Abstract
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) (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.
Highlights
Several recent modeling studies have estimated the global radiative forcing caused by black carbon (BC) deposited to snow and sea-ice (Hansen and Nazarenko, 2004; Jacobson, 2004; Hansen et al, 2005; Flanner et al, 2007, 2009; Koch et al, 2009a; Rypdal et al, 2009; Skeie et al, 2011)
Applying the dynamic effective medium approximation (DEMA) (Stroud and Pan, 1978; Chylek and Srivastava, 1983) we showed that the 460 nm effective mass absorption cross-section of BC increases by a factor of ∼1.8–2.1 for ranges of BC inclusion size distribution, volume fraction, and snow effective radius expected for natural snow surfaces
The simulated global-mean fraction of BC residing within ice grains of surface snow is 32–73 %, smaller than the global deposition fraction because meltwater scavenging, sublimation, and freezing of liquid water preferentially remove internal BC or transfer it to the external state
Summary
Several recent modeling studies have estimated the global radiative forcing caused by black carbon (BC) deposited to snow and sea-ice (Hansen and Nazarenko, 2004; Jacobson, 2004; Hansen et al, 2005; Flanner et al, 2007, 2009; Koch et al, 2009a; Rypdal et al, 2009; Skeie et al, 2011). Sources of uncertainty and variability in this forcing include the distribution of BC deposition, particle optical properties, ice effective grain size, meltwater scavenging efficiency, snow coverage, and coincident absorption by other impurities such as dust (e.g., Flanner et al, 2007). Of global BC/snow studies, only Jacobson (2004) considered internal BC/ice mixing, achieved by partitioning BC into components residing external to ice grains and as large individual cores enclosed by ice shells. No global studies have included the influence of polydisperse BC within ice grains, a more realistic scenario explored here
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