Academician talks about the real cause of smog

Tang Xiaoyan: Academician of the Chinese Academy of Engineering, environmental science expert. He has served as the vice chairman of the Chinese Environmental Society and the director of the Environmental Science Center of Peking University. He has won the first prize of National Science and Technology Progress Award once, the second prize of National Science and Technology Progress Award 3 times, and the "He Liang He Li" Science and Technology Progress Award once. The founder of the major of atmospheric environmental chemistry in my country, he has done many pioneering and creative systematic work in the frontier field of environmental chemistry in atmospheric ozone, acid rain and atmospheric fine particulate matter (aerosol) chemistry.

 Fog and haze are the terms of meteorology. The cause of haze weather is not only pollution. Haze has existed in ancient times. There are many small particles in the air, such as dust, foam brought by waves, pollen, etc., especially The soil particles brought up by the wind are all originally existing in nature. These small particles float in the atmosphere, and the surface will absorb a certain amount of water vapor. The gravity is not large and it is not easy to settle. It will float to the sky and absorb a large amount of water and become the condensation nucleus of clouds or fog. Some fine particles gather near the ground. Under the influence of ground humidity, they will absorb, refract or reflect sunlight, causing visibility to drop. At this time, these fine particles participate in the formation of haze. When there are a lot of pollutants such as sulfur dioxide, nitrogen oxides and volatile organic compounds in the atmosphere, the sulfuric acid and sulfate, nitric acid and nitrate and organic matter formed by them will cause visual range obstacles and haze due to their effect on light. Increasing degree and harm. 

The difference between fog and haze doesn't seem to be big, it's just a matter of humidity. However, the issue of their discrimination limits has caused academic discussion and controversy in the meteorological circle for a period of time. In short, according to the current definition of the Meteorological Department, the humidity of the atmosphere is lower than 80%, and the light effect of particulate matter reduces the visibility to less than 10 kilometers. The grayish visual range obstruction is called haze, but when the humidity is high enough More than 90%, then it becomes light fog, and when the water vapor is higher, it becomes fog. The fog is easier to disperse, but the haze is difficult. Because the water content of the fog is large, it is greatly affected by the changes of the weather. When the sun comes out and encounters dry air, the fog can easily disperse, so the fog generally does not exist all day. But haze is more troublesome, because it does not contain much water vapor, the humidity is below 80%, and the space where the haze is located is generally below 3000 meters, if there is an inversion layer in the sky, and there is no wind in the horizontal direction Or when the wind is very small, the air is convective up and down or the horizontal flow is not smooth, the haze cannot be dispersed. 

Pollution is the internal cause of severe haze, while meteorological conditions are the external cause. Smog is related to emissions from energy use. We know that pollutants such as sulfur dioxide, nitrogen oxides, ammonia, and volatile organic compounds are all gases. Why do they become particulate matter in the atmosphere and eventually form What about Cheng Haze? Because the atmosphere is oxidizing, the gaseous pollutants emitted after energy combustion can be said to be primary pollutants. After entering the atmosphere, a very complex chemical reaction occurs and secondary pollutants are produced. Because the valence state of primary pollutants is low, such as sulfur dioxide and nitric oxide are all in reduced valence state, and there are a large number of free radicals in the atmosphere, low-valence gas can easily react with free radicals to be oxidized to form a high-valence state. The sulfuric acid, nitric acid, sulfate and nitrate, etc., organic matter also has this reaction. 

