Ground based in situ observations of particle_number_size_distribution at Birkenes (NO0001R) using dmps. These measurements are gathered as a part of the following projects NILU, EMEP, GAW-WDCA, EUSAAR 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)