A system for characterising particles in the form of an aerosol in an ambient gas, comprising: - a sampler capable of sampling the ambient gas likely to contain particles in aerosol form; - a device for characterizing said particles by laser-induced plasma spectrometry, the device comprising: • one chamber, • means for pumping the gas present in the chamber in order to create a vacuum in this chamber, • a system for generating, from the gas coming from the sampler, a jet of said particles in the chamber, • a laser capable of emitting a laser beam in the form of pulses, with which is associated an optical device arranged to focus said laser beam in the chamber, perpendicularly to a direction of propagation of the jet of particles, in order to create, in a focal volume, a plasma by the interaction between the laser beam and the particles (N NP) of the jet, said plasma emitting other particles, characteristics of the interaction between the laser beam and said particles of the jet, • at least one detection device (DD, DD') comprising a means for collecting the particles emitted by the plasma and a means (MAS, MAS') for carrying out a spectrometric analysis of these particles; characterised in that the system comprises: - at least one means (MD, MD') known as dilution to reduce the concentration of the particles in aerosol form in the ambient gas sampled by the sampler, so that the focal volume comprises at most only one individual particle. - a sampler capable of sampling the ambient gas likely to contain particles in aerosol form; - a device for characterizing said particles by laser-induced plasma spectrometry, the device comprising: • one chamber, • means for pumping the gas present in the chamber in order to create a vacuum in this chamber, • a system for generating, from the gas coming from the sampler, a jet of said particles in the chamber, • a laser capable of emitting a laser beam in the form of pulses, with which is associated an optical device arranged to focus said laser beam in the chamber, perpendicularly to a direction of propagation of the jet of particles, in order to create, in a focal volume, a plasma by the interaction between the laser beam and the particles (N NP) of the jet, said plasma emitting other particles, characteristics of the interaction between the laser beam and said particles of the jet, • at least one detection device (DD, DD') comprising a means for collecting the particles emitted by the plasma and a means (MAS, MAS') for carrying out a spectrometric analysis of these particles; characterised in that the system comprises: • one chamber, • means for pumping the gas present in the chamber in order to create a vacuum in this chamber, • a system for generating, from the gas coming from the sampler, a jet of said particles in the chamber, • a laser capable of emitting a laser beam in the form of pulses, with which is associated an optical device arranged to focus said laser beam in the chamber, perpendicularly to a direction of propagation of the jet of particles, in order to create, in a focal volume, a plasma by the interaction between the laser beam and the particles (N NP) of the jet, said plasma emitting other particles, characteristics of the interaction between the laser beam and said particles of the jet, • at least one detection device (DD, DD') comprising a means for collecting the particles emitted by the plasma and a means (MAS, MAS') for carrying out a spectrometric analysis of these particles; characterised in that the system comprises: - at least one means (MD, MD') known as dilution to reduce the concentration of the particles in aerosol form in the ambient gas sampled by the sampler, so that the focal volume comprises at most only one individual particle.