Seminar by: Stephen Warren (University of Washington, Seattle, USA)
Light-absorbing impurities in Arctic snow, and their effect on snow albedo
Abstract: Absorption of radiation by ice is extremely weak at visible and near-ultraviolet wavelengths, so small amounts of light-absorbing impurities (LAI) 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. We collected 1600 snow samples 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. Snow was also collected at 67 sites in western North America and at 84 sites in northern China. Black carbon (BC) was also measured on the Antarctic Plateau and on East Antarctic sea ice.
The snow is melted and filtered; the filters are analyzed in a spectrophotometer to infer the mixing ratio of 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. In a global survey, we find BC mixing ratios in snow ranging over 4 orders of magnitude from 0.2 ng/g in Antarctica to 1000 ng/g in northeast China; the Arctic and North American values are intermediate between these extremes. Chemical analyses, input to a receptor model, indicate that the major source of BC in most of the Arctic is biomass burning, but industrial sources dominate in Svalbard and the central Arctic Ocean. In northeast China, BC is the dominant LAI, but in Inner Mongolia soil dominates.
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. This process was quantified through field studies in Greenland, Alaska, and Norway, where we found that only 10-30% of the BC is removed with meltwater. In the percolation zone of South Greenland at the end of July, the subsurface snow had 2 ng/g but the top 5 cm had 10-20 ng/g. In much of the Arctic, the snow cover, even at its maximum depth in April before melting begins, is thin and patchy; in these regions the albedo is determined more by snow thickness than by impurities.
References:
Warren, S.G., 2019: Light-absorbing impurities in snow: a personal and historical account. Front. Earth Sci., 6, doi:10.3389/feart.2018.00250.
S.J. Doherty, S.G. Warren, T.C. Grenfell, A.D. Clarke, and R.E. Brandt, 2010: Light-absorbing impurities in Arctic snow. Atmos. Chem. Phys., 10, 11647-11680.
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The GeoHyd seminars are normally held in Auditorium 2 in the Geology Building, Blindern (map) but it is also possible to follow some of the seminars on Zoom.
Videolink to the lectures will be sent to the e-mail list geohyd-alle@geo and geohyd-info@geo. If you want to subscribe to the geohyd-info@geo mailing list send an email to Anita Sørlie.
About the seminar series
The talks in GeoHyd Lunch Seminar Series are offered by the Section for Geography and Hydrology (GeoHyd), Dept. of Geosciences, University of Oslo. The seminars are announced as lunch seminars so bring your lunch if you want to.
The seminars are open for everyone interested, and especially students are welcome.
The Lunch Seminar Team
– Anita and Chong-Yu