An ultrafine particle monitor (UFPM) was designed and constructed to obtain size-resolved particle number concentrations of atmospheric aerosols in air quality monitoring networks. The UFPM employs a corona-jet charger for particle charging, a DMA for electrical mobility classification, and an aerosol electrometer for particle quantification based on electrical charge. It does not require a radioactive source or an alcohol-based particle detector. A sensor model of the UFPM was derived, which relates the measured electrical current to the particle number concentration. A data inversion algorithm based on the Tikhonov-regularization was implemented. It yields the number concentration in six size classes between 20 and 720 nm from a 10-min-measurement with an upper limit of 3 x 10(6)cm(-3). A method was developed to calculate the kernel matrix from the data of comparison measurements with the UFPM and a reference mobility particle size spectrometer such as an SMPS. In experimental comparisons, the number concentrations of laboratory and atmospheric aerosols measured by the UFPM were within a range of +/- 20% of the reference concentration determined by an SMPS. (C) 2013 Elsevier Ltd. All rights reserved.