A new type of instrument for the size classification of airborne particulate matter is described. This apparatusoperates on an aerodynamic principle and effects ordered size separation of both liquid and solid particles. Calculationsof the spatial separation as a function of size for spherical particles of unit density are in accord withpreliminary experimental data for diameters between 10 and 100 &mgr;. Application of this principle to aerosol size-frequencyanalyses is discussed briefly.