Abstract-The concentrations of 210Pb,21oBi and 210Po in rain and snow were determined. Extremely high concentrations of 210Pb, e.g. 190 dpm/ l , were observed for snow pro duced from convective clouds by the north-west monsoon on 3 Feb. 1972, at Assabu, Hokkaido.The high concentration is due to the active convection in the continental air mass which contains much radon and has rather low absolute humidity. The 210Bi/ 210Pb ratio was higher in rain in summer than in snow in winter . By using the two models, CRM (continuous removal model) and SRM (sudden removal model), the residence times of aerosols in the atmosphere are calculated from the activity ratios, 210Bi/21opb and 210Po/210Pb . The residence time from the 210Bi/210Pb ratio is apt to be shorter for the snow by the winter monsoon. This is due to the addition of aged aerosols which may come from the lower stratosphere to the troposphere. By resolving the simultaneous equations, the percentage of aged aerosols and the true residence time of tropospheric aerosols are estimated to be 1 to 5 % and 2 to 7 days for the snow produced by the winter monsoon. The residence time, however, varies widely owing to the origin and history of the air mass.

Total precipitation including rain or snow and dry fallout was collected once a month from February 1970 to March 1972 at five sampling stations in Hokkaido located from the coast of the Sea of Japan to inland. The monthly deposition rate of Pb-210 waa determined together with other chemical constituents. The results are summarized aa follows: (1) The deposition rate and the concentration of Pb-210 are remarkably high in winter.(2) A linear relation between the deposition rate of Pb-210 and the amount of rainfall is found in both summer and winter respectively. (3) As compared with the production rate of Pb-210, namely the exhalation rate of Rn-222 from the continent, the deposition rate of Pb-210 to the unit area at the coast of the Sea of Japan in winter is 2-4 times the exhalation rate of Rn-222 from the continent. (4) The budget calculation of Pb-210 in the atmosphere shows that about 40% of aerosols transported from the continent are caught by snow and deposited over the Sea of Japan and the Japan Islands in winter. Therefore, it is concluded that the Japan Islands play an important role as a collector of the continental aerosols in winter.Tellus XXVII (1975). 5

The snow caused by the winter-monsoon in Japan was analyzed for chemical constituents, radionuclides and stable isotopes. The chloride in the snow was correlated fairly well with not only sodium of maritime origin but also calcium, sulfate and radionuclides for samples collected during a single snowfall. The rainout process seems to be prevailing over the washout process for the removal of the chemical constituents from the convective cloud. The air mass responsible for making 1 kg of snow water was calculated to be about 2000 m3 STP from the radon daughter nuclides 210Pb, 210Bi and 210Po in the snow collected at Assabu, Hokkaido. The isotopic composition of the snow is not controlled directly by the temperatures of the sea surface and of the air where the snow is forming as suggested by Isono et al. (1966) but controlled by the relation among water vapor contents in the continental air and in the air transported to the Pacific and the amount of water vapor evaporated from the Japan Sea. The consistency between these results was quantitatively examined by making the budget diagram of water and chemical substances during the winter-monsoon season.

Total precipitation including rain or snow and dry fallout was collected once a month from February 1970 to March 1972 at five sampling stations in Hokkaido located from the coast of the Sea of Japan to inland. The monthly deposition rate of Pb‐210 was determined together with other chemical constituents. The results are summarized as follows: (1) The deposition rate and the concentration of Pb‐210 are remarkably high in winter. (2) A linear relation between the deposition rate of Pb‐210 and the amount of rainfall is found in both summer and winter respectively. (3) As compared with the production rate of Pb‐210, namely the exhalation rate of Rn‐222 from the continent, the deposition rate of Pb‐210 to the unit area at the coast of the Sea of Japan in winter is 2–4 times the exhalation rate of Rn‐222 from the continent. (4) The budget calculation of Pb‐210 in the atmosphere shows that about 40% of aerosols transported from the continent are caught by snow and deposited over the Sea of Japan and the Japan Islands in winter. Therefore, it is concluded that the Japan Islands play an important role as a collector of the continental aerosols in winter.

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