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.
Highlights
Condensation of vapor to particles and coagulation between particles are major process leading to change in particle size distribution
We developed a kinitic method like Gopalakrishnan and Hogan Jr. (2011) to calculate coagulation coefficients focusing on the concentration distribution of particle
Concentration distribution of the collision particles around the scavenging particle with a horizontal axis of r and 1/r are shown in Figures 3 and 4, respectively
Summary
Condensation of vapor to particles and coagulation between particles are major process leading to change in particle size distribution. In a free molecular regime ( Kn → ∞ ) the coagulation rate can be expressed by cross section of particle and relative thermal speed between particles, in contrast by diffusion flux of particles in continuum regime ( Kn → 0 ). Fuchs (1964) proposed an expression of coagulation coefficient β by means of “Flux-matching” method in which particle diffusive transport and particle motion like free molecular were combined. A novel kinetic numerical model focusing on both diffusive nature of particles far from other particle and like free molecular nature was developed
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