AIR HYGIENE REPORT no. 10
Contents

II HEAVY METALS

1 Bryophytes

  1.1 Introduction
  1.2 Monitoring design

1.2.1 Method selection
1.2.2 Species selection
1.2.3 Site selection
1.2.4 Chemical analyses

  1.3 Multi-national surveys
  1.4 National surveys
  1.5 Regional surveys
  1.6 Urban and industrial surveys
  1.7 Transplants

1.1 Introduction

Bryophytes include mosses and liverworts but most literature on air pollution monitoring centres on mosses. Bryophytes can indicate the presence of elements and their concentration gradients. The use of bryophytes constitutes an effective method in air pollution monitoring for many reasons:

• Many species are widely distributed and grow in a range of habitats.
• Bryophytes are small and easy to handle.
• Most of them are evergreen and can be surveyed all year round.
• Bryophytes lack a cuticle and a root system and obtain nutrients as particulates and in solution directly from atmospheric deposition. They have good bioaccumulating ability, particularly for heavy metals, where metal concentrations reflect deposition without the complication of additional uptake via a root system.
• Comparisons of fresh samples with herbarium specimens enable retrospective analysis of metal pollution.
• The ability of bryophytes to accumulate elements in very high concentrations aids chemical analyses of the tissues and may facilitate the detection of elements present in very low concentrations in the environment.
• The annual growth increment is usually easier to detect in mosses than lichens and mosses are often believed more desirable for temporal studies (this is particularly true of Hylocomium splendens).

Some studies have been carried out to compare the effectiveness of different biological samplers as biomonitors. Kansanen and Venetvaara (1991) compared the capability of two mosses, an epiphytic lichen, pine bark samples, pine needle litter, earthworms, and moths in assessing airborne chromium and nickel dust near a ferrochrome and stainless steel works in Finland. The two mosses and the lichen were the most effective biomonitors at low and moderate depositions. None of the biomonitors worked effectively at high deposition loads. Moss and epiphytic lichens were found to be the best indicators for zinc (Zn) and aluminium (Al) in a study of biological indicators around the Rautaruukki steel works in northern Finland (Mukherjee and Nuorteva, 1994).

Most methods in heavy metal monitoring employ bryophytes as bioaccumulators and involve sample collection followed by laboratory analysis techniques to detect actual levels. Bioindication of heavy metal deposition by the use of bryophyte distribution techniques and physiological effects is rare. References

This section is subdivided depending on the geographical scale of the survey, into multi-national, national, regional and urban/industrial areas. National studies are mostly part of wider multi-national monitoring programmes. Urban/industrial area studies deal with urban areas and point source emissions. Spatial studies and temporal studies are considered.

Table 2.1 summarises the surveys reviewed in this section. The type of survey, species used and metals analysed are compared.

The majority of studies have been multi-element investigations and few have been restricted to a particular metal except where different methods of analysis were required. Transplantation techniques are discussed separately. Comment is limited to monitoring metal deposition from the atmosphere using bryophytes and will not examine in detail the effects of metals on bryophytes. References

Since Burton's review in 1986, approaches to moss monitoring of heavy metal deposition have changed little in principle. Refinements have been made in monitoring design, in terms of standardised sampling and reduction of error and variance. As may be expected new developments in chemical analysis have occurred.

1.2 Monitoring design

The importance of planning when initialising a biomonitoring programme of trace-element air pollution is emphasised by Wolterbeek and Bode (1995). Successful cryptogram monitoring is achieved when contaminant burden is readily distinguished from background levels in the plants. Certain important parameters have been considered in heavy metal monitoring utilising bryophytes, which are discussed below. It is noteworthy that many of the following comments are also applicable to the monitoring of aerial deposition of heavy metals using lichens unless specificity to mosses is expressed. References

A major choice lies between using techniques which employ indigenous species or transplanted species. This will ultimately affect the type of species selected and to some extent the chemical analytical techniques employed.

Comparisons of techniques utilising indigenous and transplanted samplers are summarised in Table 2.2 (taken from Gailey and Lloyd, 1993).

Factors which should be considered in methodology selection include finance and resources, desired accuracy of results, study time-scales, size of study area, extent and type of pollution.

In metal deposition biomonitoring, species selection criteria include the availability of the species, its tolerance, its bioaccumulation characteristics and ease of sampling (Wolterbeek and Bode, 1995). Additionally, the species utilised and its effectiveness will depend to an extent on the elements to be monitored.

