Basics and Definitions

An aerosol is a suspension of particulate matter in air. It comprises airborne droplets, solid particles, or both all will be referred to as particles in this paper. Because of mechanical particle fransport, an aerosol is intrinsically unstable (i.e., particles tend to deposit onto exposed surfaces) also, small aerosol particles at high concentrations tend to aggregate. 

The particle diameters given in this paper refer to the geometrie diameter of a unit-density sphere, if not otherwise specified. 

Marcel Dbkker, Inc. 270 Madison Avenue, New York, New York 10016 

Most frequently, an aerosol is characterized by its particle size distribution. Usually this distribution is reasonably well approximated by a log-normal frequency function (Fig. 4A). If the distribution is based on the logarithm of the particle size, the skewed log-normal distribution is transferred into the bell-shaped, gaussian error curve (see Fig. 4B). Consequently, two parameters are required to describe the particle size distribution of an aerosol the median particle diameter (MD), and an index of dispersion, the geometric standard deviation (Og). The MD of the log-normal frequency distribution is equivalent to the logarithmic mean and represents the 50% size cut of the distribution. The geometric standard deviation is derived from the cumulative distribution (see Fig. 4C) by 

If all particles of an aerosol are of imiform size, an aerosol is termed monodis-perse. Because in reality perfect monodispersity does not exist, an aerosol is termed monodisperse if the geometric standard deviation of the particle distribution is smaller than 1.2. For deposition analyses, all particles of a monodisperse aerosol are considered to behave as if they had exactly the same diameter as the MD of the size distribution. In polydisperse aerosols, particles of widely differing sizes are present, and Og is larger than 1.2. 

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This paper is organized as follows In chapter 2 some basics and definitions are provided. Chapter 3 describes the commonly applied procedure to calculate the reliability of electronic components. Electronic components with time-dependent failure rate are presented in chapter 4, followed by the consequences in chapter 5. 

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