Atmospheric Chemistry and Physics Discussions
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10.5194/acpd-7-7767-2007
http://www.atmos-chem-phys-discuss.net/7/7767/2007/
http://www.atmos-chem-phys-discuss.net/7/7767/2007/acpd-7-7767-2007.html
http://www.atmos-chem-phys-discuss.net/7/7767/2007/acpd-7-7767-2007.pdf
7767
7817
2007-06-04
Atmospheric radiative effects of an in-situ measured Saharan dust plume and the role of large particles
S. Otto
sebastian.otto@dlr.de
M. de Reus
T. Trautmann
A. Thomas
M. Wendisch
S. Borrmann
Remote Sensing Technology Institute (IMF), DLR Oberpfaffenhofen, Germany
Institute for Atmospheric Physics, Johannes Gutenberg University Mainz, Germany
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
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 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 ω<sub><I>o</I></sub>=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 atmosphere.
<br><br>
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.
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