A complete parameterisation of the relative humidity and wavelength dependence of the refractive index of hygroscopic inorganic aerosol particles

Calculations of aerosol radiative forcing require knowledge of wavelength-dependent aerosol optical properties, such as single scattering albedo. These aerosol optical properties can be calculated using Mie theory from knowledge of the key microphysical properties of particle size and refractive index, assuming that atmospheric particles are well-approximated to be spherical and homogeneous. We provide refractive index determinations for aqueous aerosol particles containing the key atmospherically relevant inorganic solutes of NaCl, NaNO₃, (NH₄)₂SO₄, NH₄HSO₄ and Na₂SO₄, reporting the refractive index variation with both wavelength (400 – 650 nm) and relative humidity (from 100% to the efflorescence value of the salt). The accurate and precise retrieval of refractive index is performed using single particle cavity ring-down spectroscopy. This approach involves probing a single aerosol particle confined in a Bessel laser beam optical trap through a combination of extinction measurements by cavity ring-down spectroscopy and elastic light scattering measurements. Further, we assess the accuracy of these refractive index measurements, comparing our data with previously reported data sets from different measurement techniques but at a single wavelength. Finally, we provide a Cauchy dispersion model that parameterizes refractive index measurements in terms of both wavelength and relative humidity. Our parameterizations should provide useful information to researchers requiring an accurate and comprehensive treatment of the wavelength and relative humidity dependence of the inorganic component of atmospheric aerosol.