Abstract
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 effective resolutions – molecular and 30 μm, respectively – were used to determine the SSA of similar natural snow samples. Except for very fresh snow, the two methods give identical results, with an uncertainty of 3%. This implies that the surface of aged natural snow is smooth up to a scale of about 30 μm and that if smaller structures are present they do not contribute significantly to the overall SSA. It furthermore implies that for optical methods a voxel size of 10 μm is sufficient to capture all structural features of this type of snow; however, fresh precipitation appears to contain small features that cause an under-estimation of SSA with tomography at this resolution. The methane adsorption method is therefore superior to computed tomography for very fresh snow having high SSA. Nonetheless, in addition to SSA determination, tomography provides full geometric information about the ice matrix. It can also be advantageously used to investigate layered snow packs, as it allows measuring SSA in layers of less than 1 mm.
Highlights
Snow, after sedimentation of the snow flakes on the surface, has a very high initial porosity and sinters rapidly
In extension to the work done by Legagneux et al (2002), we checked the “a-priori error” of the method i.e. we evaluated the error on the measured Specific surface area (SSA) values resulting (i) from errors of the experimental devices used for the analysis and (ii) from errors on the physico-chemical data used for the evaluation of the adsorption isotherms
We present the first direct comparison of methods based on two different physical processes and find an agreement within 3% between the SSA values measured by adsorption of methane (SSABET) and μCT (SSAμCT), for snow having an SSA below 700 cm2 g−1
Summary
After sedimentation of the snow flakes on the surface, has a very high initial porosity and sinters rapidly. SSA was found to be a monotonously decreasing parameter apart from volume fraction (Flin et al, 2004; Schneebeli and Sokratov, 2004; Legagneux and Domine, 2005; Taillandier et al, 2007) in the course of snow metamorphism. This is in contrast to the traditionally used grain size, which has a nonmonotonous behaviour in the transition from fresh snow to rounded grain snow to depth hoar (Colbeck et al, 1990). SSA is one of the key parameters in snow physics and chemistry and a precise and unambiguous measurement is necessary
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