Variability of the infrared complex refractive index of African mineral dust: experimental estimation and implications for radiative transfer and satellite remote sensing
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 origi...
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| Format: | Article |
| Language: | English |
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Copernicus Publications
2014-10-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | http://www.atmos-chem-phys.net/14/11093/2014/acp-14-11093-2014.pdf |
| _version_ | 1811325920251740160 |
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| author | C. Di Biagio H. Boucher S. Caquineau S. Chevaillier J. Cuesta P. Formenti |
| author_facet | C. Di Biagio H. Boucher S. Caquineau S. Chevaillier J. Cuesta P. Formenti |
| author_sort | C. Di Biagio |
| collection | DOAJ |
| description | 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 (<i>n</i>) and imaginary (<i>k</i>) 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 <i>n</i> and <i>k</i> 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.
<br><br>
For the five analysed dust samples, aerosol intensive optical properties
relevant to radiative transfer (mass extinction efficiency, <i>k</i><sub>ext</sub>,
single scattering albedo, ω, and asymmetry factor, <i>g</i>) 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 <i>k</i><sub>ext</sub>, ω, and <i>g</i> 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. |
| first_indexed | 2024-04-13T14:40:52Z |
| format | Article |
| id | doaj.art-a3ee8a78377846ed92b8f4c748c26284 |
| institution | Directory Open Access Journal |
| issn | 1680-7316 1680-7324 |
| language | English |
| last_indexed | 2024-04-13T14:40:52Z |
| publishDate | 2014-10-01 |
| publisher | Copernicus Publications |
| record_format | Article |
| series | Atmospheric Chemistry and Physics |
| spelling | doaj.art-a3ee8a78377846ed92b8f4c748c262842022-12-22T02:42:54ZengCopernicus PublicationsAtmospheric Chemistry and Physics1680-73161680-73242014-10-011420110931111610.5194/acp-14-11093-2014Variability of the infrared complex refractive index of African mineral dust: experimental estimation and implications for radiative transfer and satellite remote sensingC. Di Biagio0H. Boucher1S. Caquineau2S. Chevaillier3J. Cuesta4P. Formenti5LISA, UMR CNRS 7583, Université Paris Est Créteil et Université Paris Diderot, Institut Pierre Simon Laplace, Créteil, FranceIPSL/LOCEAN, UMR 7159-IRD-CNRS-UPMC-MNHN, Institut de Recherche pour le Développement, Bondy, FranceIPSL/LOCEAN, UMR 7159-IRD-CNRS-UPMC-MNHN, Institut de Recherche pour le Développement, Bondy, FranceLISA, UMR CNRS 7583, Université Paris Est Créteil et Université Paris Diderot, Institut Pierre Simon Laplace, Créteil, FranceLISA, UMR CNRS 7583, Université Paris Est Créteil et Université Paris Diderot, Institut Pierre Simon Laplace, Créteil, FranceLISA, UMR CNRS 7583, Université Paris Est Créteil et Université Paris Diderot, Institut Pierre Simon Laplace, Créteil, FranceExperimental 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 (<i>n</i>) and imaginary (<i>k</i>) 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 <i>n</i> and <i>k</i> 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. <br><br> For the five analysed dust samples, aerosol intensive optical properties relevant to radiative transfer (mass extinction efficiency, <i>k</i><sub>ext</sub>, single scattering albedo, ω, and asymmetry factor, <i>g</i>) 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 <i>k</i><sub>ext</sub>, ω, and <i>g</i> 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.http://www.atmos-chem-phys.net/14/11093/2014/acp-14-11093-2014.pdf |
| spellingShingle | C. Di Biagio H. Boucher S. Caquineau S. Chevaillier J. Cuesta P. Formenti Variability of the infrared complex refractive index of African mineral dust: experimental estimation and implications for radiative transfer and satellite remote sensing Atmospheric Chemistry and Physics |
| title | Variability of the infrared complex refractive index of African mineral dust: experimental estimation and implications for radiative transfer and satellite remote sensing |
| title_full | Variability of the infrared complex refractive index of African mineral dust: experimental estimation and implications for radiative transfer and satellite remote sensing |
| title_fullStr | Variability of the infrared complex refractive index of African mineral dust: experimental estimation and implications for radiative transfer and satellite remote sensing |
| title_full_unstemmed | Variability of the infrared complex refractive index of African mineral dust: experimental estimation and implications for radiative transfer and satellite remote sensing |
| title_short | Variability of the infrared complex refractive index of African mineral dust: experimental estimation and implications for radiative transfer and satellite remote sensing |
| title_sort | variability of the infrared complex refractive index of african mineral dust experimental estimation and implications for radiative transfer and satellite remote sensing |
| url | http://www.atmos-chem-phys.net/14/11093/2014/acp-14-11093-2014.pdf |
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