On the three charge regimes of bipolar charge conditioners

Abstract

Many aerosol instruments utilize bipolar charge conditioners (neutralizers), including scanning mobility particle sizers (SMPS). The charge distribution from a bipolar charge conditioner must be known to calculate size distributions from SMPS scans. Therefore, a reproducible and known “steady-state” charge distribution is desired to improve measurement accuracy. In this work, we show that although a steady-state charge develops within a common Kr-85 bipolar charge conditioner (TSI 3077A), gaseous ions are readily convected into tubing downstream of the charge conditioner, causing significant deviation from steady-state in less than a second. The “downstream ions” are predominantly positive since negative ions are more readily lost to tubing walls due to their higher diffusivity. Others have previously studied this potential effect, but there is disagreement among them. This study resolves the disagreements, and to the authors’ knowledge, we are the first to: quantify the surprising significance of this effect (mean charge at 239 nm changes by up to a factor of 4); show it occurs rapidly (milliseconds); and demonstrate that a true steady-state distribution is more asymmetric than classical theory (mean charge at 239 nm is >3⁢x higher in magnitude). Interaction time of the particles with the remaining free ions downstream of the charge conditioner is shown to be the main consideration for this effect, as demonstrated by varying flow rate, tube length and tube diameter and achieving similar charging results as a function of this time. We perform advanced numerical modeling of charging, convection and diffusion of particles and ions, and show good quantitative agreement with experimental data. These results are further supported by similar charging results measured from a bipolar charge conditioner that has a different internal geometry and ion source than the 3077A charger. To increase consideration of this critical charging effect and enhance understanding of bipolar charging in general, 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) discharging 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 with the aim of improving accuracy in aerosol instruments such as the SMPS