Development of a Two-Stage Virtual Impactor System for High Concentration Enrichment of Ultrafine, PM2.5, and Coarse Particulate Matter

Abstract

A two-stage particle concentration enrichment system was developed to provide highly concentrated particles at low flow rates, for applications in areas such as toxicity studies of particulate matter (PM) as well as for increasing the signal-to-noise ratio in online particle sampling instruments. The current system is an extension of the Versatile Aerosol Concentration Enrichment System (VACES) developed at University of Southern California and operates by placing a second-stage miniature virtual impactor (VI) downstream of the VACES. Particles are sequentially enriched through each stage. Laboratory evaluations were conducted using various types of polydisperse particles to simulate typical ambient PM components as well as monodisperse polystyrene latex (PSL) particles. The system's configuration was tested by adjusting the intermediate flow rate, which is the intake flow of the second-stage VI (or minor flow of the first-stage VIs), for which 15 L/min was determined to be optimal in terms of maximizing the overall concentration enrichment. Particle size distributions before and after concentration enrichment were compared using a scanning mobility particle sizer. Overall, our results indicate that the sampled particles were relative consistently enriched by factors of 100–120 (i.e., a concentration enrichment efficiency 75–85% of the ideal value) based on both PM mass and number concentrations, and along with similar physical properties of the size distribution (i.e., mode, median). Continuous and time-integrated field tests using urban ambient PM also showed consistent enrichment factors (by roughly 100–120 times) for number and mass concentrations, black carbon, and PM-bound polycyclic aromatic hydrocarbons. Copyright 2013 American Association for Aerosol Research