Measuring the specific surface area of snow with X-ray tomography andgas adsorption: comparison and implications for surface smoothness

[摘要] Chemical and physical processes, such as heterogeneous chemicalreactions, light scattering, and metamorphism occur in the naturalsnowpack. To model these processes in the snowpack, the specificsurface area (SSA) is a key parameter. In this study, two methods,computed tomography and methane adsorption, which have intrinsicallydifferent effective resolutions – molecular and 30 μm,respectively – were used to determine the SSA of similar naturalsnow samples. Except for very fresh snow, the two methods giveidentical results, with an uncertainty of 3%. This implies that thesurface of aged natural snow is smooth up to a scale of about30 μm and that if smaller structures are present they donot contribute significantly to the overall SSA. It furthermoreimplies that for optical methods a voxel size of 10 μmis sufficient to capture all structural features of this type ofsnow; however, fresh precipitation appears to contain small featuresthat cause an under-estimation of SSA with tomography at thisresolution. The methane adsorption method is therefore superior tocomputed tomography for very fresh snow having high SSA.Nonetheless, in addition to SSA determination, tomography providesfull geometric information about the ice matrix. It can also beadvantageously used to investigate layered snow packs, as it allowsmeasuring SSA in layers of less than 1 mm.

[学科分类] 大气科学