INERTIAL SEPARATION OF ULTRAFINE PARTICLES USING A CONDENSATIONAL GROWTH VIRTUAL IMPACTION SYSTEM/

A system for the separation of ultrafine particles (i.e., particles sm aller than 0.1 mu m) has been developed and evaluated. Ultrafine parti cles are first grown by means of supersaturation to a size that can be easily separated in a virtual impactor. Thus, inertial separation of ultrafine particles occurs without subjecting them to a high vacuum. T he condensational growth/virtual impaction system has been evaluated u sing monodisperse 0.05 and 0.1 mu m fluorescent PSL particles, as well as polydisperse ultrafine ammonium sulfate and potassium nitrate aero sols. The generated aerosols were first drawn over a pool of warm wate r (50 degrees C) where they became saturated. Subsequently, the satura ted aerosol was drawn through a cooling tube (8 degrees C) where parti cles grew due to supersaturation to sizes in the range 1.0-4.0 mu m. B y placing a virtual impactor with a theoretical 50% cutpoint of 1.4 mu m downstream of the condenser, ultrafine particles were separated fro m the majority (i.e., 90%) of the surrounding gas. The sampling Bow ra te of the virtual impactor was 8 L/min and its minor-to-total Bow rati o was 0.1. For these operating conditions, the particle collection eff iciency of the virtual impactor averaged to about 0.9 for particle con centrations in the range 7 x 10(4)-5 x 10(5) particles/cm(3). Particle losses through the system were found less than 5%. Increasing the par ticle concentration to levels in the range 10(6)-10(7) particles/cm(3) resulted in a decrease in the collection efficiency of the virtual im pactor to about 50-70%, presumably due to the smaller final droplet si ze to which the ultrafine particles grew for the available supersatura tion. (C) 1996 American Association for Aerosol Research.