Chapter 2 U- and Th-Series Nuclides in the Atmosphere: Supply, Exchange, Scavenging, and Applications to Aquatic Processes
Introduction
The atmosphere is an important pathway for the exchange and transfer of selected U- and Th-series nuclides between land and various aquatic reservoirs. This makes the study of the atmospheric supply of these U- and Th-series nuclides, and their distribution and removal from the atmosphere, important to investigate various aquatic processes. This is particularly relevant for the latter part of the 238U-series, the so-called radon (222Rn)-daughter series with prominent atmospherically derived progeny (Figure A1). These radionuclides are useful in quantifying the rates of both atmospheric and aquatic processes, such as gas and particulate exchange, mixing, scavenging and sedimentation. In addition, the U- and Th-series nuclides can also serve as tracers of aerosol provenances and can help track their atmospheric trajectories and sources to aquatic systems.
This chapter builds on two comprehensive reviews on atmospheric 222Rn and daughter products (Turekian et al., 1977) and natural radionuclides in the atmosphere (Turekian and Graustein, 2003). Here we include progress made in these fields during recent years, as well as perspectives on their use in aquatic systems. Thus the scope of the chapter includes the use of U- and Th-series nuclides particularly 222Rn and its daughter products in the study of scavenging processes in the atmosphere, gas exchange/transfer across the air–water interface and particle-associated processes in selected reservoirs of the hydrosphere, such as snow/ice, lakes and coastal ocean. This chapter is complementary to the chapters by Chabaux et al. dealing with freshwater processes, Rutgers van der Loeff and Geibert on scavenging processes in seawater, Chase on sedimentary processes, Stewart et al. on bioaccumulation of U–Th nuclides, and McKee dealing with estuarine processes.
Section snippets
Sources of U- and Th-Series Nuclides to the Atmosphere
The 238U- and 232Th-series nuclides are introduced into the atmosphere primarily through the following pathways.
Atmospheric Distributions
The distribution of U- and Th-series nuclides in the atmosphere is governed by their supply, atmospheric circulation/mixing, removal by scavenging/deposition and radioactive decay. The supply, as mentioned in Section 2, is dominated by continental sources and in situ production in the atmosphere.
Aerosol Scavenging Residence Times Using 222Rn and Its Daughters
The 222Rn daughters (210Pb, 210Bi and 210Po) interact with aerosols leading to their differential scavenging depending on their respective chemistry. This differential removal of nuclides in this sub-series provides an approach to estimate their atmospheric residence times, those of their chemical homologues and of the fine particles with which they associate (Poet et al., 1972; Moore et al., 1973; Martell and Moore, 1974). The residence times can be derived based on models of different
Sedimentation and trace element accumulation
Among the U-series nuclides of atmospheric origin, 210Pb has found extensive applications to determine chronologies of a variety of aqueous system deposits and thereby provide a time frame for records stored in them. 210Pb has been successfully used to date sediments from lakes and coastal ocean, marshes, coastal bays and shelf and snow. The principles of the 210Pb method are presented in detail in Krishnaswami and Lal (1978), with applications reviewed in Appleby and Oldfield (1992), and
Radon exchange models across the air–sea interface
The 222Rn produced from dissolved 226Ra in surface seawater escapes to the atmosphere as the air over the ocean is essentially devoid of 222Rn, and such escape of 222Rn causes 222Rn-226Ra disequilibrium (Figure 2). The nature and extent of the 222Rn-226Ra disequilibrium have been used to derive air–water gas exchange parameters (Broecker and Peng, 1971, Broecker and Peng, 1974, Broecker and Peng, 1982; Cochran, 1992).
The basis for estimating evasion of gases from the surface ocean to the
Perspectives and Future Directions
Since the work of Moore et al., 1973, Moore et al., 1974, atmospheric radon and its progenies have been used widely to tag aerosols and time their scavenging process in the atmosphere. These applications were a direct result of efforts to understand the transport and fate of nuclear weapon debris in the atmosphere and provided some of the key results on the mixing proportions and residence times of atmospheric aerosols. Since then, the scope of applications using these nuclides has been
Acknowledgements
The genesis of this review was made possible during a study fellowship visit awarded to TMC by the Hanse Wissenshaftskolleg, Delmenhorst, Germany, during the spring of 2006. We acknowledge Dr R. Rengarajan for his help in providing experimental details and additional references. We also acknowledge the thorough assistance from the Editor S. Krishnaswami in a more substantial compilation of the manuscript, and the comprehensive and sage editorial advice from Editor J. K. Cochran.
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