Dr. Stephen G. Warren, Professor of Atmospheric Sciences and Earth & Space Sciences, Faculty – Astrobiology Program, Adjunct Faculty Member, Quaternary Research Center, University of Washington, Seattle, Washington

Abstract: Absorption of radiation by ice is extremely weak at visible and near-ultraviolet wavelengths, so small amounts of light-absorbing impurities in snow can dominate the absorption of sunlight at these wavelengths, reducing the albedo relative to that of pure snow and leading to earlier snowmelt. For this study about 1600 snow samples were collected in Alaska, Canada, Greenland, Svalbard, Norway, Russia, and the Arctic Ocean, on tundra, glaciers, ice caps, sea ice, frozen lakes, and in boreal forests. Snow was collected mostly in spring, when the entire winter snowpack was accessible for sampling.

The snow is melted and filtered; the filters are analyzed in a spectrophotometer to infer the concentration of black carbon (BC) and the fraction of absorption due to non-BC light-absorbing constituents. The non-BC impurities, principally brown (organic) carbon, are typically responsible for ~40% of the visible and ultraviolet absorption. Median BC amounts (in ppb) are: Greenland 3, Arctic Ocean 7, Arctic Canada 8, subarctic Canada 14, Svalbard 13, northern Norway 21, western Arctic Russia 26, northeastern Siberia 17. Chemical analyses of filters and meltwater, input to a receptor model, indicate that the major source of BC in most parts of the Arctic is biomass burning, but industrial sources dominate in Svalbard and the central Arctic Ocean.

When the snow surface layer melts, much of the BC is left at the top of the snowpack rather than carried away in meltwater, thus causing a positive feedback on snowmelt. In the percolation zone of South Greenland at the end of July, the subsurface snow had 2 ppb but the top 5 cm had 10-20 ppb.

The BC content of the Arctic atmosphere has declined markedly since 1989, according to the continuous measurements of near-surface air at Alert (Canada), Barrow (Alaska), and Ny-Alesund (Svalbard). Correspondingly, our recent BC concentrations for Arctic snow are lower than those reported by Clarke and Noone for 1983-4. It is therefore doubtful that BC in Arctic snow has contributed to the rapid decline of Arctic sea ice in recent years.

In some regions, particularly the Canadian Arctic islands and the Arctic coast of northeast Siberia, the snow cover, even at its maximum depth in April before melting began, was thin and patchy; in these regions the albedo is determined more by snow thickness than by impurities.

The reduction of snow albedo by BC is typically only 1-2%, which is significant for climate but difficult to detect experimentally, because snow albedo depends on several other variables. Albedo reduction can be computed in a radiative transfer model, using the BC amount from the filter measurement together with snow grain size, but the model requires experimental verification. We use an artificial snowpack with large soot content so as to produce a large signal on albedo. Using a small snowmaking machine, a snowpack of area 75 square meters and depth 15 cm is made in a period of 4 hours, deposited over a natural snowpack. A soot suspension is maintained in a sonicated bath, which can be entrained into the water stream. Two snowpacks are made side-by-side, with and without added soot. For a soot content of 1 ppm, 3 grams of soot is dispersed into 3 tons of snow. The spectral albedos of the two snowpacks are in agreement for near-infrared wavelengths beyond 1 micrometer, but diverge at shorter wavelengths, as expected. The albedo reduction suggests a mass-absorption cross-section larger than that expected for an external mixture.

Satellite remote sensing will not be useful to detect BC in Arctic snow, for several reasons, particularly because thin snow has the same spectral signature as sooty snow.