What are the action mechanisms of urban green space to reduce rainfall runoff pollutants?

(1) Urban radiation and illumination: Urban radiation and illumination are affected by air pollutants, the atmospheric turbidity increases, and the solar radiation reaching the urban ground decreases; Scattered radiation in urban areas has also weakened, but it is not as obvious as direct radiation from the sun; Sunshine hours and illuminance in urban areas have decreased. (2) Temperature state in the city: The temperature state of the city is another important feature of the urban microclimate, that is, there is a so-called "heat island" effect. This is because the pollutants above the city increase the atmospheric backward radiation and reduce the effective radiation, which has the function of heat preservation; The evaporation of urban underlying surface consumes less heat, and the net radiation obtained is mainly used to heat the air; The low wind speed in urban areas reduces the horizontal transfer of heat; Heating function of urban artificial heat source; Heat exchange with organisms. (3) Wind conditions in the city: When the airflow passes through the city, due to the obstruction of buildings, some airflow is forced to rise on the windward side, the wind speed over the city increases, and the turbulence is strengthened. Study on the role of urban green space in improving urban microclimate II. The role of 1 in regulating microclimate (1) in regulating air temperature. Because the canopy of trees and shrubs blocks, absorbs and reflects solar radiation, and the transpiration of vegetation absorbs some heat, which together with the branches and leaves of trees and shrubs blocks the rapid circulation of air, eases the diurnal variation of temperature and reduces the diurnal amplitude, so a better microclimate environment of green space can be formed. The microclimate of green space is relatively stable and the temperature difference is small. The average and maximum temperatures in green areas are lower than those in non-green areas, while the minimum temperatures are higher than those in green areas. (2) Increase the humidity. Plants absorb a lot of water from the soil, and leaves do a lot of transpiration during the growing season, which volatilizes a lot of water, accounting for 99.8% of the water absorbed from the roots. A tree can evaporate 450L of water a day, and transferring 230,000 kcal (lcal = 4. 1858j) can increase the energy of air temperature, which is equivalent to five air conditioners operating together 19h, thus improving the air humidity. (3) Ventilation and wind protection. Forest belt can reduce wind speed, reduce wind damage and play a protective role. A 30-meter-wide shelterbelt can reduce the wind speed to 10% ~ 50% within a distance of 20 ~ 25 times the tree height. Urban belt-shaped green space, including urban roads and waterfront green space, is a green ventilation channel of the city, especially when the direction of belt-shaped green space is consistent with the dominant wind direction in summer-the air flow in the suburbs of the city can be introduced into the central area of the city under the action of wind, creating good conditions for hot urban ventilation. 2.2 the role of alleviating the "urban heat island" effect "urban heat island" refers to the phenomenon that the temperature in urban areas is higher than that in suburbs and rural areas, which is a unique climate feature in urban areas. The urban heat island effect is mainly caused by the change of urban underlying surface properties and the concentrated emission of a large amount of heat in the city. In addition, the emission of urban buildings and polluted gases makes heat accumulate in urban areas, which also promotes the generation of urban heat island effect. The decrease of green space is the main reason of urban "heat island effect". Many studies show that the pattern of urban green space can affect the distribution of urban heat island, and increasing green space can weaken the heat island effect. 2.3 Indirect energy-saving effect of urban green space The shielding and reflection of urban green space on solar radiation, its own physiological transpiration and the blocking effect of canopy on airflow all have a positive impact on the stability of microclimate. American scholar H.Akbari and others put forward feasible strategies and methods (see figure 1) in the study of "cold roof" and shade trees on energy saving and air quality improvement, and thought that "cold roof" and shade trees could reduce energy consumption, greatly reduce the damage to the ozone layer and promote energy saving and emission reduction. Therefore, urban green space can directly and indirectly improve the microclimate effect. It can improve the urban ecological environment through various ways, and has a good role in promoting the long-term sustainable development of the city. It is an indispensable and important ecological guarantee part to play the role of the city.