ABSTRACT
Airborne particles form a colloid consisting of a gaseous dispersion medium and dispersed particles, which may be solid or liquid. Liquid particles are always spherical, since the surface tension is much larger than frictional forces. Solid particles show a variety of shapes such as spheres, regular polyhedrons, straight and curved fibers, flake, and irregular particles. If physical properties of the particle, like density or index of refraction, are unknown, equivalent diameters can also be useful for measuring spheres. The best method of sampling is designated as isokinetic sampling. It involves a thin-walled sampling tube having its axes parallel to the wind and the mean gas velocity in the tube equal to the wind velocity. No change of the aerosol concentration will occur, if the wind velocity is much larger than the sedimentation velocity of the particles. If the flow through the nozzle is too high, the sampling rate of large particles is too small and vice versa.
