Motion Of Fine Particles In Atmospheric Boundary Layer Over Complex Underlying Surface

Aerosol plays an important role in the climate system and atmospheric environment.As one of the major air pollutants,fine particles(PM2.5)receive increasing attention with the development of urbanization and economy in China.However,the transport and deposition of fine particles in the atmospheric boundary layer over complex underlying surfaces are still challenging research problems.In this dissertation,a framework to describe transport and deposition of fine particles within and above forest and urban canopies using large eddy simulation(LES)is presented.The dry deposition model of ultrafine particles(UFP)is first developed and validated in LES model.A hybrid spectral/finite-volume algorithm of scalars is modified in the urban canopy flow with obstacles.An extended nonperiodic domain large-eddy simulation approach is introduced into the study of atmospheric boundary layer,which enables the simulation of scalar plumes in a larger domain.This framework is validated against field and wind-tunnel measurements,showing accuracy and robustness when applying to forest and urban canopies.Next,the turbulence and motion of fine particles over forest and urban canopies are studied using the LES model.(1)The effects of leaf area index(LAI)and leaf area density(LAD)profile on UFP deposition onto forest canopies are evaluated.Mean particle concrentration profile and total deposition are insensitive to LAD profile but show strong dependence on LAI.Meanwhile,the subcanopy(or ground)deposition cannot be neglected in sparse canopy.(2)The effects of forest clearings and edges on turbulence and UFP deposition onto forest vegetation and ground are studied.The length of clearings has a great impact on the forest edge flow.The edge effect increases the deposition in the front area of forest canopy.However,the increase of local leaf area density decreases the flow velocity inside canopies,thereby reducing the deposition efficiency.Although the total deposition onto trees is insensitive to the heterogeneity of forest,the clearings can promote the ground deposition compared to homogenous forest.(3)The effects of trees on air quality inside street canyons are explored at different scales and with different emission scinarios.For local emissions,the aerodynamic effect of trees leads to a redistribution of particle concentration inside canyons and an increase of the mean concentration.For background pollutant,the concentration inside canyons tends to be homogeneous and insensitive to the existence of trees.Although the deposition effect of trees plays a small effect in a single canyon scale,it reduces the buildup of background pollution at the urban scale,which improves the air quality inside deep canyons.Finally,a simple analytical model for UFP deposition velocity above forest canopy and a semi-empirical model for urban street air quality with the consideration of trees are developed.Both of them show good agreement with LES results.