Molecular dynamics simulations of liquid water microphysics in nano-aerosol droplets

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 thermodynamic representations). To gain insight into the thermodynamic and kinetic properties of liquid water in nano-aerosol droplets, we carried out molecular dynamics (MD) simulations of aerosol particles composed of liquid water, sodium chloride, and pimelic acid with diameters between 1 and 10 nm. Specifically, we characterized atomistic-level structure and water dynamics in well-mixed and phase-separated system with different sizes, NaCl salinities, and surface organic excess as a function of distance from the time-averaged or instantaneous water-air interface. We also examined the validity of Köhler theory on the energetics of water uptake as a function of droplet size and salinity. Our study shows that the instantaneous interface scheme provides more detailed information on the droplet-air interface and reconciles experimental observation and MD simulation results on water dynamics at interface. In addition, we find that Köhler theory accurately predicts water uptake under moderate salinities and organic loadings but requires further extension to account for droplet-size-dependent phase separation effects in systems with high salinity and organic loading.