Sea salt particles are constantly produced from ocean surfaces by wave-wind interactions and removed by deposition and precipitation scavenging. These particles constitute the background aerosol for light scattering in the marine boundary layer. In this work, the thermodynamic and optical properties of sea salt aerosol particles generated from seawater samples are measured at 25°C as a function of relative humidity, using a single-particle levitation technique. Water activities, densities, and refractive indices of aqueous solution droplets containing a single salt NaCl, Na2SO4, MgCl2, or MgSO4 are also reported as a function of concentration. The light-scattering properties of the sea salt aerosol are modeled by the external mixture of these four salt systems selected to approximate the sea salt composition. Good agreements are obtained. It follows that in either visibility reduction or radiative forcing calculations, both freshly produced and aged sea salt aerosols may be modeled by external mixtures of the appropriate inorganic salts, whose solution properties are now available in the literature.