## 2.8.4. Black and organic carbon and mineral dust within snow[¶](#black-and-organic-carbon-and-mineral-dust-within-snow "Permalink to this headline")
----------------------------------------------------------------------------------------------------------------------------------------------------

Particles within snow originate from atmospheric aerosol deposition (\\(D\_{sp}\\) in Table 2.3 (kg m\-2 s\-1) and influence snow radiative transfer (sections [2.3.2.1](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/Surface_Albedos/CLM50_Tech_Note_Surface_Albedos.html#snow-albedo), [2.3.2.2](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/Surface_Albedos/CLM50_Tech_Note_Surface_Albedos.html#snowpack-optical-properties), and [2.3.2.3](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/Surface_Albedos/CLM50_Tech_Note_Surface_Albedos.html#snow-aging)). Particle masses and mixing ratios are represented with a simple mass-conserving scheme. The model maintains masses of the following eight particle species within each snow layer: hydrophilic black carbon, hydrophobic black carbon, hydrophilic organic carbon, hydrophobic organic carbon, and four species of mineral dust with the following particle sizes: 0.1-1.0, 1.0-2.5, 2.5-5.0, and 5.0-10.0 \\(\\mu m\\). Each of these species has unique optical properties ([Table 2.3.5](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/Surface_Albedos/CLM50_Tech_Note_Surface_Albedos.html#table-single-scatter-albedo-values-used-for-snowpack-impurities-and-ice)) and meltwater removal efficiencies ([Table 2.8.1](#table-meltwater-scavenging)).

The black carbon and organic carbon deposition rates described in Table 2.3 are combined into four categories as follows

(2.8.23)[¶](#equation-8-34 "Permalink to this equation")\\\[D\_{bc,\\, hphil} =D\_{bc,\\, dryhphil} +D\_{bc,\\, wethphil}\\\]

(2.8.24)[¶](#equation-8-35 "Permalink to this equation")\\\[D\_{bc,\\, hphob} =D\_{bc,\\, dryhphob}\\\]

(2.8.25)[¶](#equation-8-36 "Permalink to this equation")\\\[D\_{oc,\\, hphil} =D\_{oc,\\, dryhphil} +D\_{oc,\\, wethphil}\\\]

(2.8.26)[¶](#equation-8-37 "Permalink to this equation")\\\[D\_{oc,\\, hphob} =D\_{oc,\\, dryhphob}\\\]

Deposited particles are assumed to be instantly mixed (homogeneously) within the surface snow layer and are added after the inter-layer water fluxes are computed (section [2.8.3](#water-content)) so that some aerosol is in the top layer after deposition and is not immediately washed out before radiative calculations are done. Particle masses are then redistributed each time step based on meltwater drainage through the snow column (section [2.8.3](#water-content)) and snow layer combination and subdivision (section [2.8.7](#snow-layer-combination-and-subdivision)). The change in mass of each of the particle species \\(\\Delta m\_{sp,\\, i}\\) (kg m\-2) is

(2.8.27)[¶](#equation-8-38 "Permalink to this equation")\\\[\\Delta m\_{sp,\\, i} =\\left\[k\_{sp} \\left(q\_{liq,\\, i-1} c\_{sp,\\, i-1} -q\_{liq,\\, i} c\_{i} \\right)+D\_{sp} \\right\]\\Delta t\\\]

where \\(k\_{sp}\\) is the meltwater scavenging efficiency that is unique for each species ([Table 2.8.1](#table-meltwater-scavenging)), \\(q\_{liq,\\, i-1}\\) is the flow of liquid water into layer \\(i\\) from the layer above, \\(q\_{liq,\\, i}\\) is the flow of water out of layer \\(i\\) into the layer below (kg m\-2 s\-1) (section [2.8.3](#water-content)), \\(c\_{sp,\\, i-1}\\) and \\(c\_{sp,\\, i}\\) are the particle mass mixing ratios in layers \\(i-1\\) and \\(i\\) (kg kg\-1), \\(D\_{sp}\\) is the atmospheric deposition rate (zero for all layers except layer \\(snl+1\\)), and \\(\\Delta t\\) is the model time step (s). The particle mass mixing ratio is

(2.8.28)[¶](#equation-8-39 "Permalink to this equation")\\\[c\_{i} =\\frac{m\_{sp,\\, i} }{w\_{liq,\\, i} +w\_{ice,\\, i} } .\\\]

Values of \\(k\_{sp}\\) are partially derived from experiments published by [Conway et al. (1996)](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/References/CLM50_Tech_Note_References.html#conwayetal1996). Particles masses are re-distributed proportionately with snow mass when layers are combined or divided, thus conserving particle mass within the snow column. The mass of particles carried out with meltwater through the bottom snow layer is assumed to be permanently lost from the snowpack, and is not maintained within the model.

Table 2.8.1 Meltwater scavenging efficiency for particles within snow[¶](#id15 "Permalink to this table")  
| Species
 | \\(k\_{sp}\\)

 |
| --- | --- |
| Hydrophilic black carbon

 | 0.20

 |
| Hydrophobic black carbon

 | 0.03

 |
| Hydrophilic organic carbon

 | 0.20

 |
| Hydrophobic organic carbon

 | 0.03

 |
| Dust species 1 (0.1-1.0 \\(\\mu m\\))

 | 0.02

 |
| Dust species 2 (1.0-2.5 \\(\\mu m\\))

 | 0.02

 |
| Dust species 3 (2.5-5.0 \\(\\mu m\\))

 | 0.01

 |
| Dust species 4 (5.0-10.0 \\(\\mu m\\))

 | 0.01

 |