Puckett (1988) reviewed the applicability and mechanisms of mosses and lichens as biomonitors of metal deposition. Ectohydric mosses with no differentiated water conducting system, enabling direct absorption over the entire plant surface, are more appropriate than endohydric mosses possessing differentiated water conducting systems and cuticle-like surfaces. The use of epigeic mosses (mosses growing naturally on the ground) has been recommended in Scandinavia for assessment of heavy metal deposition on a regional scale (Steinnes et al., 1993). Pleurocarpous species, otherwise known as the carpet-forming mosses or feather mosses, are probably the most commonly utilised group (Table 2.1). However, element concentrations in carpet forming mosses may be elevated by soil-blown dusts. Such contamination and possible misinterpretation of results would be particularly heightened in seasonally arid countries (Ruhling, 1995).

Markert and Weckert (1989) investigated the suitability of Polytrichum formosum as a passive biomonitor of heavy metal deposition. Moss samples from the forest under study showed seasonal variation in metal content. The authors concluded that sample collection of this species should be undertaken in the last week of September if comparable results between regional surveys are to be obtained. This study highlighted that species type and sampling period should be considered prior to initiating a moss monitoring programme.

The signal-to-noise ratio was suggested by Wolterbeek et al. (1996) as a means of assessing the quality of a biomonitoring survey. The authors used large-scale biomonitoring surveys of trace-element air contamination to illustrate the utility of such an approach. The investigators used survey variance as the signal and the survey noise was defined by measurement of local variance per site. The most significant conclusion from this study was that the 'selection of the biomonitor species should be based on minimisation of the signal-to-noise ratio rather than on minimisation of the noise level of the survey'.

The density and location of sampling sites will depend very much on the type of survey. Larger scale surveys covering larger areas will obviously require more sites than studies investigating point emission sources (Table 2.1). In the latter, sites are frequently spaced along transects or gradients in relation to the pollution source. Intensity of sampling sites should be adequate to detect gradual changes along the study area. If indigenous species are to be utilised, the number and location of sites will depend on the natural distribution of the species. If transplantation techniques are used, choice of sites is at the discretion of the investigator.

At the sampling site, attention should be given to the substrate since this may affect the elemental composition of the study species. Other considerations include safety and ease of access of the site.

To overcome noise variation the development of strict criteria in site selection is necessary. For example in large-scale surveys in Europe, sampling sites were selected in areas remote from roads, large population centres and industrial plants in order to identify areas susceptible to long-range transported pollutants (Ruhling, 1995). Samples from forest ecosystems are usually taken from openings in the canopy, not directly exposed to throughfall precipitation. However, some species are found in sheltered areas where they may not be freely exposed to aerial deposition and measured levels in the sample may not adequately indicate pollutant levels. This highlights the need to choose locations where exposure to atmospheric pollutants is not reduced.

Sources of heavy metals other than atmospheric deposition which can contribute to metal concentrations in moss samples include (Steinnes, 1992):

• Natural cycling process, particularly the deposition of emissions from marine sources.
• Root uptake in vascular plants and subsequent leaching from the tree canopy, understorey vegetation and leaf litter.
• Windblown mineral particles.

Ideally an accurate interpretation of results from large-scale moss surveys should include an assessment of the contribution from other sources (Brumelis and Nikodemus, 1995). This can be undertaken using multivariate analyses techniques such as factor analysis (e.g. Sloof and Wolterbeek, 1991). References

In bioaccumulation monitoring studies, the standardisation of sample collection, preparation and analytical techniques has been recommended (Puckett, 1988). In terms of collection this could include the general area of collection (e.g. forest), the specific area of collection (e.g. position on tree) and the quality and quantity of sample. Sample preparation varies for example in washing and drying procedures. Differences also exist in the analytical chemical methods adopted.

Results are more useful when background elemental levels are obtained (Seaward, 1995). Generally, a large number of elements is chosen for analysis because the benefits of obtaining large amounts of data outweigh the extra effort, especially when the extent of fieldwork is independent of the number of elements chosen for analysis (Wolterbeek and Bode, 1995). Contamination during collection should be avoided. Replication of samples is recommended for accurate results. Consistency of measurement units aids comparative studies.

