THE EFFECT OF INCLUDED PARTICULATES ON THE SPECTRAL ALBEDO OF SEA-ICE

Sediments and other particulates are often entrained into sea ice form ed over shallow shelves in the Arctic, causing significant changes in the albedo of the ice and in the amount of shortwave radiation absorbe d and transmitted by the ice. A structural-optical model was used in c onjunction with a four-stream radiative transfer model to examine the effects of such particulates on the optical properties of sea ice. Alb edo data from well-characterized ice with moderate particulate loading were combined with model calculations to infer a spectral absorption coefficient and effective size for the particulates. Results indicate that sediment particles contained in the ice have an effective radius (R) of similar to 9 mu m, assuming absorption coefficients similar to those of Saharan dust. With these values, model predictions are in clo se agreement with spectral albedo observations over a broad range of p articulate loading. For particle size distributions commonly observed in sea ice, the calculations indicate that particles with R > 30 mu m have little effect on the bulk optical properties of the ice. The albe do data also suggest that even apparently ''clean'' ice contains trace amounts (5-10 g m(-3)) of particulates that reduce albedos by as much as 5-10% in the visible part of the spectrum. The calculations show t hat particulates in sea ice primarily affect radiative transfer at vis ible wavelengths, whereas apparent optical properties in the near-infr ared tend to be governed by ice structure rather than by the presence of particulates. Particle-bearing layers occurring below similar to 20 -30 cm are found to have little effect on albedo, although they can st ill have a substantial effect on transmission. Estimates of total part icle loading cannot be obtained from reflectance data without some add itional information on particle size, vertical distribution, and ice s tructure.