The trace metal recycling component in the North-western Mediterranean

Pergamon

PII: SO025326X(98)00160-4

b’fnrurr f’dhfim Bu//e/b~, Vol. 36. No. 4, pp. 273-277, ,998 0 IYY8 Elsevier Science Ltd. All rights rescrvcd Printed in Great Britain 0025-326X/Y8 $ IY.OO+O.(WI

The Trace Metal Recycling Component in the North-western Mediterranean CHRISTOPHE MIGON and EMMANUEL NICOLAS Laboratoire de Physique et Chimie Marines, Universit& Paris 6, INSU CNRS, La Darse, BP 8, 06238 Villefranche-sur-mer Cedex, France

Cd, Cu, Ph and Zn concentrations were measured in a north-western Mediterranean areas in three different reservoirs (atmosphere, marine surface microlayer and subsurface seawater) with the aim of comparing their respective metal contents. Although the surface microlayer is strongly enriched in trace metals, enrichment factors normalized to sodium indicate that the recycling component (i.e. sea-salt particles emitted by the sea surface and re-introduced into the atmosphere) are negligible compared with direct atmospheric loadings, which are very high in the north-western Mediterranean. The importance of the recycling component decreases in the sequence Cd > Ph > Cu > Zn. 0 1998 Elsevier Science Ltd. All rights reserved Keywords: trace metals; atmospheric aerosol; marine surface microlayer; sea-salt particles; recycling processes; north-western Mediterranean. The long-range transport of matter by the atmospheric pathway is responsible for an efficient spreading of elements to the sea (Bergametti, 1988; Duce et al., 1991). Owing to its relatively reduced dimensions as well as the numerous and intense land-based emission sources along its shores, atmospheric deposition is a major source of particulate and dissolved material in the north-western Mediterranean (Bethoux et ~rl., 1990; Sandroni and Migon, 1997) and it has been shown that other sources such as rivers are significantly less efficient (Martin et al., 1989; Guieu et al., 1991; Migon, 1993). The marine surface microlayer is systematically enriched in suspended matter (Armstrong and Elzerman, 1982; Hardy et al., 19X5; Marty et al., 1988) and contains enriched levels of many trace metals (dissolved and particulate) relative to hulk seawater (Roman0 and Laborde, 1987). Indeed, atmospheric trace metals deposited onto the seawater surface are partly concentrated in the microlayer. Hunter and Liss (1981) have considered that the enrichment of dissolved tract metals in this reservoir is mediated by association with the surface-active organic components

of the sea surface. Recycling from seawater, i.e. the movement of atmospheric matter to the sea surface and its incorporation into sea-spray and its return to coastal or continental environments, may thus be an important pathway for elements (e.g. radionuclides Pattcnden et al., 1983). The rcsidencc time of particles in the microlayer depends on surface tension forces and may range between 1.5 ,and 15 h for natural urban air metallic particles deposited on the surface of seawater microcosms (Hardy et al., 1985). This enriched reservoir might affect the composition of marine atmospheric aerosols, because of the ability of rising bubbles to transport particulate trace metals and organically complexed metals from bulk seawater to the surface (Lion and Leckie, 1981). One can thus expect a recycling of tract metals from the sea-spray to the atmosphere. The present paper compares the trace metal content in the three reservoirs (atmosphere, microlayer and subsurface seawater) and cstablishcs that direct atmospheric inputs are significantly more important than recycled ones in the north-western Mediterranean.

Methodology Atrnoq.dwric uerosol Sampling. Atmospheric samples were collected at a sampling site (Vignola) on the west Corsican coast, close to the Sanguinaires islands (Fig. 1). 48 aerosol samples were continuously collected from 26 January to 16 May 1994 and from 9 September to 28 November 1994, with a sampling duration ranging typically between 3 and 6 days. A hlter holder (Sartorius SM 165 10) protected by a PVC funnel was positioned at the top of a 2 m-high wooden mast. The filters were Sartorius SM 11106 (cellulose acetate, diameter 47 mm, porosity 0.45 urn). The flow was approximately 1 m3 hh’. Analysis. Samples and blank filters were mineralized in closed Teflon bottles and heated in a microwave oven. The rcagcnts were Merck Suprapur 273

