Quantifying the Three Regimes of Bipolar Charging to Improve Accuracy of SMPS Measurements

ROBERT T. NISHIDA, Tyler J. Johnson, Jason S. Olfert, University of Alberta

     Abstract Number: 518
     Working Group: Instrumentation and Methods

Abstract
Many aerosol instruments utilize bipolar charge conditioners (neutralizers), including the widely-used scanning mobility particle sizer (SMPS). The bipolar charge distribution from a neutralizer must be known to calculate size distributions from SMPS scans. Therefore, a reliable ‘steady-state’ charge distribution is desired to improve accuracy of measurements. In this work, we show that although a ‘steady-state’ charge develops within a common Kr-85 neutralizer (TSI 3077A), gaseous ions are readily convected into tubing downstream of the neutralizer, causing significant deviation from steady-state within milliseconds. The ‘downstream ions’ are predominantly positive since negative ions are more readily lost to tubing walls due to their higher diffusivity. By example, we charge monodisperse spheres (225 nm) to an asymmetric, steady-state with mean charge per particle of -1 immediately at the neutralizer outlet. By varying downstream tubing lengths and flow rates, we show the mean charge shifts more positive in polarity to -0.4 as residence time with ‘downstream ions’ increases to ~1 s (e.g. 33 cm tube at 0.3 L/min), where it aligns with the classical charge distribution of Wiedensohler (1988). We also perform advanced numerical modelling of charging, convection and diffusion of particles and ions and show good quantitative agreement with experimental data. We thereby quantify three distinct regimes of bipolar charging, namely: the 1) charging regime (i.e. as charge develops from its initial to steady-state), 2) steady-state regime, and 3) dis-charging regime (i.e. effect of ‘downstream ions’). These three regimes of bipolar charging have wide implications for designing instrumentation, interpreting measurements and validating charging models. Finally, we present the design of a new charge conditioner which allows operation in any of the three regimes enabling new charge distributions to be achieved. The charge conditioner can be reliably operated in a steady-state regime to improve accuracy in aerosol instruments such as the SMPS.