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? Causes and control of urban heat island

Introduction: "Urban heat island effect", also known as "atmospheric heat pollution phenomenon", refers to the fact that the temperature in urban areas is higher than that in suburbs, and the high-temperature urban areas are surrounded by low-temperature suburbs, forming an island shape with high inside and low outside on the temperature distribution map indicated by isotherms, so it is called "urban heat island effect". Environmental experts believe that 80% of the "heat island effect" is attributed to the reduction of green space and wetlands, while the urban heat emission factor only accounts for 20%, so the control focus of urban heat island effect is the construction of green space and wetlands. Satellite thermal infrared remote sensing technology can effectively and comprehensively detect the temperature characteristics of the underlying surface and monitor the changing trend of urban thermal environment periodically and dynamically, which is an effective means to study the urban heat island effect.

Keywords: urban heat island effect, urban thermal environment, causes, control

First, the urban heat island effect

Urban heat island effect, also known as air thermal pollution phenomenon, refers to the fact that the urban temperature is higher than that of suburbs, and the high-temperature urban area is surrounded by low-temperature suburbs, forming an island shape with high inside and low outside on the temperature distribution map represented by isotherm, so it is called urban heat island effect.

The urban heat island effect makes the annual average temperature in the city higher than that in the suburbs 1? The temperature is above℃, and the temperature in some areas of the city sometimes reaches 6? Above 30 degrees Celsius. Under the background of rapid global urbanization, urban heat island has become one of the factors affecting urban environment, posing a serious threat to urban public health, air quality and energy consumption.

Second, the cause analysis of urban heat island effect

The formation of urban heat island effect is related to many factors, and urbanization is the main reason for the formation of urban heat island phenomenon.

1 change of underlying surface

The formation of urban microclimate is closely related to the artificial underlying surface of the city. Due to the different underlying surfaces of cities and suburbs, their respective thermal environments and thermal balances are formed (Figure 1, Figure 2).

Figure 1 suburban heat balance Figure 2 Urban heat balance

Urban surface is inorganic, and more and more surfaces are covered by buildings, concrete and asphalt. The decrease of green space and water surface weakens evaporation and the atmosphere cannot cool down. At the same time, these artificial structures have small heat capacity, and under the same solar radiation conditions, the temperature rises faster than the natural green space and water surface.

2, the influence of artificial heat source

A large number of man-made heat emissions in cities are also an important cause of heat island. Factory production, transportation and residents' life all need to burn all kinds of fuels, and emit a lot of heat every day.

3. Urban air pollution

Nitrogen oxides, carbon dioxide and dust emitted by motor vehicles, industrial production and residents' life in cities will absorb the thermal radiation of artificial buildings in cities, resulting in greenhouse effect, thus further raising the atmospheric temperature. Under the influence of urban heat island phenomenon, the temperature in urban areas is higher than that in rural areas around suburbs. The hot air in urban areas rises in the sky and spreads around, and the cooler air in surrounding suburbs flows into urban areas, thus forming a unique urban heat island circulation. The pollutants emitted by suburban factories can be blown from low altitude to the urban area with the heat island circulation, which increases the concentration of pollutants in the city and further enhances the urban heat island effect.

4. Density and load of urban buildings

High-density buildings increase the direct absorption and rebound absorption of solar radiation. There are many high-rise buildings in the city, which affects the natural flow of wind and is one of the reasons for the urban heat island effect.

5. Urban energy consumption

A large amount of energy consumption in cities raises the temperature of the surface atmosphere and enhances the heat island effect. Especially in summer, the air conditioning load of buildings greatly enhances the heat island effect, which in turn increases the energy consumption of air conditioning and forms a vicious circle.

In addition, the urban completion rate, geometric shape, urban scale and urban geographical location are also obviously related to the heat island effect.

Third, control the urban heat island effect

Green space and water surface are the most effective factors to effectively alleviate the urban heat island effect and adjust and improve the urban microclimate environment. Environmental experts believe that the "heat island effect" 80? % of the factors are attributed to the reduction of green space and wetlands, and the urban heat emission factor accounts for only 20? %? Therefore, the control focus of urban heat island effect is the construction of green space and wetland.

