biomonitoring of trace element pollution

Biomonitoring of trace element air pollution: principles, possibilitiesand perspectives

Bert Wolterbeek*

Delft University of Technology, Interfaculty Reactor Institute (IRI), Department of Radiochemistry, Nuclear Environmental Studies, Mekelweg 15,

2629 JB Delft, The Netherlands

Received 15 March 2001; accepted 9 January 2002

‘‘Capsule’’: Programs on air pollution biomonitoring should accommodate input from physiology, emission control,analytical chemistry, ecology and epidemiology.

Abstract

This paper discusses the biomonitoring of trace element air pollution. Much attention is given to both lichens and mosses as thedominant plant species used in biomonitoring surveys. Biomonitoring is regarded as a means to assess trace element concentrationsin aerosols and deposition. This implies that the monitor should concentrate the elements of interest and quantitatively reflect its

elemental ambient conditions. Environmental impact on the biomonitor’s behaviour is viewed as resulting in changes in the dose–response relationships.

The current literature is briefly reviewed, for plant’s behaviour modelling, for laboratory studies on physiological processes

responsible for accumulation, retention and release, and for field work on quantification of dose–response relationships. Monitor-ing of elemental atmospheric availability is presented as deriving its relevance from presumed impact on both ecosystem perfor-mance and human health; source apportionment is regarded as an important parallel result for purposes of emission regulatory

management. For source apportionment, the paper argues in favor of multi-elemental determinations, supplemented by informa-tion on organic compounds and elemental chemical forms. Furthermore, the discussion points towards more explicit coupling ofbiomonitoring data to knowledge and databases on both emission registration, ecosystem performance and human health. This

means that multidisciplinary programs should be set up, which accommodate expert inputs from biomonitoring, emission controlprograms, analytical chemistry, ecology, and epidemiology. # 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Air pollution; Biomonitoring; Quantification; Trace element; Impact; Health

1. Introduction

Concern about atmospheric pollutants underlies theefforts to establish control programmes in many coun-tries. Policies may be both source-oriented (e.g. tech-nology-based emission management) and effect-oriented(e.g. risk assessment). In most countries, various reg-ulatory instruments are combined into a co-ordinatedcontrol programme. In practice, controlling (anthro-pogenic) air pollutants is a very complex problem:sources and emissions have to be identified, analyticalmethods have to be evaluated, risks have to be assessed,critical emissions have to bew controlled, and economical

aspects have to be integrated (Sloof, 1993; Wolterbeekand Freitas, 1999).

The necessary information on air pollutants can beobtained by dispersion modelling (source-orientation, apriori known emission sources) and by field measure-ments of the immission (receptor/effect orientation). Inmany countries, dispersion modelling has gained moreand more interest, also based on economic reasons:technical field measurements require equipment andmanpower and are generally associated with high costs(Van Duijvenbooden, 1992; Ham, 1992). Immissionmeasurements, however, should be regarded as neces-sary and indispensable: they may be used to validatedispersion models, and the data obtained may indicatethe presence of sources which are not known or registered(Wolterbeek and Freitas, 1999). Immission measure-ments require long-term sampling at large numbers of

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* Tel.: +31-15-2787053; fax: +31-15-278906.

E-mail address: [email protected] (B. Wolterbeek).

sampling sites. Such measurements using technicalequipment have been few, mainly due to the high costs,and the lack of sufficiently sensitive and inexpensivetechniques which permit the simultanuous measurementof many air contaminants (Puckett, 1988). It is here thatbiomonitoring comes in.

This paper addresses biomonitoring as a techniqueused in the context of programmes on trace element airpollution. Attention is given to principles, to possibi-lities, and eventually, to the biomonitoring technique’sperspectives in future studies. Throughout the paper,emphasis is on lichens and mosses, since they are thedominant plant species used in environmental surveyson trace element air pollution.

