26 Sep 2022
26 Sep 2022

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

Xiaohan Li1 and Ian C. Bourg1,2 Xiaohan Li and Ian C. Bourg
  • 1Department of Civil and Environmental Engineering, Princeton University, Princeton, NJ, USA
  • 2High Meadows Environmental Institute, Princeton University, Princeton, NJ, USA

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 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 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 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 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 (fs) 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*.

Xiaohan Li and Ian C. Bourg

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • CC1: 'Comment on egusphere-2022-826', Anthony Wexler, 03 Oct 2022
    • AC1: 'Reply on CC1', Xiaohan Li, 14 Oct 2022
  • RC1: 'Comment on egusphere-2022-826', Robert McGraw, 14 Nov 2022
    • AC2: 'Reply on RC1', Xiaohan Li, 17 Nov 2022
  • RC2: 'Comment on egusphere-2022-826', Anonymous Referee #2, 25 Nov 2022

Xiaohan Li and Ian C. Bourg

Xiaohan Li and Ian C. Bourg


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Short summary
Aerosol particles with sizes smaller than 50 nm impact cloud formation and precipitation. Representation of this effect is hindered by limited understanding of the properties of liquid water in these particles. Our simulations of aerosol particles containing salt or organic compounds reveal that water becomes a less cohesive phase at droplet sizes below 4 nm. This effect causes important deviations from theoretical predictions of aerosol properties, including phase state and hygroscopic growth.