The Wavelength‐Dependent Complex Refractive Index of Hygroscopic Aerosol Particles and Other Aqueous Media: An Effective Oscillator Model

Abstract

AbstractWe present an effective oscillator model for the wavelength‐dependent complex refractive index of weakly absorbing aqueous solutions. The model is derived using the causal connection between the real and imaginary parts of the refractive index that is described through the Kramers‐Kronig relation. Using cavity‐enhanced Raman spectroscopy, we measure both the real and imaginary parts of the refractive index of several aqueous solutions whose optical properties are relevant to seawater, aqueous sea‐salt particles, and aqueous organic aerosol. It is demonstrated that the effective oscillator model accurately describes both the real and imaginary parts of the refractive index of aqueous solutions across a wide range of water activities and optical wavelengths. Finally, through a comparison with measurements, we verify that mixing rule calculations utilizing oscillator parameters from solutions containing a single solute and water can be used to predict the optical properties of aqueous solutions containing multiple solutes.