Increasing the Earth's albedo: the Köhler equation at sea

Abstract

Increasing marine haze and clouds has been considered as a possible means of increasing the Earth's albedo. This would reduce solar heating and global warming, counteracting the effects of the anthropogenic increase in greenhouse gases. One proposed method of doing so would inject small droplets of seawater or condensation nuclei into the marine boundary layer, creating artificial haze and cloud. The equilibrium size of such droplets is described by the Köhler equation that includes the vapor pressure reduction attributable to the solute according to Raoult's law and the vapor pressure increase of a small droplet as a result of surface tension according to Kelvin. Here we apply this classic result to small droplets in the marine boundary layer, where the partial pressure of water vapor is less than the equilibrium vapor pressure because it is in equilibrium with the saline ocean. We calculate the equilibrium size of a droplet containing dissolved ions and find that the radius of a droplet of seawater shrinks greatly before it achieves equilibrium.