Ground based in situ observations of optical_particle_size_spectrometer at Birkenes II (NO0002R). These measurements are gathered as a part of the following projects NILU and they are stored in the EBAS database (http://ebas.nilu.no/). Parameters measured are: pm10_mass in pm10 (mass_concentration_of_pm10_ambient_aerosol_particles_in_air), pm1_mass in pm1 (mass_concentration_of_pm1_ambient_aerosol_particles_in_air), pm25_mass in pm25 (mass_concentration_of_pm2p5_ambient_aerosol_particles_in_air)