Abstract: An apparatus which produces an aerosol having a narrow size distribution. e apparatus includes a supply means for providing a primary aerosol stream having a wide size distribution. Concentration means are positioned to receive the stream and this concentration means includes means for reducing volume flow rate without disregarding a proportionate fraction of the particles to provide a reduced flow rate stream. Venturi means are provided to monitor the stream and transfer the stream. Impactor means are provided for receiving the stream and introducing a core of clean air into the center of the stream. Means are provided for adjusting the relative velocities of the stream and the core to exclude particles having an inertia below a predetermined size, thereby eliminating smaller particles. Outlet impactor means are positioned to receive the stream and the core, for removing particles larger than a predetermined size.