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[ACC29-P04] Measuring near-infrared reflectance of SIGMA-A ice cores, Greenland
Keywords:Greenland, ice core, near-infrared reflectance
The specific surface area (SSA) of snow particles is a parameter that determines the microstructure of the snowpack 1). Measuring SSA is several methods but measuring near-infrared reflectance is suitable for many samples such as ice cores.
An ice core was drilled in 2017 at the SIGMA-A site (78°03′06″N, 67°37′42″W, 1490 m a.s.l) 2) in northwestern Greenland. The SIGMA-A ice core is approximately 61 m long and restoring the past approximately 100 years. Also, this core has 243 ice layers (average 14 mm) of thickness greater than 1 mm formed recrystallization 3). This study measured the near-infrared reflectance of the SIGMA-A ice core.
The near-infrared reflectance measurement system used for the measurement was located at -20℃ in the cold laboratory, in a dark room (2.4x1.8x2.0 (m)) with the inside walls surrounded by white boards. A near-infrared imaging camera (Nikon D7000) was attached to the ceiling of the darkroom, and a 1.5 m long ice core stage was placed in the center of the field of view. The Nikon D7000 removes the near-infrared removal filter and then installs a filter that removes wavelengths below 850 nm. Four near-infrared light sources (S8100 Energy Power; wavelength: 850 nm) installed across the stage were illuminated to the wall of the darkroom so that the stage was uniformly lighted by the scattered light. Taking and saving images can be wirelessly by Nikon Camera Control Pro2 from a PC. The captured RAW file format images were imported using ImageJ software, and using the DCRraw Reader software, one-dimensional intensity data of approximately 4000 pixels corresponding to a length of 80 cm was acquired. 4 types of standard reflector with known reflectance (Tokyo Keiki; Zenith Lite) and a correction board (Panel Depot; New Color Board) with known reflectance was photographed with a Nikon D7000, and the equation relating digital number (DN) to reflectance was obtained. After that, the ice core that surface was shaved with a microtome was photographed at the same position as the standard reflector, and the near-infrared reflectance of the ice core was obtained by linear approximation.
As a result, the near-infrared reflectance of the surface snow and the firn (near close off depth) was 70% and 40% respectively, which is higher than the previous study 4). On the other hand, the reflectance of 243 layers of ice sheets was 20%, which was almost the same value as the previous study 4). The average snow grain size of the surface snow measured by loupe (number of samples) is 2.1 mm (n=30). And, according to Aoki (2009) 5), the reflectance at the wavelength of 850 nm is about 70%, so the reflectance of near-infrared reflectance in this study is considered to be reasonable.
[References]
1) Matzl & Schneebeli, 2006: Journal of Glaciology, 52(179), 558-564.
2) Matoba et al., 2018: Bulletin of Glaciological Research, 36, 15-22.
3)Kawakami et al., 2020: Summaries of JSSI & JSSE Join Conference on Snow and Ice Research -2020 online, 33.
4)Ando et al., 2018: Summaries of JSSI & JSSE Join Conference on Snow and Ice Research -2018 in Sapporo, 65.
5)Aoki, 2009:tenki, 56, 1, 5-16.
An ice core was drilled in 2017 at the SIGMA-A site (78°03′06″N, 67°37′42″W, 1490 m a.s.l) 2) in northwestern Greenland. The SIGMA-A ice core is approximately 61 m long and restoring the past approximately 100 years. Also, this core has 243 ice layers (average 14 mm) of thickness greater than 1 mm formed recrystallization 3). This study measured the near-infrared reflectance of the SIGMA-A ice core.
The near-infrared reflectance measurement system used for the measurement was located at -20℃ in the cold laboratory, in a dark room (2.4x1.8x2.0 (m)) with the inside walls surrounded by white boards. A near-infrared imaging camera (Nikon D7000) was attached to the ceiling of the darkroom, and a 1.5 m long ice core stage was placed in the center of the field of view. The Nikon D7000 removes the near-infrared removal filter and then installs a filter that removes wavelengths below 850 nm. Four near-infrared light sources (S8100 Energy Power; wavelength: 850 nm) installed across the stage were illuminated to the wall of the darkroom so that the stage was uniformly lighted by the scattered light. Taking and saving images can be wirelessly by Nikon Camera Control Pro2 from a PC. The captured RAW file format images were imported using ImageJ software, and using the DCRraw Reader software, one-dimensional intensity data of approximately 4000 pixels corresponding to a length of 80 cm was acquired. 4 types of standard reflector with known reflectance (Tokyo Keiki; Zenith Lite) and a correction board (Panel Depot; New Color Board) with known reflectance was photographed with a Nikon D7000, and the equation relating digital number (DN) to reflectance was obtained. After that, the ice core that surface was shaved with a microtome was photographed at the same position as the standard reflector, and the near-infrared reflectance of the ice core was obtained by linear approximation.
As a result, the near-infrared reflectance of the surface snow and the firn (near close off depth) was 70% and 40% respectively, which is higher than the previous study 4). On the other hand, the reflectance of 243 layers of ice sheets was 20%, which was almost the same value as the previous study 4). The average snow grain size of the surface snow measured by loupe (number of samples) is 2.1 mm (n=30). And, according to Aoki (2009) 5), the reflectance at the wavelength of 850 nm is about 70%, so the reflectance of near-infrared reflectance in this study is considered to be reasonable.
[References]
1) Matzl & Schneebeli, 2006: Journal of Glaciology, 52(179), 558-564.
2) Matoba et al., 2018: Bulletin of Glaciological Research, 36, 15-22.
3)Kawakami et al., 2020: Summaries of JSSI & JSSE Join Conference on Snow and Ice Research -2020 online, 33.
4)Ando et al., 2018: Summaries of JSSI & JSSE Join Conference on Snow and Ice Research -2018 in Sapporo, 65.
5)Aoki, 2009:tenki, 56, 1, 5-16.