Seasonal variations of residence time and upper atmospheric contribution of aerosols studied with Pb-210, Bi-210, Po-210 and Be-7

All daily precipitation samples amounting to 196 in total were collected during the period from January 1990 to June 1991 at a station in Hakodate, Japan, and precisely analyzed for radon daughters, Pb-210, Bi-210 and Po-210. The samples collected after July 1990 were also determined for Be-7. The mean concentration and deposition rate of Pb-210 in 1990 were 0.20 Bq/1 as a mean and 288 Bq/m2/ yr, respectively, and they were a few times greater in winter due to the northwest winter monsoon. The total mean activity ratios with 95% confidence limits were 0.467 ± 0.024 and 0.0624 ± 0.0133 for Bi-210/Pb-210 and Po-210/Pb-210, respectively. The residence time of aerosols calculated from the Po-210/Pb-210 ratio was always longer than that calculated from the Bi-210/Pb-210 ratio. The difference is chiefly due to old aerosols derived from the upper atmosphere (probably the stratosphere), of which contribution for Pb-210 has been estimated to be 6% as a mean, or 12% in spring and 2% in early winter. Subtracting the upper atmospheric component, we have obtained the residence time of tropospheric aerosols carrying Pb-210 to be 4.8 ± 0.3 days for a whole year or 5.9 ± 0.7 days and 3.6 ± 0.5 days for the periods from February to April and from May to June, respectively. The deposition rate of Be-7 was correlated well with the upper atmospheric component of Pb-210. The larger deposition rate of Be-7 suggests that the study area is extremely effective for the removal of aerosols of stratospheric origin.