What are the effects of alleviating Beijing's heat island effect?

Urban heat island effect in Beijing and its mitigation measures

Urban heat island effect is an important factor affecting urban environmental quality, which has attracted more and more attention. Especially in recent years, the transformation of global warming environment and the strong urban heat island effect have made the urban thermal environment worse, especially in summer, the high temperature and summer heat have begun to seriously affect people's lives and work. As the capital of China and an international metropolis, Beijing's heat island effect has also developed to a considerable extent. Urban greening can change the thermal characteristics of urban underlying surface and reduce heat accumulation. Therefore, urban greening plays a key role in reducing the heat island effect. The influence of Beijing urban green space on urban heat island has changed the nature of the underlying surface, thus affecting the heat balance of the surface. Green space reflects and absorbs part of solar radiation, which reduces the net radiation on the surface. Most of the net radiation is used for plant transpiration and converted into chemical energy in photosynthesis, which greatly reduces the heat that raises the environmental temperature. The photosynthesis of a large area of green space can absorb carbon dioxide in the air, weaken the urban greenhouse effect and further alleviate the heat island effect. 1. Influence of urban green space on thermal characteristics of underlying surface. Different underlying surfaces have different thermal properties, so the rate of temperature rise and fall is also different. Asphalt pavement and cement pavement have high temperature rise rates (4.9 degrees Celsius and 4.0 degrees Celsius, respectively), and the lowest temperature rise rate of the underlying surface of green space is 0.9 degrees Celsius and 0.7 degrees Celsius; And the heating rate of asphalt and cement is higher than the cooling rate, so heat is easy to accumulate, while the cooling rate of green space is lower than the heating rate, so heat is easy to spread. It can be seen that the underlying surface of green space has the best effect of alleviating the heat island effect, while other underlying surfaces such as asphalt and cement are not only unfavorable for reducing the heat island effect, but also important factors for enhancing the heat island effect. 2. The study on the relationship between urban heat island intensity and urban green coverage shows that green coverage is inversely proportional to urban heat island intensity, and the higher the green coverage, the lower the urban heat island intensity. Covered by green and strong,

The radiation ratio relationship of secondary heat island shows that when the coverage rate reaches above 30%, the green space will obviously weaken the heat island; When the coverage rate is more than 50%, the effect of reducing heat island in green space is extremely obvious, and the living comfort is satisfactory. For example, in parks such as the Temple of Heaven, the green coverage rate is as high as 85%, and there is no secondary heat island in the whole region, thus eliminating the heat island phenomenon. The analysis in embassy district, university district (such as Peking University and Tsinghua) and Fangzhuang district with good greening effect shows that there are few large-scale strong heat islands in this area due to the obvious weakening effect of green space on heat islands when the greening conditions are good. The coverage rates of West Huang Zhuang and Dashilan are only 4.45% and 13.26%. There are few green plants and concentrated heat, forming a high-intensity heat island area. 3. The radiation intensity of urban green space to heat island decreases. After the green coverage rate reaches a certain level, the heat lost by transpiration of green space is higher than the obtained solar radiation energy, the temperature decreases, the air cools and sinks, the ground air pressure increases, and the airflow blows from green space to non-green space, forming a local circulation, which leads to the "permeability phenomenon" of green space on the ground, which objectively plays a role in reducing the environmental temperature. Measures and suggestions to reduce urban heat island effect: Urban greening, water surface and urban wind are effective factors to reduce urban heat island effect. Increasing urban green space and improving the thermal characteristics of urban underlying surface are effective and feasible ways to reduce urban heat island effect. 1. Strengthening urban greening and improving the overall level of urban greening are the key measures to reduce urban heat island effect. Garden plants in urban green space constantly absorb a lot of heat from the environment through transpiration, which reduces the temperature of the ambient air. Every hectare of green space can absorb 8 1.8 MJ of heat from the surrounding environment every day, which is equivalent to the function of 189 units 1 kW air conditioner. Garden plants can absorb a lot of carbon dioxide in the air through photosynthesis, inhibit the greenhouse effect, release oxygen and purify the air. One hectare of green space can absorb 1.8 tons of carbon dioxide on average every day. 2. Establish the urban ecological corridor system scientifically according to the dominant wind direction in Beijing,

