Microphysics of liquid water in sub-10 nm ultrafine aerosol particles

. 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 conditions. Knowledge of the microphysics of liquid water in these droplets remains limited, 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 5 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 loadings as a function of distance from the time-averaged Gibbs dividing interface or instantaneous water-air interface. We define a sphericity factor ( ϕ ) that can shed light on the phase-mixing state of 10 nanodroplets, and we reveal an unexpected dependence of mixing state on droplet size. Our results also evidence an ion concentration enhancement in ultrafine aerosols, which should modulate salt nucleation kinetics in ultrafine sea salt aerosols, and provide detailed characterization of the influence of droplet size on surface tension and on water self-diffusivity near the interface. Analysis of water evaporation free energy and water activity demonstrates the validity of the Kelvin equation and Köhler theory at droplet sizes larger than 4 nm under moderate salinities 15 and organic loadings and the need for further extension to account for ion concentration enhancement in sub-10 nm aerosols, droplet-size-dependent phase separation effects, and a sharp decrease in the cohesiveness of liquid water in sub-4 nm droplets. Finally, we show that an idealized fractional surface coating factor ( f s ) can be used to categorize and reconcile water accommodation coefficients ( α ∗ ) observed in MD simulations and experimental results in the presence of organic coatings, and we resolve the droplet-size dependence of α ∗ from liquid water. that have the potential to help resolve the challenges identified in previous studies. First, we use two methods 80 of interface characterization – the time-averaged Gibbs dividing interface (Lau et al., 2015) and the instantaneous interface (Willard and Chandler, 2010) – to compare different perspectives on the properties of the interface. Second, we characterize a broad range of structural, dynamic, and energetic properties to help reveal links between these properties. We re-examine the validity of the Kelvin equation and Köhler theory and, more broadly, of treating nano-droplet water as a bulk aqueous phase. We find that the instantaneous interface characterization enables a novel characterization of the droplet phase state and helps resolve apparent contradictions between experimental observation and MD simulation results on interfacial water dynamics. We also find that the Kelvin equation and Köhler theory accurately describe water microphysics under moderate salinity and organic loadings in nanometer-scale droplets. However, these theories underestimate water activity at droplet diameters below 4 nm and cannot account for the droplet size effect on ion concentration enhancement in NaCl-bearing droplets and on phase separation 90 in droplets bearing surface-active organic compounds. Overall, our results indicate that the properties of nano-aerosol water deviate strongly from those of bulk water in particles smaller than 4 nm. One implication of this finding is that molecular-scale insight is required to accurately represent the initial stages of droplet nucleation and growth.