25 Jan 2023
25 Jan 2023
Status: this preprint is open for discussion.

Collision-sticking rates of acid–base clusters in the gas phase determined from atomistic simulation and a novel analytical interacting hard-sphere model

Huan Yang1, Ivo Neefjes1, Valtteri Tikkanen1, Jakub Kubečka2, Theo Kurtén3, Hanna Vehkamäki1, and Bernhard Reischl1 Huan Yang et al.
  • 1Institute for Atmospheric and Earth System Research/Physics, Faculty of Science, University of Helsinki, P.O. Box 64, FI-00014, Finland
  • 2Department of Chemistry, iClimate, Aarhus University, Langelandsgade 140, 8000 Aarhus C, Denmark
  • 3Institute for Atmospheric and Earth System Research/Chemistry, Faculty of Science, University of Helsinki, P.O. Box 55, FI-00014, Finland

Abstract. Kinetics of collision-sticking processes between vapor molecules and clusters of low volatile compounds govern the initial steps of atmospheric new particle formation. Conventional non-interacting hard-sphere models underestimate the collision rate by neglecting long-range attractive forces, and the commonly adopted assumption that every collision leads to the formation of a stable cluster (unit mass accommodation coefficient) is questionable for small clusters, especially at elevated temperatures. Here, we present a generally applicable analytical interacting hard-sphere model for evaluating collision rates between molecules and clusters, accounting for long-range attractive forces. In the model, the collision cross section is calculated based on an effective molecule-cluster potential, derived using Hamaker's approach. Applied to collisions of sulfuric acid or dimethylamine with neutral bisulphate-dimethylammonium clusters composed of 1–32 dimers, our new model predicts collision rates 2–3 times higher than the non-interacting model for small clusters, while decaying asymptotically to the non-interacting limit as cluster size increases, in excellent agreement with a collision rate theory-atomistic molecular dynamics simulation approach. Additionally, we calculated sticking rates and mass accommodation coefficients (MAC) using atomistic molecular dynamics collision simulations. For sulfuric acid, unit MAC is observed for collisions with all cluster sizes at temperatures between 200 K and 400 K. For dimethylamine, we find that MACs decrease with increasing temperature and decreasing cluster size. At low temperatures, the unit MAC assumption is generally valid, but at elevated temperatures MACs can drop below 0.2 for small clusters.

Huan Yang et al.

Status: open (until 08 Mar 2023)

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Huan Yang et al.


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Short summary
We present a new analytical model for collision rates between molecules and clusters of arbitrary sizes, that accounts for long-range interactions. The model is verified against atomistic simulations of typical acid-base clusters participating in atmospheric new particle formation. Results show that accounting for long-range interactions leads to 2–3 times higher collision rates for small clusters, indicating the necessity of including such forces in atmospheric new particle formation modelling.