Water activity is an important macroscopic property of actosol particles and dioplets in the atmosphere as well as aqueous solutions in many other fields of physical chemistry This study, focuses on relating water activity, described using osmotic coefficients, to the microscopic water structure in systems of atmospheric relevance, namely, aqueous solutions of each of the four electrolytes NaCl, (NH1)(2),SO4, NH4Cl and Na2SO4 The osmotic coefficients of these compounds is reported in literature based on thermodynamic measurements, decrease as a function of molality for dilute solutions and increase as a function of mobility for concentrated solutions. At an intermediate mobility, a minimum value of the osmotic coefficient is observed We explain this behavior by describing osmotic coefficients as the product or two concentration-dependent effects. incomplete electrolyte dissociation and variations in the microphysical water structure The degree of dissociation in electrolyte solutions can be obtained directly from literature or derived from reported pK values, and in this work the water structure is quantified using low-wavenumber Raman spectroscopy We use the band at 180 cm(-1) in Raman spectra of aqueous electrolyte solutions, which has been assigned to the displacement of the central oxygen atom in a tetrahedral hydrogen bonding environment composed of five H2O units The abundance of such translationally restricted water molecules is essential in describing the local microphysical stucture of water, and the height of the band is used to estimate the amount of such translationally restricted water molecules in solution. We were able to qualitatively reproduce and explain literature values of osmotic coefficients for the four studied electrolytes Our results indicate that the effect of electrolyte dissociation, which decreases as a function of molality, dominates in dilute solutions, whereas changes in water structure are more significant at higher concentrations.