AAAR 34th Annual Conference
October 12 - October 16, 2015
Hyatt Regency
Minneapolis, Minnesota, USA
Abstract View
Direct Ultraviolet Photoionization and Charge Recombination of Aerosol Nanoparticles
Robert Nishida, Simone Hochgreb, ADAM M BOIES, University of Cambridge
Abstract Number: 737 Working Group: Instrumentation and Methods
Abstract Aerosol nanoparticles are more efficiently charged by direct ultraviolet photoionization than with other charging mechanisms such as corona discharge. The charged nanoparticles can more easily be manipulated for detection, capture, or control in many aspects of aerosol and materials science. For example, newly generated particles can be charged to prevent/promote agglomeration or to control the formation of nanostructures.
Photoionization behaviour is not well characterized for different particle types or larger sizes into the accumulation mode. Existing models for photoionization yield include empirical ‘constants’ which vary for particle size and type. Mechanisms of particle charge recombination with gaseous ions are well understood, but models often neglect particle and ion wall losses and 3D effects. The aim of this work is to understand the fundamental aerosol charging and charge recombination mechanisms from the free molecular to continuum regimes.
Equations capturing both photoionization and recombination of ions/particles are modelled in 3D computational fluid dynamics for the first time. The model incorporates ion/particle advection/diffusion, wall losses, electric field transport. Upwards of 50 simultaneous species transport equations are solved to allow the resolution of local charge distribution and average charges per particle for multiple charge states.
The model is compared with experimental results for charge efficiency for a range of particle types (e.g. soot, silver), diameters, and concentrations showing good quantitative agreement. The effects of adsorbed volatiles on photoionization are explored by incorporating a catalytic stripper for a range of soot types. Results demonstrate the limitations of fundamental photoionization theory and previous continuum scale models.