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Atmospheric radiative effects of an in situ measured Saharan dust plume and the role of large particles

by S. Otto, M. de Reus, T. Trautmann, A. Thomas, M. Wendisch, S. Borrmann
Atmospheric Chemistry and Physics ()

Abstract

This work will present aerosol size distributions measured in a Saharan dust plume between 0.9 and 12km altitude during the ACE-2 campaign 1997. The distributions contain a significant fraction of large particles of diameters from 4 to 30 µm. Radiative transfer calculations have been performed using these data as input. Shortwave, longwave as well as total atmospheric radiative effects (AREs) of the dust plume are investigated over ocean and desert within the scope of sensitivity studies considering varied input param- eters like solar zenith angle, scaled total dust optical depth, tropospheric standard aerosol profiles and particle complex refractive index. The results indicate that the large particle fraction has a predominant impact on the optical properties of the dust. A single scattering albedo of ωo=0.75−0.96 at 550nm was simulated in the entire dust column as well as 0.76 within the Saharan dust layer at ∼4km altitude indicat- ing enhanced absorption. The measured dust leads to cooling over the ocean butwarming over the desert due to differences in their spectral surface albedo and surface temperature. The large particles absorb strongly and they contribute at least 20% to the ARE in the dusty atmosphere. From the measured size distributions modal parameters of a bimodal lognormal column volume size distribution were deduced, resulting in a coarse median diameter of ∼9µm and a column single scattering albedo of 0.78 at 550 nm. A sensitivity study demonstrates that variabilities in the modal parameters can cause completely different AREs and empha- sises the warming effect of the large mineral dust particles.

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