Real refractive index: Dependence on relative humidity and solute composition with relevancy to atmospheric aerosol particles

A closure evaluation of the hygroscopic response in refractive index (n) was completed with measurements and a semi-empirical model using aqueous solutions containing inorganic salts, dicarboxylic acids, and their mixtures. Measurements of n were made for subsaturated aqueous solutions at select mass fractions of solute with an ellipsometer and a refractometer. The semi-empirical model was based on the Zdanovskii-Stokes-Robinson (ZSR) mixing rule, the partial molar refraction (PMR) method and previously published thermodynamic data. Closure was obtained within 0.75%, as represented by the average absolute error, for n values obtained from the ellipsometry/ refractometry measurements and the ZSR-PMR model for aqueous solutions. It was observed that the hygroscopic response of n on relative humidity could be described by a linear equation. Sensitivity of the top of the atmosphere aerosol radiative forcing to changes in n is also discussed. Finally, strengths and limitations for the ellipsometer, the refractometer, and the ZSR-PMR method, are discussed.