2. Our interest in trace element air pollution

2.1. Ecosystem performance

Longest standing attention for widespread air pollu-tion stems from our concern over its ecological effects.The environmental impact of atmospheric depositionhas been studied for more than a century; probably thefirst effect that was described on a scientific basis wasthe decline of epiphytic lichens in areas with high levelsof atmospheric pollution. Ever since Nylander’s (1886)classical report on the epiphytic lichens of Paris and itssurroundings, extensive studies have been performed inmany areas (Hawksworth, 1971; Barkman, 1958; DeWit, 1976). The decline of forest tree vitality caused byatmospheric pollution has been known for a long time(Crowther and Ruston, 1911; Wieler, 1913), and theoccurrence of forest decline over large parts of Europe isnow well documented (Van Breemen, 1990). In analogyto the lake and streamwater acidification, first describedfor Scandinavia (Oden, 1968), widespread effects ofatmospheric deposition (‘‘acid-rain’’) were described forecosystems such as freshwater bodies, forest tree layers,heathland, chalk, grass and moorlands (Westhoff andVan Leeuwen, 1959; Heil, 1984; Bobbink and Willems,1987; Arts, 1990), and for the decline of many plantspecies (Van Ree and Meertens, 1989; Jansen and VanDobben, 1987).

Although the research effort has led to a greatlyimproved scientific understanding of the abiotic effectsof atmospheric deposition, especially in the fields ofatmospheric chemistry (Asman, 1987), soil chemistry(Mulder, 1988) and water chemistry (Van Dam et al.,1990), many of the biotic effects are still poorly under-stood, particularly in the terrestrial environment:although many changes in vegetation are now generallyattributed to atmospheric deposition, dose–effect rela-tionships are usually poorly known (Heij et al., 1991).Reviews on monitoring, behaviour and impact of ter-restrial trace element pollution can be found in Farago

(1994), Markert (1993a, b), Adriano (1986), Bowen(1979), Lepp (1981a, b) and Martin and Coughtrey(1982).

2.2. Human health

In addition to the ongoing concern for ecosystemperformance as such, attention has been and becomesincreasingly more directly focused on human health.This may be ascribed to the generally recognized impactof ecosystem performance on human well-being; fur-thermore, health-care has also been progressively devel-oping towards approaches which include our nutritionand our social and environmental surroundings (Vanden Broucke and Hofman, 1993).

As a result, throughout the world, epidemiologicalstudies were set up on air pollution and mortality ratesand respiratory health effects, initially mostly on airparticulates, ozone, acid rain, NOx and sulphur oxides(Lebowitz, 1996; Dockery et al., 1993; Spix et al., 1993;Castillejos et al., 2000). For The Netherlands, the lattermay be illustrated by the steady increase in clinical–epidemiological medical services from the 1980sonwards (Vandenbroucke and Hofman, 1993), and bycurrent Dutch epidemiological studies, which also focuson the relationships between atmospheric black smoke(diesel engines) and nitrogen dioxide (all motorizedvehicles) on the one hand and both total mortality andcancer incidence on the other (Dockery et al., 1993,Bausch-Goldbohm, personal communication).

Today, epidemiological studies progressively includeevaluations of possible relationships between healtheffects and the (soluble) elemental concentrations inparticulates (Roemer et al., 2000; Carter et al., 1997;Heinrich et al., 1999; Badman and Jaffe, 1996; Laden etal., 1999, 2000). Wappelhorst et al. (2000), in a study inthe German–Czech–Polisch Euroregion Neisse, areamong the first to relate disease incidences with ele-mental biomonitoring outcomes. This growing attentionto atmospheric pollutants other than the ‘‘tradionional’’air pollutants (sulphur oxides, ozone, nitrogen oxides),such as heavy metals, polycyclic aromatic compoundsand halogenated organic compounds, all differingwidely with respect to their environmental and healthimpact properties, is also shown by their presence as‘‘black list substances’’ in the priority action lists in alarge number of countries (see Wiederkehr, 1991).

