Processes, sources and particle size distributions

Abstract

Rational control policies for particulate air pollutants ultimately must consider the particle size distribution. This point is illustrated by a simple example of the inadequacy of regulations based on limitation of total particulate mass emissions from primary sources. A general model for the evolution of the atmospheric aerosol size distribution is discussed for the particular case of an urban area. Conservation equations for particulate matter and nucleating vapour are developed. An important element in the development of the general model is knowledge of the particle size distribution associated with various primary sources of particulate matter. Such primary sources are classified according to the basic aerosol generation process of homogeneous and heterogeneous nucleation and comminution.

Aerosols from primary sources in which particle generation occurs by homogeneous nucleation are considered to be well aged aerosols, which by virtue of coagulation have reached asymptotic limit or “self preserving” distributions. As a practical matter, however, randomization will alter the asymptotic limit distributions from primary sources. Inherent complications in attempts to characterize the particle size distributions of aerosols formed by heterogeneous nucleation are discussed. Aerosols generated by comminution are known to approach asymptotic limit distributions which in certain cases have the log normal form, although randomization again will alter such asymptotic forms.