Ground based in situ observations of particle_number_concentration at East Trout Lake (CA0102R) using cpc. These measurements are gathered as a part of the following projects GAW-WDCA, NOAA-ESRL and they are stored in the EBAS database (http://ebas.nilu.no/). Parameters measured are: particle_number_concentration in aerosol (number_concentration_of_particles_in_dry_aerosol_particles_in_air), particle_number_concentration in aerosol (number_concentration_of_particles_in_dry_aerosol_particles_in_air), particle_number_concentration in aerosol (number_concentration_of_particles_in_dry_aerosol_particles_in_air)