Particle Size

Particle size generally refers to the diameter of a particle.  Both particle volume and and area are used in some applications.  The particle diameters of interests in  studying atmospheric aerosol behavior range from molecular clusters of about 10-3 mm to cloud droples and dust particles as large as 100 mm.  In air pollution applications, particles larger than 1 mm are called coarse mode. Particles that are smaller than 1 micron (submicron particles) can be divided into the accumulation mode, from 0.1 to 2.0 mm, and the ultrafine mode consisting of particles that are smaller than 0.1 mm. There is such a wide range in the particle sizes of interest in the atmosphere, (varying in size by as much as105  and  in mass by as much as 1015 ), that some specialized graphical and mathematical tools have been developed in order to represent the whole size distribution of interest in atmospheric studies. The most generally useful of these representations will be discussed later in this unit. The size distribution of atmospheric aerosol results from dynamic processes that affect both natural and manmade aerosol.  There are usually two modes in the mass distribution: the coarse mode corresponding to particles larger than 2 to 3 mm in diameter and the accumulation mode between about 0.1 and 2.5 mm.  Distribution like this have been repeatedly observed in many different locations.  For liquid  or spherical particles, the particle size is easily characterized by diameter.  However, atmospheric aerosol are frequently non-spherical.  There are several application specific ways of determining "equivalent" particle diameters but the most commonly  is the aerodynamic diameter.   Aerodynamic diameter is the diameter of a sphere with a density of 1 that has the same terminal settling velicity as the irregularly shaped particle being measured.

Terminal settling velocity is determined using the formula:

where Dp is the particle density, g is the gravitational acceleration ( 980 cm/sec²) , dp is the particle diameter, m is the viscosity of air. 

The animation below shows a spherical particle with a density of 1 (blue) and an irregular particle with the same density, and therefore the same settling velocitiy (green).  The aerodynamic diameter of the green and blue particles are the same.  The red particle also has a density of 1 but it falls at a much slower rate. Therefore it must have a smaller aerodynamic diameter than the other two particles.

Three particles of the same density but one has a different aerodynamic diameter

The optical properties of aerosol are closely related to particle size because interaction of small particles with light is a sensitive function of particle size and optical properties of the airbonre particles.  Light scattering, a propertiy of interest in air pollution science because of its affect on visibility, depends on the ratio of diameter to the wavelength of incident light,

d_p/ l, where d_p is  the particle diameter, and l the wavelength of the incident lightlight.

Atmospheric chemical and physical properties lead to the accumulation of particles in the size range of that produces the most severe effects on visibility.  The particle size range that scatters the maximum amount of light per unit mass of material is near 0.2 mm depending somewhat upon other optical properties of the particle.