Is the higher the high-rise residence, the better the air quality?

Dr Zhai Zhicai, director and associate professor of the Department of Environmental Engineering, School of Biochemistry, Jiaxing University, conducted a detailed interview on the distribution of air pollution on the ground and its impact on high-rise residential buildings. In small and medium-sized cities with good environment, the higher the high-rise buildings, the better the air quality, which has no scientific basis. Professor Zhai's answer to the reporter's question is concise and to the point.

According to reports, in small and medium-sized cities with good air quality, or in areas with less dense high-rise buildings, compared with low-rise houses, high-rise houses often have lower dust concentration and better air quality.

Professor Zhai analyzed that the so-called air pollutants mainly include suspended particulate matter (PM) and other gaseous pollutants (mainly SO2 and NO2). Among them, suspended particles mainly include coal, coal gangue, cinder, fly ash, sand, earthwork, lime, lime soil and so on. Under the action of airflow, fine particles will be suspended in the air, but due to the action of gravity, they will eventually fall back to the ground. For example, if the dust raised on the ground caused by car driving is not carried away by the wind, it will form a high concentration layer in the air near the ground after repeated circulation and settlement; In this high concentration layer, the closer to the ground, the higher the concentration will be, and it will not be rashly displaced to an altitude of 30 meters. Therefore, generally speaking, the density of suspended particles in the air around high-rise buildings decreases with the increase of height. In other words, the higher the property, the cleaner the air. In addition, other gaseous pollutants in air pollution, such as NO2, mainly come from the exhaust of factories and automobiles. These pollutants often have the highest concentration when they are discharged; With the continuous diffusion of air flow, it becomes thinner and thinner, and the pollution level becomes lower and lower. Because the emission of automobile exhaust and other pollution is mainly on the ground, the pollution degree of low-rise residential buildings near the ground is often more serious than that of high-rise residential buildings. The "dust layer" in the extraordinary zone enjoys this. It can be seen that under normal circumstances, the higher the floor, the better the air quality in high-rise buildings.

However, the air quality in all high-rise buildings is not normally distributed with the increase of floors. According to Professor Zhai, there are 20-30-storey high-rise buildings in a metropolis like Shanghai. Due to the influence of air flow and building microenvironment, the so-called "dust layer" may also appear. Under the action of street wind "canyon effect", many high-rise houses have "high-density areas of pollutants". In this area, the normal airflow is blocked by floors, forming a "vortex" in the middle of each high-rise residential building, which makes the dusty airflow not move smoothly, but "wander" up and down in a certain interval between high-rise buildings and stay for a long time, which also produces the so-called "dust layer". However, the "dust layer" does not necessarily exist in the ninth to 1 1 floors of high-rise buildings. They can be described as happy-go-lucky Because "its specific height is influenced by many factors, and there are differences in different regions, cities, urban areas and even different communities, which are related to dust density, surrounding building height and airflow turbulence characteristics". In other words, even in areas or cities with dense high-rise buildings, the "grey layer" height of different buildings is not the same.

There is no scientific theoretical basis for the rumor that the "dust layer" is completely concentrated on the 9 ~1floor of high-rise buildings (that is, the height of about 30 meters). In addition to high-rise buildings in densely populated areas, high-rise residential buildings may also form a "dust layer", resulting in relatively high air dust density. There are also some special external environments that will also affect the distribution of residential air quality, resulting in the air quality of high-rise residential buildings being worse than that of low-rise residential buildings. Professor Zhai said, for example, there are fixed air pollution sources such as power plants around the residence, so the concentration of polluted waste gas just discharged from high altitude is relatively high, and the pollution to the surrounding air is more serious, and it is diluted with continuous decline and diffusion. In this case, the air quality in the lower floors of residential buildings is obviously better than that in the upper floors. In view of this, experts advise readers to pay more attention to the surrounding geographical environment and greening facilities when choosing buildings. If possible, try to choose buildings by the river and water, because water has a good purification effect on the air. Generally, buildings near water have less dust and relatively high air quality.

There is less and less sunshine in Shencheng and more and more foggy days. Before World Meteorological Day this year, the Urban Environmental Meteorological Center of Shanghai Meteorological Bureau released a research result, saying that since the beginning of 2 110 to 140 every year, the number of days with low visibility weather such as smog in this city accounts for about one third of the whole year.

Aerosol and ozone, although the main components of these smog are invisible to the naked eye, are constantly threatening people's lives and health. When talking about the relationship between smog and health, experts also clarified several misunderstandings about smog: the higher you live, the fresher the air does not mean; Getting up early every day to participate in exercise is not absolutely beneficial to body and mind; The higher the air quality level, the more blue sky and white clouds you can see.

In the face of misunderstanding, people need to have a deeper understanding of the surrounding air environment.

