A Virtual Impactor with Clean Air Core for the Generation of Aerosols with Narrow Size Distributions

Abstract

An improved virtual impactor with interchangeable nozzles was designed and fabricated. One impactor nozzle, of 1.98-mm diameter, was calibrated with monodisperse polystyrene latex spheres. The results show that the separation efficiency of this impactor can be predicted on a semiempirical basis. With a clean air core in the center of an aerosol flow, this impactor has a better separation characteristic than the conventional virtual impactor. Aerosols with narrow size distributions were obtained by passing Arizona road dust particles through this impactor. The mass median diameter (MMD) of these aerosols varies from 10.0 to 3.3 μm with the geometric standard deviation (GSD) ranging from 1.16 to 1.48. The mass concentrations (Ma) of these aerosols ranges over several orders of magnitude from 0.03 to 13 mg/m3. Aerosols with the MMD values ranging from 2.0 to 0.98 μm were obtained with this virtual impactor followed by a real impactor. The GSD values of these aerosols vary from 1.44 to 1.28 and the M a range from 0.07 to 3.57 mg/m3. The effects of varying total flow rate (Q t), clean air flow ratio (q), and minor flow ratio (r) were investigated. As Q i decreases, the MMD becomes larger and the GSD is decreased. With increasing q, the distribution curve becomes narrower and the Ma decreases. In addition, the fine particle contamination increases as r increases.