Marine

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MPB/915/GRi.-iIF Fig. 1 Location (Vignola)

OT the coastal atmospheric and the Ligurian marine sampling

(HN03, HCl and HF). The mineralization protocol is described elsewhere (Migon et al., 1997). After cooling, the residue was made up to 10 ml with Milli-Q water (Millipore). After mineralization, filters were analysed by graphite furnace atomic absorption spectrophotometry (GFAAS), using a Perkin Elmer 3100, with EDL lamps. Standard metal solutions were provided by Merck. Detection limits were 0.1 ug l- ’ for Cd and 1 ug 1-r for Cu, Pb and Zn. Reproducibility ranged between 0.5 and 5% (with five replicate determinations). Blank values were very low and reproducible. The quality of the data was controlled with the analysis of certified rcfcrencc materials provided by the National Research Council, Canada (MESS-2, BCSS-I and PACS-1, i.e. estuarinc sediment samples). The discrepancy was always less than 10%. Bulk data, methodological details and analysis of certified reference materials can bc found in Sandroni and Migon (1997). Marirte surface microlayer Sampling. Only two samples were collected during

the campaign PHYCEMED (May 1983) with the R.V. Suroit, bearing in mind that the aim of the present sampling was to provide indications of the trace metal concentration levels in the surface microlayer and not to describe and budget this specific reservoir. The spatio-temporal variability of metallic concentrations should be negligible compared with the order of magnitude of direct atmospheric inputs, as discussed below. Figure 1 shows the location of the sampling station. The screen method was used for the collection of large-volume samples (20 1 hh’). The sampling methodology is described in Saliot and Marty (1986). 274

sampling site (r).

station

The only difference in the present work concerns the screen, which was made of Nylon (0.36 mm diameter mesh with 1.25 mm square openings). It was immersed vertically and rcmovcd horizontally from the water surface, sampling a seawater film approximately 0.44 mm thick. Samples were stored in precleaned Teflon bottles, which wcrc themselves packed in scaled polyethylene bags and kept at 15°C until analysis. All sample manipulations on board wcrc carried out in a laminar flow hood. Analysis. Samples were not filtered. They were acidified at pH 2 in order to extract adsorbed tract metals from suspended matter. Prior to analysis, Zn samples were UV irradiated (Lamps Philips HUV 5 Low Density, 253.7 nm, 5 W, irradiation time approximately 8 h) in order to extract organically bound Zn. The Cd, Cu and Pb samples were not irradiated, taking into account that UV treatment is not strictly necessary analytical methodology employing the present (Nurnberg, 1984). We assume that after our extraction procedure, the metallic residues remaining in the suspcndcd matter arc ncgiigible. Cd, Cu and Pb wcrc immediate/y analysed hy differential pulse anodic stripping voltammetry (DPASV). The mcasuremcnts were performed with Tacusscl polarographic equipment (PRG-5 polarograph, fitted with two rotating Edi-type electrodes, with automatic Polaromate), according to the methodology described in Laumond et al. (1984). Zn was analysed in the laboratory by DPASV on a hanging Hg drop electrode. The measurements were performed with an EG&G Princeton Applied Research 264A polarographic analyscr in conjunction with a 303 static Hg drop electrode. For further details, readers are referred to Ruiz-Pin0 el al. (1991).

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Although it is not an absolute proof of accuracy (the possibility of sample contamination is not cxcludcd), the quality of the analysis can be evaluated with a comparison of GFAAS and DPASV methods (see Eaumond et ui., 1984). Subsurface seawater Sampling. During the PHYCEMED campaign (1983), 10 unfiltered marine samples were collected with precleaned Teflon-coated Go-Flo samplers (General Occanics) at 10 m depth. For the methodology for sample storage and manipulations on board see above. Sampling protocol and further details are given in Copin-Montegut et al. (1986). Analysis. Analyses were carried out ashore by DPASV according to the same methodology as for the marine surface microlayer (see Copin-Montegut et al. (1986)).