3. Assessing atmospheric trace elements

Information on atmospheric trace elements can beobtained by modelling of their atmospheric dispersionand deposition, based on a-priori known emissionsources (source orientation), and by measurements ofactual atmospheric occurrences and/or deposition

12 B. Wolterbeek / Environmental Pollution 120 (2002) 11–21

(receptor-orientation) (Sloof, 1995; Wolterbeek andFreitas, 1999). In many countries, the dispersion mod-elling approach is attracting more and more interest,also due to economical reasons: the receptor-orientedmeasurements require expensive equipment and man-power, and are generally associated with high costs.Generally, dispersion modelling is based on meteor-ological observations, particle size distributions andtheir deposition properties, surface roughness condi-tions and emmission registrations (Van Jaarsveld, 1989).The receptor measurements, however, should be regardedas necessary and indispensable: they may be used tovalidate dispersion models, and the data obtained mayindicate the presence of sources and/or emissions whichwere not known or registered. In order to ensure thetemporal and spatial representativeness of in-fieldmeasurements, sampling is required on a long-termbasis, and at a large number of sites (Slanina et al.,1990). Such measurements of air particulate matter anddeposition, using technical equipment, have been few,mainly due to the high costs, and the lack of sufficientlysensitive and inexpensive techniques permitting simul-taneous measurements of many air contaminants at alarge number of stations. Measurements are mostlyperformed on restricted geographical scales (Alves et al.,1998a, b), or are dedicated to the assessment of theimpact of specific sources (Sverdrup et al., 1991). In arestricted set-up in the Dutch Rotterdam harbour area,De Bruin and Wolterbeek (1984) tried to identify geo-graphical source positions, making use of a wind tra-jectory approach, in which elemental filter data wereobtained and correlated to wind direction.

Providing that certain criteria are fulfilled, biomoni-toring can be an efficient supplement or even replace-ment for the earlier-mentioned type of investigations,and also permits larger-scaled multiple-sites programs,the latter shown by the running series of NORDICmoss surveys (Ruhling and Steinnes, 1998).

4. Biomonitors

4.1. The biomonitoring concept

Biomonitoring, in a general sense, may be defined asthe use of bio-organisms/materials to obtain (quantita-tive) information on certain characteristics of the bio-sphere. The relevant information in biomonitoring (e.g.using plants or animals) is commonly deduced fromeither changes in the behaviour of the monitor organism(impact: species composition and/or richness, physi-ological and/or ecological performance, morphology) orfrom the concentrations of specific substances in themonitor tissues. With proper selection of organisms,the general advantage of the biomonitoring approachis related primarily to the permanent and common

occurrence of the organism in the field, even in remoteareas, the ease of sampling, and the absence of anynecessary expensive technical equipment.

4.2. Selection criteria

Referring to the determination of the biomonitor’selemental content, organisms may be further selected onbasis of their accumulative and time-integrative behav-iour (see also Wolterbeek and Bode, 1995; Wolterbeeket al., 1996a, b). In the literature, biomonitoring speciesfor trace element air pollution are often selected onbasis of criteria such as specificity (Ruhling, 1994; whichmeans that accumulation is considered to occur fromthe atmosphere only), accumulation ratio’s (Sloof, 1993;Puckett, 1988), or a well-defined representation of asampling site (Wolterbeek and Bode, 1995). Wolterbeeket al. (1996a) reasoned that selection should be con-sidered on basis of the differences between local andsurvey variances. Of course, the almost implicit criterionfor selection is the biomonitors common occurrence. Inearlier work, air pollution was indexed by geographicalvariances in biodiversity and biomonitor’s species rich-ness (Barkman, 1958; De Wit, 1976; Martin andCoughtrey, 1982); more recent work is aimed at clari-fication of the impact of variable levels of atmosphericpollution on the biomonitor’s responses. In this sense,much study is devoted to processes such as photo-synthesis, respiration, transpiration, element accumula-tion etc. (Garty, 1993; Branquinho et al., 1999). Thelatter studies share two different objectives: first,knowledge is gathered on the (ecological/physiological)impact of atmospheric air pollution, and second, byrelating impact to response in terms of elemental accu-mulation, knowledge is gathered on the dose–responserelationships for the biomonitor of interest.

