Intermediate air ions are charged nanometer-sized aerosol particles with an electric mobility of about 0.03-0.5 cm(2) V-1 s(-1) and a diameter of about 1.5-7.5 nm. Intensive studies of new particle formation provided good knowledge about intermediate ions during burst events of atmospheric aerosol nucleation. Information about intermediate ions during quiet periods between the bursts remained poor. The new mobility analyzer SIGMA can detect air ions at concentrations of mobility fractions of about 1 cm(-3) and enables studying intermediate ions during quiet periods. It became evident that intermediate ions always exist in atmospheric air and should be considered an indicator and a mediator of aerosol nucleation. The annual average concentration of intermediate ions of one polarity in Tartu, Estonia, was about 40 cm(-3) while 5% of the measurements showed a concentration of less than 10 cm(-3). The fraction concentrations in logarithmic 1/8-decade mobility bins between 0.1 and 0.4 cm(2) V-1 s(-1) often dropped below 1 cm(-3). The bursts of intermediate ions at stations separated by around 100 km appeared to be correlated. The lifespan of intermediate ions in the atmosphere is a few minutes, and they cannot be carried by wind over long distances. Thus the observed long-range correlation of intermediate ions is explained by simultaneous changes in air composition in widely spaced stations. A certain amount of intermediate ion bursts, predominantly of negative polarity, are produced by the balloelectric effect at the splashing of water drops during rain. These bursts are usually excluded when speaking about new particle formation because the balloelectric particles are assumed not to grow to the size of the Aitken mode. The mobility distribution of balloelectric ions is uniform in shape in all measurements. The maximum is located at a mobility of about 0.2 cm(2) V-1 s(-1), which corresponds to the diameter of particles of about 25 nm. (C) 2012 Elsevier B.V. All rights reserved.