Results and Discussion Mean total metallic concentrations and their variability in the three reservoirs (atmosphere, marine microlayer and subsurface seawater) are given in Table 1. Metal concentration levels in the atmosphere have been compared with those of other regions elsewhere (Sandroni and Migon, 1997). Trace metal concentrations in the sea surface microlayer are frequently IO- to lOOO-fold those found in subsurface waters (Hardy et al., 1985). Although enrichment of the microlayer relative to subsurface waters has been discussed for several non-Mediterranean regions, to our knowledge there is a lack of data for the Mcditerranean Sea. The two surface microlayer samples from the PHYCEMED campaign (1983) cannot account for the natural variability of elemental concentrations in this environment, but should be considered as elements to understand the role of the recycling component. The atmospheric sampling station is presumably sheltered from close external anthropogenic emissions. Thus, atmospheric concentration levels presented here should be representative of the background composition of western Mediterranean marine areas, with respect to continent-based coastal regions, which can TABLE Trace

-___ Atmosphere ngm 3 Cd Pb Na

1

metal concentrations in the three reservoirs (atmosphere, marine surface microlayer and subjacent seawater).

0.11 (0.01-0.58) 8.8 (4.07-38.2) 1655

Trace

metal Surface

concentrations microlayer L%-’

4.8 (3.9-2.5) 2.6 (1.2-6.5) 10.8 x 106

Bulk seawater (-lOm)ngt 87 (ho- 136) 140 (98-174) 10.8 x 10”

’

TABLE Enrichment

factors atmospheric

Enrichment standardized

1.3 1.4 2.2 5.5

x x x x

factors to sodium EFsc;,

EF,,n, Cd CU Pb Zn

2

in Cd, Cu, Pb and Zn slandardized to Na in the aerosol and in the surface microlayer.

10s 10’ 10” 10’

857 25 100 45

150 560 000 22 000 I 222 220

be subjected to significant urban and/or industrial emission sources (Sandroni and Migon, 1997). Table 2 provides enrichment factors calculated for Cd, Cu, Pb and Zn normal&d to Na (see Flament (1985) and Migon (1988) for calculations of enrichment factors), for the atmospheric aerosol (EF*,,) and the sea surface microlayer (EFs,J. The ratio EFAt,I EFSea clearly shows that the direct metal inputs (i.e. from the continent) are far more important than those coming from the marine surface (i.e. recycled inputs). Thus, although 1. the surface microlayer very significantly concentrates trace metals (Armstrong and Elzerman, 1982), frequently by lo- to lOOO-fold with respect to concentrations of the subsurface water a few centimetres below (Hardy et al., 1985) and airborne sea-salt particles may be enriched by a factor of 10” to lo4 (Weisel et al., 1984) and 2. the marine source is very close to the aerosol sampling site, i.c. reloading of the atmosphere with sea-salt particles is rapid and continuous, the recycling component is negligible in the western Mediterranean basin, which is directly subjected to fast and mediumrange transport from the continental emission sources. The homogeneity of the north-western Mediterranean aerosol is mainly the result of the reduced dimensions of this marine region as well as the atmospheric mixing by diffusion (Sandroni and Migon, 1997) at least for anthropogenic elements, which are not usually brought to the sea by pulsed inputs. The open wcstcrn Mediterranean Sea is within the range of coastal anthropogenic emissions, and the direct atmospheric inputs are more important than the metal recycling from the sea to the atmosphere. The latter is particularly true for Zn, probably because of relatively high atmospheric fluxes (Guieu et al., 1991; Migon et al., 1997). The ratio EFA,,/EFs,, is relatively low for Cd and low atmospheric fluxes could be the cause (Dulac et al., 1989; Migon et ul., 1997), in addition to relatively high concentration levels in the surface microlayer. Cu exhibits the lowest concentrations in the surface microlayer and the Lowest EFs,;,. The ability of Cu to form organic complexes is well known (Donat and Van 27.5

