Langevin Dynamics Calculation of Brownian Coagulation Coefficient for Spherical Equal-size Aerosol Particles in Transient Regime

Abstract

Coagulation coefficient of aerosol particles due to Brownian motion is an important issue to describe change in particle size distribution. Motion of aerosol particles is diffusive in continuous region (small Knudsen number; Kn), or like free molecular motion of gaseous molecular in free molecular region (large Kn). Fuchs (1964) presented an expression of coagulation coefficient in transition regime by a so-called “Flux Matching” method. In his method, transportation of particles inside of the “limiting sphere” is assumed to be like free molecular, or diffusive outside of the sphere. These days, some researchers presented coagulation coefficient of aerosol particles by direct calculation of motion of aerosol particles. They employed Langevin dynamics equation to represent the stochastic motion of aerosol particles. In this study, we developed new model to calculate the coagulation coefficient. Our model employed spherical calculation space in which one scavenging particle is in the center of it: the calculation sphere moves together with the motion of the scavenging particle. The coagulation coefficient can be calculated from the mean time between collisions and the concentration of collision particles. By using the above numerical model, we have calculated the coagulation coefficient of spherical particles of from 4 nm to 100 nm in diameter.