Gradually establish a reasonable ecological corridor system in urban areas, introduce cool and clean air from the periphery (ecological hinterland) into the city, and effectively alleviate the heat island effect in the city. At the same time, it can promote the material and energy flow between the city and the periphery, and restore and improve the ecosystem. However, the influencing factors of urban wind are complex and are largely limited by atmospheric circulation. The layout of the city, the orientation of urban buildings and the direction of roads can all affect the direction and intensity of urban wind to a certain extent. Therefore, the introduction of urban wind should be systematically planned on the basis of urban overall construction planning. 3. Reasonable increase of urban water surface can not only effectively alleviate the urban heat island effect, but also be an important part of urban ecosystem. However, Beijing is a city with a serious shortage of water resources. In the case of serious shortage of urban water resources, according to Beijing's natural conditions and urban characteristics, it is of great significance to gradually increase the water area by using reclaimed water in conditional areas for building an urban space with perfect ecology and beautiful environment. 4. It is urgent to alleviate the heat island effect in the old city. At present, the heat island effect is most concentrated in the core area within the second ring road, which is not only high in intensity, but also distributed in contiguous areas. The environmental situation in this area not only affects Beijing's urban development and people's life, but also directly affects the city image. In the transformation of this area, we should pay full attention to the construction of ecological environment in this area and handle the relationship between developing economy and establishing a good ecological environment. 5. Establish large-scale centralized green space to reduce the influence of heat island. Only when the green space monomer reaches a certain scale can a low temperature zone centered on green space be formed. At present, these low-temperature areas can provide valuable outdoor activities for urban residents in the case of high temperature and serious heat island effect in the whole city. The research shows that the internal thermal radiation intensity of concentrated green space with a scale of more than 3 hectares and a green coverage rate of more than 60% is obviously reduced (the thermal radiation intensity is 4), which is basically the same as that of suburbs, that is, a low temperature zone centered on urban central green space can be formed. 6. Pay attention to the uniformity of green space distribution and improve the urban thermal body condition. With the increase of the distance from green space, the radiation reduction effect of green space on heat island is obviously attenuated, and far green is difficult to get close to heat. Therefore, the scale of urban green space should not only have a considerable number, but also be evenly and reasonably distributed. 7. Adopt reasonable greening structure to improve the benefit of green space per unit area and reduce heat island. The cooling effect of green space with multi-layer planting structure of arbor and shrub is the best, followed by arbor and shrub green space, and lawn green space is the lowest. 8. Beijing's overall urban planning adheres to the principle of decentralized group layout. According to the distribution characteristics of urban heat field in Beijing, the planning of green space system should proceed from the strategy of sustainable development, rationally lay out centralized green space, establish ecological corridors and increase water surface. , carry out key and targeted green space construction, change the situation of urban greening work, and realize "planning and building green". Establishing a perfect urban green space system is to improve the urban thermal environment.

Beijing is hot! The "heat island effect" is the culprit.

For everyone who studies and works in Beijing, the sultry summer is always the most uncomfortable season of the year. Perhaps you will find it strange that Beijing, with its high latitude, is always very hot in summer. After years of research, experts found that the "heat island effect" is one of the culprits of Beijing's persistent high temperature.

What kind of physical phenomenon is "heat island effect"? How has it profoundly changed the environment and temperature in Beijing? Professor Liu Shuhua, from the Department of Atmospheric Science of Peking University Institute of Physics, who has deeply studied the relationship between the "heat island effect" and Beijing's climate, told reporters about the mystery of the "heat island effect". The surface composition of modern cities makes energy absorption slower, and the whole city is like a "heat island".

It turns out that in this modern metropolis where we live, the surface is composed of cement, concrete and asphalt roads, while the surface in the suburbs is composed of vegetation and soil. The thermal balance characteristics between them are quite different.

The special surface of the city makes it absorb more sunlight than the soil, and the increasingly popular electrical equipment such as air conditioners, millions of cars and various man-made heat are also excessively absorbed.

Therefore, the heat storage capacity of urban surface is much larger than that of suburbs. In addition, the atmosphere over the city is turbid and the content of greenhouse gases is high, which obviously affects the loss of long-wave radiation on the ground and leads to the greenhouse effect. Therefore, the temperature of the city with dense buildings is obviously higher than that of the surrounding suburbs, making the city like a "heat island".

Liu Jiaoshou told reporters that due to the characteristics of the surface, the surface temperature rises, and the heated air will form a hot air flow. Under the influence of atmospheric balance, the air in suburbs and other urban fringe areas will sink to supplement the areas where the air rises, so there will be dynamic heat island circulation. Heat island circulation has a great influence on urban precipitation. Usually, when the temperature rises, the air forms an updraft, which is easy to form convective weather, leading to an increase in urban precipitation intensity. The representative of this weather in Beijing is frequent thunderstorms in the afternoon.

