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| Titel |
Reevaluation of Mineral aerosol radiative forcings suggests a better agreement with satellite and AERONET data |
| VerfasserIn |
Y. Balkanski, M. Schulz, T. Claquin, S. Guibert |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 7, no. 1 ; Nr. 7, no. 1 (2007-01-10), S.81-95 |
| Datensatznummer |
250004346
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| Publikation (Nr.) |
 copernicus.org/acp-7-81-2007.pdf |
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| Zusammenfassung |
| Modelling studies and satellite retrievals do not agree on the
amplitude and/or sign of the direct radiative perturbation from
dust. Modelling studies have systematically overpredicted mineral dust
absorption compared to estimates based upon satellite retrievals. In
this paper we first point out the source of this discrepancy, which
originates from the shortwave refractive index of dust used in
models. The imaginary part of the refractive index retrieved from
AERONET over the range 300 to 700 nm is 3 to 6 times smaller than that
used previously to model dust. We attempt to constrain these
refractive indices using a mineralogical database and varying the
abundances of iron oxides (the main absorber in the visible). We first
consider the optically active mineral constituents of dust and compute
the refractive indices from internal and external mixtures of minerals
with relative amounts encountered in parent soils. We then compute the
radiative perturbation due to mineral aerosols for internally and
externally mixed minerals for 3 different hematite contents, 0.9%,
1.5% and 2.7% by volume. These constant amounts of hematite
allow bracketing the influence of dust aerosol when it is respectively
an inefficient, standard and a very efficient absorber. These values
represent low, central and high content of iron oxides in dust
determined from the mineralogical database. Linke et al. (2006)
determined independently that iron-oxides represent 1.0 to 2.5% by
volume using x-Ray fluorescence on 4 different samples collected over
Morocco and Egypt. Based upon values of the refractive index retrieved
from AERONET, we show that the best agreement between 440 and 1020 nm
occurs for mineral dust internally mixed with 1.5% volume weighted
hematite. This representation of mineral dust allows us to compute,
using a general circulation model, a new global estimate of mineral
dust perturbation between –0.47 and –0.24 Wm−2 at the top of
the atmosphere, and between –0.81 and –1.13 Wm−2 at the
surface for both shortwave and longwave wavelengths. The anthropogenic
dust fraction is thought to account for between 10 and 50% of the
total dust load present in the atmosphere. We estimate a top of the
atmosphere forcing between –0.03 and –0.25 Wm−2, which is
markedly different that the IPCC range of –0.6 to
+0.4 Wm−2 (IPCC, 2001). The 24-h average atmospheric heating by
mineral dust during summer over the tropical Atlantic region
(15° N–25° N; 45° W–15° W) is in the
range +22 to +32 Wm−2 τ−1 which compares well with the
30±4 Wm−2 τ−1 measured by Li et al. (2004) over that
same region. The refractive indices from Patterson et al. (1977) and
from Volz (1973) overestimate by a factor of 2 the energy absorbed in
the column during summer over the same region. This discrepancy is due
to too large absorption in the visible but we could not determine if
this is linked to the sample studied by Patterson et al. (1997) or to
the method used in determining the refractive index. |
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