ReviewReviews on atmospheric selenium: Emissions, speciation and fate
Introduction
Selenium (Se) is an essential trace element for human and animal health and vegetation. However, its overabundance and depletion may cause serious biological and ecological problems, such as Se toxicosis and chronic Keshan disease (due to Se depletion) (Fordyce et al., 2000; Wang and Gao, 2001). One of the most important features of Se is the very narrow margin between nutritionally optimal and potentially toxic dietary exposures for vertebrate animals (Wilber, 1980). The duality depends on its concentration and chemical forms (Cutter and Cutter, 2004).
Se exists in the atmospheric, marine and terrestrial environments, where it may be transported and transformed via different chemical and physical pathways. The distribution of Se is greatly inhomogeneous, resulting in the relative Se enrichment and depletion in the different environments. In particular, elevated levels of Se in the aquatic environment and in terrestrial plants such as lichens and mosses remote from anthropogenic emission sources have been documented, indicating that atmospheric deposition may be an important source of contamination (Beavington et al., 2004; Bennett, 1995; Cutter and Church, 1986; Cutter and Cutter, 2004; Haygarth et al., 1991; Kagawa et al., 2003).
According to the latest evaluation of the global Se budget, approximately 13,000–19,000 tons of Se is cycled through the troposphere annually (Mosher and Duce, 1987). Se is emitted into the atmosphere from a variety of natural and anthropogenic sources, the former accounting for 50–65% of total emissions at the global scale (Mosher and Duce, 1987). However, since the onset of industrialization, anthropogenic emissions have greatly increased compared to natural sources. As a result, the scientific community is paying increasing attention to atmospheric Se and its sources, transport, diffusion, transformation and deposition, in an effort to constrain its importance in the environment.
In 1984, Ross made the first systematic summary in his research report ‘Atmospheric Selenium’. An atmospheric Se model was presented which incorporated speciation and chemical transformations, along with a depositional budget for regions between 30°N and 90°N (Ross, 1984, Ross, 1985, Ross, 1990). However, the model raised a number of ambiguous questions concerning the Se atmospheric cycle due to the lack of available data at that time. For example, the model treats the atmosphere as a single-homogeneous reservoir where a mass balance exists between the sources and sinks. However, new data from field investigations and experiments have since become available that may be used to further constrain the atmospheric processes involving Se. Thus, it now seems an appropriate time to summarize and update our current understanding of the atmospheric Se cycle and its environmental impact. Special attention is paid to atmospheric speciation and its speculated atmospheric pathway and processes. In addition, thermodynamic and kinetic data concerning atmospheric Se speciation are reviewed, which might be used to improve our understanding of Se behaviour in the atmospheric environment.
Section snippets
Atmospheric Se sources and global fluxes
As for other metals, the atmospheric Se sources can be divided into two categories: natural and anthropogenic emissions. Mosher and Duce (1987), Nriagu (1989) and Nriagu and Pacyna (1988) suggest that the natural sources include crustal weathering (wind blown soil dust), volcanoes, sea salt, and the continental and marine biospheres, while the anthropogenic sources mainly comprise combustion (coal, oil, wood, biomass, incineration, etc.), nonferrous metal melting, manufacturing and utilization
Physical and chemical speciation of atmospheric Se
Atmospheric Se may undergo various physical and chemical transformations before being deposited on the ground (Chiaradia and Cupelin, 2000; Cutter and Cutter, 2001; Martens and Suarez, 1999). Because of their significantly different atmospheric behaviour, at least three species of Se should be examined explicitly for emission inventories: (1) volatile organic Se (dimethyl selenide, DMSe; dimethyl diselenide, DMDSe; methane selenol, MeSeH; dimethyl selenl sulphide, DMSeS, etc.); (2) volatile
Atmospheric transport, transformation and removal of selenium
Once emitted into the atmosphere, Se is subjected to a variety of physical, chemical and photochemical processes and/or interactions. Very few works have addressed the atmospheric pathways and processes that cover the entire atmospheric emission-to-deposition Se cycle. Because of the similarity between Se and S chemistry, and the observed correlations of their concentrations in atmospheric aerosol, Se behaviour in the atmosphere can be estimated using S atmospheric chemistry. However, Ross
The promising contribution of Se isotopes
Se isotope geochemistry started in the early 1960s with the pioneering work of Krouse and Thode (1962). Advances in analytical techniques for Se isotopic measurement in the 1990s (TIMS) and 2000s (MC-ICP-MS) made possible the study of natural samples with low Se concentrations (see Johnson, 2004 for a review). Experimental work by Krouse and Thode (1962), Rees and Thode (1966) and later by Herbel et al., 2000, Herbel et al., 2002, Johnson et al. (2000), Ellis et al. (2003) and Johnson and
Conclusions
Although measurements of Se levels in ambient air and precipitation are now readily available, many questions still remain unanswered. Very few field measurements have been made which reveal the correlation between the concentrations of different Se species and the concentrations of various atmospheric oxidants and reductants. Moreover, experimental investigation of the kinetics of Se species under atmospheric conditions still need to be conducted, particularly on those that concern the
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