Articles | Volume 9, issue 6
The Cryosphere, 9, 2323–2337, 2015

Special issue: Interactions between climate change and the Cryosphere: SVALI,...

The Cryosphere, 9, 2323–2337, 2015

Research article 11 Dec 2015

Research article | 11 Dec 2015

Soot on Snow experiment: bidirectional reflectance factor measurements of contaminated snow

J. I. Peltoniemi1,2, M. Gritsevich1,2,8, T. Hakala1, P. Dagsson-Waldhauserová5,6,7, Ó. Arnalds6, K. Anttila1,3, H.-R. Hannula4, N. Kivekäs3, H. Lihavainen3, O. Meinander3, J. Svensson3,9, A. Virkkula3, and G. de Leeuw2,3 J. I. Peltoniemi et al.
  • 1Finnish Geospatial Research Institute (FGI), Geodeetinrinne 2, Masala, Finland
  • 2Department of Physics, University of Helsinki, P.O. Box 64, Helsinki, Finland
  • 3Finnish Meteorological Institute, P.O. Box 503, Helsinki, Finland
  • 4Finnish Meteorological Institute Arctic Research Centre, Tähteläntie 62, Sodankylä, Finland
  • 5Agricultural University of Iceland, Faculty of Environmental Sciences, Hvanneyri, Iceland
  • 6University of Iceland, Faculty of Physical Sciences and Faculty of Earth Sciences, Reykjavik, Iceland
  • 7Czech University of Life Sciences Prague, Faculty of Environmental Sciences, Department of Ecology, Prague, Czech Republic
  • 8Ural Federal University, Institute of Physics and Technology, Ekaterinburg, Russia
  • 9Department of Environmental Science, University of Helsinki, Helsinki, Finland

Abstract. In order to quantify the effects of absorbing contaminants on snow, a series of spectral reflectance measurements were conducted. Chimney soot, volcanic sand, and glaciogenic silt were deposited on a natural snow surface in a controlled way as a part of the Soot on Snow (SoS) campaign. The bidirectional reflectance factors of these soiled surfaces and untouched snow were measured using the Finnish Geodetic Institute's Field Goniospectropolariradiometer, FIGIFIGO.

A remarkable feature is the fact that the absorbing contaminants on snow enhanced the metamorphism of snow under strong sunlight in our experiments. Immediately after deposition, the contaminated snow surface appeared darker than the natural snow in all viewing directions, but the absorbing particles sank deep into the snow in minutes. The nadir measurement remained the darkest, but at larger zenith angles, the surface of the contaminated snow changed back to almost as white as clean snow. Thus, for a ground observer the darkening caused by impurities can be completely invisible, overestimating the albedo, but a nadir-observing satellite sees the darkest points, underestimating the albedo. Through a reciprocity argument, we predict that at noon, the albedo perturbation should be lower than in the morning or afternoon. When sunlight stimulates sinking more than melting, the albedo should be higher in the afternoon than in the morning, and vice versa when melting dominates. However, differences in the hydrophobic properties, porosity, clumping, or size of the impurities may cause different results than observed in these measurements.

Short summary
Light-absorbing impurities change the reflectance of snow in different ways. Some particles are heated by the Sun and they sink out of sight. During the process, snow may look darker than pure snow when observed by nadir, but at larger view zenith angles the snow may look as white as clean snow. Thus an observer on the ground may overestimate the albedo, while a satellite underestimates the albedo. Climate studies need to examine how the contaminants behave in snow, not only their total amounts.