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| Titel |
Impact of brown and clear carbon on light absorption enhancement, single scatter albedo and absorption wavelength dependence of black carbon |
| VerfasserIn |
D. A. Lack, C. D. Cappa |
| 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. 9 ; Nr. 10, no. 9 (2010-05-06), S.4207-4220 |
| Datensatznummer |
250008423
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| Publikation (Nr.) |
 copernicus.org/acp-10-4207-2010.pdf |
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| Zusammenfassung |
| The presence of clear coatings on atmospheric black carbon (BC) particles is
known to enhance the magnitude of light absorption by the BC cores. Based on
calculations using core/shell Mie theory, we demonstrate that the
enhancement of light absorption (EAbs) by atmospheric black carbon
(BC) when it is coated in mildly absorbing material (CBrown) is reduced
relative to the enhancement induced by non-absorbing coatings
(CClear). This reduction, sensitive to both the CBrown coating
thickness and imaginary refractive index (RI), can be up to 50% for 400 nm
radiation and 25% averaged across the visible radiation spectrum for
reasonable core/shell diameters. The enhanced direct radiative forcing
possible due to the enhancement effect of CClear is therefore reduced if
the coating is absorbing. Additionally, the need to explicitly treat BC as an
internal, as opposed to external, mixture with CBrown is shown to be
important to the calculated single scatter albedo only when models treat
BC as large spherical cores (>50 nm). For smaller BC cores (or fractal
agglomerates) consideration of the BC and CBrown as an external mixture
leads to relatively small errors in the particle single scatter albedo of
<0.03. It has often been assumed that observation of an absorption Angström
exponent (AAE)>1 indicates absorption by a non-BC aerosol. Here, it is shown
that BC cores coated in CClear can reasonably have an AAE of up to 1.6, a
result that complicates the attribution of observed light absorption to
CBrown within ambient particles. However, an AAE<1.6 does not exclude
the possibility of CBrown; rather CBrown cannot be confidently
assigned unless AAE>1.6. Comparison of these model results to various
ambient AAE measurements demonstrates that large-scale attribution of
CBrown is a challenging task using current in-situ measurement methods.
We suggest that coincident measurements of particle core and shell sizes
along with the AAE may be necessary to distinguish absorbing and non-absorbing
OC. |
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