城市热岛效应和气溶胶浓度的动力、热力学分析

Dynamic and thermodynamic analysis of the urban heat island effect and aerosol concentration

在能量平衡方程中引入气溶胶的吸收和散射作用,并与三维行星边界层运动方程组相耦合,根据温度分布显式求解运动场,探讨三维行星边界层内温度、运动、气溶胶浓度分布特征.结果表明,城市人为热释放直接决定了城市热岛效应的强度,城市面积越大,城市热岛效应的强度也越强,城市面积固定时,城市越分散,城市热岛效应的强度越弱,这为城市建设多采取卫星城的方式提供了一定的理论支撑.气溶胶的散射作用要大于吸收作用,其对城市热岛效应的强度主要起削弱作用,当气溶胶浓度较大时,吸收作用更显著一些,此时城市热岛效应的强度会有一定的增强,但是幅度不大.当城市热岛效应的强度增强时,其所驱动的环流也会增强,造成城区中心气溶胶浓度略有下降.

With the accelerating urbanization and deteriorating urban environment,the interaction between urban heat island（UHI）effect and air pollutants has been attracting more attention.On one hand,the circulation induced by the UHI effect could rearrange the distribution of the air pollutants in the planetary boundary layer（PBL）.On the other hand,air pollutants could influence the UHI effect by absorbing and scattering effects on radiation processes.Using a theoretical model,the relationship between UHI effect and air pollutants is analytically solved in two types of city distributions：a single big city and urban agglomeration.Dividing the long-wave radiation absorption spectrum into strong and weak parts,then the radiation processes could be expressed as functions of the temperature distribution.Setting thetemperature declines exponentially with height,then the three-dimensional PBL motion could also be expressed as functions of the temperature distribution.If fixing the air pollutants emission intensity,the radiation absorption coefficients could be determined.Then substituting the coefficients into the temperature equation,it is easy to solve the temperature equation and to calculate the three-dimensional PBL motion which in turn could influence the air pollutants concentration.Therefore,the UHI effect and air pollutants are coupled through the theory.The UHI effect is directly determined by the anthropogenic heat release intensity.The larger a city is,the stronger the UHI is.When the area of a city is given,the more scattering the city is,the weaker UHI is.This provides certain theoretical support for the multi-city conurbation approach in city construction and development.The situation is similar to the air pollutants concentration.Stronger emission intensity corresponds to higher air pollutants concentration.Same emission intensity causes more serious pollution in a single big city than in urban agglomeration.It also implies that the multi-city conurbation could bring better air quality.With strengthening emission intensity,the UHI effect intensity decreases to a minimum value then increasing slowly.The scattering effect of the aerosol could decrease the temperature anomalies by reducing the amount of solar radiation on the lower boundary.However,the absorption heating effect could surpass the scattering cooling effect when pollutants concentration is high enough.The UHI intensity would intensify with a moderate range under this circumstance.Stronger UHI intensity could drive a stronger PBL circulation cell which could redistribute and transport more pollutants far from the emission center hence reducing the pollutants concentration there.Comparing with the relieving effect of the anthropogenic heat release on pollutants concentration,its driving effect on the UHI is more significant.Above results show that the interaction between the UHI effect and air pollutants concentration could be coupled through a simple theoretical model.On one hand,air pollutants could scatter and absorb the radiation hence weakening and strengthening the UHI intensity.On the other hand,the stronger circulation cell induced by the stronger UHI effect could transport more pollutants from the emission center and relieve the contamination extent of the air.Both the UHI intensity and pollution extent are weaker in the urban agglomeration than in a single big city which might provide certain theoretical guide for the city construction and development.