[en] The paper gives a technical description of an aerosol monitor for determining low concentrations of artificial radioactive aerosols in the atmosphere. The principle of measurement consists of determining the natural beta aerosol radioactivity concentration and subtracting it from the gross beta radioactivity concentration, based on a simultaneous alpha and beta measurement. The performance of the monitor is demonstrated with laboratory experiments and experiences related to the Chernobyl accident. The paper also describes further developments in the field using modern nuclear detectors