The influence of the particle concentration on the photoelectric charging process is studied in experiments and in numerical calculations. For this purpose the mean particle charge and mobility distribution of the particles after the exposure to UV irradiation were determined experimentally as a function of the particle concentration. Model calculations were performed to account for the experimental results and to show the effect of the lifetime of ions on the resulting charge distribution. The results show a reasonable agreement. The study demonstrates that the re-attachment of negative ions produced by released photoelectrons plays an important role as additional charging mechanism for the final charge distribution on the particles. A reduction of the charging efficiency is already observed when the particle concentration is in the range of 10(5)-10(6) cm(-3) and the residence time of the ions in the aerosol exceeds a few tens of milliseconds.