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| Titel |
Effects of internal mixing and aggregate morphology on optical properties of black carbon using a discrete dipole approximation model |
| VerfasserIn |
B. V. Scarnato, S. Vahidinia, D. T. Richard, T. W. Kirchstetter |
| 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 ; 13, no. 10 ; Nr. 13, no. 10 (2013-05-16), S.5089-5101 |
| Datensatznummer |
250018658
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| Publikation (Nr.) |
 copernicus.org/acp-13-5089-2013.pdf |
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| Zusammenfassung |
According to recent studies, internal mixing of black carbon (BC) with other
aerosol materials in the atmosphere alters its aggregate shape, absorption of
solar radiation, and radiative forcing. These mixing state effects are not
yet fully understood. In this study, we characterize the morphology and
mixing state of bare BC and BC internally mixed with sodium chloride (NaCl)
using electron microscopy and examine the sensitivity of optical properties
to BC mixing state and aggregate morphology using a discrete dipole
approximation model (DDSCAT). DDSCAT is flexible in simulating the geometry
and refractive index of particle aggregates. DDSCAT predicts a higher mass
absorption coefficient (MAC), lower single scattering albedo (SSA), and
higher absorption Angstrom exponent (AAE) for bare BC aggregates that are
lacy rather than compact. Predicted values of SSA at 550 nm range between
0.16 and 0.27 for lacy and compact aggregates, respectively, in agreement
with reported experimental values of 0.25 ± 0.05. The variation in
absorption with wavelength does not adhere precisely to a power law
relationship over the 200 to 1000 nm range. Consequently, AAE values depend
on the wavelength region over which they are computed. The MAC of BC
(averaged over the 200–1000 nm range) is amplified when internally mixed
with NaCl (100–300 nm in radius) by factors ranging from 1.0 for lacy BC
aggregates partially immersed in NaCl to 2.2 for compact BC aggregates fully
immersed in NaCl. The SSA of BC internally mixed with NaCl is higher than for
bare BC and increases with the embedding in the NaCl. Internally mixed BC SSA
values decrease in the 200–400 nm wavelength range, a feature also common
to the optical properties of dust and organics. Linear polarization features
are also predicted in DDSCAT and are dependent on particle size and
morphology.
This study shows that DDSCAT predicts complex morphology and mixing state
dependent aerosol optical properties that have been reported previously and
are relevant to radiative transfer, climate modeling, and interpretation of
remote sensing measurements. |
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