Marine

den Berg, 1992; Antelo et al., 199.5; Spokes et al., 1996). Organic matter accumulates at the surface microlayer and thus presumably affects the mobility of Cu and its transfer to the subsurface water layer. Phytoplankton may play a major role in the depletion of the surface waters and, probably, in the surface microlayer: primary producers excrete organic ligands that have a high affinity for Cu (Korazac et ul., 1989; Zhou et al., 1989; Elbaz-Poulichet et al., 1994) and the concentration of complexed Cu should vary significantly according to seasonal variations of biological activity (Shine and Wallace, 1995). More globally, the efficiency of phytoplankton uptake should partly determine trace metal concentrations in surface waters (Morel et al., 1991) particularly for metals, such as Cu, .having a role in the growth of phytoplankton. Cu concentrations in the microlayer might be very variable in time, therefore, and the variations in the subsurface water might be of different range. It is noteworthy that the concentrations found in the marine microlayer depend strongly on the mcasurement methodology. The thickness of the water microlayer considered obviously determines the dilution of the superficial organic film. It is thus expcctcd to find significantly different concentrations in comparable marine areas (e.g. in the North Sea, Flament (1985)). However, a variation by a factor of 100 on microlaycr metal concentrations would not signihcantly modify the relative importance of the recycling contribution, at least for Cu, Pb and Zn. Likcwisc, trace metal content in bulk particulate (after acidification and UV irradiation - see Marine sutface microlayer section) should not significantly modify the prcscnt results. The quantities and types of anthropogcnic chemicals entering the Mediterranean atmosphere (and thus the surface microlayer) continue to increase (Bethoux et al., 1990). Moreover, the spatial variability of metal concentrations in the microlayer has not been taken into account here, and Carlson (1983) reported cvidcncc, in certain cases, of decreasing enrichment in the microlayer with increasing wave state. Nevertheless, taking into account the dcgrcc of accuracy of the present calculations, the enrichment of the microlayer between 1983 and 1994, as well as its spatial heterogeneity, should not significantly modify our conclusions, E:F’,,,,,, and EFsc,, being different by several orders of magnitude (from 2 for Cd up to 6 for Zn; see Table 2). These results are in agreement with the lack of correlation observed in the atmospheric aerosol at the coastal station of Cap Ferrat between trace metals originating from continental sources and sodium (Migon, 1988): the trace metal content brought by air masses coming from marine areas (i.e. recycled pollutants) is always negligible compared with that loaded by continental air masses (i.e. direct atmospheric loadings). In certain sets of data, an inverse correlation was observed; this indicated that marine inputs arc 276

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characterized by very low trace metal content despite recycling processes, contrary to continental inputs enriched in anthropogenic elements (Migon, 1988). Sea-salt aerosols exhibit large mass median diameter (i.e. on average 5.9 pm vs 0.7 urn for atmospherically transported Cd and Pb Dulac et al. (1989)) which significantly reduces their residence time and their range and, finally, their influence in atmospheric reloading processes. Airborne metal concentrations are found to bc very low in some remote areas. For example, mean Cu, Pb and Zn concentrations in the atmospheric aerosol are 0.12 ng rn-’ and respectively 0.044 ng m p3, 0.17 ng rn-’ at Enewetak Atoll (tropical Pacific Arimoto et al. (1987)). In such regions the emission sources are very far away and the recycling component may play a more important role in trace metal biogeochemical cycles.

Conclusion In the north-western Mediterranean the emission sources are abundant and arc close to the open sea. As a consequence, direct atmospheric inputs are very important, compared with the recycling component which dots not contribute significantly. At lower spatio-temporal scales, the natural variability of metal concentrations in the diffcrcnt reservoirs may wcakcn the dominance of the atmospheric contribution, but, globally, this variability should not invalidate the marked prepondcrancc of direct atmospheric inputs. On the contrary, in remote regions (e.g. tropical south Pacific or polar areas), the airborne metal concentrations decrcasc by several orders of magnitude from the emission sources to the oceanic zone and it is expected that the recycling component predominates. This work was par-tly supported Project (MTP 2-MATER/Contribution Marine Science and Technology 960051).

by the Mediterranean Targeted 003), within the EC program (MAST, contract MAS 3-CT

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