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| Titel |
What is the real role of iron oxides in the optical properties of dust aerosols? |
| VerfasserIn |
X. L. Zhang, G. J. Wu, C. L. Zhang, T. L. Xu, Q. Q. Zhou |
| 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 ; 15, no. 21 ; Nr. 15, no. 21 (2015-11-03), S.12159-12177 |
| Datensatznummer |
250120136
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| Publikation (Nr.) |
 copernicus.org/acp-15-12159-2015.pdf |
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| Zusammenfassung |
| Iron oxide compounds constitute an important component of mineral dust
aerosols. Several previous studies have shown that these minerals are strong
absorbers at visible wavelengths and thus that they play a critical role in
the overall climate perturbation caused by dust aerosols. When compiling a
database of complex refractive indices of possible mineral species of
iron oxides to study their optical properties, we found that uniformly
continuous optical constants for a single type of iron oxide in the
wavelength range between 0.2 and 50 μm are very scarce, and that
the use of hematite to represent all molecular or mineral iron-oxides types
is a popular hypothesis. However, the crucial problem is that three
continuous data sets for complex refractive indices of hematite are employed
in climate models, but there are significant differences between them. Thus,
the real role of iron oxides in the optical properties of dust aerosols
becomes a key scientific question, and we address this problem by
considering different refractive indices, size distributions and more
logical weight fractions and mixing states of hematite. Based on the
microscopic observations, a semi-external mixture that employs an external
mixture between Fe aggregates and other minerals and partly internal mixing
between iron oxides and aluminosilicate particles is advised as the optimal
approximation. The simulations demonstrate that hematite with a spectral
refractive index from Longtin et al. (1988) shows approximately equal
absorbing capacity to the mineral illite over the whole wavelength region
from 0.55 to 2.5 μm, and only enhances the optical absorption
of aerosol mixture at λ < 0.55 μm. Using the data set from
Querry (1985) may overestimate the optical absorption of hematite at both
visible and near-infrared wavelengths. More laboratory measurements of the
refractive index of iron oxides, especially for hematite and goethite in the
visible spectrum, should therefore be taken into account when assessing the
effect of mineral dust on climate forcing. |
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