Climatic effects of soot contamination in surface snow have been studied since the 1980s. Numerous field studies have been conducted in the cryosphere by collecting snow samples from surface snow pile and measuring the soot content. Long-term records of soot deposition in mid-latitude zones have mostly been limited to those from ice cores (cylindrical samples of snow and ice) obtained in the Himalayan mountain ranges and the European Alps. In the mid-latitude westerlies, these sampling locations are on the upwind side of the coastal regions of China, where current soot emissions are estimated to be the highest in the world. Those data are therefore not necessarily appropriate in analyzing the effects of soot particles emitted in China. In some observations, soot contents in snow and rainwater which were directly collected by bottles with a funnel were reported, although the longest observation period has only been three and half years.

The researcher has for the first time analyzed and clarified changes over the past 20 years in surface deposition of elemental carbon (soot) particles at Sapporo City and Rishiri Island in Hokkaido located in northern Japan, in collaboration with Hokkaido University, the University of Yamanashi, the Hokkaido Research Organization, and the Nagoya City Institute for Environmental Science. They utilized a stored archive of membrane filters that had been used in the acid deposition study by an environmental research institution of local government, and developed a method of extracting and analyzing the soot particles retained on these filters. The revealed deposition of soot particles over the past 20 years had large interannual variation and it significantly increased when massive Asian Dust swept over Japan in 2000 and 2001. It is presumed that large amounts of air pollutants including soot were transported, along with the Asian Dust, to northern Japan and deposited there in those years. Soot deposited on the snow and ice reduces the solar reflectivity and changes the surface energy balance, and thus affects the climate. The large interannual change in the soot deposition found in this study suggests that the use of observational data from short-term sampling may lead to erroneous results in validating climate models that deal with snow-darkening effects.