1, vigorously promote urban three-dimensional greening

Plants have high absorption rate and low transmittance to solar radiation; Plants can take away a lot of heat through transpiration and improve air humidity; In particular, trees can greatly reduce radiant heat by blocking sunlight. The reflectance of plants to solar radiation is generally 10? %~20? %? The reflectivity of infrared ray is as high as 70? More than%. The research shows that when the green coverage rate of an area reaches 30%, the intensity of heat island is obviously weakened; The green coverage rate is over 50%, and the phenomenon of heat island mitigation is extremely obvious. Plants can produce good environmental benefits through photosynthesis, which will play a very important role in alleviating the global greenhouse effect and repairing the ozone hole.

Urban greening should be carried out with reasonable collocation of trees, shrubs and grass, especially the planting of native tree species. Blindly planting lawns and precious trees's unreasonable practices should be abandoned. Paper reference network. Due to the shortage of urban land, urban greening should develop in the direction of three-dimensional greening in addition to horizontal forest planning. There are a large number of cement or concrete roofs, walls, interchanges and slopes in cities, and these exposed surfaces provide the possibility of three-dimensional greening. Developing three-dimensional greening is an important method to alleviate the urban heat island effect.

The roof of a building is an important interface between the building and the atmosphere. And the roof area in the city accounts for 50% of the whole city area? %? About. Increasing the coverage area of roof green space can not only reduce the energy consumption of air conditioning, but also adjust the outdoor temperature, which has the function of slowing down the urban heat island effect and has remarkable ecological benefits.

2. Protect natural wetlands and strive to build artificial wetlands.

The phenomenon of urban heat island can be alleviated to some extent because of the large heat capacity of water body, large evaporation of water, moderate temperature rise and drop, and the existence of water surface in cities. Urbanization has led to the reduction or disappearance of a large number of urban farmland, ponds, lakes, inland rivers, swamps and other wetlands. Impermeable concrete buildings, roads and squares lead to a large amount of precipitation flowing directly through the drainage pipe network. With the decrease or disappearance of wetlands and the loss of natural precipitation in vain, cities lose the opportunity to take away urban heat and reduce the heat island effect through evaporation, and the possibility that water can indirectly reduce the heat island effect by promoting the growth of trees. On the basis of vigorously protecting the original wetlands (including suburban wetlands), building artificial wetlands in cities is an effective way to reduce the heat island effect. In addition, actively improve the impervious underlying surface and use permeable materials (permeable bricks, etc.). Data show that permeable cement concrete pavement can reduce the pavement temperature by about 5℃. Therefore, the permeable pavement in the city plays an obvious role in improving the urban thermal environment and forming local wind, just like the evaporation of urban water and the transpiration of greening system.

3. Construct a reasonable urban planning structure.

Pay attention to the rational layout of urban construction and implement the strategy of sustainable development in urban planning.

There is a certain correlation between urban heat island effect and urban scale. Some scholars believe that urban expansion is the main reason for the formation of urban heat island effect. High population density areas are also high building density areas and high energy consumption areas, which inevitably form high temperature areas. According to research, The urban heat island effect of 65438+ ten thousand people can reach 0.32℃, and the urban heat island effect of 1 ten thousand people can reach 0.9 1℃. Therefore, it is necessary to plan the city scale and control the urban population within a certain range. At the same time, it is necessary to control the urban population density and building density and height, choose a well-ventilated terrain for urban construction, adjust measures to local conditions, rationally arrange various functional areas of the city, increase air flow space, and try to make the main roads consistent with the direction of prevailing winds in summer.

4. Reduce equipment energy consumption and rationally plan urban energy.

By improving the components of energy consumption equipment and updating the energy use method, energy use efficiency can be improved and energy loss can be reduced. Control scattered and inefficient small heat sources in urban areas, vigorously promote central heating, and improve energy utilization; Consider central air conditioning as much as possible. Reduce building heating, ventilation and air conditioning? Exhaust the heat in the system to the air. We can improve building air conditioning technology, actively explore and popularize new energy-saving technologies (such as ground source heat pump technology), vigorously carry out building energy conservation, and formulate corresponding policies and regulations to promote the implementation of energy-saving measures.

Green energy sources, such as wind energy, solar energy, bioenergy and geothermal energy, can be properly utilized when the economy is reasonable and the technology is feasible. It not only saves energy, but also converts some solar energy into other forms of energy, reducing the energy directly converted into heat energy from radiant energy, that is, reducing the influence of solar radiation on urban heat island phenomenon.