Haze pollution has nothing to do with floor height.

Compared with the blue sky and white clouds, the main performance of haze days is like a layer of fog that can never be dispersed, and its main atmospheric component is aerosol or ozone.

With the development of the city, the products of modern society such as motor vehicles and chemical plants are increasing, and automobile exhaust and industrial waste gas are discharged into the atmosphere in large quantities, resulting in a large number of aerosol particles. In order to avoid the hazards caused by the precipitation of these fine particles, many people consciously buy high-rise buildings when buying a house. However, the idea that "the higher you live, the fresher the air" seems to be of little help to experts.

"There is no necessarily positive or negative correlation between smog pollution and height." The person in charge of the Urban Environmental Meteorological Center of the Municipal Meteorological Bureau said that the flow of the atmosphere determines the accumulation position of suspended particles. In many cases, the higher the altitude, the less pollutants in the air, but when the temperature inversion occurs, the air will become "top-heavy".

According to reports, the inversion phenomenon, that is, the low-level atmospheric temperature has a high density and the high-level atmospheric temperature has a high density. At this time, atmospheric stratification is prone to rolling up and down, that is, "convection" movement, which can transport pollutants near the ground to high altitude or even far away. In this way, when the inversion phenomenon occurs, the higher the floor people live in, the more vulnerable they are to the erosion of polluted air.

Smog is more common in the morning and evening rush hours and the early stage of exercise, and has always been regarded as a part of a healthy life. However, living in a city with frequent smog days, experts advise people to look at the outdoor weather before making a decision before going out for exercise. "When the distant sky is not clear and the line of sight is shrouded in smog, maybe you should stop exercising for your health." Meteorologists remind people that high-concentration haze days generally appear in the morning and evening rush hours of the day. Especially in the morning rush hour, if the atmospheric mobility is poor, a large number of particles will gather in the air. "Large particles can be automatically' filtered' by the human body, but some small particles can directly penetrate the skin and enter the body."

This is not grandstanding, but the conclusion of scientific research: as the particle size shrinks, the human parts it can reach are the mouth, nostrils and bronchi. When the particle diameter is 2 microns or less, they can even penetrate into bronchioles and alveoli.

In view of the increasing trend of haze days, Song Weimin, director of the teaching and research section of the School of Public Health of Fudan University, said at yesterday's Meteorological Day report that, theoretically, for every increase of 10mg/m3 of PM2.5 (2.5 microns in diameter), the total mortality rate of urban residents will increase by 0.36%, and the mortality rate of cardiovascular diseases will increase by 0.4 1%.

The pollution index does not include fine particles.

"There are * * * blue days in this city this year ..." People often like to use similar language when describing the air quality in a period of time. But in fact, good air quality does not correspond to blue sky and white clouds one by one.

The reporter learned that last year, the number of days with excellent ambient air quality in this city was 328, with an excellent rate of 89.6%, but foggy days are still common. "Due to the problem of monitoring standards, the air quality rate is different from what the blue sky people imagined." Meteorologists said.

A few days ago, Wang Jian, deputy inspector of the Pollution Prevention Department of the Ministry of Environmental Protection, also revealed that although fine particles have become the main pollutants in China's atmosphere, China's air pollution index has not included them. At present, the air pollution index has only three pollutant indicators: sulfur dioxide, nitrogen dioxide and inhalable particulate matter. Therefore, when people come to the conclusion that the air quality is excellent, they live in an environment where aerosols are flying all over the sky.

The haze swallowed up the brilliant sun.

In fact, the frequent occurrence of smog not only makes people's breathing more "dangerous", but also has another obvious side effect-"swallowing" sunlight.

For Shencheng, let alone the blue sky, even the sunshine is getting less and less. "Aerosol particles will refract sunlight, so the higher the aerosol content in the air, the higher the refractive index of sunlight, and the sunlight that finally reaches the ground will be greatly reduced." The person in charge of the Urban Environmental Meteorological Center of the Municipal Meteorological Bureau said so.

Just in the last two days, scientists all over the world have come to the conclusion that in the past 50 years, the sunlight reaching the earth's surface has dropped by 10% to 20%, and the earth is getting darker and darker. The reason is that the particulate matter in the air cannot be shirked. Such as soot and sulfide, these pollutants will not only reflect sunlight back into space, but also change the optical characteristics of clouds, thus making polluted clouds more reflective.

[Noun explanation] What is smog?

Haze is also called atmospheric brown cloud. In the Standard for Surface Meteorological Observation, the definition of haze weather is: "A large number of extremely fine dry dust particles float evenly in the air, making the air with horizontal visibility less than 10 km generally turbid, making bright objects in the distance slightly yellow and red, and making dark objects slightly blue."

(The above answers were published on 20 13-09- 17. Please refer to the current actual purchase policy. )

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