An approach is developed to retrieve the complex index of refraction by coupling two well known instruments: an aerodynamic particle size spectrometer (APS) probe for measuring aerosol size distributions and an integrating nephelometer for measuring total light scattering coefficients. The retrieval is realized by an iterative least squares minimization of the fractional error between the nephelometer-measured light scattering coefficients and those calculated from the APS-measured size distributions based on the Mie theory for spherical particles. High-resolution data collected during two field experiments conducted at two locations with distinct environments in China are analyzed. The results show that light scattering coefficients, aerosol size distributions, and refractive indices all vary substantially with time. Further examination of their dependence on relative humidity suggests that instead of being monotonic change with relative humidity, the refractive index often fluctuates when the relative humidity changes. This nonmonotonic variation of refractive index with relative index suggests concurrent change of relative humidity and other chemical compositions. Possible errors in the retrieval are also discussed.