5. Develop "ecological thermal insulation" building skin.

Building energy-saving technology usually uses white or light color to treat the "reflective insulation layer" on the building surface, the "insulation layer" to increase the thermal resistance of the building envelope, and the "sunshade insulation layer" to block solar radiation with artificial materials, all of which directly transfer solar radiation to the outside, which not only meets the indoor thermal comfort and building energy saving, but also brings about the increase of urban heat island strength. Therefore, at present, conventional means such as reflective decorative materials should be weakened in the control of heat island, and a complex with ecological elements such as green vegetation and water medium should be adopted and placed on the building surface to form an ecological building skin. The absorbed solar radiation heat will be dissipated by evaporation after physical and biochemical reactions. This can reduce the solar radiation reflection and heat transfer of the building skin to the surrounding environment, fundamentally improve the thermal environment outside the building and weaken the urban heat island effect.

6. Standardize the urban transportation system.

Advocate public transportation, control the number of private cars, and gradually use clean liquefied gas instead of oil as the main fuel for urban buses and taxis. At present, foreign countries have developed a new technology using electricity as motor vehicle power, which will have a positive impact on urban heat island and urban climate environment. At the same time, a large number of urban traffic is divided into layers and vehicles are diverted to promote the dispersion of a large number of accumulated heat, greenhouse gases and suspended particles in the city, reduce the dust cover effect and reduce the heat island effect.

Four. Research Status and Suggestions

Research status of 1. heat island effect

Over the years, many scholars have done a lot of research work on urban heat island phenomenon. Many scholars at home and abroad have pointed out the influencing factors of urban temperature field and urban heat island intensity through the analysis of observation data, and put forward the correlation model between urban heat island intensity and some factors. Paper reference network. Many scholars have extensively discussed some physical processes in urban energy balance and achieved some beneficial results. In recent years, researchers in China have studied the relationship between the range and intensity of urban heat island and these factors from the aspects of urban population density, the number and distribution of green space, the built-up area, man-made heat, air pollution and topography, and also studied the relationship between the distribution of urban heat field and land use and cover.

Conventional research on urban heat island is mainly based on monitoring, and the factors affecting urban temperature field and urban heat island intensity are analyzed according to the monitoring results. The conventional monitoring method is a combination of line observation and fixed-point observation. Because the observation can't be carried out synchronously and the density of observation points is not high, this method can't reflect the ground thermal radiation comprehensively and synchronously. Satellite thermal infrared remote sensing technology can effectively and comprehensively detect the temperature characteristics of the underlying surface and monitor the changing trend of urban thermal environment periodically and dynamically, which is an effective means to study the urban heat island effect. Commonly used thermal infrared remote sensing technologies include meteorological satellite NOAA? AVHRR, Landsat TM, etc. Compared with the meteorological satellite NOAA? AVHRR？ Ground resolution (1? km？ × 1? Kilometers), Landsat? TM6？ With 120? m？ × 120? m？ Ground resolution, TM6 for areas requiring accurate analysis? Is a better choice. In addition, the urban heat island simulation software developed by the Urban and Building Environment Laboratory of Kyushu University in Japan-AU? SSSM? Tools? ,? It can simulate the thermal environment of typical areas and provide reference for reasonable urban planning. Paper reference network.

2. Some suggestions

First of all, the study of heat island effect should start with the investigation and collection of basic data, monitor the direction and size of heat flow in different areas of the city on the spot, and divide the building climate zones. Secondly, monitoring stations are set up in different areas of the city and under different building conditions, especially in areas with significant heat island phenomenon, so as to obtain the changing mode of heat island phenomenon in key areas of the city. These on-site monitoring data are very important for establishing and revising the mathematical model of local cities and building climate communities. Finally, according to the measured values, the energy balance model between local buildings and urban environment can be established, including the organic connection model between local urban climate community, urban underlying surface, artificial heat removal and heat island effect.

Verb (abbreviation of verb) conclusion

In 2 1 century, environmental and energy problems have affected urban form and people's social life, and the influence of heat island effect on urban thermal environment needs to be solved urgently. Through the study of heat island effect, it is of great significance to grasp the relationship between urban thermal effect distribution and urban development, promote urban planning and ecological environment construction, promote urban sustainable development and improve the quality of human settlements.