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.