Mobility spectra of air ions have been measured at a rural site in Estonia during several periods. The annual average mobility spectrum of natural small air ions is presented. The concentrations of two groups of air ions with mobilities 0.32-0.5 cm2/(V s) and 0.5-2.5 cm2(V s) are not correlated; this fact indicates the different nature of the ions of the two groups. The air ions with mobilities 0.5-2.5 cm2/(V s) are interpreted as cluster ions and the air ions with mobilities 0.32-0.5 cm2/(V s) as charged aerosol particles that can be created in the process of ion-induced nucleation. A half-year average mobility spectrum of the large ions with mobilities 3.2 x 10(-4) - 1.5 x 10(-1) cm2/(V s) is presented. The spectrum is well interpreted on the basis of the average size distribution of aerosol particles and on the theory of diffusion charging of the particles.