meteorological conditions for the formation of haze. When the cold air blows, the haze phenomenon disappears. It is because the stability of the atmosphere decreases and the pollutants diffuse away. Therefore, the folks have this experience: to let the haze clear, wait for the cold air to come. The reason is very simple.

Many people think that the cause of the haze is very complicated, and there are also reasons for it. One is that "dry fog" such as haze is often accompanied by the usual "wet fog". When the atmospheric stability is high, the moisture on the ground will easily form fog near the ground under the condition that the temperature difference between morning and night changes greatly. When the water mist dissipates in the afternoon, the haze still exists, and it is easy for people to take the two seriously. Second, the haze will come "hand in hand" with the photochemical smog. Under high atmospheric stability and sunny weather conditions, the exhaust emissions of many cars in cities are likely to cause photochemical smog. Both it and the haze have the effect of fog, which makes it difficult for people to distinguish. In fact, the haze has its own causes.

The formation of haze is inseparable from meteorological conditions, and the laws of meteorology are not shifted by people's will. Therefore, the prevention and control of haze is mainly to reduce the emission of suspended particulate matter and avoid the emission of pollutants in areas where haze is prone to weather conditions. The first is to improve the fuel structure, reduce the use of fossil fuels, especially coal, in cities, and avoid haze just like other smog. The effect is significant. The second is to rationally adjust the layout of industries and the direction of arterial traffic to facilitate the diffusion of air pollutants and avoid accumulation in urban areas. Third, in areas prone to haze, stricter environmental quality standards should be implemented. China’s current air quality standards impose a secondary standard for urban areas. The total suspended particulate matter (TSP) allows 0.30 mg cubic meters per day on average, which can be inhaled. Particulate matter (PM10) is 0.15 mg/m3, which is much higher than the content in the general clean atmosphere (20μg/m3). This standard is still too low for areas prone to haze, not to mention the fact that it is fine and easier Haze particles (d<2.5 microns) have not been effectively monitored. Therefore, in severe haze weather, there is a paradox that the atmospheric environmental quality evaluated according to the standard is still good.

Atmospheric aerosols play an important role in air pollution. Aerosols are dispersed in the atmosphere like ordinary colloids. All kinds of fog, including water fog, haze, and smog, are caused by various aerosols, or the synergistic effects of aerosols have similar characteristics: make the atmosphere turbid and reduce visibility .

Tips: Atmospheric aerosol
Aerosols are suspended colloids formed by solid particles or liquid droplets with gas as the dispersion medium. Atmospheric aerosols are suspended colloids that use the atmosphere as a dispersion medium. In other words, an aerosol includes two components, solid particles or liquid droplets and gas, which together form a suspending colloid. For example, mist is an aerosol formed by water droplets dispersed in the air, and smoke is an aerosol formed by solid particles dispersed in the air. All kinds of fog, including water, haze, and smog, are caused by various aerosol amines, or are closely related to aerosols.

