Abstract: An apparatus for classifying polydisperse aerosols includes aerosol and sheath gas conduits for conducting a sample aerosol and a sheath gas toward a merger area. At the merger area the sheath gas and about ten percent of the sample aerosol merge, then travel through a differential mobility analyzer (DMA) and along a tubular electrode of the DMA. Selected particles, i.e. particles having electrical mobilities within a narrow range, pass through a collection aperture of the electrode. The DMA output, an aerosol consisting of the selected particles, is provided to a condensation particle counter or other device for determining the aerosol concentration. The remainder of the sample aerosol is conducted away from the merger area along a bypass flow conduit. The bypass flow and an improved aerodynamic design provide for a slit at the merger area that is sufficiently narrow to minimize unwanted electric field penetration at the slit and DMA entrance.