The choice of analytical method will depend on the purpose of the respective survey. Some analytical methods are non-destructive (e.g. neutron activation) and are useful for repetitive surveys such as baseline studies. Samples can also be archived and used at a later date for additional analysis. Destructive techniques include atomic absorption spectrometry and inductively coupled plasma analysis.

Steinnes et al. (1993) compared methods previously used in heavy metal deposition studies in Norway (instrumental neutron activation analysis (INAA) and atomic absorption spectrometry (AAS)) with other multi-element techniques. The alternative analytical techniques investigated were inductively coupled plasma emission spectrometry (ICP-ES) and inductively coupled plasma mass spectrometry (ICP-MS). INAA produced much higher values for Na, Al and Fe compared with ICP-ES and ICP-MS. The former technique measures total content while the other techniques are based on leaching procedures. On consideration of the methods available for moss monitoring in Norway, Steinnes et al. conclude that ICP-ES works well for Fe, Zn, Pb and Cu, to a lesser extent for V and Ni and but is not satisfactory for Cr, Cd and As. ICP-MS analysis proves a good method for all of the above except As and Cd where less satisfactory results were observed. In conclusion, ICP-MS was regarded as a valid alternative to INAA/AAS analysis of Hylocomium splendens samples within the Norwegian monitoring programme at the time. References

A similar study was conducted by Frontasyeva et al. (1994). Epithermal neutron activation analysis (ENAA) was compared with conventional INAA and ICP-MS. ENAA produced promising results for expansive multi-element analysis of mosses used in monitoring atmospheric deposition.

1.3 Multi-national surveys

Techniques using indigenous moss populations to identify and monitor geographical patterns in heavy metal atmospheric pollution are well established in Europe. Precise element concentrations are often not reported and techniques are applied as a practical tool in establishing and characterising deposition sources. Such long-term, larger scale monitoring is extremely useful and also enables transboundary ameliorative action to be taken. Most programmes are ongoing, allowing comparisons over time and space to be made.

In a joint Nordic project, Rasmussen et al. (1988) used moss analysis as a means of identifying sources of airborne pollutants and mapping metal deposition in northern Europe (Denmark, Finland, Norway and Sweden). In 1985 Hylocomium splendens and Pleurozium schreberi samples were collected from openings in coniferous forest or young plantations, not directly exposed to throughfall precipitation. The three youngest fully developed segments of the moss were used for analysis. Atomic absorption, neutron activation or ICP techniques were used to determine various metal concentrations. Lead (Pb), arsenic (As), cadmium (Cd) and vanadium (V) concentrations in samples showed a steep gradient from south to north Fennoscandia, with highest levels in the south decreasing towards the north. Nickel (Ni), chromium (Cr), copper (Cu) and iron (Fe) and to an extent zinc (Zn) concentrations showed weaker gradients. This pattern was attributed to long-range transport of air pollutants from the densely populated areas in the south. In non-forested areas such as alpine and agricultural regions, metals originating from soil dust such as As, Cr, Cu, Fe and V were present in high concentrations in the collected moss samples. The importance of larger local emission sources was also revealed. References

Ruhling (1995) reported on a comparable study in northern Europe carried out in 1990-91 as part of a large-scale heavy metal monitoring programme covering 21 European countries. By mapping results, conclusions on heavy metal deposition in northern Europe were drawn and comparisons with past studies made. Sources of long-range transported air pollution were identified andregional atmospheric deposition of heavy metals was characterised. Almost ten locally important emission sources of heavy metals and the extent of these emissions were established. Metals showed similar gradients from south to north as indicated by Rasmussen et al. (1988). Heavy metal concentration levels have displayed a decrease over the last two decades. The authors attribute this to improved filter techniques and response to new legislation. These studies clearly emphasised the effectiveness of moss survey technique as a biological tool in long-term, large-scale monitoring initiatives. References

The UN ECE Convention on Long-range Transboundary Air Pollution (1993) produced a manual for integrated monitoring with a programme phased between 1993 to 1996. The overall purpose of the programme is monitoring and assessing effects from air pollutants in the environment. Numerous countries are involved in the programme but Sweden was appointed lead country and Finland took responsibility for data handling. Nordic countries have a high profile within the framework particularly in the development of methods. Monitoring metal concentration of mosses is included in the programme. Detailed sampling procedures were prescribed in the manual. A general outline is provided below:

• Three composite samples should be collected from either Pleurozium schreberi or Hylocomium splendens species.
• Samples should be retrieved from areas not exposed to direct throughfall water - ideally in forests or open heathland or peatland.
• Sampling period should be early summer.
• Contamination should be minimised during sampling.
• Only green shoots from the three most recent years should be included in the analysis.
• Chemical analysis should be undertaken using AAS /ICP/neutron activation.
• Metals As, Cd, Cr, Cu, Fe, Ni, Pb, Zn should be monitored.

