INTRODUCTION

In discussions of light scattering and deposition in previous chapters, the size distribution function, n( v), was considered a given quantity. However, the deposition process itself results in the loss of particles preferentially with respect to size, thereby changing n( v). In addition, processes occurring within the gas, including coagulation and gas-to-particle conversion, modify n( v). This occurs in the production of titania pigment, in the evolution of the atmospheric aerosol, and at sources of combustion aerosols such as incinerators and pulverized coal combustion units. The next few chapters deal with the internal processes that modify the particle size distribution. In this chapter, we consider collisions among coalescing spherical particles, the process we define as coagulation.

Aerosols are unstable with respect to coagulation. The reduction in surface area that accompanies coalescence corresponds to a reduction in the Gibbs free energy under conditions of constant temperature and pressure. The prediction of aerosol coagulation rates is a two-step process. The first is the derivation of a mathematical expression that keeps count of particle collisions as a function of particle size; it incorporates a general expression for the collision frequency function. An expression for the collision frequency based on a physical model is then introduced into the equation that keeps count of collisions. The collision mechanisms include Brownian motion, laminar shear, and turbulence. There may be interacting force fields between the particles. The processes are basically nonlinear, and this leads to formidable difficulties in the...