Po-210 and Pb-210 as atmospheric tracers and global atmospheric Pb-210 fallout: a Review

Over the past ∼5 decades, the distribution of 222Rn and its progenies (mainly 210Pb, 210Bi and 210Po) have provided a wealth of information as tracers to quantify several atmospheric processes that include: i) source tracking and transport time scales of air masses; ii) the stability and vertical movement of air masses iii) removal rate constants and residence times of aerosols; iv) chemical behavior of analog species; and v) washout ratios and deposition velocities of aerosols. Most of these applications require that the sources and sink terms of these nuclides are well characterized. Utility of 210Pb, 210Bi and 210Po as atmospheric tracers requires that data on the 222Rn emanation rates is well documented. Due to low concentrations of 226Ra in surface waters, the 222Rn emanation rates from the continent is about two orders of magnitude higher than that of the ocean. This has led to distinctly higher 210Pb concentrations in continental air masses compared to oceanic air masses. The highly varying concentrations of 210Pb in air as well the depositional fluxes have yielded insight on the sources and transit times of aerosols. In an ideal enclosed air mass (closed system with respect to these nuclides), the residence times of aerosols obtained from the activity ratios of 210Pb/ 222Rn, 210Bi/ 210Pb, and 210Po/ 210Pb are expected to agree with each other, but a large number of studies have indicated discordance between the residence times obtained from these three pairs. Recent results from the distribution of these nuclides in size-fractionated aerosols appear to yield consistent residence time in smaller-size aerosols, possibly suggesting that larger size aerosols are derived from resuspended dust. The residence times calculated from the 210Pb/ 222Rn, 210Bi/ 210Pb, and 210Po/ 210Pb activity ratios published from 1970’s are compared to those data obtained in size-fractionated aerosols in this decade and possible reasons for the discordance is discussed with some key recommendations for future studies. The existing global atmospheric inventory data of 210Pb is re-evaluated and a ‘ global curve’ for the depositional fluxes of 210Pb is established. A current global budget for atmospheric 210Po and 210Pb is also established. The relative importance of dry fallout of 210Po and 210Pb at different latitudes is evaluated. The global values for the deposition velocities of aerosols using 210Po and 210Pb are synthesized.