a Numerical Study of the Efficiency with which Aerosol Particles Collide with Simple Planar Ice Crystals.
Abstract
In this dissertation a theoretical investigation is presented on the interaction of simple planar ice crystals with aerosol particles in air of various conditions. The purpose of this study is to provide a fuller elucidation of the importance of platelike ice crystals in the overall atmospheric aerosol particle budget. In order to accomplish this task, two theoretical models, the trajectory model and the flux model, are discussed which permit the computation of the efficiency with which aerosol particles of radius 0.001 (LESSTHEQ) r(,p) (LESSTHEQ) 10 (mu)m are collected by simple ice crystal plates idealized by thin oblate spheroids having an axis ratio = 0.05, and radii 50 (LESSTHEQ) a(,c) (LESSTHEQ) 640 (mu)m. The calculations considered various atmospheric pressures, temperatures and relative humidities. Particle capture due to inertial impaction, Brownian diffusion, thermophoresis, diffusiophoresis and electrostatic forces are considered. It is shown that, analogous to water drops, ice crystals exhibit a minimum in collision efficiency within a specific size interval of aerosol particles. The minimum is strongly affected by the relative humidity of the ambient air. The trajectory model utilizes numerically obtained velocity and gradient fields to determine the effect of inertial impaction, phoresis, and surface electric charge on the collection of particles r(,p) (GREATERTHEQ) 0.1 (mu)m. It is shown that the collision efficiency of particles r(,p) (GREATERTHEQ) 1.0 (mu)m is considerably affected by the flow field around the ice crystal. Trajectory analysis with our model predicts that electrically charged and uncharged aerosol particles are preferentially captured at the rim of the ice crystal. Electrically neutral ice crystals of N(,Re) (LESSTHEQ) 50 capture particles only on the upstream surface. Ice crystals which are electrically charged collect electrically charged aerosol particles also by rear capture if N(,Re) < 0.5. The flux model derived from Fick's first and second "laws" computes the efficiency with which aerosol particles 0.001 (LESSTHEQ) r(,p) (LESSTHEQ) 1.0 (mu)m are captured by single planar ice crystals due to Brownian diffusion, phoresis, and electrostatic forces. The enhancement of particle flux due to flow around a nonstationary collector is included through use of ventilation coefficients. It is shown that Brownian diffusion dominates the capture process if r(,p) < 0.01 (mu)m. For aerosol particles of 0.01 < r(,p) < 0.1 (mu)m the collection is controlled by phoretic forces, while electric forces significantly affect the collection process in the size range 0.01 < r(,p) < 1 (mu)m. Our theoretical results are found to agree satisfactorily with the laboratory studies presently avilable.
 Publication:

Ph.D. Thesis
 Pub Date:
 1980
 Bibcode:
 1980PhDT........23M
 Keywords:

 Physics: Atmospheric Science;
 Aerosols;
 Efficiency;
 Ice;
 Numerical Analysis;
 Particle Collisions;
 Particle Interactions;
 Analysis (Mathematics);
 Brownian Movements;
 Electrostatic Charge;
 Mathematical Models;
 Thermophoresis;
 Geophysics