Accurate Quantification of Single Aerosol Particle Microphysical Properties Using Broadband Light Scattering...

The wavelength-dependent refractive indices of aerosol particles, n(λ), are essential quantities for several areas of atmospheric science. Broadband light scattering (BLS) is a technique that has the potential to determine n(λ) and particle size at the single particle level by measuring variations in light scattering intensity with wavelength. However, a significant barrier to the use of BLS is the time-consuming analysis of measured spectra, caused primarily by the need to simulate large numbers of BLS spectra. We introduce a new approach to fitting BLS spectra for single, levitated, nonabsorbing aerosol particles that reduces the time required for analysis by minimizing the number of spectra that require calculation. The method is tested by comparing BLS measurements with concurrent cavity ring-down spectroscopy (CRDS) measurements of extinction cross section for two benchmark nonabsorbing aerosol species: (i) 1,2,6-hexanetriol (a semivolatile organic species) and (ii) aqueous particles containing the hygroscopic salt ammonium sulfate. The accuracy of the BLS-retrieved values for aerosol particle sizes and n(λ) is verified by their use in simulations that reproduce the simultaneously measured BLS spectra and CRDS-derived extinction cross sections. For particles of radius 0.8-2.5 μm interrogated in our experiments, particle radii and the real component of the complex refractive index are retrieved with typical precisions of 1.4 nm and ∼10^-3, respectively, across the 380-800 nm wavelength range of the measurements.