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Title: Enhanced Solar Energy Absorption by Internally-mixed Black Carbon in Snow Grains

Abstract

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) (Chylek 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 {approx}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.more » 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.« less

Authors:
; ; ; ;
Publication Date:
Research Org.:
Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
1042528
Report Number(s):
PNNL-SA-85219
Journal ID: ISSN 1680-7316; KP1703020; TRN: US201212%%813
DOE Contract Number:  
AC05-76RL01830
Resource Type:
Journal Article
Journal Name:
Atmospheric Chemistry and Physics
Additional Journal Information:
Journal Volume: 12; Journal Issue: 10; Journal ID: ISSN 1680-7316
Country of Publication:
United States
Language:
English
Subject:
14 SOLAR ENERGY; ABSORPTION; AEROSOLS; APPROXIMATIONS; CARBON; CLIMATE MODELS; COATINGS; CONVECTION; DEPOSITION; DUSTS; FREEZING; GRAIN SIZE; INTERSTITIALS; METAMORPHISM; NUCLEATION; OPTICAL PROPERTIES; PUMPING; SNOW; SOLAR ENERGY; SUBLIMATION; SULFATES; enhanced; solar energy; absorption; internally-mixed; black carbon; snow grains

Citation Formats

Flanner, M G, Liu, Xiaohong, Zhou, Cheng, Penner, Joyce E, and Jiao, C. Enhanced Solar Energy Absorption by Internally-mixed Black Carbon in Snow Grains. United States: N. p., 2012. Web. doi:10.5194/acp-12-4699-2012.
Flanner, M G, Liu, Xiaohong, Zhou, Cheng, Penner, Joyce E, & Jiao, C. Enhanced Solar Energy Absorption by Internally-mixed Black Carbon in Snow Grains. United States. https://doi.org/10.5194/acp-12-4699-2012
Flanner, M G, Liu, Xiaohong, Zhou, Cheng, Penner, Joyce E, and Jiao, C. 2012. "Enhanced Solar Energy Absorption by Internally-mixed Black Carbon in Snow Grains". United States. https://doi.org/10.5194/acp-12-4699-2012.
@article{osti_1042528,
title = {Enhanced Solar Energy Absorption by Internally-mixed Black Carbon in Snow Grains},
author = {Flanner, M G and Liu, Xiaohong and Zhou, Cheng and Penner, Joyce E and Jiao, C},
abstractNote = {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) (Chylek 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 {approx}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.},
doi = {10.5194/acp-12-4699-2012},
url = {https://www.osti.gov/biblio/1042528}, journal = {Atmospheric Chemistry and Physics},
issn = {1680-7316},
number = 10,
volume = 12,
place = {United States},
year = {2012},
month = {5}
}