16 May 2022
16 May 2022
Status: this preprint is open for discussion.

Modelling approaches for atmospheric ion-dipole collisions: all-atom trajectory simulations and central field methods

Ivo Neefjes1, Roope Halonen2, Hanna Vehkamäki1, and Bernhard Reischl1 Ivo Neefjes et al.
  • 1Institute for Atmospheric and Earth System Research / Physics, Faculty of Science, University of Helsinki, P.O. Box 64, FI-00014, Finland
  • 2Center for Joint Quantum Studies and Department of Physics, School of Science, Tianjin University, 92 Weijin Road, Tianjin 300072, China

Abstract. Ion-dipole collisions can facilitate the formation of atmospheric aerosol particles, and play an important role in their detection in chemical ionization mass spectrometers. Conventionally, analytical models, or simple parametrizations, have been used to calculate rate coefficients of ion-dipole collisions in the gas phase. Such models, however, neglect the atomistic structure and charge distribution of the collision partners. To determine the accuracy and applicability of these approaches at atmospheric conditions, we calculated collision cross sections and rate coefficients from all-atom molecular dynamics collision trajectories, sampling the relevant range of impact parameters and relative velocities, and from a central field model using an effective attractive interaction fitted to the long-range potential of mean force between the collision partners. We considered collisions between various atmospherically relevant molecular ions and dipoles, as well as charged and neutral dipolar clusters. Based on the good agreement between collision cross sections and rate coefficients obtained from molecular dynamics trajectories and a generalized central field model, we conclude that the effective interactions between the collision partners are isotropic to a high degree, and the model is able to capture the relevant physico-chemical properties of the systems. In addition, when the potential of mean force is recalculated at the respective temperatures, the central field model exhibits the correct temperature dependence of the collision process. The classical parametrization by Su and Chesnavich [J. Chem. Phys., 76, 5183–5185, 1982], which combines a central field model with simplified trajectory simulations, is able to predict the collision rate coefficients and their temperature dependence quite well for molecular systems, but the agreement worsens for systems containing clusters. Based on our results, we propose the combination of potential of mean force calculation and central field model as a viable and elegant alternative to brute force sampling of individual collision trajectories over a large range of impact parameters and relative velocities.

Ivo Neefjes et al.

Status: open (extended)

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  • RC1: 'Comment on egusphere-2022-93', Kai Leonhard, 16 May 2022 reply

Ivo Neefjes et al.

Ivo Neefjes et al.


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Short summary
Collisions between ionic and dipolar molecules and clusters facilitate the formation of atmospheric aerosol particles, which affect global climate and air quality. We compared often used classical approaches for calculating ion-dipole collision rates with robust atomistic computer simulations. While classical approaches work for simple ions and dipoles only, our modelling approach can also efficiently calculate reasonable collision properties for more complex systems.