| Atmospheric aerosol pollutants accompanied by development of society has a mass production,which have a lot of adverse effects on climate change,atmospheric environment and human health.The migration of airborne pollutants in the atmosphere is one of the hot topics in the field of environmental protection.The velocity of aerosol particle migration is a major parameter to study the diffusion and deposition of aerosol particles.Nowadays,the theoretical and experimental studies on aerosol particle diffusion mostly conducted on the canopy layer of atmospheric boundary layer.However,it's of great research significance that the transportation of sand particles on the higher altitudes such as roughness sub-layer covered peaks.In this study,it was investigated that the transportation and deposition of the aerosol particles released from the point source of the rough sub-layer over the typical topographic model in a wind tunnel,and the effects of particle properties,meteorological factors(wind speed,rainfall)and topography(different peak height)on particle diffusion behavior and deposition law in this height range were investigated.The following conclusions were drawn:1.Thermostatic Hotline Anemometer was used to determine the flow field of typical terrain in wind tunnel.The results showed that the wind profile accorded with the exponential law under the experimental conditions,that is,the background flow field in the wind tunnel conformed to the characteristics of the atmospheric boundary layer flow field.In addition,the wind speed profile and turbulence intensity of flow field are affected by topography and wind speed in the range of roughness sub-layer,but the extent of influence is much smaller than that of canopy layer.2.The Particle Dynamic Analysis technique(PDA)was used to determine the velocity distribution of particles in roughness sub-layer of typical terrain.It showed that the probability distribution of longitudinal and vertical velocity in the range of heights accorded with the Gaussian distribution.When the wind speed increased from 1 m/s to 2 m/s,the range of particle longitudinal velocity increases from 0.5?1.3 to 1?2.5 m/s and the range of frequency decreases from 0.12?0.17 to 0.05?0.09,the range of particle longitudinal velocity increases and the frequency of velocity decreased.It indicated that the increasing wind speed could make the longitudinal motion of particles diffuse and the instability of motion increase.Besides,the probability distribution of particle vertical velocity presented that when the wind speed is 1 m/s,the particle vertical velocity range in the range of roughness sub-layer over peak1 is-0.5?0 m/s,indicates all the particles moved downward motion,while when the wind speed increased and the peak was higher,a plenty of particles appeared positive in velocity,with a large number of particles moving upward.It showed that the vertical motion of particles in range of roughness sub-layer is the result of the joint action of wind speed and topography.3.By comparing the particle mean longitudinal and vertical velocity of in the range of roughness sub-layer of smooth surface and typical terrain.It was found that the particle mean longitudinal velocity in hilly terrain is smaller than that in smooth surface.however,the particle longitudinal velocity on the leeward side of the peak was increasing because of the vortex,especially at peak2(the larger peak),when the wind speed was 2 m/s,the particle velocity under the hilly terrain even exceeded the particle velocity under the smooth surface.From probability distribution of particle vertical velocity,it can be concluded that the particles at peak1were still affected by the rising force of air disturbance,while the particles at peak2contained descending motion and the ascending motion.And the particle force was relatively balanced,the final state of stability.4.The dry and wet deposition flux of point-source released aerosol particles in typical terrain was investigated by weighing method.It is found that in dry deposition,deposition flux decreased with the downwind distance and the increase of wind speed.The higher deposition flux on the upwind slope of the peaks was due to the interception of aerosols here.In wet deposition,the rainfall can significantly promote aerosol deposition compared with dry deposition,and aerosol deposition was more at low altitude in the terrain.This is because rainfall scour redistributes the aerosol deposition from sloping terrain,and rainfall drives aerosols from high areas to low altitude.5.The dry deposition rate was calculated according to the surface aerosol concentration and deposition flux.The results showed that the velocity increases from 1.1×10-4to 2.5×10-3m/s at 1 m/s of wind speed and 1.8×10-3?5.9×10-3m/s at 2 m/s of wind speed,which stated the dry deposition velocity increased with the downwind distance and the increasing of wind speed.therefore,larger wind speeds had a positive effect on aerosol diffusion.Because it brought enhanced turbulence,reduced the atmospheric stability,increased particle diffusion and reduced deposition in near-source. |
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