mean free path, $$\lambda$$

https://doi.org/10.1351/goldbook.M03778
The average distance a molecule travels between collisions. For a molecule, λ = ( 2 π n d m 2 )-1, where n is the number of molecules per unit volume and dm is their mean diameter. For O2 at one atmosphere and 25°C, this distance is only 9.7 x 10E-6 cm; at 10E-6 atmospheres and 25°C it is 9.7 cm. For an aerosol particle, the mean free path, λ B in the Stokes region (see Stokes law) is given by: λ B = 3 k T m m B where m is the mass of the particle, k is the Boltzmann constant (1.381 x 10E-23 J K-1), T is the temperature (K) and B is the mobility.
Sources:
Green Book, 2nd ed., p. 56 (https://dev.goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
PAC, 1990, 62, 2167. 'Glossary of atmospheric chemistry terms (Recommendations 1990)' on page 2201 (https://doi.org/10.1351/pac199062112167)