Pb-210, Bi-210, and Po-210 in meteoric precipitation and the residence time of tropospheric aerosol

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The concentrations of ²¹⁰Pb, ²¹⁰Bi and ²¹⁰Po in rain and snow were determined. Extremely high concentrations of ²¹⁰Pb, e.g. 190dpm/l, were observed for snow produced 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/²¹⁰Pb 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.