Light absorption and scattering by aggregates: Application to black carbon and snow grains

Liou, Kuo‐Nan; Takano, Y.; Yang, Ping

doi:10.1016/j.jqsrt.2011.03.007

Cited by 111 publications

(129 citation statements)

References 47 publications

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“…Over most of the parameter space relevant to global BC/snow forcing the DEMA enhancement ratio falls into a relatively narrow range of 1.8-2.1. Similarly, Liou et al (2011) calculated a 1.9-fold increase in co-single-scatter albedo for solitary BC inclusions residing in large ice particles. Encouragingly, this enhancement was nearly identical for BC inclusions in spheres, hexagonal plates, and rough-surface plates (their Fig.…”

Section: Dynamic Effective Medium Approximationmentioning

confidence: 87%

“…Numerous studies have explored the optics of BC residing within cloud droplets and ice particles (e.g., Danielsen et al, 1969;Chýlek et al, 1984Chýlek et al, , 1996Fuller, 1995;Videen and Chýlek, 1998;Chuang et al, 2002;Jacobson, 2006;Liou et al, 2011), with mean estimates of absorption enhancement ranging from factors of 1.5-3, depending on the hydrometeor size and shape, inclusion optical properties, inclusion size distribution, spatial distribution of inclusions within the hydrometeor, and model representation. Although enhancement can exceed a factor of 20 for inclusions near the droplet surface and along certain axes relative to incident light, orientation-averaged enhancement for nearsurface inclusions is actually smaller than that for randomlyor centrally-located inclusions (Chýlek et al, 1996;Fuller et al, 1999).…”

Section: Enhanced Absorptionmentioning

confidence: 99%

“…For an ice grain radius of 100 µm and monodisperse BC size distribution, they calculated an absorption enhancement factor of about 1.7 for realistic BC volume fractions, and cited BC particle size distribution as the largest source of uncertainty, a result we reaffirm here. Liou et al (2011) explored the optical properties of solitary BC inclusions within three different ice habits, showing an average absorption enhancement of about 1.9 that was insensitive to ice shape. Particles attached to the outside of weakly-absorbing spheres cause enhancements that are negligible to weak (∼1.0-1.3) (e.g., Fuller et al, 1999;Liou et al, 2011).…”

Section: Enhanced Absorptionmentioning

confidence: 99%

“…Liou et al (2011) explored the optical properties of solitary BC inclusions within three different ice habits, showing an average absorption enhancement of about 1.9 that was insensitive to ice shape. Particles attached to the outside of weakly-absorbing spheres cause enhancements that are negligible to weak (∼1.0-1.3) (e.g., Fuller et al, 1999;Liou et al, 2011).…”

Section: Enhanced Absorptionmentioning

confidence: 99%

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Enhanced solar energy absorption by internally-mixed black carbon in snow grains

et al. 2012

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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 0.05-10 9 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) ) 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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Section: Dynamic Effective Medium Approximationmentioning

confidence: 87%

Section: Enhanced Absorptionmentioning

confidence: 99%

Section: Enhanced Absorptionmentioning

confidence: 99%

Section: Enhanced Absorptionmentioning

confidence: 99%

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Enhanced solar energy absorption by internally-mixed black carbon in snow grains

et al. 2012

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“…A summary of the size and shape characteristics of the surface snow particles is given in Table 3. Fierz et al (2009), but it is adopted in both observational studies (Fujiyoshi and Wakahama, 1985) and snow models (Jin et al, 2008;Liou et al, 2011;Yang et al, 2013).…”

Section: Near-surface Snow Stratigraphymentioning

confidence: 99%

Measurements and modelling of snow particle size and shortwave infrared albedo over a melting Antarctic ice sheet

et al. 2015

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Abstract. The albedo of a snowpack depends on the singlescattering properties of individual snow crystals, which have a variety of shapes and sizes, and are often bounded in clusters. From the point of view of optical modelling, it is essential to identify the geometric dimensions of the population of snow particles that synthesize the scattering properties of the snowpack surface. This involves challenges related to the complexity of modelling the radiative transfer in such an irregular medium, and to the difficulty of measuring microphysical snow properties. In this paper, we illustrate a method to measure the size distribution of a snow particle parameter, which roughly corresponds to the smallest snow particle dimension, from two-dimensional macro photos of snow particles taken in Antarctica at the surface layer of a melting ice sheet. We demonstrate that this snow particle metric corresponds well to the optically equivalent effective radius utilized in radiative transfer modelling, in particular when snow particles are modelled with the droxtal shape. The surface albedo modelled on the basis of the measured snow particle metric showed an excellent match with the observed albedo when there was fresh or drifted snow at the surface. In the other cases, a good match was present only for wavelengths longer than 1.4 µm. For shorter wavelengths, our modelled albedo generally overestimated the observations, in particular when surface hoar and faceted polycrystals were present at the surface and surface roughness was increased by millimetre-scale cavities generated during melting. Our results indicate that more than just one particle metric distribution is needed to characterize the snow scattering properties at all optical wavelengths, and suggest an impact of millimetre-scale surface roughness on the shortwave infrared albedo.

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Black carbon radiative forcing over the Tibetan Plateau

Liou

et al. 2014

Geophysical Research Letters

111

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We estimate the snow albedo forcing and direct radiative forcing (DRF) of black carbon (BC) in the Tibetan Plateau using a global chemical transport model in conjunction with a stochastic snow model and a radiative transfer model. The annual mean BC snow albedo forcing is 2.9 W m À2 averaged over snow-covered plateau regions, which is a factor of 3 larger than the value over global land snowpack. BC-snow internal mixing increases the albedo forcing by 40-60% compared with external mixing, and coated BC increases the forcing by 30-50% compared with uncoated BC aggregates, whereas Koch snowflakes reduce the forcing by 20-40% relative to spherical snow grains. The annual BC DRF at the top of the atmosphere is 2.3 W m À2 with uncertainties of À70-85% in the plateau after scaling the modeled BC absorption optical depth to Aerosol Robotic Network observations. The BC forcings are attributed to emissions from different regions.

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Light absorption and scattering by aggregates: Application to black carbon and snow grains

Cited by 111 publications

References 47 publications

Enhanced solar energy absorption by internally-mixed black carbon in snow grains

Enhanced solar energy absorption by internally-mixed black carbon in snow grains

Measurements and modelling of snow particle size and shortwave infrared albedo over a melting Antarctic ice sheet

Black carbon radiative forcing over the Tibetan Plateau

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