An Experiment Relating Black Carbon Content to Reduction of Snow Albedo

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Black carbon (BC) in snow and ice has been identified as one of the largest uncertainties in estimates of global radiative forcing for climate change. In this project artificial snowpacks will be made by freezing of water droplets produced by a snowmaking machine in an open field, using water with and without added quantities of a commercial soot, in concentrations about 100 times natural soot levels so as to obtain a large signal on albedo. The reason for conducting the experiment outdoors rather than in a laboratory is to obtain uniform illumination, to avoid elimination of edge-effects, and to be able to measure albedo rather than narrow-field-of-view reflectance. The optically effective snow grain size will be determined from the measured near-infrared albedo; matching the measured visible albedo will then require addition of BC to the radiative transfer model. The BC content of the artificial snowpacks will be measured by filtering the meltwater; the filters will be analyzed by a laboratory spectrophotometer as is done for filters from samples of natural snow. The BC from the filter measurement will be compared to that inferred from the albedo measurement.A series of experiments will be carried out, varying (a) the type of soot, to investigate different size distributions; (b) the amount of soot, to investigate the nonlinearity of albedo reduction; and (c) the grain size of the artificial snow, because the albedo reduction by a given concentration of soot is predicted to be greater in coarse-grained snow than in fine-grained snow. In conjunction with the experiments, parameterizations will be developed for use in climate models, using soot properties consistent with the experimental results. For models that resolve layers of the surface snow, single-scattering quantities will be parameterized for three spectral bands. For models in which snow albedo is a specified lower boundary condition for the atmospheric model, average albedos will be computed as functions of snow grain size and BC concentration, for the two broad bands commonly used in general circulation models (GCMs), as well as the total solar albedo, for incident solar spectra under both clear sky and cloudy sky. Simple analytical functions will be fit to the model results.Broader impacts of this work are in the potential to advance our understanding of the radiative forcing of BC in snow and its climatic consequences and to improve the representation of this process in models used for climate and mitigation studies.