Nonspherical particles classification

Relation (7.113) is very general. First, it applies to any two-component system at chemical equilibrium, and to any classification procedure we have chosen to identify the two quasicomponents. Second, because of the application of the Kirkwood-Buff theory of solutions, we do not have to restrict ourselves to any assumptions on the total potential energy of the system. Furthermore, the quantities appearing here depend on the spatial pair correlation functions go, R), even though we may be dealing with nonspherical particles. 

Terminal velocities can then be found directly from eqs (20) and (21). Based on this method, the terminal fall velocities in air of nonspherical particles in Groups A, B, and D of the Geldart classification have been calculated as functions of temperature and pressure (Table 2 and Figs. 3 and 4). 

The size classification of aerosol particles is greatly facilitated if the particles are assumed to have a spherical shape. The size is then defined by their radius or diameter. The assumption clearly is an idealization, as crystalline particles come in various geometrical, but definitely nonspherical, shapes and amorphous particles are rarely perfectly spherical. At sufficiently high relative humidities, however, water-soluble particles turn into concentrated-solution droplets, which are essentially spherical, and therein lies the most convincing justification for the assumption of spherical particles. 

Polar stratospheric clouds have been classified into two broad types, so-called Type I and Type II (Table 4.1). Type I PSCs have been further subdivided into Type la and Type Ib. Type la PSCs have traditionally been identified as crystals of nitric acid trihydrate, HNO, 3 H2O, denoted NAT, that form once temperatures fall below about 195 K. Type lb PSCs consist of supercooled ternary solutions of HNO3/H2SO4/H2O, also forming at about the same temperature threshold. Type II PSCs are largely frozen water ice, nonspherical crystalline particles, that form at temperatures below the ice frost point. The ice frost point, for example, at 3 X 10 Torr H O is 191 K. Despite the above classification, the composition of PSCs is still uncertain (Toon and Tolbert, 1995). 

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