A database of 8615 hourly averaged air ion mobility spectra in the range of 0.00041--3.2 cm2 V-1 s-1 was measured at Tahkuse Observatory, Estonia, during 14 months in 1993--1994. The average mobility spectrum over the whole period shows distinct peaks of small and large ions. Intermediate ions with mobilities of 0.034--0.5 cm2 V-1 s-1 are of low concentration of about 50 cm-3 in the average spectrum. They experience occasional bursts of up to about 900 cm-3 during 6--10 hours at daytime. The number of burst events recorded during 14 months was 101, with maximum frequency in spring and minimum frequency in winter. Physically, large and intermediate ions can be called aerosol ions, and small ions can be called cluster ions. The principal component analysis was applied to detect the structure of an air ion mobility spectrum. As a result, the mobility spectrum in the range of 0.00041-- 3.2 cm2 V-1 s-1 (diameters of 0.36--79 nm) was divided into five classes: small cluster, big cluster, intermediate, light large, and heavy large ions. The boundaries between the classes are 1.3 cm2 V-1 s-1 (diameter of 0.85 nm), 0.5 cm2 V-1 s-1 (1.6 nm), 0.034 cm2 V-1 s-1 (7.4 nm), and 0.0042 cm2 V-1s-1 (22 nm). The five principal components that are closely correlated with the respective ion classes explain 92% of total variance. The classification of aerosol ions is in accord with the three-modal structure of the size spectrum of submicron aerosol particles. ¿ 2000 American Geophysical Union