The average distance a molecule travels between collisions. For a molecule, , where is the number of molecules per unit volume and is their mean diameter. For O2 at one atmosphere and , this distance is only ; at atmospheres and it is . For an aerosol particle, the mean free path, in the Stokes region (see Stokes law) is given by: where is the mass of the particle, is the Boltzmann constant (), is the temperature () and is the mobility.
PAC, 1990, 62, 2167 (Glossary of atmospheric chemistry terms (Recommendations 1990)) on page 2201