1.4 National surveys

Sweden is very experienced in moss monitoring techniques and has been using mosses as a means of studying heavy metal deposition every five years since 1975 (Bernes, 1990). Sampling and analysis is carried out in much the same way as the studies mentioned above. Mapping levels of metals in mosses has illustrated the geographical distribution and long-term changes in metal fallout. The amount of fallout is quantitatively estimated from the growth rate of Hylocomium, which grows at the same rate in northern and southern Sweden according to the equation:

metal deposition (mg m^-2 y^-1) = metal content in moss (mg kg^-1) x average growth rate (0.15 kg m^-2 y^-1)

This assumes, however, that mosses possess 100% efficiency in metal uptake from the atmosphere. Comparisons of fallout estimates with heavy metal values in precipitation were good for Pb, Cd, Cu, Zn and V. Correlations with levels of Cr and Ni were less satisfactory and this was attributed to mosses concentrating these metals from other sources besides atmospheric deposition (Bernes, 1990). The author concluded that in point emission source studies, where metals tend to be deposited in particulate form rather than through precipitation, moss analysis provides an adequate picture of total fallout. References

Schaug et al. (1990), Steinnes et al. (1994) and Berg et al. (1995) used the moss technique for mapping atmospheric deposition patterns of metals in Norway. Pilot studies carried out in the 1970s determined the most appropriate species and analytical techniques. Hylocomium spendens was chosen for a number of reasons, most of which are mentioned in Section 1.1. The investigators reported on results from national surveys carried out in 1977, 1985 and 1990. Elements monitored, number of sites and date of study are provided in Table 2.1. As the studies progressed the sampling network was altered to enable more accurate descriptions of areas of high deposition rates (i.e. the south). The Norwegian surveys differed from the multi-national ones discussed earlier in that many more elements were investigated. This was achieved by modification of the analytical techniques.

This ongoing programme enabled national spatial and temporal patterns in heavy metal deposition to be presented as isopleth maps. Although some variation occurred over the survey years, statistical analysis enabled the following pollution sources to be defined (elements in brackets derived their main contribution from some other factor) (from Berg et al., 1996):

• Long-range transported pollutants from other parts of Europe: V, (Cu), Zn, (Ga), As, Cd, Mo, Cd, Sb, Se, Ag, Hg, Tl, Pb, Bi.
• Mineral particles, mainly windblown dust from local soil: Li, Al, (V), Cr, Fe, (Co), Ga, Y, La, Th, U.
• Local point sources: Co, Ni, Cu.
• Marine environment: (Li), B, Na, Mg, Ca, Sr.
• Root uptake in vascular plants from soil, and subsequent transfer to mosses by leaching from living or dead plant material: (Mg), (Ca), Mn, (Cu), (Zn), Rb, (Sr), Cs, Ba.

Most long-range transported elements present in southern Norway had decreased by 1990 to 70 to 50% of 1977 levels.

Spatial and temporal trends of atmospheric deposition of mercury (Hg) in Norway were investigated using moss samples and peat samples, respectively (Steinnes and Andersson, 1991). The distribution of Hg in moss is different from the previously studied elements in that it did not appear to be associated with long-range atmospheric transport or point sources. Hg showed a much less pronounced south-north gradient than other elements and the authors suggested that dry deposition of Hg may be important at northern latitudes.

Moss techniques were reported by Kuik and Wolterbeek (1995) in the Netherlands as part of a larger European study. This 1992 study was compared to previous accounts using epiphytic lichens samples from the same area in 1986 to 1987. Average moss concentrations in the 1992 study were significantly lower than those observed in lichens in 1986/87. Application of Monte Carlo-Assisted Factor Analysis to the data proved an effective method of determining and characterising different heavy metal sources throughout the country. Major sources were classified as:

• Crustal material: Al, Ce, Cr, Fe, La, Sc, Sm, Th.
• Sea aerosol: Na, Br, I.
• Zinc industry: Zn, Pb, Hg, As, Co, Sb.
• Metallurgical processes or refuse incineration: Sb, V, Se, Co, Pb, Hg, Fe, Ni, La.
• Oil combustion and processing of oil products: Cr, Ni, Co, Fe, V.