5. Biomonitoring: dose–response relationships

5.1. The dose and the response

To avoid any lengthy discussion on terminology ofwhat is meant by monitors, indicators, collectors etc.[see Puckett (1988) or Garty (1993) for clear overviews],in the present paper the term biomonitoring is handled ina more general way: ‘‘we use the biomonitor organismto get information on elemental deposition and/oratmospheric levels, thereby including impact informa-tion, because we have to try and quantify the dose–response relationships as far as we are able to’’. Thelatter means that, although the information on impactalso serves its own additional purposes (Markert et al.,2000), if we regard the elemental levels in the bio-monitor as a response to ambient elemental levels (air,deposition=the dose), and if we restrict ourselves to the

B. Wolterbeek / Environmental Pollution 120 (2002) 11–21 13

context of the dose–response relationship, impact onbiomonitor physiology should be seen as relevantbecause it may cause changes in the nature of this dose–response relationship [see Garty (1993) for a review onthe impact on lichen physiology of metals such as Pb,Fe, Cu, Zn, Cd, Ni, Cr, Hg]. Throughout the years, thedose–response relationship has received considerableatttention, initially in a rather implicit manner, but inlater years research efforts have become more and morespecifically aimed at the precise characterization ofbiomonitor’s responses.

5.2. The quantification problem

Apart from the reported regional distribution patternsof elemental concentrations in biomonitor materials(Ruhling and Tyler, 1973; Sloof, 1993; Steinnes et al.,1992; Freitas et al., 1997) with implicitly presumedpositive correlations between dose and response, theearly work on dose–responses was mostly on the corre-lations between elemental levels in biomonitors andtheir distance to metal point sources (Nieboer et al.,1973; Leblanc et al., 1974; Nieboer and Richardson,1981; Puckett, 1988). In later work, averaged elementalcontents of filter-trapped air particulate materials ordeposition were compared with biomonitor’s averagedmetal concentrations: Saeki et al. (1977), Andersen et al.(1978) and Pilegaard (1979) reported parallelisms andlinear relations with lichens, Sloof (1993) and Jeran etal. (2000) found positive correlations between metals inair particulates and transplanted lichens, Hanssen et al.(1980), Ross (1990), Berg et al. (1995), and Berg andSteinnes (1997) observed significant correlationsbetween wet deposition and moss metal concentrations.

Parallel to these phenomenological approaches wasthe work carried out to gain knowledge on the processesresponsible for accumulation, retention and releases ofelements in the biomonitor materials. These studieshave included much laboratory investigations, and haveshown that metal retention may be by particulateentrapment, physio-chemical processes such as ionexchange, as well as by passive and active cellularuptake (Ruhling and Tyler, 1968, 1970; Tyler, 1970;Taylor and Whitherspoon, 1972; Nieboer and Richard-son, 1981; Puckett, 1988; Tyler, 1989; Gjengedal andSteinnes, 1990). In addition, the Chernobyl accident hasprovided much information on accumulation efficienciesand release rates in biomonitors: fall out has been meas-ured in air particulate matter, deposition and bio-monitors (Devell et al., 1986; De Vries and Van derKooij, 1986; Papastefanou et al., 1988, 1989; Raes et al.,1990; Van Den Berg et al., 1992; Sloof, 1993). Retentionefficiencies and release rates could be calculated from thetime course of radioactivity levels in biomonitors afterthe short-term atmospheric influx (Ellis and Smith, 1987;Smith and Ellis, 1990; Sloof and Wolterbeek, 1992).

