Title: Space charge and ion transport in aerosol neutralization: Toward a concentration-dependent alternative to the $N_it$ product Show Chinese title

Title: 空间电荷和气溶胶中性化中的离子传输：一种浓度依赖的$N_it$产品替代方案

Abstract: In this study, we quantify how charged particle concentration affects the neutralization rate of aerosol particles, focusing on the role of ion dynamics shaped by internal electric fields arising from net space charge. Conventional neutralizer performance is typically evaluated using the $N_it$ product, which assumes quasi-neutral conditions and neglects electric fields from small charge imbalances. We demonstrate that internal electric fields become increasingly important at high aerosol concentrations and significantly influence neutralization dynamics. We develop a coupled ion--aerosol transport model in a two-dimensional axisymmetric geometry that includes ion generation, convection, diffusion, recombination, attachment to aerosols, and wall loss, with self-consistent electric fields obtained from the Poisson equation. Results show that even small net charges generate electric fields that enhance ion drift and accelerate neutralization, effects not captured by traditional $N_it$-based approaches. Using a neutralization time metric, we find that neutralization becomes slower with increasing aerosol number concentration $N_p$, higher initial particle charge $q_0$, and smaller particle diameter $d_p$ when space charge is absent. When space charge is included, the influence of $q_0$ and $d_p$ diminishes, while $N_p$ becomes the dominant factor governing neutralization behavior. Accordingly, we propose a concentration-dependent analytical expression for mean charge relaxation that captures coupled ion--aerosol transport and space charge effects. The modeling framework presented here is applicable to laboratory instruments, industrial processes, and atmospheric environments where electrostatic interactions govern aerosol behavior. Show Chinese abstract

Abstract: 在本研究中，我们量化了带电粒子浓度如何影响气溶胶粒子的中和速率，重点研究由净空间电荷产生的内部电场所塑造的离子动力学作用。 常规中和器性能通常使用$N_it$产品进行评估，该方法假设准中性条件并忽略了小电荷不平衡产生的电场。 我们证明，在高气溶胶浓度下，内部电场变得越来越重要，并显著影响中和动力学。 我们开发了一个二维轴对称几何中的耦合离子-气溶胶传输模型，包括离子生成、对流、扩散、复合、附着到气溶胶以及壁面损失，自洽电场通过泊松方程获得。 结果表明，即使小的净电荷也会产生增强离子漂移并加速中和的电场，这些效应未被传统的$N_it$基础方法捕捉到。 使用中和时间指标，我们发现当没有空间电荷时，中和速度随着气溶胶数浓度$N_p$的增加、初始粒子电荷$q_0$的提高以及粒子直径$d_p$的减小而变慢。 当考虑空间电荷时，$q_0$和$d_p$的影响减弱，而$N_p$成为决定中和行为的主要因素。 因此，我们提出了一种与浓度相关的平均电荷弛豫的解析表达式，该表达式能够捕捉离子-气溶胶传输和空间电荷效应的耦合。 本文提出的建模框架适用于实验室仪器、工业过程和大气环境，其中静电相互作用支配气溶胶行为。