The heat island circulation in Beijing has further worsened air pollution, even in the western suburbs.

It is noteworthy that heat island circulation not only increases precipitation, but also aggravates air pollution, and the pollution generated by cities further affects cities through circulation. Liu Jiaoshou introduced the manifestation of heat island effect under the special terrain of Beijing surrounded by mountains on three sides. During the day, the mountainous area in the northwest of the suburbs with the same height receives more energy than the plain area, and the temperature rises rapidly, and an updraft will form above it, which will drive the plain airflow to form a south wind, thus affecting the mountainous area. At night, the heat loss in mountainous areas with the same height is faster than that in plain areas. Coupled with the man-made heat island effect, it will turn to the north wind after 9: 00 pm, first from the mountainous area, and gradually affect the city.

Therefore, it is the north wind all year round in Beijing, which gradually turns to the south wind after sunrise during the day and then turns to the north wind at night, which has caused the problem of regional climate formation. These pollutants are sent to mountain areas for storage in the daytime in late autumn and winter. When there is no big weather system, the pollutants will be sent back by circulation at night 10. For five consecutive days in late autumn and early winter, Beijing's pollution index reached five.

Liu Jiaoshou pointed out that if the city is surrounded by plains in an ideal state, the development of the city will definitely lead to the enhancement of heat island circulation. Due to the special possibility of Beijing's topography, the temperature difference and air pressure difference between Beijing's urban and suburban areas have caused the formation of heat island circulation. According to the numerical simulation and observation of Hu Xiaoming, Master of Atmospheric Science Department of Peking University Institute of Physics, the ground temperature difference between urban center and suburb is 6℃, and the long-term temperature difference is 1-2℃. The temperature difference between different urban areas in Beijing is related to the economic development and population in this area. Generally speaking, the temperature in the west is higher than that in the east. The average temperature in Haidian in July is 14, which is about 1℃ higher than that in Chaoyang.

To reduce the heat island effect, it is necessary to reduce the ground area with large heat capacity, and it is even more indispensable to increase greening.

He was asked whether the heat island effect played a key role in various factors leading to the high temperature disaster in Beijing. Liu Jiaoshou said that from the perspective of global warming, it has only increased by about 0.4℃ in ten years, so as far as Beijing is concerned, the impact of global temperature change is negligible. The high temperature disaster in Beijing is mainly the result of the superposition of high temperature year and Beijing heat island effect in the high and low temperature natural cycle. Therefore, it is urgent to take effective measures to control the heat island effect.

Liu Jiaoshou emphasized that in order to slow down the urban heat island effect, cities must first reduce the ground area of urban heat capacity. At present, the general buildings in Beijing are all cement structures and the pavement is asphalt pavement, which absorbs a lot of heat. Among them, the black reflectivity of asphalt road is very low, so the heat absorbed by asphalt road is much higher than that of light color in the same area.

If the roofs in urban areas are all green, or a rest place, such as a swimming pool, can not only reduce the increase of heat, but also increase entertainment facilities. In Tokyo, Japan, many roofs are used as tennis courts, swimming pools or gardens to effectively reduce the occurrence of heat island effect, and these roofs are rarely used. Research shows that if the vegetation area in Beijing increases by 30%, the temperature will drop by 2-3℃. At present, buildings in Beijing account for more than 30% of the ground, and most old roofs can be effectively afforested, so it is entirely possible to increase vegetation by 30%.

For pavement, we should consciously promote the honeycomb structure, because the use of honeycomb floor can effectively change the heat balance of the ground, naturally open up the circulation of water and heat on the ground, and reduce the surface temperature by 3-4℃ during the evaporation of water on the ground. Removing fences, changing building orientation and improving ground ventilation effect are effective means to reduce heat island effect.

Liu Jiaoshou has studied urban residential areas and found that there are tens of thousands of people living in more than 50 buildings, which are densely arranged. In addition, the area of green space has decreased. Because the building is a cement body that absorbs a lot of heat, the heat island effect is very obvious, and the building area is reduced by increasing the height. If ten buildings are built, most of the original buildings will become green spaces, which will increase the ground ventilation, optimize the surface heat balance and dynamic effect, and the heat island effect will decrease rapidly.

He told reporters that ventilation is also closely related to the urban heat island effect. In coastal cities, if the streets are perpendicular to the coastline, you can make full use of the sea breeze.