Investigations On The Removal Characteristics Of Urban Aerosols By Wet Scavenging And Deposition

Particulate emission and haze pollution have led to serious urban atmospheric problems.It not only causes the atmospheric visibility reduced and people's exposure level increased but also particles deposited on the surface of urban building will influence thermal properties of building walls and the adjacent air quality.The main removal pathways of aerosol in the urban atmospheric environment are wet scavenging and dry deposition.As a result of the increase of both building density and the change of building cluster structures in urban areas,ambient airflow patterns within urban boundary layer are different from that in the countryside.The existing studies about the estimation method of wet scavenging and dry deposition are not appropriate for describing the processes which take place in the urban area because of the local turbulent motion of the airflow.In the present study,theoretical analysis and numerical simulation are employed to explore the aerosol diffusion and elimination in the process of aerosol scavenging by rain and dry deposition according to the characteristic of urban airflow.The theoretical estimation of wet scavenging in the laminar flow regime and the influence of urban turbulent flow on the scavenging process are investigated and discussed,and the features of aerosol particle dry deposition are discussed simutenuously on the condition of haze pollution and traffic emission.Theoretical analysis of aerosol scavenging by raindrops at the terminal velocities in the laminar flow should be conducted firstly in order to explore the influence of turbulent fluctuation on the collection of aerosols by wet scavenging.Considering there exist large discripencies between many previous theoretical results and field observations for the calculations or estimations of aerosol collection efficiency,the assumption of thin concentration boundary layer and expansion method of boundary layer were applied to solve the convection diffusion equation,and then the collection efficiencies of inertialess aerosols?i.e.,submicron aerosols?by below-cloud scavenging were determined.The results show that the flow field around the raindrop falling at its terminal?9?This work was supported by the National Natural Science Foundation of China?Grant Nos.41275157 and 51578121?and the Innovation Foundation of Shanghai Education Commission?Grant No.14ZZ073?.velocity can be assumed as Hadamard–Rybczynski flow for a small raindrop and potential flow for a large raindrop when estimating the collection efficiency.It is also approved that the boundary layer expansion method can be efficiently used to simplify the velocity profiles in the concentration boundary layers of raindrops when seeking a theoretical expression of collection efficiency.The diffusional deposition results of aerosols captured by raindrop falling at its terminal velocity are obtained through solving convection diffusion equation,which prove the imperfection of previous studies of viscous flow.The Sherwood numbers?Sh?are all proportional to RpPe^1/2 for the two airflow pattens.That is to say the collection efficiency of submicron aerosols by either a large or a small raindrop is roughly proportional to Pe^-1/2.Comparing the new correlations of the present results with the experimental data and theoretical curves of previous studies,the interception effect can be neglected for submicron aerosol captured by raindrops,and the collection efficiency of submicron aerosols by a raindrop falling at its terminal velocity can be simply estimated by E = 3.34Pe^-1/2.The roughness of underlying surface in the city is relatively high and then the turbulent intensity of urban airflow is large.The estimation of the scavenging effect of aerosol in the previous studies on the condition of laminar flow could lead to large error in the calculation.The specific influence of turbulence effect on aerosol scavenging by rain can not obtain by analytical algorithm.Therefore the background turbulence effects which interfere and distort particle trajectories are simulated and analyzed by employing CFD numerical simulations.The results indicate that the collection efficiencies of inertial particles obtained by numerical simulations in the laminar flow region agree well with the experimental data in previous studies.When additional turbulence intensity is exerted on the flow field,the random effect of airflow velocity will influence the collection of aerosols by raindrops.The transportations of particles in the turbulent flow are affected by the flow fluctuation and the randomness of particle trajectory is then increased,so the probability of aerosol being captured by a raindrop is also increased.Compared with that under the laminar flow condition,the capture of the particles by raindrops is totally changed in the turbulent flow.Therefore,the calculation formula of particle capture efficiency in the laminar flow is invalid for the cases in turbulent collection.To improve the estimation of efficiency,an alternative concept of capture probability is introduced to re-define new capture efficiency calculation formula which is applicable for the cases of turbulent flow.The simulated results show that the distributions of capture probability for different particle diameters are all affected by the background turbulence intensity.The collection efficiency is significantly influenced by turbulence intensity in the range of particle diameter less than 5 ?m.The collection efficiency is increased with the increase of turbulent intensity.The results also indicate that the factors of influencing the turbulent collection efficiency include the raindrop's diameter,the particle size and particle density.The collection efficiency of particle is increased with decreasing of the values of the three parameters.Dry deposition is one of the main ways to remove aerosol particle from urban air except wet scavenging process.The increase of building intensity and height in the urban area makes building have huge total surface area,which provides a potential condition for particle deposition.The concentratioin distribution of aerosol particle is uniform in the period of haze pollution.The particle deposition features on the building surface of individual building and building complex were investigated by using computational fluid dynamics in the above condition.The numerical results demonstrate that effect of building configuration on the flow field reflected by the width of windward side?W?.The influence of W on the flow field is greater than that of building height.The particle deposition velocities and deposition fluxes on the windward side of building are all found to be a downward trend with the increase of W,which is opposite on the leeward side.Furthermore,it is also investigated that the effect of background wind speed on the features of particle deposition.The particle deposition velocities increase with the increase of background wind speed.And the more background wind speed is,the more obvious the dilution effect of airflow on the diffusion of aerosol particles is.The particle concentration and particle deposition flux on the building surface decrease with increasing background wind speed.The inertial effect of particle is larger than that of the same diameter particle when the background wind speed becomes higher.The particle diameter corresponding to minimum deposition flux is smaller caused by the synthetic action between inertial mechanism and particle diffusion mechanism.The flow patterns around the building clasters depend on the building layout of the clasters and building density,which will influence the particle deposition features on the building surface.The numerical results show that the particle deposition velocity around the windward side of the building is larger than that around side surface and deposition velocity around leeward side is smallest both the regular arrangements and the staggered arrangements.Comparing two densities building complex in the regular arrangements with the staggered arrangements respectively,it can be found that in the regular arrangements the smaller building density is,the larger particle depositon flux on the building surface is.On the contrary the variation tendency is opposite in the staggered arrangements.Aerosol particles in the vehicular emissions are also the main contributor to urban aerosol pollution.In order to fully understand the effects of the geometric configuration of street canyon on particle deposition feature onto the surface of frontage buildings,the influences of five values of aspect ratio H/W?i.e.the ratio of building height to street width?and two building arrangements on the flow field and pollution concentration field are simulated and discussed.The results show that the particle concentration in the regular street canyon is increasing with the increase of H/W.But the particle concentration in the staggered street canyon appears maximum value of H/W = 2.0.The mean concentration of the former is larger than that of the latter when H/W exceeds 1.0.The regular and staggered arrangements are both adoptable when H/W is less than 1.0.The staggered arrangements should be adopted on the condition of H/W > 1.0 and avoided the most unfavorable street canyon from the view of concentration.The distribution regularities of deposition flux on the surfaces of frontage buildings appear higher on the bottom of surface and lower on the top of building surface for the regular and staggered street canyons.The average deposition fluxes on the leeward side of regular and staggered buildings increase with increasing the aspect ratio H/W.But the maximum particle deposition flux on the windward side of regular street canyon appears at the street canyon of H/W = 3.0.The particle deposition flux on the windward side of staggered street canyon is negligible because it is very low compared with that occurred in regular cases.