Water activities, densities, and refractive indices over extended concentration ranges at 25¿C are reported for solution droplets containing a single salt of either (NH4)2SO4, NH4HSO4, (NH4)3H(SO4)2, Na2SO4, NaHSO4, or NaNO3, which are common constituents of atmospheric aerosols. The extensive data reported here are obtained from experiments using the single-particle levitation technique recently developed for measuring the thermodynamic and optical properties of microdroplets. These data should find application in mathematical models predicting the dynamic behavior, visibility reduction, and radiative effects of atmospheric sulfate and nitrate aerosols. ¿ American Geophysical Union 1994