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| Titel |
Enhanced solar energy absorption by internally-mixed black carbon in snow grains |
| VerfasserIn |
M. G. Flanner, X. Liu, C. Zhou, J. E. Penner, C. Jiao |
| 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 ; 12, no. 10 ; Nr. 12, no. 10 (2012-05-30), S.4699-4721 |
| Datensatznummer |
250011176
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| Publikation (Nr.) |
 copernicus.org/acp-12-4699-2012.pdf |
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| Zusammenfassung |
| Here we explore light absorption by snowpack containing black carbon
(BC) particles residing within ice grains. Basic considerations of
particle volumes and BC/snow mass concentrations show that there are
generally 0.05–109 BC particles for each ice grain. This
suggests that internal BC is likely distributed as multiple inclusions
within ice grains, and thus the dynamic effective medium approximation
(DEMA) (Chýlek and Srivastava, 1983) is a more appropriate optical representation
for BC/ice composites than coated-sphere or standard mixing
approximations. DEMA calculations show that the 460 nm absorption
cross-section of BC/ice composites, normalized to the mass of BC, is
typically enhanced by factors of 1.8–2.1 relative to interstitial BC.
BC effective radius is the dominant cause of variation in this
enhancement, compared with ice grain size and BC volume fraction. We
apply two atmospheric aerosol models that simulate interstitial and
within-hydrometeor BC lifecycles. Although only ~2% of the
atmospheric BC burden is cloud-borne, 71–83% of the BC deposited to
global snow and sea-ice surfaces occurs within hydrometeors. Key
processes responsible for within-snow BC deposition are development of
hydrophilic coatings on BC, activation of liquid droplets, and
subsequent snow formation through riming or ice nucleation by other
species and aggregation/accretion of ice particles. Applying
deposition fields from these aerosol models in offline snow and
sea-ice simulations, we calculate that 32–73% of BC in global
surface snow resides within ice grains. This fraction is smaller than
the within-hydrometeor deposition fraction because meltwater flux
preferentially removes internal BC, while sublimation and freezing
within snowpack expose internal BC. Incorporating the DEMA into a
global climate model, we simulate increases in BC/snow radiative
forcing of 43–86%, relative to scenarios that apply external optical
properties to all BC. We show that snow metamorphism driven by
diffusive vapor transfer likely proceeds too slowly to alter the mass
of internal BC while it is radiatively active, but neglected processes
like wind pumping and convection may play much larger roles. These
results suggest that a large portion of BC in surface snowpack may
reside within ice grains and increase BC/snow radiative forcing,
although measurements to evaluate this are lacking. Finally, previous
studies of BC/snow forcing that neglected this absorption enhancement
are not necessarily biased low, because of application of
absorption-enhancing sulfate coatings to hydrophilic BC, neglect of
coincident absorption by dust in snow, and implicit treatment of
cloud-borne BC resulting in longer-range transport. |
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