Optical aerosol measurement systems can be classified into single-particle counting techniques and ensemble techniques. In single-particle counting instruments, such as optical aerosol spectrometers, each particle is analyzed individually. This allows for the accurate measurement of the number concentration and the size distribution of the measured aerosol. Current aerosol spectrometers suffer from a limited concentration range due to coincidence errors. In contrast, ensemble techniques, such as photometers, are suitable for high-concentration measurements. One major drawback of such instruments is that they only provide information about one aerosol property, which is typically the mass concentration. A new ensemble technique for the simultaneous measurement of two aerosol properties, namely particle size and concentration, was developed and experimentally verified. The results show that the median particle size (D₅₀) of a monodisperse aerosol can be determined with a deviation of about 10 %, and the concentration with a deviation of about 30 %. With individual instrument calibration, the accuracy can be further improved to approximately 5 % with respect to particle size and 10 % with respect to concentration. The method may be either implemented as a standalone measurement device or, as an add-on for e. g. optical particle spectrometers in the form of a high-concentration mode.