Aerosol Apparent Mean Free Path

Since aerosol particles are continually undergoing molecular bombardment, their paths are smooth curves rather than segments of straight lines. It still is possible to define an apparent mean free path for the aerosol particles (Fuchs, 1964). This is the distance traveled by an average particle before it changes its direction of motion by 90°. The apparent mean free path represents the distance traveled by an average particle in a given direction before particle velocity in that direction equals zero. But this is just the stop distance. 

Certain quantities associated with the Brownian motion and the dynamics of single aerosol particles are shown as a function of particle size in Table 9.5. All tabulated quantities in Table 9.5 depend strongly on particle size with the exception of the apparent mean free path A,p, which is of the same order of magnitude right down to molecular sizes, with atmospheric values Xp 10-60nm. 

Transition and Free Molecular Regime When the mean free path of the diffusing aerosol particle is comparable to the radius of the absorbing particle, the boundary condition at the absorbing particle surface must be corrected to account for the nature of the diffusion process in the vicinity of the surface. The correction is appreciable when the apparent free path of the aerosol particles kp is of the same order of magnitude as the radius Rp as we will see this occurs for Rp <0. /im. 

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