Trace Elements in Antarctic Aerosol

Unlike Hg and Se, most trace elements are emitted as fine particles which can be transported in atmospheric air masses over distances ranging from hundreds to several thousand kilometres. Antarctica is thus the best region in which to follow global trends in background concentrations of airborne trace elements. Moreover, as discussed in the previous chapter, atmospheric transport and deposition of Fe, Mn and other trace elements play an important role in the chemistry and productivity of the Southern Ocean. Research on the elemental composition of Antarctic aerosol began in the 1970s (e.g. Zoller et al. 1974; Parungo et al. 1979) and, although new works were published more recently (Harvey et al. 1991; Artaxo et al. 1992; Hara et al. 1996; Zreda-Gostyn-ska et al. 1997), it is difficult to compare the results of different studies. The amount and composition of airborne particles in Antarctica change with seasons and between coastal and inland sites; moreover, different sampling and analytical procedures (bulk analysis or microprobe analysis of single particles) were adopted. Artaxo et al. (1992) combined bulk analytical techniques with microanalytical techniques to study the elemental composition of aerosol in the Antarctic Peninsula. The aerosol was dominated by sea-salt particles and small amounts of soil dust and sulphate; some trace elements such as Cr, Ni, Cu, Pb and Zn showed high enrichment factors with respect to soil composition, possibly indicating local or regional contamination from

Fig. 33. Average concentrations of trace elements (ng m-3) in the coarse fraction (2-15 |m) of aerosol particles collected at King George Island (Artaxo et al. 1992) and the Victoria Land coast (Mittner et al. 1994) during summer

Br er Cu Fe Mn Ni Ti Zn anthropogenic sources. A rough comparison (Fig. 33) between average element concentrations measured in coarse particles (2-15 |m in diameter) collected during summer (13 December-8 February) at King George Island (Artaxo et al. 1992) and at Terra Nova Bay (2.3-8.0 |m in diameter; Mittner et al. 1994) indicates a higher input of most elements in the Antarctic Peninsula region than at the northern Victoria Land coast.

Hara et al. (1996) analysed individual aerosol particles (1.6-5.4 |m size range) collected 3 m above ground at Syowa Station. They found remarkable seasonal variations in sulphates, MSA, nitrate, NaCl, KCl, silicates and trace element concentrations. Metallic elements were divided into two categories according to their behaviour: those which are components of soil particles (Ti,V, Fe, Mn, Ga, Zr, Sn and Pb) and those, including elements such as Cr, Co, Cu, Ba and Pb, which were detected in the form of chlorides, oxides or nitrates. The Pb detected in the latter chemical forms was ascribed to anthropogenic sources. Knowing to what extent metal concentrations measured in Antarctic aerosols are of anthropogenic origin is important for modelling their transport within air masses and preparing proper strategies for emission reduction. Unfortunately, it is difficult to evaluate trace metal inputs from human activity (in Antarctica and/or elsewhere in the Southern Hemisphere), due to the fact that these inputs are "masked" by much higher, natural local sources of elements such as sea salt, volcanic and biogenic emissions, soil and rock dust particles. Identification is difficult even for fly ash particles, released during the burning of fossil fuels, which appear under the scanning electron microscope as typical hollow spheres or as smooth surfaces. Many other airborne particles, such as cosmic dust ablated in the atmosphere and particles produced during explosive volcanic activity, may become spherical through melting processes and have long been recognised in sediments and snow from Antarctica and other remote regions of the Earth (Murray 1876; Thiel and Schmidt 1961; King and Wagstaff 1982).

Fig. 33. Average concentrations of trace elements (ng m-3) in the coarse fraction (2-15 |m) of aerosol particles collected at King George Island (Artaxo et al. 1992) and the Victoria Land coast (Mittner et al. 1994) during summer

According to a recent inventory of global anthropogenic emissions of trace elements (Pacyna and Pacyna 2001), in the mid-1990s the combustion of coal, oil and gasoline was the major source of airborne Cr, Hg, Mn, Ni, Sb, Se, Sn, Tl and V, while non-ferrous metal production was the largest source of atmospheric As, Cd, Cu, In and Zn. Estimates of global anthropogenic and natural emissions (Nriagu 1989; Pacyna and Pacyna 2001) suggest that anthropogenic emissions of Pb, V, Cd and Ni prevail on natural emissions, those of Cu, Hg, Mo, Sb and Zn are comparable, while global natural sources of As, Cr and Se are by far more significant than anthropogenic sources (Fig. 34). However, these should be considered rather tentative estimates, especially for natural sources.

