According to the invention, particles of an aerosol are firstly charged in a unipolar manner in a diffusion charger (10). They are subsequently led through a diffusional precipitator (20) in which a part of the particles is precipitated. The diffusional precipitator current is measured and a value for the number concentration is evaluated from the current. A single diffusional precipitator may be used for this. According to preferred embodiment, a means for measuring the influence current and/or an aerosol electrometer are additionally present, by way of which one may measure a complete current. The latter additionally permits the evaluation of the average particle size. Additionally, one may determine an elementary carbon total quantity from the average particle size with the help of a measurement of the photoelectric charging carried out in parallel.