So far, however, hardly any work has been done tobridge the laboratory findings and the results obtainedin field work: Ellis and Smith (1987) tried to interpret137Cs lichen data by dynamic flux modelling, Schwartz-man et al. (1991) applied ion-exchange models forexplaining 210Pb and Pb uptake in lichens, and Garty(1993) wrote a review on metal accumulation in lichensunder both laboratory and field conditions. Sloof (1995)compared metal accumulation in lichen transplants withthat in synthetic rags, and compared transplant out-comes with data obtained with in situ growing lichens,and Reis et al. (1999) modelled lichen behaviourtowards ambient air particulate and deposition metalavailability, thereby including a memory loss concept,based on uptake and release processes, which describethe lichen transplant’s progressive reflection of its newambient conditions.

6. Biomonitor behaviour

6.1. General

Lichens and mosses may be considered as the mostcommonly applied biomonitor organisms. This is largelybased on their lack of any roots comparable with higherplants, which makes that both are thought to obtaintheir mineral supplies from aerial sources and not fromthe substratum (Puckett and Burton, 1981; Martin andCoughtrey, 1982; Brown and Brown, 1990; Ruhling andSteinnes, 1998). However, uptake or other contributionsfrom the substrate may occur. For lichens, Goyal andSeaward (1981a) demonstrated possible metal uptake byrhizines, Prussia and Killingbeck (1991) found differ-ences in lichen metal content associated to differences insubstrata, De Bruin and Hackenitz (1986) found metalconcentrations which did not differ between lichens andtheir bark substrata, and Sloof and Wolterbeek (1993)suggested further study on possible bark-lichen inter-ferences for Cd, Mn and Zn. For mosses, Kuik andWolterbeek (1995) found relatively high crustal con-tributions to the moss levels of elements such as Al, Sc,La and further lanthanides, and Brown and Brown(1990) suggested that the increase in cation exchangecapacity from moss apex to base is part of its naturalbalance of elements, which in turn is affected by theproximity of the soil.

Both lichen and moss species are suggested to accu-mulate elements largely by passive processes (Sloof,1995; Gjengedal and Steinnes, 1990). Furthermore, bio-monitor’s field data are often interpreted as related tothe entrapment of relatively insoluble metal-rich parti-culates rather than by the retention of soluble elements(Brown and Brown, 1990), which supports their reportedhigh retention efficiencies (Garty et al., 1979; Ruhlingand Steinnes, 1998). The finding that older lichen and


moss parts show the higher metal concentrations has ledto the additional assumption that the plants provide anhistorical and interative recording of the metal supply inthe environment. For lichens, the central older parts ofthe thallus, carrying most fruiting bodies, have beenshown to have higher metal concentrations (Brown andBrown, 1990; Sloof and Wolterbeek, 1992). For mosses,the carpets are build up during a period of 3–5 years,and their metal content is generally considered to reflectthe atmospheric deposition during that period (Ruhlingand Steinnes, 1998).

However, much more research emphasis has beenplaced on metal acquisition by biomonitors than on thepossible metal redistribution within or losses fromthe plants. For mosses, Tyler (1990), Brown and Wells(1990) and Wells and Brown (1990) indicated metaltransfer into the cell protoplasts, and Brown and Brown(1990) demonstrated metal-specific rates of intracellulartransport from old to newly grown tissues. Taylor andWitherspoon (1972) reported rainfall-related losses ofdeposited 137Cs carrying particles, and eventually cal-culated particles ‘‘weathering’’ half-lives for both mos-ses and lichens of about 50 days. For lichens, Goyal andSeaward (1981b) reported free metal movement fromrhizinae to the upper thallial surfaces and vice versa.Losses were mostly indicated on basis of (Chernobyl)radiocaesium data (Taylor and Whitherspoon, 1972;Ellis and Smith, 1987; Nagy and Konya, 1991; Sloofand Wolterbeek, 1992; Papastefanou et al., 1992);uptake and release processes are also part of models onlichen behaviour used by Schwartzmann et al. (1991)and Reis et al. (1999).