1.5 Regional surveys

More intensive regional based investigations using natural growing mosses as biomonitors of heavy metal pollution have been undertaken. Results tend to be classified into zones within the region allowing identification of sources. Description of the heavy metal status within the region can be made. References

The epiphytic moss Polytrichum juniperinum and the bark of Azardirachta indica produced comparable results when used to determine the atmospheric metal in a north-eastern region of Nigeria (Kakulu, 1993). Three pollution zones for Pb and Zn (high, medium and low) were evident within the region (Table 2.3). Pb and Zn levels in moss samples showed ranges of 10 to 241 µg g^-1 and 28 to 123 µg g^-1 dry weight, respectively. Cd, Fe and Mn moss levels where highest in the big towns. For example, mean Cd, (µg g^-1) Mn, (µg g^-1) and Fe (mg g^-1) in the high pollution zones were 0.41, 97 and 12.7, respectively, whereas mean metals concentrations in dry weight moss samples in the low pollution zones were 0.1, 37.8 and 5.6, respectively. Ni and Cu did not show significant concentration gradients between the larger towns (Jos, Maiduguri and Bauchi) and smaller towns and villages. The author concluded that Pb, Zn and Fe were responsible for the greatest heavy metal pollution burden in the study area. Sources of these and other metals in the high pollution zone were attributed to fossil fuel burning due to industrialisation, automobile exhaust emissions and incineration of domestic wastes due to urbanisation. Vehicle emissions, small metal works and incineration of domestic wastes accounted for the presence of metals in the less polluted areas.

Godzik and Grodzinska (1991) used Pleurozium schreberri and Hylocomium splendens to gauge the heavy metal burden in Mazurian Landscape Park, Mazurian Lake District, Poland, in comparison to the relatively polluted Ojcow National Park and the 'cleaner' Bialowieza National Park. Metal levels in Mazurian Landscape Park were significantly lower than in the other parks. For example, the Pleurozium from the Ojcow National Park was found to accumulate 14 times as much lead and cadmium. The authors concluded that the relatively unpolluted character of the landscape park in combination with its unique flora and fauna is justification for its promotion to a higher conservation, national park status. This is a primary example of the application of moss monitoring techniques as a practical management tool. Another relevant outcome of this study was the observation of Hylocomium splendens as a better copper accumulator than Pleurozium schreberri.

Winner (1988) reviewed studies related to metal concentrations and mosses in North America. These studies were mainly regionally based. References

1.6 Urban and industrial surveys

Moss techniques have been applied to measure heavy metal levels and trends within and around urban and industrial areas. These studies can analyse temporal and/or spatial trends in heavy metal deposition and results are generally expressed as pollution gradients. Within the gradient, metal levels in the local moss populations are seen to decrease with increasing distance from the suspected source.

In Sweden, long-term metal fallout, particularly of Cr has been mapped using moss analyses as part of the monitoring programme operated by Vänersborg-Trollhättan Regional Air Quality Association. Samples of the carpet moss Hypnum cupressiformae from approximately thirty sites in the vicinity of the works have been collected every three years since 1973 (Bernes, 1990). In the early 1970s chromium levels at Trollhättan reached 20,500 mg/kg dry weight moss in the immediate vicinity of the plant. Less polluted areas of Sweden showed moss levels of 3 mg/kg at this time. Chromium emissions from Trollhättan fell in the late 1970s but this coincided with the initiation of chromium alloy manufacture at Vargön. Since then, high emissions from Vargön have been reduced and the Trollhättan plant has been closed down. Bernes (1990) reported other similar monitoring programmes in Sweden.

Gupta (1995) analysed three moss species in an assessment of non-point sources of heavy metal contamination in Shillong, Meghalaya State, north-eastern India. Plagiothecium denticulatum (from stones and cemented surfaces) and Bryum argenteum (from asbestos roofs) were collected from four sites within the urban area and four from the outskirts. Sphagnum sp. was collected from one site where it was found in a forest edge, two km away from the city centre. Results (Table 2.4) showed that Sphagnum sp. reflected Cd and Zn concentrations better than the other two even in the suburban areas. P. denticulatum appeared to be the best accumulator of lead. All species accumulated manganese effectively. An urban-suburban gradient was obtained for lead and zinc in P. denticulatum and for cadmium in B. argenteum. This study provided a good baseline dataset for future moss monitoring in India, where data is lacking. It will allow comparative studies between other urban areas.