There are two types of smoke that should attract people's attention, one is called sulfuric acid smoke and the other is called photochemical smoke. A typical sulfuric acid smoke incident occurred in London in 1952. London was originally the world-famous "fog city". This is due to meteorological reasons, and water mist itself is not a pollutant. Until the 1950s, the city used coal as fuel from life to production, and a large amount of smoke and sulfur dioxide was discharged, mixed in the droplets of water mist, and easily converted into sulfuric acid mist. Severe sulfuric acid smoke appeared in London from December 5 to 8, 1952. The mass concentration of sulfur dioxide in the atmosphere was as high as 3.75 mg/m3, and the mass concentration of smoke and dust was as high as 44 mg/m3, which caused more than 4,000 people in just 4 days. Death. Two months later, more than 8,000 people died. It was the London smog incident that shocked the world. After investigation and research, we affirmed that it is the result of the combined effect of sulfur dioxide and smoke. In 1956, the United Kingdom implemented the "Clean Atmosphere Act" to reduce the source of particulate matter from industrial and living sources. Since then, the particulate matter in London's atmosphere has gradually decreased. Although the fog has not stopped, there have been no serious smog incidents. From December 3 to 7, 1962, weather conditions similar to those in 1952 occurred again, and there was another heavy smog. At that time, the concentration of sulfur dioxide in the atmosphere was still higher than that of the 1952 event. However, the concentration of particulate matter was greatly reduced, which caused far-reaching harm. Smaller than in 1952, the death toll was 340, only 1/12 of the 1952 incident. Why is this? It turns out that pure sulfur dioxide gas is not very toxic. Once it is converted into sulfuric acid aerosol, the toxicity is greatly enhanced. The conversion of sulfur dioxide to sulfuric acid is carried out on the surface of particles in the atmosphere. Therefore, reducing soot particles actually reduces the conversion of sulfur dioxide to sulfuric acid. Facts have proved that it is difficult to control the generation of sulfuric acid fumes and reduce the emission of sulfur dioxide alone. It is necessary to take measures to eliminate smoke and dust at the emission source at the same time.

Another type of smog is called "photochemical smog", which is generated by gaseous substances under the action of sunlight. It first appeared in Los Angeles, USA in 1944. At that time, it was sunny and bright, and there were large patches of light blue smoke, like a light yarn drifting in the low altitude. Smoke has caused many people to have red and swollen eyes, inflammation and pain in their throats, worsening of respiratory diseases, and even pulmonary edema and disordered thinking. A large number of vegetable crops have been damaged, which has become a public nuisance incident that attracts attention. Since then, similar situations have often occurred in Los Angeles. This kind of smog is called photochemical smog, which is still not familiar to the world. How is it formed? Studies have proved that photochemical smog is formed by a series of chemical reactions of organic compound gases such as nitrogen oxides (NOX) and hydrocarbons under strong sunlight. There are three conditions here. One is the presence of nitrogen oxides, including nitrogen monoxide and nitrogen dioxide. The latter is easily decomposed by light to produce ozone (O3), a strong oxidant, and nitrate aerosols. Nitrogen dioxide is the "pioneer" of photochemical smog. The formation of smog starts from its photolysis. The second is the presence of organic gases such as hydrocarbons. They undergo a series of chemical reactions with ozone to generate secondary pollutants such as peroxyacetyl nitrate (PAN) and other oxidants. PAN is strongly irritating to the eyes. Third, there is strong sunlight. The light that causes the photolysis of nitrogen dioxide must have sufficient energy. Only ultraviolet rays and visible light close to violet light have the energy for photolysis, and strong sunlight meets this condition. When these three conditions are met, coupled with inversion weather, photochemical smog may occur. At this time, in addition to ozone and PAN, the reaction products also form aerosols containing a large amount of nitrates and other organic compounds. Together, they form light blue smoke. People can see the drift of this smoke.

Now, we need to investigate the sources of organic compounds such as NOX and hydrocarbons in the atmosphere. They exist at low altitudes in some areas and are mainly caused by man-made emissions. Industry, especially petroleum and petrochemicals, and automobiles are the main sources of emissions. Photochemical smog has appeared in cities where China's petrochemical industry has developed. Automobiles are a common source of emissions. Exhaust gas contains incompletely combusted hydrocarbon gases and NOX with a high concentration. There are more and more cars in cities, and exhaust emissions are very large. Photochemical smog has gradually appeared in cities around the world, and there is no exception in China. Take the Pearl River Delta region as an example, this kind of smog can already be seen from time to time. Although there is no incident, the harm still exists. We must not take it lightly and must pay close attention and take preventive measures. The first is to control pollution sources, improve vehicle fuel efficiency, reduce harmful gas emissions, improve road conditions, and prevent harmful gases from accumulating in the urban atmosphere. The second is to strengthen environmental monitoring. A large amount of ozone is produced when photochemical smog is formed, and ozone becomes a marker of smog, which can be used to determine the possible formation and severity of smog and warn people to take measures to avoid danger.

