Abstract
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 noninteracting 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 studied the statistics of collisions of sulfuric acid molecules in a vacuum using atomistic molecular dynamics simulations. We found that the effective collision cross section of the H2SO4 molecule, as described by an optimized potentials for liquid simulation (OPLS). 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 of 2.2 at 300 K compared with 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, based on the attractive part of the atomistic intermolecular potential of mean force.
Highlights
New particle formation from condensable vapours provides an important contribution to the composition of aerosols in the atmosphere which affects air quality as well as the Earth’s climate
We studied the statistics of collisions of sulfuric acid molecules in a vacuum using molecular dynamics simulations and compared our results against simple theoretical models
We found that the effective collision cross section of two H2SO4 molecules, as described by the optimized potentials for liquid simulation (OPLS) force field, is significantly larger than the hard-sphere diameter assigned to the molecule based on the liquid density of sulfuric acid
Summary
New particle formation from condensable vapours provides an important contribution to the composition of aerosols in the atmosphere which affects air quality as well as the Earth’s climate. It is well known that acid–base clusters, in particular clusters containing sulfuric acid and ammonia, or amines, are very relevant in nucleation and growth of particles that can serve as cloud condensation nuclei (Almeida et al, 2013) Such molecules are not necessarily spherical and, despite being charge neutral, exhibit long-range attraction due to interactions between permanent dipoles, permanent and induced dipoles, or induced dipoles (Israelachvili, 2011). Systematic discrepancies have been found between experimental particle formation rates and values predicted from kinetic modelling and cluster dynamics simulations, where hard-sphere collisions are assumed. We use atomistic molecular dynamics simulations to study the statistics of collisions between sulfuric acid molecules in a vacuum, determine the collision rate coefficient, and calculate the enhancement factor over kinetic gas theory.
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