At present, we have not paid enough attention to VOC (Volatile Organic Compounds) in the research on the causes of smog. The VOC and nitrogen oxides emitted into the atmosphere are the key substances that enhance the oxidation of the atmosphere. Due to their non-linear relationship, the ratio of the effects of the two will be different in different regions. For example, studies have pointed out that the increase in atmospheric oxidation in Beijing urban areas, the effect of VOC is even greater than nitrogen oxides, while the suburbs are the opposite. VOC is a combination of hydrocarbons. Most of its production is related to combustion. It can be said that any combustion will produce VOC, so it is closely related to the use of energy. Organic matter exists in coal, oil, and natural gas, and refining and chemical industries are all emissions. Large households, even straw burning, paint and catering, etc. also generate VOC. For a long time, people have always believed that coal combustion is the main cause of haze, ignoring the pollution caused by chemical industry. The focus of attention is mainly on smoke and sulfur dioxide, and nitrogen oxides and VOC are ignored. If the understanding is clear, natural gas cannot be regarded as a clean energy. 

The combustion process in boilers and automobiles will produce fine particulate matter, PM2. 5 There are primary emissions and secondary emissions. The primary emissions are the particulate matter directly produced by combustion, and the secondary emissions are the particulate matter generated by the chemical reaction and nucleation reaction of the gas generated by combustion in the atmosphere. The existing dust removal device in the power plant , At most, it can only remove the particulate matter above PM1, and the particulate matter below PM1 cannot be removed. 

Automobiles emit a lot of VOCs. The amount of VOCs has a lot to do with the oil product and whether the combustion is sufficient. The best conversion result of organic matter is to become carbon dioxide and water, but the combustion process of the engine is not fully completed before it is emitted, especially for automobiles. When driving at low speeds or idling in urban areas. Some countries have special laws that require cars to be turned off when they encounter a red light. 

Some people think that cars in cities emit a lot of nitrogen oxides. In fact, this is not the case. The production of nitrogen oxides has the greatest relationship with the temperature of combustion. The speed of cars in urban areas will not be too fast, and the combustion temperature of the engine will not be too high. The reaction conversion with oxygen is relatively small. Where is there a lot of nitrogen oxides? On highways, cars drive fast on highways and the engine temperature is high, so the more nitrogen oxides they produce. Diesel vehicles emit more emissions. 

One way to control the exhaust gas produced by a car is to install a "three-way catalytic converter" on the car. It is generally installed on the car now, but it will age and the effect will be weakened after a long time, so the nitrogen oxide emission of a new car is more than Old cars should be low. Hong Kong now requires that the "three-way catalytic converter" of automobiles must be replaced every two years, but most provinces in my country do not have this requirement. Sometimes when I go to Beijing’s North Fifth Ring Road, there are many people on the side of the road holding signs for car inspection. Now this matter has become an industry. No matter if your car is qualified or not, they can help you pass the inspection. Now there is the phenomenon of renting a "three-way catalytic converter". The car owner can spend money to rent a new one to deal with the car inspection, and then return it to others after the inspection. Behind this phenomenon is the management problem. The current management is extensive, not fine management. 

Don't underestimate the restaurant street. There are many VOCs, high sulfur dioxide, and high nitrogen oxides. Therefore, the issue of energy utilization is very complicated. Especially for northern cities, the problem of burning of loose coal for heating in winter, how to manage the loose coal used in the suburbs of heating areas every winter? Some governments don't care about these small ones, but only focus on such key sources as large boilers in power plants and metallurgy. The scattered coal burns and the scattered pollution is very serious. So I think there must be production and consumption in the energy revolution, and we must pay attention to these small things that we did not pay attention to before. 

What I particularly like to talk about is a city in Brazil. The mayor of their city designed a special road passage when planning the city, and its width only allowed one. The bus comes to a bus with two-way lanes. Financial institutions, government agencies, corporate headquarters, etc. are only allowed to be built on this main road and side roads. This road can only be used by buses, and all light vehicles can only stop in the suburbs or along the avenue when they come, and then people change to buses to enter the city. In addition, in order to ensure good air circulation, the houses on both sides of the main road are relatively low, and the tall buildings are built behind them, not in the densest places with cars. The city has 2.2 million residents, and more than 70% of them are used by public transportation. This state has been implemented from the 1960s to the present, and it has been decades. The urban environment is cleaner than Rio and Sao Paulo.