In an attempt to develop a reference generator for the number size distribution function in the submicron size range we carried out a theoretical and experimental analysis of Brownian coagulation of liquid droplets in a continuously fed well stirred tank reactor operated in the steady state regime where the particle collision time scale is small compared to the mean residence time of the aerosol in the reactor. The feed aerosol, characterised by the flow of total particle volume (mass flow) and average particle size, is generated in a turbulent jet of finite length inside the reactor by homogeneous nucleation of a saturated stream of an organic vapour and coagulation growth. In a time scale large compared to the typical collision time the aerosol size distribution has reached a steady state. The steady state population balance equation taking into account size dependent Brownian coagulation and particle removal was solved numerically. It was found that for the conditions realized in the experimental system the asymptotic form of the size distribution function for large particle volumes can be approximated by a single function determined by two scaling groups formed of the process parameters. The relevant process parameters are the inflowing aerosol volume, the residence time and the reactor volume. In the size range between 70 nm and 250 nm coagulation is the only relevant mechanism controlling the distribution function. The experimental data are in good agreement with these theoretical findings. This section of the steady state size distribution could serve as a reference for the number size distribution of submicron particles. (C) 2012 Elsevier Ltd. All rights reserved.