Measuring the specific surface area of snow with X-ray tomography and gas adsorption: comparison and implications for surface smoothness
1WSL, Swiss Federal Institute for Snow and Avalanche Research SLF, Davos, Switzerland
2Paul Scherrer Institute, 5232 Villigen PSI, Switzerland
3University of Berne, 3012 Bern, Switzerland
Abstract. Chemical and physical processes, such as heterogeneous chemical reactions, light scattering, and metamorphism occur in the natural snowpack. To model these processes in the snowpack, the specific surface area (SSA) is a key parameter. In this study, two methods, computed tomography and methane adsorption, which have intrinsically different spatial resolutions –molecular and 30 μm, respectively – were used to determine the SSA of identical natural snow samples. The two methods give identical results, with an uncertainty of 3%. This implies that the surface of natural snow is smooth up to a scale of about 30 μm and that for optical methods a voxel size of 10 μm is sufficient to capture all structural features of natural snow. This smoothness can be physically explained by calculating sublimation and surface diffusion on the snow particles. The methane adsorption method is superior to computed tomography for very fresh snow, but thin layers typical for natural snowpacks can not be resolved. Computed tomography can measure SSA in layers of less than 1 mm thickness, and is therefore advantageous in layered snowpacks.