Behavior of atmospheric 7Be and 210Pb at Kumamoto, Japan

Concentrations of natural radionuclides, 7Be and 210Pb, in the atmosphere and depositions to the ground as wet and dry were measured at Kumamoto, Japan. The atmospheric concentration of 7Be ranged between 1.04 and 6.81 mBq m-3, showing an increased level in fall and winter and a decreased level in summer. A similar seasonal variation as 7Be was observed on atmospheric 210Pb concentrations which showing concentrations between 0.17 and 1.71 mBq m-3. The total deposition between 7Be and 210Pb showed a high correlation, suggesting the same removal process for the major fraction of these nuclides though quite different origins, 7Be produced in the upper atmosphere and 210Pb produced in the lower atmosphere as decay products of 222Rn. Wet deposition occupies considerable fractions of the total deposition; 92.3% in 7Be and 75.1% in 210Pb. A relation between 7Be and 210Pb deposition velocity showed that the deposition velocity for wet fraction is distributed around a unity line, which indicates rain removes 7Be and 210Pb equally from atmosphere, while that for dry fraction is distributed to larger deposition velocity in 210Pb, which implies more effective removal of 210Pb than 7Be. This would suggest that particle size of dry fraction is sifted to larger distribution. (author)