Molecular Dynamics Simulations of the Microphysics of Liquid Water in Nano-aerosol Droplets

XIAOHAN LI, Ian Bourg, Princeton University

     Abstract Number: 401
     Working Group: Aerosol Physical Chemistry and Microphysics

Abstract
Ultrafine aerosol particles with sizes smaller than 50 nm have been shown in recent studies to serve as a large source of cloud condensation nuclei (CCN) that can promote additional cloud droplet formation under supersaturation condition. Investigation of the microphysics of liquid water in such droplets remains arduous, particularly in the sub-10 nm particle size range, due to experimental and theoretical challenges associated with the complexity of aerosol components and the small length scales of interest (e.g., difficulty of precisely sampling the liquid-air interface, questionable validity of mean-field theoretical representations). Here we carried out molecular dynamics (MD) simulations of aerosol particles with diameters between 1 and 10 nm and characterized atomistic-level structure and water dynamics in well-mixed and phase-separated system with different particle sizes, NaCl salinities, and organic surface excess values as a function of distance from the time-averaged Gibbs dividing interface or instantaneous water-air interface. We use a sphericity factor developed in this study to quantitatively represent the phase-mixing state of nanodroplets and characterize its strong dependence on droplet size. Our study also evidences and quantifies an ion concentration enhancement in ultra fine aerosols, which suggests a size-dependence to salt nucleation kinetics in ultrafine sea salt aerosols, and provides detailed characterization of the influence of droplet size on surface tension and on water self-diffusivity near the interface. Besides, analysis of water evaporation free energy and water activity demonstrates the validity of Kelvin equation and Köhler theory at droplet sizes larger than 4 nm under moderate salinities and organic loadings and the need for further extension to account for ion concentration enhancement of sub-10 nm aerosols, droplet-size-dependent phase separation effects, and the sharp variation of surface tension of sub-4 nm droplets. Finally, we use the surface coating factor defined in this study to categorize and reconcile the water accommodation coefficient results from MD simulations and experiment results, and resolve the size dependence and its relationship to chain-structured organic coatings.