An Investigation Of The Influence Mechanism Of Atmospheric Aerosols On Urban Boundary Layer

With the rapid development of China's economy and urbanization,severe particulate matter pollutions,also known as haze,have appeared in most regions of China.These high-concentration pollutants not only affect the social life,but also threaten the human's health.Therefore,scientists both at home and abroad have made great efforts to explore the cause of these haze events.They found that apart from the high emissions of primary aerosols and gas precursor and the fast formation of secondary aerosols,the interactions between aerosols and planetary boundary layer(PBL)are also important in triggering the haze episode.Aerosols affect the surface energy balance through radiation effect,therefore directly or indirectly change the thermal structure of boundary layer.On the other hand,the boundary layer determines the atmospheric vertical diffusion ability and the change of planetary boundary layer height(PBLH)controls the variation of pollutant concentrations,forming the aerosol-PBL feedback system.Herein,at the basis of the multi-year observations of Bei Jing City site of the Institute of Atmospheric Physics,coupling with a large-eddy simulation model,we elucidate the influence mechanism of atmospheric aerosols on PBLH through both observations and modeling.Furthermore,how aerosol loading,scattering ability and the vertical aerosol distribution affect the development of PBL is systemically investigated.Statistics of the long-term observations of 2010-2017 shows a decline of aerosol optical depth(AOD)and the absolute value of aerosol radiative forcing,indicating the effective control of the Chinese government policy.The planetary boundary layer height(PBLH)shows an upward trend.For polluted conditions,the atmospheric stratification is stable and the water vapor is accumulated within the boundary layer.The PBLH decreases with the increase of AOD,suggesting the high concentration of aerosols seriously suppresses the boundary layer.To accurately resolve the properties of the boundary layer meteorological and physical variables,we used a large-eddy simulation model named as DALES coupling with radiation and land surface modules to simulate a stagnant weather case over Beijing.Results show that DALES performed satisfiedly in reproducing the diurnal variation of the variables,demonstrating the ability of DALES in simulating the boundary layer thermodynamic structures.We observe an enhanced upper-atmosphere heating and a lower-layer cooling in the scenario of increasing absorption aerosol loadings.In the sensitivity experiment of aerosol scattering ability,the enhancement of aerosol scattering property leads to a vigorous convection through weakening the upper-atmosphere heating and lower-layer cooling,demonstrating that the absorption aerosols suppress PBL more than the scattering aerosols.In order to systematically investigate the influence of aerosol scattering ability on PBL development,a series of sensitivity experiments were designed based on the thermodynamic structure of this typical stagnant weather condition.The impact of scattering and absorption aerosols on PBL are found to be closely related to the vertical position of aerosol layer.The local emission of absorption aerosols below the residual layer acts as a stove effect,heats the underneath stable boundary layer and residual layer,promotes the development of PBL.The absorption aerosols above the residual layer heat the upper inversion layer,strengthen the atmospheric stability,and strongly inhibit the PBL,which is defined as dome effect.The impact of scattering aerosols depends on the loadings rather than the vertical position because they have no heat input to the thermal structure of the boundary layer.Only the umbrella effect backscattering the solar radiation weakens the radiation reaching to the ground surface and inhibits the PBL.However,this inhibition effect is weaker than the dome effect.It is,therefore,found that there exists a transition height h in the atmosphere,above which,the absorption aerosols inhibit PBL more than the scattering aerosols(i.e.,dome effect > aloft umbrella effect),while below which,the scattering aerosols dominate the suppression of PBL rather than absorption aerosols(i.e.,stove effect < surface umbrella effect).This height is highly related to the height of the residual layer.The aerosol dome and umbrella effects can be interpreted as a “double-inhibition” effect on the formation mechanism of North China Plain haze event.For the aerosol suspending on the North China Plain region,i.e.transport aerosols,we emphasize the importance of controlling the coal and straw burning which producing high amount of absorption pollutants(i.e.,Black carbon,BC;Brown carbon,Br C)over the southern area.For the local region of North China Plain,measures such as vehicle restrictions and desulfurization of coal burning should be strengthened to reduce the emission of scattering aerosols and precursors.The precise and coordinated control schemes proposed based on the “double-inhibition” effect have important guiding significance for the prevention of air pollutions.For the transport aerosols(i.e.,aerosols above the residual layer or free atmosphere aerosols),it is found that there is a dome effective height z for the effect of absorption aerosols on the PBL.Below z,absorption aerosols exihibit as the dome effect,suppress the PBL.This dome inhibition weakens as the evaluation of aerosol layer.When the aerosol layer is higher than z,the inhibition of aerosol layer on the boundary layer maintains a constant value,which is defined as the virtual dome effect.The scattering aerosols show the same inhibition performance regardless of their vertical position(from residual layer to ?),but to a weaker extent than the virtual dome effect,denoted as aloft umbrella effect.Aerosol stove,dome,virtual dome,and surface/aloft umbrella effects visualize the impacts of different types of aerosols located at different heights on the evolution of PBL,which deepens our understandings on the aerosol-PBL interactions.