Aerosol Measurement: Principles, Techniques, and Applications. Second Edition, PA Baron, K Willeke, eds., New York: John Wiley and Sons, Inc., 2001 Sep; :83-97
Aerosols, by their nature, are somewhat unstable in the sense that concentration and particle properties change with time. These changes can be the result of external forces, such as the loss of larger particles by gravitational settling, or they may be the result of physical and chemical processes that serve to change the size or composition of the particles. This chapter addresses the latter category or processes. They all involve mass transfer to or from a particle. This transfer may be the result of molecular transfer between the particle and the surrounding gas, for example, condensation, evaporation, nucleation, adsorption, and chemical reaction, or it may result from interparticle mass transfer, such as by coagulation. Processes that cause physical or chemical changes in the particulate phase influence the particle size distribution of nearly all aerosols. These processes contribute in an essential way to the earth's hydrological cycle. They are involved in the formation of photochemical smog and are key to shaping the atmospheric aerosol size distribution. These processes play a significant role in many occupational aerosol exposures and in the operation of the condensation nuclei counters, described in Chapter 19. They are central to industrial aerosol processing and for the generation of test aerosols. Condensation, thermal coagulation, and adsorption are related processes that rely on the diffusion of molecules or particles to a particle's surface. Evaporation is the opposite of condensation and is governed by the same laws. Reactions may be nongrowth processes that change the composition or density of an aerosol particle with little or no change in particle size. Because the processes discussed in this chapter are related and may be occurring simultaneously, it is necessary to look at each process seperately in order to obtain an accurate picture of the changes that occur. Furthermore, these processes depend on particle size in a complex way, so I adopt a single particle approach for much of the analysis that follows. I rely on the concepts of mean free path and diffusion coefficient, which are defined in Chapter 4.