Possible albedo reduction due to light absorbing impurities in snowpack observed at various sites T. Aoki 1,2 , M. Niwano 2 , S. Matoba 3 , T. Tanikawa 2 , Y. Kodama 1 and Y. Hirozawa 4 1 National Institute of Polar Research, 2 Meteorological Research Institute, JMA, 3 Institute of Low Temperature Science, Hokkaido Univ., 4 Toshiba Corp. Sapporo SIGMA-A in Greenland 1