Mobility spectra of air ions have been measured at a rural site in Est
onia during several periods. The annual average mobility spectrum of n
atural small air ions is presented. The concentrations of two groups o
f 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 proc
ess of ion-induced nucleation. A half-year average mobility spectrum o
f 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 av
erage size distribution of aerosol particles and on the theory of diff
usion charging of the particles.