The formation of hollow sea-salt particles from the evaporation of drops of seawater

Some non-volatile materials dissolved in water drops will, upon evaporation of the water, produce a hollow shell of the material. Though known for years in the chemical engineering literature, this fact was not appreciated by those working with the marine sea-salt aerosol. Our laboratory experiments show that a majority of seawater drops in free fall in air with a relative humidity less than about 60% evaporate, become salt-saturated, and change phase to produce hollow sea-salt particles. Some hollow salt particles have been found in marine air, though further investigation is required to show the extent of this phenomenon.Expressions have been derived from which both the size of the original drop of seawater and its salt supersaturation at the time of phase change can be calculated from the inner and outer radii of the salt shell. If hollow particles are common in the marine aerosol, then instruments that measure particle size cannot be used to obtain particle mass.