A measuring system with a measuring system inlet and a measuring system outlet for investigating concentrated, larger aerosol particles of an aerosol in the gas phase, comprising a multi-stage aerosol particle concentrator for separating fine particles and a portion of the gas phase of the aerosol , as well as a measuring chamber for analysing the larger aerosol particles , wherein: i) the measuring chamber comprises at least one measuring instrument for the qualitative and/or quantitative determination of the aerosol particles , in particular in real time, and ii) the aerosol particle concentrator comprises a first (4-1) and at least one last (4-x) virtual impactor stage each with a sample inlet , side outlet and sample outlet , wherein the virtual impactor stages separate a larger portion of the aerosol with fine particles and concentrate the larger aerosol particles in the smaller portion of the aerosol , wherein the aerosol particle concentrator - comprises an aerosol suction pump with an aerosol inlet and aerosol outlet for generating a negative pressure in the virtual impactor stages , wherein the side outlet of the first virtual impactor stage (4-1) is connected to the aerosol inlet of the aerosol suction pump , characterized in that the aerosol particle concentrator comprises - a circulating flow channel in which a portion of the separated aerosol with fine particles is returned in the circulating flow from the aerosol outlet to the aerosol inlet of the aerosol suction pump , wherein the side outlet of the last (4-x) virtual impactor stage is connected to the circulating flow channel . i) the measuring chamber comprises at least one measuring instrument for the qualitative and/or quantitative determination of the aerosol particles , in particular in real time, and ii) the aerosol particle concentrator comprises a first (4-1) and at least one last (4-x) virtual impactor stage each with a sample inlet , side outlet and sample outlet , wherein the virtual impactor stages separate a larger portion of the aerosol with fine particles and concentrate the larger aerosol particles in the smaller portion of the aerosol , wherein the aerosol particle concentrator - comprises an aerosol suction pump with an aerosol inlet and aerosol outlet for generating a negative pressure in the virtual impactor stages , wherein the side outlet of the first virtual impactor stage (4-1) is connected to the aerosol inlet of the aerosol suction pump , characterized in that the aerosol particle concentrator comprises - a circulating flow channel in which a portion of the separated aerosol with fine particles is returned in the circulating flow from the aerosol outlet to the aerosol inlet of the aerosol suction pump , wherein the side outlet of the last (4-x) virtual impactor stage is connected to the circulating flow channel . - comprises an aerosol suction pump with an aerosol inlet and aerosol outlet for generating a negative pressure in the virtual impactor stages , wherein the side outlet of the first virtual impactor stage (4-1) is connected to the aerosol inlet of the aerosol suction pump , characterized in that the aerosol particle concentrator comprises - a circulating flow channel in which a portion of the separated aerosol with fine particles is returned in the circulating flow from the aerosol outlet to the aerosol inlet of the aerosol suction pump , wherein the side outlet of the last (4-x) virtual impactor stage is connected to the circulating flow channel . <EOS>