2.2 x 10 7

" This assumes an aerosol concentration of 1 particle cm 3 outside the gradient region. h From Hinds (1982).

" This assumes an aerosol concentration of 1 particle cm 3 outside the gradient region. h From Hinds (1982).

diameter. The reader is referred to Hinds (1982) for a derivation of Eq. (U). For larger particles where C ~ 1 (i.e., the slip correction is negligible), the rate of diffusion varies inversely with the particle diameter. For very small particles, the rate varies with 1 D2, which contributes to making diffusion a major transport mechanism for particles 0.1 /xrn. It is this Brownian diffusion that helps to carry small particles through the boundary layer to surfaces where they may stick on impact.

The relative importance of Brownian diffusion and gravitational settling in the deposition of particles may be seen by calculating the total deposition of particles onto a horizontal surface by these two processes in a given period of time under certain conditions. Table 9.5 shows the results of such a calculation for particle diameters from 0.001 to 100 /jlm, assuming spherical particles of unit density with a constant concentration of f particle cm 3 outside the gradient region; also shown is the ratio of the number of particles deposited by diffusion compared to the number deposited by gravitational settling. At a diameter of ~0.2 /¿m, the two mechanisms become equal, with diffusion greatly exceeding gravitational settling for particles in the Aitken nuclei range. (Note that if the mass deposited were calculated, the results would be quite different.)

Other factors also come into play in laboratory systems. For example, McMurry and Rader (1985) have shown that particle deposition at the walls of Teflon smog chambers is controlled by Brownian and turbulent diffusion for particles with Dp 0.05 yu,m and by gravitational settling for particles with Dp > 1.0 yu,m. However, in the 0.05- to 1.0-/j,m range, the deposition is controlled by electrostatic effects; Teflon tends to

10"1 10° 101 102 103 104 Diameter of particle (pm)

FIGURE 9.16 Settling velocities in still air at 0°C and 760 Torr pressure for particles having a density of 1 g cm 3 as a function of particle diameter. For spherical particles of unit density suspended in air near sea level, the Stokes law applies over a considerable range of particle sizes, where the line is straight, but a correction is required at the particle size extremes (adapted from LBL, 1979).

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