Combined Lidar-Polarimeter Dust Retrievals using Spheroidal and Hexahedral Particle Shape Models

Abstract. Accurately retrieving the properties of mineral dust aerosol is critical for quantifying its impacts on climate and air quality. However, these retrievals are often hindered by overly simplistic assumptions about particle shape. To address this, we test the ability of two non-spherical particle models—a conventional spheroid model and a more recent hexahedral model from the TAMUdust2020 database—to simultaneously reproduce co-located lidar (HSRL-2) and polarimeter (RSP) observations. We performed this test on two Saharan dust events observed over the Atlantic Ocean during the ORACLES 2018 campaign. The test was conducted via a combined atmosphere and surface retrieval using the Generalized Retrieval of Atmosphere and Surface Properties (GRASP), for which we augmented the standard spheroid kernel with a newly implemented hexahedral kernel. Each shape model was then evaluated across three distinct retrieval configurations: HSRL-only, RSP-only, and a combined synergistic HSRL+RSP retrieval. This combined approach leverages the polarimeter’s high sensitivity to total column absorption and particle size with the lidar’s precise vertical profiling. Significantly, this work also presents the first reported one-step synergistic retrieval for desert dust using combined High Spectral Resolution Lidar (HSRL) and polarimeter observations.

We find the hexahedral model consistently provides physically plausible estimates of dust size, refractive indices (n, k), and single-scattering albedo (SSA), whereas the spheroid model requires unrealistic values—such as negligible absorption and an uncharacteristically low real refractive index (n ≤ 1.45)—to match the stringent constraints imposed by the synergistic lidar and polarimeter observations. The spheroid-based retrievals also led to significant cross-instrument inconsistencies, with divergent size and refractive index estimates between RSP-only and RSP+HSRL retrievals. We found that this is rooted in the spheroid model’s limited ability to reproduce high observed particle depolarization ratios within the size distributions and complex refractive indices in the range expected for coarse-mode Saharan dust. Ultimately, the hexahedral model provides a consistent and more physically realistic retrieval of coarse-mode dust properties that fits all observations within their measurement uncertainties.