Surface tension prevails over solute effect in organic-influenced cloud droplet activation

A phase-separation mechanism is proposed for the dominance of the surface tension effect over the solute effect in the observed activation of ultrafine cloud condensation nuclei. Tension among the clouds Atmospheric cloud droplets form through the spontaneous nucleation of water vapour onto aerosol particles, which act as cloud condensation nuclei. The activation of these nuclei depends on the interplay between the Raoult effect, whereby activation potential increases with decreasing water activity or increasing solute concentration, and the Kelvin effect, whereby activation potential decreases with decreasing droplet size or increases with decreasing surface tension. Surface tension may be lowered by surfactants, amphiphilic (water- and fat-loving) compounds that adsorb at the water–air interface. It is expected that surface tension lowering is rendered ineffective by a simultaneous reduction in the Raoult effect, driven by the displacement of surfactant molecules from the bulk droplet to the droplet–vapour interface. Here the authors find that in ambient air the surface tension lowering can prevail over the Raoult effect as a result of liquid–liquid phase separation, leading to substantial increases in the concentration of cloud droplets. The spontaneous growth of cloud condensation nuclei (CCN) into cloud droplets under supersaturated water vapour conditions is described by classic Köhler theory 1 , 2 . This spontaneous activation of CCN depends on the interplay between the Raoult effect, whereby activation potential increases with decreasing water activity or increasing solute concentration, and the Kelvin effect, whereby activation potential decreases with decreasing droplet size or increases with decreasing surface tension, which is sensitive to surfactants 1 . Surface tension lowering caused by organic surfactants, which diminishes the Kelvin effect, is expected to be negated by a concomitant reduction in the Raoult effect, driven by the displacement of surfactant molecules from the droplet bulk to the droplet–vapour interface 3 , 4 . Here we present observational and theoretical evidence illustrating that, in ambient air, surface tension lowering can prevail over the reduction in the Raoult effect, leading to substantial increases in cloud droplet concentrations. We suggest that consideration of liquid–liquid phase separation, leading to complete or partial engulfing of a hygroscopic particle core by a hydrophobic organic-rich phase, can explain th