Title Light-absorbing impurities in Arctic snow
Author Doherty, S.J.; Warren, S.G.; Grenfell, T.C.; Clarke, A.D.; Brandt, R.E.
Author Affil Doherty, S.J., NOAA, Joint Institute for the Study of Atmosphere and Ocean, Seattle, WA. Other: University of Washington; University of Hawaii
Source Atmospheric Chemistry and Physics, 10(23), p.11647-11680, . Publisher: Copernicus, Katlenburg-Lindau, International. ISSN: 1680-7316
Publication Date 2010
Notes In English. Includes appendix; published in Atmospheric Chemistry and Physics Discussions: 10 August 2010, http://www.atmos-chem-phys- discuss.net/10/18807/2010/acpd-10-18807-2010. html; accessed in June, 2011. 107 refs. GeoRef Acc. No: 309973
Index Terms aerosols; albedo; biomass; filters; ice; impurities; light (visible radiation); melting; meltwater; photometry; pollution; snow; solar radiation; spectra; Arctic Ocean; Greenland--Greenland ice sheet; Arctic region- -Svalbard; Arctic region; black carbon; boreal environment; carbon; energy balance; forests; Greenland; Greenland ice sheet; light; organic carbon; sea ice; seasonal variations; Svalbard; terrestrial environment; ultraviolet spectra
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 solar radiation at these wavelengths, reducing the albedo relative to that of pure snow, contributing to the surface energy budget and leading to earlier snowmelt. In this study Arctic snow is surveyed for its content of light- absorbing impurities, expanding and updating the 1983-1984 survey of Clarke and Noone. Samples were collected in Alaska, Canada, Greenland, Svalbard, Norway, Russia, and the Arctic Ocean during 1998 and 2005-2009, on tundra, glaciers, ice caps, sea ice, frozen lakes, and in boreal forests. Snow was collected mostly in spring, when the entire winter snowpack is accessible for sampling. Sampling was carried out in summer on the Greenland Ice Sheet and on the Arctic Ocean, of melting glacier snow and sea ice as well as cold snow. About 1200 snow samples have been analyzed for this study. The snow is melted and filtered; the filters are analyzed in a specially designed spectrophotometer system to infer the concentration of black carbon (BC), the fraction of absorption due to non-BC light-absorbing constituents and the absorption Angstrom exponent of all particles. This is done using BC calibration standards having a mass absorption efficiency of 6.0 m2 g-1 at 550 nm and by making an assumption that the absorption Angstrom exponent for BC is 1.0 and for non-BC light- absorbing aerosol is 5.0. The reduction of snow albedo is primarily due to BC, but other impurities, principally brown (organic) carbon, are typically responsible for ~40% of the visible and ultraviolet absorption. The meltwater from selected snow samples was saved for chemical analysis to identify sources of the impurities. Median BC amounts in surface snow are as follows (nanograms of carbon per gram of snow): Greenland 3, Arctic Ocean snow 7, melting sea ice 8, Arctic Canada 8, subarctic Canada 14, Svalbard 13, Northern Norway 21, western Arctic Russia 27, northeastern Siberia 34. Concentrations are more variable in the European Arctic than in Arctic Canada or the Arctic Ocean, probably because of the proximity to BC sources. Individual samples of falling snow were collected on Svalbard, documenting the springtime decline of BC from March through May. Absorption Angstrom exponents are 1.5- 1.7 in Norway, Svalbard, and western Russia, 2.1-2.3 elsewhere in the Arctic, and 2.5 in Greenland. Correspondingly, the estimated contribution to absorption by non-BC constituents in these regions is ~25%, 40%, and 50% respectively. It has been hypothesized that when the snow surface layer melts some of the BC is left at the top of the snowpack rather than being carried away in meltwater. This process was observed in a few locations and would cause a positive feedback on snowmelt. 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, the new BC concentrations for Arctic snow are somewhat lower than those reported by Clarke and Noone for 1983-1984, but because of methodological differences it is not clear that the differences are significant. Nevertheless, the BC content of Arctic snow appears to be no higher now than in 1984, so it is doubtful that BC in Arctic snow has contributed to the rapid decline of Arctic sea ice in recent years.
URL http://www.atmos-chem-phys.net/10/11647/2010/acp-10-11647-2010.pdf
Publication Type journal article
Record ID 65006879