It is well known that concentrations of several elements such as As, Br, Cd, Cu, In, Pb, Sb, Se,W and Zn are usually higher in the atmospheric load and in deposited materials than in rocks and soils which are the sources of the bulk of dusts (Bowen 1979). Since the 1970s (e.g. Duce et al. 1975; Weiss et al. 1978), it has been questioned whether the enrichment of trace elements in airborne

Fig. 34. Estimated relative contribution (%) of anthropogenic and natural sources to airborne trace elements

dust is a natural process or whether it is also affected by anthropogenic emissions. The elemental composition of modern dusts collected for several years in the south-western United States shows that concentrations of As, Bi, Cd, Cu, Pb, Se, Sb and Zn are much higher in at least finer-grained dusts than in the average terrestrial crust (Hinkley et al. 2002). Owing to the relatively long atmospheric residence time of fine particles, trace elements from natural or anthropogenic sources may have been acquired during their residence in air (Hinkley et al. 1997). An enrichment in metals such as Cd, Co, Cr, Cu, Mn, Pb, V and Zn has also been documented in marine aerosols with respect to sea-water, when the aerosol is formed by bubble bursting through the sea-surface microlayer (e.g. Arimoto et al. 1987; Nriagu 1989). Marine biogenic activity is an important sources of Cd in Antarctic aerosol. Surface waters in the Southern Ocean, like those in other marine areas of enhanced upwelling, have high Cd concentrations in spring, at the beginning of the algal bloom. The metal is ad/absorbed by phytoplankton, and very high concentrations occur in the liver (or digestive glands) and kidney of most Antarctic organisms (Bargagli et al. 1996b). Cd enrichment has been detected in aerosol collected at a coastal Antarctic site (Heuman 1993), and there is also evidence of the production of methylated Cd and Pb by marine bacteria in polar regions (Pongratz and Heuman 1999). However, Matsumoto and Hinkley (2001) found that, in Antarctic ice representing pre-industrial atmospheric deposition, dust and marine aerosols account for only a few percent of the Cd, I and Pb contents. The masses and proportions of these metals, and the proportion of Pb isotopes, indicate that deposition rates in pre-industrial ice match the output rate to the atmosphere by quiescent (non-explosive) degassing of ocean island volcanoes, mostly in the Southern Hemisphere.

Attempts to identify sources of trace elements to the atmosphere other than dust and salt have been hindered by uncertainties in estimates, especially of volcanic emissions. A significant amount of Hg and other trace elements are emitted as vapour species, which are usually present in greater concentrations in gas than in magma. After eruptions, trace elements condense on ash and other particles or form sublimates and agglomerates. Based on detailed records of atmospheric injection of metals from the quiescently degassing Kilauea volcano, and by combining their results with those from recent studies on other volcanoes, Hinkley et al. (1999) estimated that volcanoes probably emit a substantially smaller amount of metals (especially As, Cu and Se) than Nriagu's (1989) previous estimate. However, through fractionation processes at the melt-vapour interface, several commonly rare metals may become exceptionally abundant in the plumes of quiescent volcanoes (Hinkley et al. 1994), and this contribution may account for a significant portion of trace metal enrichment in dust deposited in Antarctic snow.

