Ground based in situ observations of particle_number_concentration at Montsec (ES0022R) using cpc. These measurements are gathered as a part of the following projects ACTRIS, GAW-WDCA and they are stored in the EBAS database (http://ebas.nilu.no/). Parameters measured are: particle_number_concentration in pm10 (number_concentration_of_pm10_aerosol_particles_in_air)