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| Titel |
Using aircraft measurements to determine the refractive index of Saharan dust during the DODO Experiments |
| VerfasserIn |
C. L. McConnell, P. Formenti, E. J. Highwood, M. A. J. Harrison |
| 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 ; 10, no. 6 ; Nr. 10, no. 6 (2010-03-31), S.3081-3098 |
| Datensatznummer |
250008274
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| Publikation (Nr.) |
 copernicus.org/acp-10-3081-2010.pdf |
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| Zusammenfassung |
| Much uncertainty in the value of the imaginary part of the refractive index
of mineral dust contributes to uncertainty in the radiative effect of
mineral dust in the atmosphere. A synthesis of optical, chemical and
physical in-situ aircraft measurements from the DODO experiments during
February and August 2006 are used to calculate the refractive index mineral
dust encountered over West Africa. Radiative transfer modeling and
measurements of broadband shortwave irradiance at a range of altitudes are
used to test and validate these calculations for a specific dust event on 23
August 2006 over Mauritania. Two techniques are used to determine the
refractive index: firstly a method combining measurements of scattering,
absorption, size distributions and Mie code simulations, and secondly a
method using composition measured on filter samples to apportion the content
of internally mixed quartz, calcite and iron oxide-clay aggregates, where
the iron oxide is represented by either hematite or goethite and clay by
either illite or kaolinite. The imaginary part of the refractive index at
550 nm (ni550) is found to range between 0.0001 i to 0.0046 i, and
where filter samples are available, agreement between methods is found
depending on mineral combination assumed. The refractive indices are also
found to agree well with AERONET data where comparisons are possible.
ni550 is found to vary with dust source, which is investigated
with the NAME model for each case. The relationship between both size
distribution and ni550 on the accumulation mode single scattering
albedo at 550 nm (ω0550) are examined and size distribution
is found to have no correlation to ω0550, while
ni550 shows a strong linear relationship with ω0550.
Radiative transfer modeling was performed with different
models (Mie-derived refractive indices, but also filter sampling composition
assuming both internal and external mixing). Our calculations indicate that
Mie-derived values of ni550 and the externally mixed dust where
the iron oxide-clay aggregate corresponds to the goethite-kaolinite
combination result in the best agreement with irradiance measurements. The
radiative effect of the dust is found to be very sensitive to the mineral
combination (and hence refractive index) assumed, and to whether the dust is
assumed to be internally or externally mixed. |
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