Environ. Sci. Technol. 1996, 30, 524-530
Atmospheric Deposition of PCBs to Moss (Hylocomium splendens) in Norway between 1977 and 1990 WENDY A. LEAD,† EILIV STEINNES,‡ AND K E V I N C . J O N E S * ,† Institute of Environmental & Biological Sciences, Lancaster University, Lancaster, LA1 4YQ U.K., and Department of Chemistry, University of Trondheim, N7055 Dragvoll, Norway
Samples of Hylocomium splendens collected in 1977, 1985, and 1990 from the same sites in remote areas of Norway have been analyzed for a range of polychlorinated biphenyl (PCB) congeners. The ∑PCB concentration (sum of the concentration of the 37 congeners screened) declined in all samples from all locations. It is probable that this decline reflects the reduction in the global use and manufacture of these compounds. Because the samples had been air-dried prior to storage, the possibility of contamination due to contact with laboratory air was investigated. It was concluded that, while some contamination may have occurred, it was largely by the tri- and tetrachlorinated groups. General trends in the moss ∑PCB composition are therefore believed to reflect broad ambient changes in the PCB concentration over time. While ∑PCB concentrations have declined, temporal changes in the congener pattern in the samples collected from the same locations were noted. For example, in the south of Norway the relative concentrations of hexa- and heptachlorinated homologue groups decreased to a greater extent than they did in the north. This observation can be interpreted as evidence for differences in congener recycling through the environment according to their volatility, and it is tentatively suggested that this may provide evidence in support of the global fractionation hypothesis.
Introduction In the 1960s and 1970s, studies were carried out to assess the background levels of polychlorinated biphenyls (PCBs) and other semivolatile organic compounds (SOCs) in both the Arctic and Antarctic (1-4). The results showed higher than expected concentrations of such compounds for cold, * Corresponding author telephone: +44 1524 593972; fax: +44 1524 593985. † Lancaster University. ‡ University of Trondheim.
524
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 30, NO. 2, 1996
remote areas. It has since been suggested that this is as a result of the global distillation of these compounds (5-8); i.e., compounds will volatilize from warm and temperate areas, will move northward in the Northern Hemisphere (even though atmospheric air flow may not always be in this direction), and will then recondense when they reach colder circumpolar regions. It has also been hypothesized that differences in the volatility and lability of the individual compounds and also in the ambient temperature will lead to a latitudinal fractionation of SOCs. This is referred to as the “global fractionation” hypothesis (7). In the case of PCBs, therefore, it might be expected that the less chlorinated congeners will move further northward and be degraded more rapidly than the higher chlorinated homologue groups, thus resulting in a temporal change in the north-south profile of the PCBs. There are many data for concentrations of PCBs and other SOCs in different environmental compartments within varying climatic areas, but there are few studies that have been carried out with the prime aim of obtaining evidence to support the global fractionation theory. This could be due to the difficulties associated with obtaining such data. For example, although the use of PCBs was restricted throughout much of Europe and North America in the late 1970s, there will still be source areas, and it will therefore be difficult to collect samples that are far removed from any possible localized sources. In order to infer possible global fractionation, temporal data are required for concentrations at different latitudes and samples that could provide such information are hard to obtain. There are also uncertainties over the PCB distribution between terrestrial and aquatic environments. Calamari et al. (8) collected plants from a range of latitudes at essentially one point in time. Their results showed altitudinal and latitudinal differences in the concentrations of compounds of varying volatility (e.g., hexachlorobenzene and hexachlorocyclohexanes) on vegetation. This observation can be interpreted as evidence for the effect of temperature and a compound’s physicochemical properties on air-vegetation partitioning, but does not in itself provide evidence to support global fractionation since they represent only a “snapshot in time” for one mediumsi.e., vegetationsand, as mentioned above, time trend data are needed for different locations simultaneously before global fractionation can be implicated. Previous studies have shown that the concentration of PCBs in temperate, industrial countries, which can be considered as global source regions, has decreased markedly since the phasing out of their usese.g., U.K. and North American studies have shown declines in soil (9, 10), herbage (11), dated peat and sediment cores (12-15), and air (16). Each of these studies has shown temporal changes, and this allows a rate of change in concentration with time to be obtained. Corresponding time trend data for the samples collected from Arctic areas, however, are extremely limited, and in order to obtain such data, either datable material, such as peat or ice cores, or archived samples from known locations are required. The latter technique has advantages in that it negates the need for the dating of samples and the corresponding errors which this would incorporate into the interpretation of results. It is unlikely, however, that
0013-936X/96/0930-0524$12.00/0
1996 American Chemical Society
FIGURE 1. Location of the 47 sample sites and division of Norway into climatic zones (adapted from ref 20).
