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Discussion papers
https://doi.org/10.5194/acp-2019-400
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-400
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 11 Jun 2019

Submitted as: research article | 11 Jun 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Rate enhancement in collisions of sulfuric acid molecules due to long-range intermolecular forces

Roope Halonen1, Evgeni Zapadinsky1, Theo Kurtén2, Hanna Vehkamäki1, and Bernhard Reischl1 Roope Halonen et al.
  • 1Institute for Atmospheric and Earth System Research/Physics, Faculty of Science, University of Helsinki, P.O. Box 64, 00014, Finland
  • 2Institute for Atmospheric and Earth System Research/Chemistry, Faculty of Science, University of Helsinki, P.O. Box 55, 00014, Finland

Abstract. Collisions of molecules and clusters play a key role in determining the rate of atmospheric new particle formation and growth. Traditionally the statistics of these collisions are taken from kinetic gas theory assuming spherical non-interacting particles, which may significantly underestimate the collision coefficients for most atmospherically relevant molecules. Such systematic errors in predicted new particle formation rates will also affect large-scale climate models. We have studied the statistics of collisions of sulfuric acid molecules in vacuum by atomistic molecular dynamics simulations. We have found that the effective collision cross section of the H2SO4 molecule, as described by an OPLS-All Atom force field, is significantly larger than the hard-sphere diameter assigned to the molecule based on the liquid density of sulfuric acid. As a consequence, the actual collision coefficient is enhanced by a factor 2.2, compared to kinetic gas theory. This enhancement factor obtained from atomistic simulation is consistent with the discrepancy observed between experimental formation rates of clusters containing sulfuric acid and calculated formation rates using hard sphere kinetics. We find reasonable agreement with an enhancement factor calculated from the Langevin model of capture, fitted to the attractive part of the atomistic intermolecular potential of mean force.

Roope Halonen et al.
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Short summary
The rate of collisions between molecules or clusters is used to determine particle formation in the atmosphere. The basic approach is to treat the colliding particles as non-interacting hard spheres. By using atomistic simulations with a realistic force field and theoretical approaches, we have shown that the actual collision rate of two sulfuric acid molecules is more than twice higher as for hard spheres. The results of this study will improve models of atmospheric particle growth.
The rate of collisions between molecules or clusters is used to determine particle formation in...
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