Now, people are familiar with acid rain. Even if it is light or light rain, people are holding umbrellas and carefully protecting their hair and clothes.

What kind of rain is acid rain? Why does rain become sour? Not every time it rains is acid rain. In the past, acid rain was indeed relatively rare. Now acid rain occurs widely all over the world because of air Pollution. People know that there is carbon dioxide (CO₂) in the atmosphere. CO₂ is commonly known as carbonic acid gas. It can combine with water to form carbonic acid and ionize hydrogen ions (H⁺), making rainwater slightly acidic. The chemical reaction formula is as follows:

CO₂+H₂O→HCO^-₃+H⁺

At this time, the lowest pH of rainwater can reach 5.7. This situation may occur in nature without human pollution, so this slightly acidic rain caused by CO₂ alone is not considered acid rain. However, this value has become a "watershed" for defining acid rain. Many rainfalls have acidity less than this value and are uneven. The scientific community defines rainfall with a pH of less than 5.6 as "acid rain". Saying "rain" here is still a popular term. In addition to rain, it also includes snow, fog, etc. They are all different forms of water, so it is more appropriate to say "precipitation". Acid rain is mainly caused by two kinds of acids, one is sulfuric acid (H₂SO₄) and the other is nitric acid (HNO₃). H₂SO₄ is converted from SO₂. The conversion process has been described in the sulfuric acid fumes. SO₂ enters the water droplets in the cloud as an aerosol, causing acid rain. SO₂ is the main cause of acid rain, and its source is closely related to the sulfur content of fossil fuels, especially coal. As the use of fossil fuels continues to increase, SO₂ emissions are increasing day by day, and acid rain has become an indisputable fact. HNO₃ is also the main cause of acid rain. It is aerosols converted from NOX, which also enters the clouds to form acid rain. NOX is generated during high-temperature combustion of fossil fuels, especially the use of petroleum, which emits more. In areas where petroleum is the main fuel, the proportion of nitric acid in acid rain is high; in areas where coal is the main fuel, nitric acid is less in acid rain than in sulfuric acid, whether in Europe or North America. Both sulfuric acid and nitric acid account for more than 90% of the total acid content of acid rain. In foreign countries, the ratio of sulfuric acid to nitric acid in acid rain is about 2:1, that is, the content of nitric acid is relatively high, which is consistent with the structure of today's world where the world's energy resources are dominated by petroleum. In China, the situation is different. Acid rain contains a lot of sulfuric acid, and the content of nitric acid is less than 1/10 of that of sulfuric acid. This is consistent with China's coal-based energy structure. Some people say that nature itself also produces and exists SO₂ and NOX, and they account for most and most of the amount in the global atmosphere. They will also form sulfuric acid and nitric acid to cause acid rain, so acid rain is not necessarily caused by the use of fossil fuels. . Specific analysis should be made for this view. Indeed, sulfides and nitrogen oxides are produced in nature due to volcanic eruptions, forest fires and the decomposition of organic matter by microorganisms. The high temperature during lightning will also produce a large amount of NOX. Estimates indicate that natural sources account for more than 90% of the sources of SO₂ in the atmosphere, and anthropogenic sources account for less than 10%; natural sources of NO₂ account for 60% and anthropogenic sources account for 40%. Such a large natural source will undoubtedly make a "contribution" to acid rain. However, natural source acid gases are dispersed in the global atmosphere and become the background value of atmospheric acid gas mass fraction, which is very low. According to monitoring, the mass fraction of SO₂ in the atmospheric troposphere is only 1×10-9, which is even lower in the stratosphere. The natural source of NOX is far less large than SO₂, about 1/10 of SO₂, and its mass fraction background value in the atmosphere is much lower than SO₂. Using fossil energy, although the anthropogenic emissions are less than natural sources, they are mainly scattered in the lower atmosphere in local areas. Its mass score is 2 to 3 orders of magnitude higher than the background value. Obviously, the "contribution" to acid rain is much greater than that of natural sources. 

