mean free path, λ

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 $d m$ is their mean diameter. For O₂ at one atmosphere and $25 °C$ , this distance is only $9.7 × 10 −6 cm$ ; at $10 −6$ 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 ( $1.381 × 10 −23 J K −1$ ), $T$ is the temperature ( $K$ ) and $B$ is the mobility.

Source:

PAC, 1990, 62, 2167 (Glossary of atmospheric chemistry terms (Recommendations 1990)) on page 2201

Green Book, 2nd ed., p. 56