[A-CC25] Glaciology

[ACC25-P09] Intercomparison of specific surface area of snow grains measured using the HISSGraS, IceCube and BET method

*Teruo Aoki^1,2, Akihiro Hachikubo³, Motoshi Nishimura¹, Ryo Inoue², Tomonori Tanikawa⁴ (1.National Institute of Polar Research, 2.SOKENDAI (The Graduate University for Advanced Studies), 3.Environmental and Energy Resources Research Center, Kitami Institute of Technology, 4.Meteorological Research Institute, Japan Meteorological Agency)

Keywords:specific surface area, snow grain size, HISSGraS, IceCube, BET method

Specific surface area (SSA) of snow grains is a measure of snow grain size, which governs the snow albedo and describes the snow metamorphism process. Various techniques including optical method, X-ray microtomography, gaseous adsorption (Brunauer–Emmett–Teller: BET) method and so on have been proposed to measure the SSA. However, there were no portable field instruments with high accuracy so far. We developed the Handheld Integrating Sphere Snow Grain Sizer (HISSGraS), whose basic measurement principle is that the reflected light from the target snow surface illuminated by diode laser at the wavelength of 1.3 µm is measured using an integrating sphere. It is the same as that of the IceCube (A2 photonics, France), but the HISSGraS can directly measure the snow surface or snow pit face. The HISSGraS’s weight is 1.6 kg which is much lighter than the IceCube's weight of 8 kg. Furthermore, the two conversions methods from reflectance to SSA are formulated by a radiative transfer model calculation using spherical and non-spherical snow grain shape models. We observed the wide range of SSAs for different types of natural snow and artificial ice grains using the HISSGraS, IceCube and BET method in a cold laboratory at the temperature of -20 °C in Kitami Institute of Technology. Those measured SSA values are compared. The BET method is in principle the most accurate among the three techniques. The SSAs measured with the HISSGraS with spherical snow shape model agree well with those with the IceCube over a wide range of SSA. This is because both instruments use the same measurement principle together with the same spherical snow shape model although different integrating spheres in size and material are employed. The SSAs measured with the HISSGraS with spherical snow shape model and IceCube agree with those by BET method for SSA < 60 m² kg^-1, whereas the HISSGraS- and IceCube-derived SSAs are lower than the BET measurements for SSA > 60 m² kg^-1. This tendency is almost same for the HISSGraS measurements using non-spherical snow grain shape model. The possible cause is that the SSA dependence of reflectance at the wavelength of 1.3 µm decreases for the SSA beyond 60 m² kg^-1. Finally, we found that the SSA measured with the HISSGraS directly contacting the target snow surface with very low snow density could be underestimated compared to the BET measurement. When the same snow is compacted to the snow density higher than 200 kg m^-3, both instruments agree with each other. In case of very low snow density, this is likely that many photons emitted from a laser could escape outwards instead of returning to the integrating sphere as they are scattered by the snow gains.