Mean Free Path of an Aerosol Particle

The particle motion can be characterized by a mean thermal speed, Cp, 

FIGURE 8.11 A two-dimensional projection of the path of (a) an air molecule and (b) the center of a 1 ptm particle. Also shown is the apparent mean free path of the particle. 

To obtain the mean free path Xp, we recall that in Section 8.1, using kinetic theory, we connected the mean free path of a gas to measured macroscopic transport properties of the gas such as its binary diffusivity. A similar procedure can be used to obtain a particle mean free path kp from the Brownian diffusion coefficient and an appropriate kinetic theory expression for the diffusion flux. Following an argument identical to that in Section 8.1, diffusion of aerosol particles can be viewed as a mean free path phenomenon so that 

Certain quantities associated with the Brownian motion and the dynamics of single aerosol particles are shown as a function of particle size in Table 8.5. All tabulated quantities in Table 8.5 depend strongly on particle size with the exception of the apparent mean free path kp, which is of the same order of magnitude right down to molecular sizes, with atmospheric values 10 to 60 nm. 

TABLE 9.5 Characteristic Quantities in Aerosol Brownian Motion 

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FIGURE 9.10 Removal rale of particles as a function of time for the conditions of Figure 9.9. 9.5.3 Mean Free Path of an Aerosol Particle... 

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