10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Low-cost Sensing of Aerosol Active Surface Area by Direct Ultraviolet Photoionization
ROBERT NISHIDA, Tyler J. Johnson, Adam M Boies, John Saffell, Simone Hochgreb, University of Cambridge
Abstract Number: 1594 Working Group: Low-Cost and Portable Sensors
Abstract Direct ultraviolet (UV) photoemission yields electrical current measurements proportional to aerosol active surface area which is of interest for emissions and environmental monitoring. We have demonstrated a low-cost, proof-of-concept device for using UV photoionization and detection electronics to yield direct, real-time total active surface area measurements of some ultrafine particle materials (Nishida, Boies, & Hochgreb, 2018). However, the fundamental mechanisms of photoemission must be understood to accurately interpret measurements in the field.
The particle mobility diameter may be used to describe an active particle surface which interacts with gas molecules producing a drag force for both spheres and agglomerates. Photoionization measurements yield signals proportional to the active surface, however, for the same aerosol material, previous research has found a morphology dependency distinct from mobility diameter which disagrees with existing photocharging theory (Keller, Fierz, Siegmann, Siegmann, & Filippov, 2001; Zhou, You, Tan, & Zachariah, 2013). This work aims to isolate the effect of particle morphology in order to improve photoelectric charging theory for accurate interpretation of in-field measurements.
In this work, photoemission theory is evaluated by analysing photoelectric current measurements for a range of aerosol particle types, sizes, concentrations, and morphologies. Neutrally charged, sintered silver spheres and aggregates are classified with an aerodynamic aerosol classifier (AAC) and subsequently measured with a scanning mobility particle sizer (SMPS) and photoionization sensor in parallel. This configuration allows a direct comparison of aerosols of the same material and electrical mobility, but with different morphologies. Results confirm that total photoelectric yield of both silver aggregates and spheres is a linear function of active surface area, however, it is found that silver aggregates exhibit a higher photoelectric yield than spheres of the same mobility diameter in contrast with previous research. Photoemission measurements from raw and thermo-denuded flame soot confirm surface-dependent effects on photoemission yield.
Photoemission theory is evaluated for the applicability of its experimental constants, including a single empirical constant and a particle work function for each material to account for photoionization. Results suggest photoelectric charging theory should include a morphology dependency. The method of detecting particle size and concentration parameters is evaluated in terms of experimental uncertainty and applicability in a commercial low-cost, environmental monitoring device for ultrafines.
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