Enhanced Concentration and Charging of Ultrafine Particles

Description

This work aims to improve the electrical charging, and hence the efficiency of the mobility size selection and particle collection process. Even with a unipolar charger, the fraction of sub-10nm particles that carry an electrical charge is small (a few percent), and this fraction decreases rapidly with decreasing particle diameter. Our approach is a condensationally enhanced charging and evaporation method for increased efficiency of particle charging. In contrast to other condensation approaches, our method greatly reduces the time for the entire condensation-charging-evaporation process to a few tens of milliseconds, thereby minimizing the opportunity for chemical artifacts. Our system consists of a bipolar ion source and a parallel plate growth tube equipped with an ion scavenger for sampling at 15 L/min to deliver 1.5 L/min of concentrated, charge enriched aerosol. Constructed only of stainless steel and PTFE allows for aggressive cleaning options where build-up of organic compounds must be controlled. Heat and moisture are delivered by re-circulating water filtered to prevent the accumulation of water soluble compounds on the wetted surfaces. The activated particles form 2- to 3-um droplets for enhanced charge attachment. Immediately after activation the flow is subjected to a small electric field to reduce the number of multiply charged droplets. The droplets enter the concentrator where they are aerodynamically focused by eight round orifices operating in parallel. We have demonstrated 7x concentration enhancements of particles as small as 15 nm, and a 30-50 fold enrichment in the charged fraction, as compared to bipolar charging.