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Particulates are very small particles, suspended in air, with diameters in the µm dimension.

Particulates or total suspended matter, are one of the important air pollutants, specifically the fractions of PM10 (smaller than 10 µm) and, most recently, PM2.5 (smaller than 2.5 µm). Particles below 10 µm are particularily important as a health hazard, as they are likely to be inhaled.

Depending on to their origin, particulates can also include numerous toxics: pyrogenic particulates, i.e., those generated by combustion processes (in internal combustion engines and Diesel engines in particular, but also in industrial furnaces and waste incinerators) are in this category. Examples are heavy metals such as arsen, beryllium, cadmium, nickel, lead, selenium, chromium, and mercury. Another mechanism is the adsorption of chlorinated hydrocarbons on the surface of particulates or aerosols.

The settling velocity for particles (which have a much higher density or specific gravity than air) depends primarily on their size (measured as diameter), as described by Stoke's Law. Some typical examples of settling velocities, relevant for modeling, are given in the Table below.

diameter in µm intrinsic speed mm/s settling velocity mm/s
0.01 3000.0 0.000
0.10 100.0 0.001
1.00 3.0 0.100
10.00 0.1 10.000
100.00 0.1 1000.000

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