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Comparison of the Predictions of Langevin Dynamics-based Diffusion Charging Collision Kernel Models with Canonical Experiments
Li Li, Harjindar Singh Chahl, RANGANATHAN GOPALAKRISHNAN, The University of Memphis
Abstract Number: 184
Working Group: Aerosol Physics
Abstract
Based on the prior work of Chahl and Gopalakrishnan (Aerosol Sci. Tech. 53(8): 933-957) to infer particle-ion collision time distributions using a Langevin Dynamics (LD) approach, we develop a model for the non-dimensional diffusion charging collision kernel βi or H that is applicable for 0≤ΨE≤60,0≤ΨI/ΨE ≤1,KnD≤2000 (described in J. Aerosol Sci. 140: 105481). The developed model for βi for attractive Coulomb and image potential interactions, along with the model for βi for repulsive Coulomb and image potential interactions from Gopalakrishnan et al. (J. Aerosol Sci. 64: 60-80), is tested against published diffusion charging experimental data. Current state of the art charging models, Fuchs (1963) and Wiedensohler (1988) regression for bipolar charging, are also evaluated and discussed. Comparisons reveal that the LD-based model accurately describes unipolar fractions for 10 – 100 nm particles measured in air (Adachi et al., 1985), nitrogen and argon but not in helium (Adachi et al., 1987). Fuchs model and the LD-based model yield similar predictions in the experimental conditions considered, except in helium. In the case of bipolar charging, the LD-based model captures the experimental trends quantitatively (within ±20%) across the entire size range of 4 – 40 nm producing superior agreement than Wiedensohler’s regression. The latter systematically underpredicts charge fraction below ~20 nm in air (by up to 40%) for the data presented in Adachi et al. (1985). Comparison with the data of Gopalakrishnan et al. (2015), obtained in UHP air along with measurements of the entire ion mass-mobility distribution, shows excellent agreement with the predictions of the LD-based model. This demonstrates the capability to accommodate arbitrary ion populations in any background gas, when such data is available. Wiedensohler’s regression, derived for bipolar charging in air using average ion mass-mobility, also describes the data reasonably well in the conditions examined. However, both models failed to capture the fraction of singly and doubly charged particles in carbon dioxide warranting further investigation.
We thank The University of Memphis High Performance Computing Cluster for providing computational resources to carry out this research. Partial support for this work was provided by US National Science Foundation (NSF) PHY Grant Award Number 1903432 under the Directorate of Mathematical & Physical Sciences. Published as J. Aerosol Sci. 140: 105481.