Chapter 2 U- and Th-Series Nuclides in the Atmosphere: Supply, Exchange, Scavenging, and Applications to Aquatic Processes

Publisher Summary

This chapter summarizes the use of U- and Th-series nuclides particularly ²²²Rn and its daughter products in the study of scavenging processes in the atmosphere. The chapter also includes progress made in these fields during recent years, as well as perspectives on their use in aquatic systems. ²³⁸U and ²³²Th are primordial radionuclides contained in rocks and minerals. Other sources of U- and Th-series nuclides to the atmosphere include volatile emissions from biogenic sources, and anthropogenic activities such as fossil fuel combustion, depleted uranium from military activities, sewage digestion, landfills, and mining activities. 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 scavenging of nuclides from the atmosphere and their subsequent deposition on Earth's surface are important processes governing their supply to lakes and ocean surface waters and provides the basis for their applications as tracers to investigate selected processes in these reservoirs. The atmospheric deposition flux of ²¹⁰Pb and other nuclides can be determined directly or by sampling natural collectors. 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. The generation of ²¹⁰Pb in the atmosphere from its gaseous grandparent ²²²Rn has led to many uses in tracing the transport and scavenging of similarly generated elements. Another application of ²¹⁰Pb has been to track the deposition of trace metals from the atmosphere to the sea surface. However, these applications require a large number of nuclide measurements, specifically high-resolution spatial and temporal measurements of ²²²Rn and its daughters in the atmosphere. Recent studies should enhance the use of U-series nuclides as tracers of atmospheric and their subsequent 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.