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Motivated by the physical picture of shape‐dependent air resistance and, consequently, shape‐induced differential sedimentation of dust particles, we searched for and found evidence of dust particle asphericity affecting the evolution and distribution of dust‐scattered light depolarization ratio (δ). Specifically, we examined a large data set of Cloud‐Aerosol Lidar with Orthogonal Polarization (CALIOP) observations of Saharan dust from June to August 2007. Observing along a typical transatlantic dust track, we find that (1) median δ is uniformly distributed between 2 and 5 km altitudes as the elevated dust leaves the west coast of Africa, thereby indicating uniformly random mixing of particle shapes with height; (2) vertical homogeneity of median δ breaks down during the westward transport: between 2 and 5 km δ increases with altitude and this increase becomes more pronounced with westward progress; (3) δ tends to increase at higher altitude (>4 km) and decrease at lower altitude (<4 >km) during the westward transport. All these features are captured qualitatively by a minimal model (two shapes only), suggesting that shape‐induced differential settling and consequent sorting indeed contribute significantly to the observed temporal evolution and vertical stratification of dust properties. By implicating particle shape as a likely cause of gravitational sorting, these results will affect the estimates of radiative transfer through Saharan dust layers.

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©2013. American Geophysical Union. All Rights Reserved. Publisher's version of record:

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Geophysical Research Letters


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Physics Commons



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