The dataset provides measurements of the mobility and size distributions of charged molecular clusters and fine aerosol particles naturally found in atmospheric air. The genesis and subsequent evolution of nanoparticles is a key to understanding the formation of atmospheric aerosols, which is an essential factor of the Earth's climate. The particles are classified according to their size and electric mobility. The particle electric mobility was immediately determined while the size was calculated as the mobility equivalent diameter of the nanoparticle. In atmospheric electricity, the charged molecules and clusters up to 1.6 nm in diameter are called small air ions, the charged aerosol particles of size 1.6–7.4 nm are called intermediate air ions, and the charged aerosol particles of size 7.4–70 nm are called large air ions. The dataset includes results of simultaneous measurements of positive and negative air ions in three stations during four years from 2003 until 2006. The small and intermediate air ions were measured at Hyytiälä, Finland (61°51'N, 24°17'E, 180 m a.s.l.), Tartu, Estonia (58.373 N, 26.727 E, 70 m a.s.l.), and Tahkuse, Estonia (58°315'N, 24°555'E, 27 m a.s.l.), while large air ions were measured only at Tahkuse. The air ion measurements are accompanied with basic meteorological data. The dataset is not designed for direct running via the Internet. Complete package of data, software, explanations, and instructions Finestion2003_06.zip should be downloaded from the website, unpacked, and operated in the personal computer of the scientist.

Full set of measurements is saved as a single file, which can be opened using MS Excel. However, the whole data table is large and analysis of data immediately by means of Excel appears to be inconvenient. Thus the dataset includes a dedicated application, which allows easily extract specific subtables, and presents some extra facilities for manipulation with the time and selecting the variables in modified order. Instructions and descriptions are presented in the document Finestion_guide.pdf, which includes references to additional documents and data files. The authors acknowledge Hilja Iher and Kaupo Komsaare for taking care of the instruments, Markku Kulmala for organizing measurements in Hyytiälä, and Sander Mirme for presenting additional meteorological data.