Lead 210, bismuth 210, and polonium 210 in the atmosphere: Accurate ratio measurement and application to aerosol residence time determination

Procedures developed for the sequential chemical separation and radiochemical analysis of 210Pb, 210Bi, 210Po, and 90Sr are described. Results from the measurements of these radioisotopes in surface air and precipitation are presented and discussed in relation to their sources and their application as tracers for the estimation of the residence times of particles in the lower atmosphere. It is concluded that the simple steady-state model does not apply to these long-lived radon daughters and that published residence time estimates based on 210Po/210Pb ratios are incorrect and too long. On the basis of 210Bi/210Pb activity ratios, correcting for the presence of an assumed secular equilibrium component of 210Pb, 210Bi, and 210Po, we estimate a mean atmospheric residence time of about 4 days for particles in the lower troposphere and about 1 week for particles in precipitation. Further tests of the steady-state assumption and other factors affecting such estimates are in progress.