Tissue analysis of the woodland epiphyte Isothecium stoloniferum was used to report atmospheric trace-element deposition in the Fraser Valley, B.C. Canada from 1960 to 1993 (Pott and Turpin, 1996). By studying herbarium samples available from the 1960s, significant reductions of varying degrees of Cd, Cr, Pb, Ni and Zn levels in moss samples from 1960 to 1993 were observed. The authors provided several reasons for the decline. There had been an obvious shift in the area from a resource and manufacturing based economy in the 1970s towards a service-based economy in the 1990s. The closure of heavy industries, new emission control legislation, reduction in fossil fuel combustion and significant decreases in leaded petrol consumption contributed to the reduction in metal deposition. Only manganese (Mn) showed a significant increase over the survey years. This was attributed to the introduction of methylcyclopentadienyl manganese tricarbonyl as an anti-knock additive to petrol in the 1970s. References

1.7 Transplants

Transplantation is an experimental technique where study plants are transferred, along with their original substrate, from unpolluted control areas to suspected or known polluted areas. The effects and responses of the transplants are subsequently examined after a measured time scale and compared to the control areas. Conclusions on pollution levels and/or nature of the pollutants with respect to exposure time can be composed. In terms of aerial metal monitoring using transplants, levels in tissues rather than effects are measured before and after the exposure period.

Pleurozium schreberri showed elevated Hg concentrations when transplanted from control sites to Roundtop Mountain and Mt. Tremblant in southern Quebec despite their distance from known point sources of mercury (Evans and Hutchinson, 1996). Mercury values in the Roundtop Mountain and Mt. Tremblant summit sites were 248.3 and 174.0 ng g^-1 representing increases of 129 and 61% respectively. This was attributed to long-range transported Hg deposition.

A similar approach is the moss-bag technique which involves using Sphagnum or Rhynchostegium species in nylon or muslin (0.07 to 0.9 mesh cm^-1) bags. Exposure times are usually shorter with this method prior to elemental analysis. This method, although applied to the terrestrial environment, is used more extensively in the detection of heavy metals in the aquatic environment. The technique is based on the high cation exchange capacity of mosses.

The effects of exploration activities associated with a niobium-mineralisation in Sarfartoq, south-west Greenland were illustrated using the moss-bag technique (Pilegaard, 1993). Samples of Sphagnum girgensohnii were collected from a remote unpolluted area in southern Sweden. Metal concentrations in the pre-exposed bags were measured (i.e. background levels). Metal levels were determined from moss bags at twelve sites, set at three different periods: before major dust producing activities; during intensive drilling activity; and after work was completed. Concentrations of Nb in particular, especially in sites close to the outcrop, were elevated during the period of highest dust production. However, examination of the indigenous flora indicated that pollution existed near the outcrop prior to drilling, with elevated concentrations in Nb, La, Ce, Th and U. This study emphasised the importance of pre-operational monitoring in assessing the scale of effects at such sites. It also provided good baseline data for further exploration works and acted as a scoping study for the sensitivity and appropriateness of methods.

Gailey and Lloyd (1993) compared the suitability of four different biomonitors in assessing short-distance and short-term changes in airborne metal contamination in Armadale, central Scotland. This formed part of an environmental epidemiology study of respiratory cancer. Sphagnum moss bags produced more consistent results than the transplanted lichen Hypogymnia physodes attached to its twig substrate. Indigenous Hynum cupressiforme provided better results than the indigenous lichen Lecanora conizaeoides. Sampling sites were positioned along a gradient from a steel foundry in the town. Metals investigated are shown in Table 2.1. All samplers showed a general decrease in metal content from the foundry, indicating a declining pollution gradient from this source. Statistical analyses of temporal data implied that meteorological factors and the steel foundry were more important pollution sources than the brickworks, the other main industry in the town.

Kirchhoff and Rudolph (1989) described a sandwich technique for the continuous monitoring of air pollutants with the bryophyte Sphagnum magellanicum, collected from a bog in Germany. The sample was washed and transferred to the field sandwiched between two layers of plastic screen and suspended, by the edges only, in a plastic holder. The method allowed the installation of a heater in the holder, permitting winter use. Results were compatible with those obtained from rainfall trapped at the same sites. References

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