Ground based in situ observations of particle_number_size_distribution at Hyytiälä (FI0050R) using dmps. These measurements are gathered as a part of the following projects ACTRIS, CREATE, EUSAAR, EMEP, GAW-WDCA and they are stored in the EBAS database (http://ebas.nilu.no/). Parameters measured are: particle_number_size_distribution in aerosol (number_concentration_of_positive_ions_in_dry_aerosol_particles_in_air)