Since we need to understand the monitor’s behaviourbefore we will be able to reliably exploit them in airpollution surveillance, the earlier data suggest that moreresearch effort is needed on the dynamics of their metalaccumulation and release processes.

6.2. Impacts on biomonitors

The ongoing attention for trace element air pollutionstems from the interests we have in ecosystem perfor-mance and human health. The consequence is that weneed means to assess trace element occurrences andlevels in air particulates and deposition. We argue thatwe can use biomonitor organisms to do this on largescales, and we like to develop biomonitoring into a fullyaccepted quantitative tool in air pollution programs.Here, one of the problems encountered is the biomo-nitor performance. The impact of trace element air pol-lution may be discussed in terms of effects on ecostems,or on human health (see Section 3), but it may also haveeffects on the biomonitor’s behaviour (see Lepp, 1981a;Pignata et al., 1999; Gonzalez and Pignata, 1999;Garty, 1993). Moreover, natural variabilities in ambientmacro and microclimate conditions, such as acidity,

temperature, humidity, light, altitude, or ambient ele-mental (nutritional) occurrences may cause the bio-monitor to exhibit variable behaviour [see Seaward etal. (1988) for altitude effects on lichen responses]. Partof this variance is shown as local variance (Wolterbeeket al. 1996a,b), but it may be clear that this variablebehaviour becomes a problem when it seriously affectsthe biomonitor in its accumulative responses.

Seasonal effects in plant elemental concentrationshave been described by Markert and Weckert (1989),Ernst (1990), and Markert (1993a, b), which, in generalterms, may be ascribed to both elemental leaching andincreased availability by rainfall (Reis, unpublished),and to seasonally varying degrees of dilution by massincrements, the latter due to seasonal variations ingrowth rates (Markert 1993a, b). At this point, metalphytotoxic effects on plant physiology should be regarded.Growth is often used as striking marker for strongphysiological disorder [Van Gronsveld and Clijsters(1994), but effects on metal accumulation already occurwhen this visible growth symptom is less pronounced oreven absent. One of the most direct effects on the cell-ular level is the alteration of the plasma membrane per-meability, which may cause excess leakage of ions (DeVos, 1991), and may have effects on metal accumulationcharacteristics (Garty et al., 1998)].

For moss, although deposition rates have been esti-mated on presumed quantitative metal retention inmoss top segments (Ruhling et al., 1987), uptake effi-ciencies were shown to be strictly ordered for a numberof metals (Ruhling and Tyler, 1970). However, compe-tition effects may significantly influence uptake andretention: high sea salt input showed effects even onthe retention of strongly absorbed metals such as Pband Cu (Gjengedal and Steinnes, 1990; Berg et al.,1995; Berg and Steinnes, 1997). Furthermore, alsostrongly acidic precipitation, generally largely asso-ciated to atmospheric SO2 (Seaward, 1980; Brown andBeckett, 1983), may yield lower moss metal concentra-tions (Gjengedal and Steinnes, 1990). In this context itshould be noted that toxic action in plants is indicatedespecially for SO2 and NO2 (De Bakker and VanDobben, 1988; Balaguer and Manrique, 1991; Garty etal., 1993).

For lichens, Demon et al. (1988, 1989) studied effectsby acidity, temperature and calcium ions on metalaccumulation in both algal and fungal components. Forboth components, hardly any effect from temperaturecould be determined for Cu and La, indicatingthe absence of metabolically controlled uptake, but thereverse was shown for As, W, Zn and Cd in the algalcomponent [see Gjengedal and Steinnes (1990) for mos-ses]. Metal uptake in the algae was generally faster atpH 7, in the fungus uptake rates were higher at pH 5. Inintact lichens, pH dependent uptake was reported byPuckett et al. (1973) and Puckett and Burton (1981).