any archived samples that are available from Arctic regions would have been collected specifically for SOC analysis; they therefore will probably not have been treated or stored under ideal conditions, and postcollection contamination or losses may have occurred (17). It is expected that plants will play an important role in the global cycling of SOCs since (1) they cover over 80% of the earth’s land surface, (2) the surface area of plants is generally much greater than the area of the ground they cover, and (3) vegetation has a high lipid fraction which is likely to accumulate lipophilic compounds such as PCBs. Mosses are thought to be ideal biomonitors for air pollution for these three reasons and also because they depend entirely on the atmosphere for delivery of nutrients and lack both cuticle and internal transport mechanisms (18). However, it has not been possible to calibrate plants to give direct air concentrations of compounds such as PCBs, so they must be used as qualitative rather than quantitative indicators (19). In this study, archived samples of the epigeic moss Hylocomium splendens, which were collected in 1977, 1985, and 1990 from sites across Norway, have been analyzed for a range of PCB congeners. The samples were collected originally to survey heavy metal deposition (20). Norway acts as an ideal platform for pollutant long-range transport (LRT) studies because of its large latitudinal variation and its extremes of climate (21, 22).
Materials and Methods Sampling. More than 500 samples of the epigeic moss H. splendens were collected from different sites across Norway in the summer of 1977 as detailed by Steinnes et al. (20). The same sites were visited and further samples taken in 1985 and 1990. After collection, all samples were air-dried at 30 °C and then stored in sealed polyethylene bags in the dark, at room temperature. Mosses from 1977, 1985, and 1990 from 47 of the sample sites (Figure 1), chosen as being
representative of Norway, were analyzed in 1993-1994 for PCBs. Solvents and Glassware. All solvents were obtained from Rathburns and were of pesticide grade. All glassware was heated at 450 °C overnight and then cleaned with acetone and hexane before use. PCB standards used in quantification were obtained from Ultra Scientific Inc. Extraction and Cleanup. Samples of 2-4 g were weighed into preextracted Whatman paper extraction thimbles. The samples were then extracted in a Soxhlet apparatus for 16 h in n-hexane. The volume of the extract was reduced to ∼2 mL using a rotary evaporator prior to cleanup by Florisil column chromatography. The extract was finally transferred to 0.5 mL of dodecane for analysis. Dry weights were calculated on a subsample of each moss by heating at 105 °C for 24 h, cooling in a desiccator, and weighing. Analysis. Extracts were analyzed using capillary gas chromatography with electron capture detection (ECD). A Hewlett-Packard 5890A instrument and a cross-linked 5% phenyl methyl silicone column (column length 50 m × 0.2 mm; film thickness 0.11 µm) were used. The GC conditions were identical to those of Alcock et al. (9). IUPAC congeners (elution order) 30, 18, 54, 28, 52, 104, 40, 61/74, 66, 155, 101, 119, 110/77, 82/151, 149, 118, 188, 153, 105, 138, 126, 187, 183, 128, 185, 202/156, 180, 170, 198, 201, 194, 205, and 209 were screened. (∑PCB refers to the sum of these congeners.) Prior to analysis, congener 209 was spiked into each sample extract to act as a retention time marker. A VG Minichrom data handling system was used to identify peaks with respect to congener 209. Analytical limits of detection ranged from 0.02 to 0.16 µg kg-1 (dry weight). QA/QC Protocol. Prior to extraction, the extraction efficiency was checked both by spiking six moss samples with a full range of congeners and by extracting a BCRcertified reference material. One analytical blank was prepared using the same extraction and cleanup procedure with every five samples. Any PCBs present in this blank were subtracted from the sample. Twenty percent of the moss samples were extracted and analyzed in duplicate. Replicate analyses of two of the samples gave a relative standard error of