Does the presence of acid gases in the atmosphere necessarily lead to acid rain? Nor is it. China is one of the regions with severe acid rain in the world, second only to Europe and North America, and is the third largest acid rain area. Acid rain in China is mainly distributed in the south, that is, south of the Yangtze River, while acid rain in the north is relatively small. For example, the huge Beijing city has no acid rain. The north also uses a lot of fossil fuels such as coal, which also emits acid gases such as SO₂. Even in some heavy industrial areas and cities, the emissions are more. The reason why acid rain has not formed is related to the alkali metals such as potassium in the ground soil and mineral components of the earth's crust ( K), sodium (Na), alkaline earth metals such as calcium (Ca) and magnesium (Mg) are more related. After they become aerosols and enter the atmosphere, they neutralize the acid in the clouds, thereby reducing the acidity of the precipitation. Therefore, the precipitation in these areas is neutral or even slightly alkaline and does not form acid rain. In the south, the rainfall is abundant and precipitation is high. The alkali metals and alkaline-earth metals in the soil and crust are mostly leached, and less alkaline substances are transferred into the atmosphere. The rich metals in the soil, such as iron (Fe), manganese (Mn), and aluminum (Al) oxides, exist in the form of clay. After they are transferred into the atmosphere, they have the effect of accelerating the conversion of SO₂ into sulfuric acid. It will be hydrolyzed to generate acid, which strengthens the acidity of precipitation.

It is very convenient and concise to use pH to represent the pH of the solution and to compare it. Understand the origin of pH, it is easy to understand that a solution with a pH of less than 7 is acidic, and the lower the pH, the stronger the acidity. A solution with a pH greater than 7 is alkaline, and the greater the pH, the stronger the alkaline.

China began to monitor acid rain in 1970, and gradually grasped the law that the formation of acid rain is restricted by emission sources and atmospheric aerosols, as well as the influence of meteorological factors such as atmospheric circulation and regional atmospheric stability. In addition to the discovery that acid rain is mainly distributed in relevant southern provinces, a prominent "acid rain belt" was also found. This acid rain belt starts from Mount Emei in Sichuan and Chongqing in the west, passes through Zunyi in Guizhou, Liuzhou in Guangxi, and Changsha in Hunan to Anhui in the east. The frequency of acid rain is relatively high, and the annual average pH of precipitation is similar to that of regions with severe acid rain in various countries.

Acid rain is not a good thing. Acid rain harms the ecological environment, corrodes buildings and metal utensils, and affects human health. But these hazards are not acute and are easily overlooked by people. Acid rain has a direct destructive effect on plant leaves and severely harms forests, pastures and crops. Acid rain mainly occurs in acid soil areas, making the soil more acidic, reducing the types and numbers of bacteria, especially nitrogen-fixing bacteria, affecting the formation and fertility of soil organic matter, deteriorating the soil aggregate structure, and reducing crop production. Acid rain acidified the lake, and fish rotted their gills, deformed and even made it difficult to survive. Due to the ecological impact of acid rain, the economic loss caused to the country is tens of billions every year, so it must be carefully studied and prevented.

The formation of acid rain has meteorological factors similar to the formation of haze and smog. In industrial and urban planning, pollutants should be prevented from accumulating in local areas. The most fundamental measure is to control the emission of pollutants in the region and reduce the amount of SO₂ and NOX. Especially for coal-fired power plants that produce more concentrated SO₂, desulfurization measures must be resolutely implemented. People should not complain that the atmospheric circulation will transport some acid gas from a distance. This complaint will affect the determination and action of acid rain prevention and control in acid rain areas. Everything started from me, and the effect was remarkable. With the improvement of the energy structure, the frequency of acid rain will gradually decrease, and the acidity will gradually weaken.