This study fits in the general framework of developing methods for the measurement of the number size distribution of airborne submicrometer particles suited for field measurements. An approach based on the combination of a Particle Size Selector (PSS) and a portable Condensation Particle Counter (CPC) is proposed in this work. The methodology relies on the successive measurement of the number concentration under 6 operational configurations (n = 0, 1, 2, 6, 12 and 18 diffusion screens). A first part of this work consisted of determining particle penetration through different sets of diffusion grids, both experimentally and theoretically. On that basis, a specific data processing was developed, assuming that aerosol size distribution follows a monomodal lognormal law. In the second part of this study, the combined PSS-CPC approach was applied to a series of test aerosols produced in the laboratory, covering a range of modal diameters from 9 to 270 nm. Compared to reference SMPS data, the output number size distributions calculated from the PSS-CPC approach were found to be within +/- 11% in terms of modal diameter, which is considered satisfactory.