Articles | Volume 17, issue 16
Research article
22 Aug 2017
Research article |  | 22 Aug 2017

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

Michael I. Cotterell, Rose E. Willoughby, Bryan R. Bzdek, Andrew J. Orr-Ewing, and Jonathan P. Reid

Abstract. 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 using 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 parameterises refractive index measurements in terms of both wavelength and relative humidity. Our parameterisations should provide useful information to researchers requiring an accurate and comprehensive treatment of the wavelength and relative humidity dependence of refractive index for the inorganic component of atmospheric aerosol.

Short summary
The optical properties of aerosols are required to analyse measurements of aerosol properties, estimate radiative forcing and interpret remote sensing retrievals. We provide comprehensive data sets of the refractive indices of the most important inorganic aerosol constituents, reporting parameterisations for the wavelength and relative humidity dependencies for each system. We assess the accuracy of this study, made on single trapped particles, against previous less comprehensive data sets.
Final-revised paper