AAAR 34th Annual Conference
October 12 - October 16, 2015
Hyatt Regency
Minneapolis, Minnesota, USA
Abstract View
Enhanced Concentration and Charging of Ultrafine Particles
NATHAN KREISBERG, Steven Spielman, Gregory Lewis, Susanne Hering, Michael J. Lawler, James N. Smith, Peter H. McMurry, Aerosol Dynamics Inc.
Abstract Number: 561 Working Group: Instrumentation and Methods
Abstract 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 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 coupled to a parallel plate growth tube equipped with an ion scavenger and concentrator. It samples at 15 L/min and outputs 1.5L/min of concentrated, charge-enhanced aerosol. Nanometer sized particles are enlarged through condensation to form droplets that acquire electrical charge more efficiently. Once activated, a small electric field reduces multiple charging. A cool-walled moderator section removes excess water vapor to reduce the exiting dew point to around 14°C. Finally, droplets are concentrated by an array of aerodynamic focusing nozzles. Constructed only of stainless steel and PTFE, the system allows for aggressive cleaning options where build-up of organic compounds must be controlled. Heat and moisture are delivered by filtered, recirculating water to minimize the accumulation of water soluble compounds on the wetted surfaces.
We have demonstrated 10-fold concentration enhancements of particles as small as 10 nm, and a 30-50 fold enrichment in the charged fraction, as compared to bipolar charging. Size recovery is within 10% of the original mobility size when sampling ammonium sulfate aerosol. The potential for chemical artifacts is being investigated with a TD-CIMS (thermal desorption chemical ionization mass spectrometer). Measurements to date show little uptake of organic species from the vapor phase with this system.