Determination of atmospheric lead-210 by alpha-scintillation counting of air filters

Alpha-counting of filters used to sample large volumes of air provides a convenient way of determining atmospheric concentrations of²¹⁰Pb. Following decay of short-lived²²²Rn and²²⁰Rn progeny, alpha activity of the filters increases as²¹⁰Pb decays to²¹⁰Po. After transient equilibrium is reached at about 3 y, alpha activity diminishes with the 22.3 y halflife of²¹⁰Pb. The degree of equilibrium between²¹⁰Pb and²¹⁰Po can be calculated subsequent to sampling, and the average concentration of²¹⁰Pb in the air during the sampling period can be computed. Contributions to the total²¹⁰Pb from ambient short-lived radon progeny are small, typically 2–4%. Using high volume air samplers with collection rates of 1.1–1.7 m³/min for 24 h periods, and using counting times of 2 h for 20 cm² filter sections, we measured alpha counts ranging from 0.0100±0.0050 to 0.200±0.0200 dps. Periodic measurements on 100 of these filters over a 4 y period yielded mean²¹⁰Pb levels with standard deviations less than ±15%. The method requires minimal sample preparation and can be used to determine past atmospheric²¹⁰Pb concentrations on filters stored for up to 20 y and more.