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Accurate determination of the aerosol complex refractive index (RI) is crucial for modeling atmospheric radiative forcing, yet it remains challenging due to limitations in conventional measurement techniques. This study demonstrates a novel and robust methodology for retrieving wavelength-dependent complex RI (m = n + ik) through integrated, simultaneous measurements of aerosol absorption, scattering, and size distribution. We combine multiwavelength photoacoustic spectroscopy (PAS) for filter-free absorption coefficients, integrating nephelometry for angular-resolved scattering, and scanning mobility particle sizing (SMPS) for high-resolution size distribution measurements. Using diethyl-hexyl-sebacate (DEHS) aerosols as a benchmark, we introduce an inverse algorithm based on Rayleigh theory in the near-IR to visible range and Modified Anomalous Diffraction Theory (MADT) in the UV wavelength regime for RI retrieval, overcoming Mie theory instabilities for RI retrieval in the so-called transition region, where particle dimensions are comparable to wavelength. The retrieved RI values agree with bulk ellipsometry measurements within Δn = 0.0011 and Δk = 0.00025. The integrated PAS-nephelometer-SMPS approach establishes a new paradigm for in situ aerosol RI determination with direct applications in climate modeling and remote sensing validation.
Rahman et al. (Wed,) studied this question.