An atmospheric ion of size and mobility intermediate between the small ion and the large ion. Whereas small and large ions are readily detected in all localities, intermediate ions have been reported only in a limited number of cases. The mobility of this class of ions lies generally in the interval from 0. 01 to 0. 1 cm s−1 (V cm−1)−1. In general, the present understanding of the nature and origin of intermediate ions is limited.