Here we explore light absorption by snowpack containing black carbon(BC) particles residing within ice grains. Basic considerations ofparticle volumes and BC/snow mass concentrations show that there aregenerally 0.05–10 BC particles for each ice grain. Thissuggests that internal BC is likely distributed as multiple inclusionswithin ice grains, and thus the dynamic effective medium approximation(DEMA) (Chýlek and Srivastava, 1983) is a more appropriate optical representationfor BC/ice composites than coated-sphere or standard mixingapproximations. DEMA calculations show that the 460 nm absorptioncross-section of BC/ice composites, normalized to the mass of BC, istypically enhanced by factors of 1.8–2.1 relative to interstitial BC.BC effective radius is the dominant cause of variation in thisenhancement, compared with ice grain size and BC volume fraction. Weapply two atmospheric aerosol models that simulate interstitial andwithin-hydrometeor BC lifecycles. Although only ~2% of theatmospheric BC burden is cloud-borne, 71–83% of the BC deposited toglobal snow and sea-ice surfaces occurs within hydrometeors. Keyprocesses responsible for within-snow BC deposition are development ofhydrophilic coatings on BC, activation of liquid droplets, andsubsequent snow formation through riming or ice nucleation by otherspecies and aggregation/accretion of ice particles. Applyingdeposition fields from these aerosol models in offline snow andsea-ice simulations, we calculate that 32–73% of BC in globalsurface snow resides within ice grains. This fraction is smaller thanthe within-hydrometeor deposition fraction because meltwater fluxpreferentially removes internal BC, while sublimation and freezingwithin snowpack expose internal BC. Incorporating the DEMA into aglobal climate model, we simulate increases in BC/snow radiativeforcing of 43–86%, relative to scenarios that apply external opticalproperties to all BC. We show that snow metamorphism driven bydiffusive vapor transfer likely proceeds too slowly to alter the massof internal BC while it is radiatively active, but neglected processeslike wind pumping and convection may play much larger roles. Theseresults suggest that a large portion of BC in surface snowpack mayreside within ice grains and increase BC/snow radiative forcing,although measurements to evaluate this are lacking. Finally, previousstudies of BC/snow forcing that neglected this absorption enhancementare not necessarily biased low, because of application ofabsorption-enhancing sulfate coatings to hydrophilic BC, neglect ofcoincident absorption by dust in snow, and implicit treatment ofcloud-borne BC resulting in longer-range transport.
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