Ambient SO2 has been considered in initial studieswith bark (Staxang, 1969; Hartel and Grill, 1972), esti-mations were based on measurements of bark S andbark acidity. In a later bark study, Wolterbeek et al.(1996a, b) examined relationships between sulphate,ammonia, nitrate, acidity and trace metals, and the dis-tance to the sea. Bark sulphate, ammonia and nitratewere interpreted as not significantly affecting bark metalretention, Ca and Hg were affected by acidity. For bark,the Ca loading in particular may determine the buffer-ing capacity with respect to incoming acidic precipita-tion (Farmer et al., 1991); further neutralization may bebrought about by alkalizing effects from atmosphericNH3 (De Bakker and Van Dobben, 1988). Based onboth moss, lichen and bark data, Wolterbeek and Bode(1995) proposed to supplement the trace metal analysisin biomonitors with the determination of pH, NH4,NO3 and SO4.

7. Besides trace elements

Today, lichens and mosses are mostly used as biomo-nitor plants for atmospheric trace elements. In addition,and especially after the Chernobyl accident, much lit-erature has emerged on the use of both plant species forthe assessment of the atmospheric levels and depositionof radionuclides. Apart from these two applications,and already in the 1950s, lichens have also been studiedextensively in the context of the deposition of nitrogenand acidity (Barkman, 1958; De Wit, 1976; Van Dobben,1993). In these studies, species diversity and richness ofoccurrence was compared with variabilities in N and Satmospheric levels, both for impact assessments andfor monitoring purposes (Van Dobben et al., 2001). Itmay also be noted here that mosses have been used forthe measurement of industrially emmitted fluorine asearly as 1950 (MacIntyre et al., 1952). From the 1960sonwards, lichens have been used in both laboratory andfield fumigation experiments on the effects of gaseouspollutants such as SO2, NO2, HF, or O3 (Pearson andSkye, 1965; Nash, 1971; Hill, 1974; Richardsonand Puckett, 1973, De Wit, 1976). In current research, alarge variety of plant species, such as wheat, barley,maize, grass, or tobacco, is investigated for theirresponses towards gaseous pollutants (Lorenzini andGuidi, 1990; Blum et al., 1997; Krupa et al., 1993), bothfor impact and monitoring purposes. Mosses wereapplied as monitors of the deposition of organic micro-pollutants from the 1980s onwards (Thomas and Herr-mann, 1980). Most of the early work concentrated onthe deposition of organochlorine compounds such aspesticides and polychlorobiphenyls (PCBs). However, inlater work, both lichens and mosses have been appliedalso in monitoring of atmospheric polycyclic aromatichydrocarbons (PAHs), which are emitted in association

with human activities such as the production of cokefrom coal, the combustion of fossil fuels, or the alumi-nium and carbon electrode industries (Thomas andHerrmann, 1980; Carlberg et al., 1983; Thomas, 1984;Thomas and Schunke, 1984; Wegener et al., 1992; Jacobet al., 1993).

Based on the earlier, and in the context of the use ofboth lichens and mosses, combined determinationsof trace elements, PAHs, and S and N compounds maybe considered: combined approaches in field work andanalyses will facilitate the interpretation of individualelements and compounds and may permit a more reli-able recognition of source fingerprints (Wolterbeek andBode, 1995).

8. Multi-elemental larger-scaled (bio)monitoring data

When multi-parameter analysis is applied, especiallyin larger-scaled surveys, the information which isobtained from (bio)monitoring may consist of manythousands of data points (Sloof and Wolterbeek, 1991;Ruhling and Steinnes, 1998). Apart from being takeninto straightforward statistics, geographical mapping ordirect plotting in series of time or distance, these datamay be processed in a variety of further mathematicalroutines to permit a condensed and strongly smoothedpresentation of results and conclusions.

