Development and evaluation of a prototype ultrafine particle concentrator

This paper presents the development and experimental characterization of a prototype ultrafine particle concentrator. In this system, ultrafine partic les pass over a pool of warm water where they become saturated with vapor, and subsequently drawn through a condenser, kept at a lower temperature, th at allows the ultrafine particles to grow to super-micrometer size by vapor condensation on their surface. In order to increase particle concentration , the grown particles are drawn through a virtual impactor with an approxim ate 50% cutpoint at 1.5 mu m. The concentrated particles from the minor flo w of the virtual impactor finally pass through a diffusion dryer that remov es the excess water on the ultrafine particles and returns them back to the ir original size and relative humidity. In its optimum configuration, the u ltrafine concentrator operates at a sampling flow rare of 106.5 or 1101min( -1) and concentrates the ultrafine particles to 3.5 or 71min(-1) by an enri chment factor of approximately 15 and 25.5, respectively. Our experimental results identified saturation of the ultrafine aerosols at 35 degrees C and cooling to 25 degrees C as the optimum temperatures for operation of the u ltrafine particle concentrator. Lower temperatures either do not concentrat e, or concentrate less efficiently the ultrafine particles. Increasing the saturation temperature to 40 degrees C and cooling to 31 degrees C does not improve the concentration enrichment achieved by the optimum configuration . Our results also indicated that the concentration enrichment does not dep end on the chemical composition of the ultrafine aerosol. Hygroscopic ammon ium sulfate, volatile ammonium nitrate, hydrophobic polystyrene latex and a ctual "real-life" indoor air ultrafine particles were all concentrated by p ractically the same factor. More importantly, the experimental results show that particle concentration occurs without any coagulation, which would ha ve distorted the size distribution of the original ultrafine aerosols. (C) 1999 Elsevier Science Ltd. All rights reserved.