Change in size distribution of ultrafine aerosol particles undergoing brownian coagulation

Abstract

Brownian coagulation in the free molecular and transition regimes has been studied. In the experiment, highly concentrated NaCl, ZnCl2, and Ag aerosol particles having geometric mean diameter of 5–40 nm and geometric standard deviation around 1.4 were passed through a metal pipe of 2.0 cm diameter in laminar state. At the inlet and the outlet of the coagulation pipe, the particle size distributions were measured by means of a differential mobility analyzer combined with a CNC and the particle number concentrations were counted independently by an ultramicroscopic system after enlarging their size in a particle size magnifier. In theoretical calculation, the coagulation rate function proposed by Fuchs was used, and the pupulation balance equation for simultaneous Brownian coagulation and diffusive deposition of aerosols in laminar pipe flow was numerically solved. It was found that the experimental results deviated from the theoretical calculation with the decrease in the particle size, that is, the enhancement of coagulation rate was observed. This will be caused by the van der Waals attractive force, which was qualitatively explained from the Marlow's theory.