Particle Size Distribution

Size is the most important single characteristic of an aerosol particle. For a spherical particle, diameter (or radius) is the usual reported dimension. When a particle is not spherical, the size can be reported either in terms of a length scale characteristic of its silhouette or of a hypothetical sphere with equivalent dynamic properties, such as settling velocity in air. For example, the aerodynamic diameter of a particle represents the diameter of a unit density (pp = 1 g/cm3) sphere having the same terminal settling velocity as the particle sampled, whatever its size, shape, or density.

When particles, at total number concentration N (particles/cm3), are measured and the number of particles dN having diameters between Dp and Dp + dDp, where dDp is a small increment of diameter, are counted, the particle size distribution n{Dp) is defined as n(Dp) = dN/dDp (reciprocal micrometers per cubic centimeters), where Dp is usually measured in micrometers. The integral of the size distribution over all sizes is the total number concentration:

The log-normal distribution is particularly useful for representing aerosol size distributions because it does not allow negative particle sizes, n{Dp) =

y/lnXno exp

where Dg is the geometric mean diameter and (Tg is the geometric standard deviation. These parameters can be determined from discrete particle count data by lno\ =

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