Reduction of air ion mobility to standard conditions

The Langevin rule of the reduction of air ion mobility is adequate in case of zero‐size ions. An alternative is the Stokes‐Millikan equation that is adequate in the limit of macroscopic charged particles. The temperature variation of air ion mobility predicted by the Stokes‐Millikan equation radically contradicts the Langevin rule. The temperature and pressure variation of air ion mobility is examined by using a new semiempirical model that describes the transition from the kinetic theory to the Stokes‐Millikan equation. The model is valid in full mobility range. It allows to calculate at first the size of an ion according to the measured mobility and then the standard mobility according to the size. The ascent of the temperature‐mobility curve on a logarithmic chart approaches the Langevin value of 1 only at very high mobilities not found in the atmosphere. The value of the ascent is 0.6 in the case of small ions of the mobility of 1.5 cm 2 V −1 s −1 which brings about a considerable error when using the Langevin rule. It is recommended to store the natural values of the mobility in databases together with the values of temperature and pressure and to definitely indicate the method when the reduced mobilities are presented in publications.