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| Titel |
Variability of the infrared complex refractive index of African mineral dust: experimental estimation and implications for radiative transfer and satellite remote sensing |
| VerfasserIn |
C. Di Biagio, H. Boucher, S. Caquineau, S. Chevaillier, J. Cuesta, P. Formenti |
| 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 ; 14, no. 20 ; Nr. 14, no. 20 (2014-10-22), S.11093-11116 |
| Datensatznummer |
250119112
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| Publikation (Nr.) |
 copernicus.org/acp-14-11093-2014.pdf |
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| Zusammenfassung |
Experimental estimations of the infrared refractive index of African mineral
dust have been retrieved from laboratory measurements of particle
transmission spectra in the wavelength range 2.5–25 μm. Five dust
samples collected at Banizoumbou (Niger) and Tamanrasset (Algeria) during
dust events originated from different Western Saharan and Sahelian areas
have been investigated. The real (n) and imaginary (k) parts of the
refractive index obtained for the different dust samples vary in the range
1.1–2.7 and 0.05–1.0, respectively, and are strongly sensitive to the
mineralogical composition of the particles, especially in the 8–12
and 17–25 μm spectral intervals. Dust absorption is controlled mainly
by clays (kaolinite, illite, smectite) and, to a lesser extent, by quartz
and calcium-rich minerals (e.g. calcite, gypsum). Significant differences
are obtained when comparing our results with existing experimental
estimations available in the literature, and with the values of the OPAC
(Optical Properties of Aerosols and Clouds) database. The different data sets
appear comparable in magnitude, with our values of n and k falling within
the range of variability of past studies. However, literature data fail in
accurately reproducing the spectral signatures of the main minerals, in
particular clays, and they significantly overestimate the contribution of
quartz. Furthermore, the real and the imaginary parts of the refractive
index from some literature studies are found not to verify the
Kramers–Kronig relations, thus being theoretically incorrect. The comparison
between our results, from western Africa, and literature data, from
different locations in Europe, Africa, and the Caribbean, nonetheless,
confirms the expected large variability of the dust infrared refractive
index. This highlights the necessity for an extended systematic
investigation of dust properties at infrared wavelengths.
For the five analysed dust samples, aerosol intensive optical properties
relevant to radiative transfer (mass extinction efficiency, kext,
single scattering albedo, ω, and asymmetry factor, g) have been
calculated, by using the Mie theory, based on the estimated refractive index
and measured particle size distribution. The optical properties show a large
sample-to-sample variability, with kext, ω, and g varying in
the range 0.05–0.35, 0.25–1.0, and 0.05–0.75. This variability is expected
to significantly impact satellite retrievals of atmospheric and surface
parameters (e.g. from the Infrared Atmospheric Sounding Interferometer,
IASI) and estimates of the dust radiative forcing. |
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