Properties: Contaminants
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Properties: Contaminants |
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To investigate the effect on the albedo of the snow-ice cover of absorbing impurities, we carried out observations of the particulate contents of the upper 10-40 cm of the system from 12 August through 20 September. The primary loading components of potential interest for the albedo of bare and snow-covered sea ice are black carbon soot, sediment entrained in the ice during formation, and potential biological production of chlorophyll and gelbstoff. First-stage analysis of our measurements produces the soot (black Carbon) - equivalent concentration, C, of all impurities in a given sample. Bulk samples of snow or ice were collected at ice stations upwind of the ship at up to 4 layers depending on the depth of the snow and the surface decomposed layer. The samples were kept frozen until they could be processed, at which time they were rapidly melted in a microwave oven. For each sample, the melt was immediately drawn through a nucleporeŽ filter (pore size 0.4 microns) via a custom-built vacuum filtration system using a NalgeneŽ hand-operated pump, and the mass of the melt water was logged. The filters were then stored in clear acrylic containers and air dried. Initial determination of C was performed on-board the ship by comparing the filters with a set of standard filters with precisely known black Carbon loading. More detailed spectrophotometric measurements will be carried out to refine our results and differentiate among the various types of inclusions sampled. From the initial analysis it is apparent that soot and sedimentary material were the dominant contributors during the transect. As shown in the accompanying graphs: (1) the peak of the distribution function for concentration was about 4 ngC/g (ppb by mass), which was also the average value for the snow layers; (2) the equivalent soot concentration showed no significant trend with location for a given sample type (snow, surface granular layer, interior ice); (3) the concentration was lowest for the snow covered surfaces, intermediate for the surface granular sites and highest for the interior ice samples. On the basis of the color of the exposed filters (gray for pure soot and dark brown for sediment), it was apparent that the elevated C values for the ice were due to the presence of marine sediments entrained in the ice. The decrease in snow albedo depends on both the soot loading and the snow grain radius (see the section on the Surface Scattering Layer). According to the studies of Warren and Wiscombe (1980), for a snow-grain radius of 100 microns, representative of the new snow, 4 ngC/g would reduce the albedo at 470 nm by approximately 0.005, a level that is approximately at the limit of detectability for precise surface-based spectral albedo observations and well below the limit for satellite observations. The maximum C value of 65 ngC/g would produce a corresponding decrease in albedo of about 0.035. In the surface scattering layer, where the average value of C was about 6 ngC/g and the grain radius was typically 1 mm, the albedo decrease is predicted to be about 0.02. |
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