The mobility spectrum of air ions has been measured at Tahkuse Observatory in Estonia for several years. The average concentration of intermediate ions with mobilities of 0.05-0.5 cm(2) V-1 s(-1) in atmospheric air is about 50 cm(-3). On the level of this low background, high concentration bursts of intermediate air ions occur occasionally. A burst can be followed by subsequent evolution of intermediate ions into larger ones. To explain the bursts of intermediate air ions, two hypotheses can be advanced: (1) A burst of neutral particles occurs due to homogeneous nucleation, and the particles are charged by the attachment of cluster ions. (2) The cluster ions grow by ion-induced nucleation in proper environmental conditions.