In air pollution studies, graphical techniques are usedto pinpoint elemental origins (Rahn and Huang, 1999);factor analytical (FA) techniques are used to resolve theelemental composition of aerosols or biomonitororganisms into a set of factors, which are interpreted asrespresenting source elemental profiles (Kuik et al.,1993a; Hopke et al., 1976; Alpert and Hopke, 1980;Henry et al., 1984; Henry, 1987; Pilegaard, 1987;Hopke, 1988; Sloof, 1993; Kuik and Wolterbeek, 1995;Reis et al., 1996). The Monte Carlo aided FA routinesdeveloped by Kuik et al. (1993a, b) permit the assess-ment of the reliability of the factor solutions, whichoffer the opportunity to manipulate the data with fullconsideration for progression of uncertainties. FA rou-tines are thus used in data clean-up procedures (e.g. theremoval of the soil dust factor) and source isolationprocedures (e.g. the isolation of a specific source pro-file); these procedures are shown to give higher-qualitysurvey outcomes (Wolterbeek et al., 1996a, b).

As may be clear, any combination of biomonitoringdata on elements and organic compounds may increasethe resolving strength of source profiling routines. Theunderlying reasoning is that a source emits an array ofcompounds and elements. The chemistry of emittedsubstances and its impact on accumulation specifics(Vongunten and Benes, 1995; Hill, 1997; Perez-Coronaet al., 1998) makes that the progressive increase in ana-lytical efforts on the determination of elemental chemical


forms in aerosols, particulates and biomonitor tissues(Cheng et al., 1994; Nimmo et al., 1998; Hsu et al., 2000;Halstead et al., 2000) should be considered as similarlyimproving our means for assessing source fingerprintsfrom biomonitoring.

9. Biomonitoring between source and impact: perspec-

tives

9.1. Status quo

In this paper, biomonitoring is regarded as a means toassess trace element concentrations in aerosols anddeposition. This means that the biomonitor should con-centrate the elements of interest, and that it shouldquantitatively reflect its ambient elemental conditions.Seen from this point of view, the relevance of elementalimpact is primarily important in terms of changes in thedose–response relationships. Any significant impact onthe behaviour of the monitor may lead to changes in theway the monitor reflects elemental availability, thusdisturbing the assumed relationships.

In current literature, reports are on the impact term,that is, changes in plant physiological parameters, arestudied extensively, without however, notation on theconsequences for the dose–response relations. In para-llel, reports, based on laboratory studies, are on physi-ological processes which are operating in elementalaccumulation, here without full exploitation of gainedknowledge in field approaches of biomonitoring. Infield situations, most quantification work is of a purelyphenomenological nature.

Essentially, biomonitoring of trace element air pollu-tion is regarded as relevant, because of the pollution’simpact on ecosystem performance and human health.To make the assessment even more significant, muchresearch effort is devoted to the extraction of sourceterms (emission profiles) from the data obtained.

9.2. Perspectives

The relative ease of sampling, the absence of any needfor complicated and expensive technical equipment, andthe accumulative and time-integrative behaviour of themonitor organisms make that biomonitoring of atmos-pheric trace elements will be continued for the foresee-able future, especially in larger-scaled surveys. Thenecessary quantitative assessment of elemental avail-ability asks for well-defined dose–response relation-ships, and knowledge on disturbances by impacts on theplant parameters on accumulation, retention and releaseprocesses. This implies that much effort should bedevoted to bridge laboratory findings and field results.

Biomonitoring may gain importance when the source-terms are reliably extracted from the data obtained. For

this, it seems that determinations should be multi-elemental, and supplemented by any additional infor-mation on emissions. Source apportionment may getmore specific when data on total concentrations of ele-ments are supplemented by information on organiccompounds and elemental chemical forms.

The assessment of atmospheric elemental availabilityderives its relevance from presumed impact on bothecosystem performance and human health; in this con-text source apportionment should be regarded as animportant parallel result for emission regulatory man-agement. Biomonitoring data should be coupled moreexplicitly to knowledge and data bases on both emissionregistration, ecosystem performance and human health.This means that multidisciplinary programs should beset up, which accomodate expert inputs from biomoni-toring, emission control programs, analytical chemistry,ecology and epidemiology.

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