Estimates of the amount of elements emitted by individual volcanoes are usually based on the parallel collection of metal-bearing particles and SO2, and results are then related to total sulphur emissions, which are monitored as

SO2 at several volcanoes worldwide. However, the proportions of emitted metals and metal-to-sulphur ratios may vary by orders of magnitude in different types of volcanoes, and temporal variations have also been found for individual volcanoes (Hinkley et al. 1999). The summit crater of Mt. Erebus, the largest (3,794 m high) and most active volcano in Antarctica, contains a con-vecting lake of anorthoclase phonolite magma, which feeds a plume of acidic gases and aerosols and explosive strombolian eruptions (Kyle et al. 1990). The emission rate of sulphur is rather low, and the alkaline magma is characterised by high emissions of halogens (HCl and HF) and aerosols, with trace elements showing an unusually large metal-to-sulphur ratio. Several studies have focused on the potential impact of Mt. Erebus emissions on the Antarctic environment (e.g. Chuan 1994; Palais et al. 1994; Sheppard et al. 1994; Zreda-Gostynska et al. 1997; Harris et al. 1999). Since an early survey on particulate and vapour-phase aerosol emissions from the plume (Germani 1980), it was found that there was an enrichment in halogens (F, Cl, Br), volatile chal-cophile elements (As, Cd, In, Sb, Se) and other elements such as Na, K, La, Ce, Sm and Th. Mt. Erebus was supposedly a source of As, Au, Br, Cs, Cu, S, Sb, Se and Zn in South Pole aerosol. By flying through the Mt. Erebus plume and measuring aerosol concentrations in 1983-1984, when small Strombolian eruptions were frequent, and during the less active period from 1985 through 1989, Chuan (1994) estimated an emission rate of up to 20 Mg day-1 for particles smaller than 50 | m. Crystalline elemental Au is a unique signature of aerosol emissions from Mt. Erebus (Meeker et al. 1991) and, during his sampling flights, Chuan (1994) found that these particles and total concentrations of aerosol at an altitude of 8 km up to 88.7° S (where sampling terminated prior to descent to the South Pole Station) were greater than background values. Zreda-Gostynska et al. (1997) found that many elements in the Erebus plume are common impurities in Antarctic snow, and that they can be detected over a wide area of the continent. In addition to Mt. Erebus and to continental sites with fumaroles, other active volcanoes occur on sub-Antarctic islands (Deception Island and South Sandwich Islands). Thus, it cannot be excluded that more reliable assessments of metal fluxes will show that volcanic emissions in Antarctica and elsewhere in the Southern Hemisphere account for a larger proportion of metals in ice and snow than presently supposed.

Although most trace elements in Antarctic aerosol have a natural origin, Pb and Cu contamination from anthropogenic sources has been widely documented in Antarctic snow (e.g. Boutron and Patterson 1987; Wolff and Suttie 1994; Barbante et al. 1998). A recent paper (Planchon et al. 2002a) indicates that in the second half of the 20th century there was an increase in Pb, Cd,Ag, Bi, Cr, Cu, U and Zn concentrations in snow from Coats Land (about 2,500 km from Mt. Erebus). Concentrations of several metals, especially Pb, Cr, Ag and U, in snow in recent years were somewhat lower than those observed in the 1970s-1980s, probably indicating a decline in atmospheric contamination by human activity. On the grounds of historical changes in ore and/or metal production and emission inventories, the increased deposition of metals on Antarctic snow was attributed to anthropogenic sources in the Southern Hemisphere, especially to non-ferrous metal mining and smelting in Chile, Peru, Zaire, Zambia and Australia.

Although the relative inputs from natural and anthropogenic sources cannot be easily discriminated, the results of research on the composition of recent snow suggest that atmospheric contamination by trace metals is becoming a global problem. Moreover, there is evidence that many elements which once occurred in extremely small concentrations in the environment are now increasingly released by human activity (Schuurmann and Markert 1997). Catalytic converters in motor vehicles are increasing global emissions of Pt and other companion elements such as Pd, Rh, Ru, Os and In. Increased concentrations of Rh, Pd and Pt with respect to ancient Greenland ice samples were measured by Barbante et al. (1999) in surface snow from the Alps, Greenland and Antarctica. Several lanthanoids, another group of scarcely investigated elements, are already employed as components of alloys for magnetic materials, for manufacturing laser crystals or as constituents of high-temperature superconductors, and their significance as environmental contaminants will probably increase in the future.

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