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| Titel |
Black carbon fractal morphology and short-wave radiative impact: a modelling study |
| VerfasserIn |
M. Kahnert, A. Devasthale |
| 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 ; 11, no. 22 ; Nr. 11, no. 22 (2011-11-24), S.11745-11759 |
| Datensatznummer |
250010218
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| Publikation (Nr.) |
 copernicus.org/acp-11-11745-2011.pdf |
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| Zusammenfassung |
| We investigate the impact of the morphological properties of freshly emitted black carbon
aerosols on optical properties and on radiative forcing. To this end, we model
the optical properties of fractal black carbon aggregates by use of
numerically exact solutions to Maxwell's equations within a spectral range
from the UVC to the mid-IR. The results are coupled to
radiative transfer computations, in which we consider six realistic case
studies representing different atmospheric pollution conditions and surface
albedos. The spectrally integrated radiative impacts of black carbon are compared
for two different fractal morphologies, which brace the range of recently
reported experimental observations of black carbon fractal structures. We also
gauge our results by performing corresponding calculations based on the homogeneous
sphere approximation, which is commonly employed in climate models. We find that
at top of atmosphere the aggregate models yield radiative impacts that
can be as much as 2 times higher than those based on the homogeneous sphere
approximation. An aggregate model with a low fractal dimension can predict
a radiative impact that is higher than that obtained with a high fractal dimension
by a factor ranging between 1.1–1.6. Although the lower end of this scale seems
like a rather small effect, a closer analysis reveals that the single scattering
optical properties of more compact and more lacy aggregates differ considerably.
In radiative flux computations there can be a partial cancellation due to the opposing
effects of different error sources. However, this cancellation effect can strongly depend on atmospheric
conditions and is therefore quite unpredictable. We conclude that the
fractal morphology of black carbon aerosols and their fractal parameters
can have a profound impact on their radiative
forcing effect, and that the use of the homogeneous sphere model introduces
unacceptably high biases in radiative impact studies.
We emphasise that there are other potentially important morphological
features that have not been addressed in the present study, such as sintering and
coating of freshly emitted black carbon by films of organic material. Finally,
we found that the spectral variation of the absorption cross section of black
carbon significantly deviates from a simple 1/λ scaling law. We
therefore discourage the use of single-wavelength
absorption measurements in conjunction with a 1/λ scaling relation
in broadband radiative forcing simulations of black carbon. |
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