Atmospheric radiative effects of an in situ measured Saharan dust plume and the role of large particles S. Otto1, M. de Reus2, T. Trautmann1, A. Thomas2, M. Wendisch2, and S. Borrmann2 1Remote Sensing Technology Institute (IMF), DLR Oberpfaffenhofen, Germany 2Institute for Atmospheric Physics, Johannes Gutenberg University Mainz, Germany
Abstract. This work will present aerosol size distributions measured in a Saharan dust
plume between 0.9 and 12 km altitude during the ACE-2 campaign
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 parameters 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 550 nm was simulated in the entire dust column
as well as 0.76 within the Saharan dust layer at ~4 km altitude
indicating enhanced absorption. The measured dust leads to cooling over the ocean but
warming 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
From the measured size distributions modal parameters of a bimodal lognormal
column volume size distribution were deduced, resulting in a coarse median
diameter of ~9 μm 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 emphasises the warming effect of the large mineral dust particles.
Citation: Otto, S., de Reus, M., Trautmann, T., Thomas, A., Wendisch, M., and Borrmann, S.: Atmospheric radiative effects of an in situ measured Saharan dust plume and the role of large particles, Atmos. Chem. Phys., 7, 4887-4903, doi:10.5194/acp-7-4887-2007, 2007.