ABSTRACT

Concentrations of ²¹⁰Pb and ⁷Be in the surface air were measured at Tsukuba Science City (36°03′N, 140°07′E), Japan, during the period of November 1987 to May 1992. The air concentrations of ²¹⁰Pb and ⁷Be ranged from 0.2 to 0.8 mBq/m³ and from 1 to 6 mBq/m³, respectively. Seasonal variation of ²¹⁰Pb concentration was similar to that of ⁷Be, showing a “two-peak” variation pattern: high concentrations appeared in the spring and the fall. This variation pattern was formed on the ²¹⁰Pb concentration generated from ²²²Rn in the surface air. This was interpreted as part of ²²²Rn emitted from the ground surface may be transported upwards to accumulate long-lived ²¹⁰Pb in the lower part of the stratosphere. It may then subside into the troposphere with the cosmogenic ⁷Be through the tropopause break, during the spring, and along the descending currents, including those which accompany the migratory anticyclones passing frequently over Japan during the spring and fall. Unusual and temporal increase in the ²¹⁰Pb concentration and in the ²¹⁰Pb/⁷Be activity ratio observed in the winter months of 1991-1992 may be attributed partly to the ²¹⁰Pb fallout originating from the 1991 eruption of Pinatubo volcano, Philippines.