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Atmospheric Environment 45 (2011) 1102e1109

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Atmospheric Environment

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Tree bark suber-included particles: A long-term accumulation site for elementsof atmospheric origin

Mickaël Catinon a,*, Sophie Ayrault b, Lorenzo Spadini c, Omar Boudouma d, Juliette Asta a,Michel Tissut a, Patrick Ravanel a

a Laboratoire LECA, UMR 5553, Equipe Perturbations Environnementales et Xénobiotiques, Univ. J. Fourier, 38041 Grenoble, Franceb Laboratoire des Sciences du Climat et de l’Environnement, UMR 8212, CEA-CNRS-UVSQ/IPSL, 91198 Gif-sur-Yvette, Francec Laboratoire LGIT, Equipe Géochimie Environnementale, Univ. J. Fourier, 38041 Grenoble, Franced Service du MEB, UFR928, Université Pierre et Marie Curie, 75252 ParisVI, France

a r t i c l e i n f o

Article history:Received 29 July 2010Received in revised form23 November 2010Accepted 26 November 2010

Keywords:BarkAir pollutionFly ashesSEM-EDXFractionationFraxinus excelsior

* Corresponding author. Tel.: þ33 4 76 51 46 54.E-mail addresses: Michael.Catinon@e.ujf-grenob

Ayrault@lsce.cnrs-gif.fr (S. Ayrault), lorenzo.spadini@boudouma@ccr.jussieu.fr (O. Boudouma), juliette.asmichel.tissut@ujf-grenoble.fr (M. Tissut), patrick.ravane

1352-2310/$ e see front matter � 2010 Elsevier Ltd.doi:10.1016/j.atmosenv.2010.11.038

a b s t r a c t

The deposition of atmospheric elements on and into the bark of 4-year-old Fraxinus excelsior L. wasstudied. The elemental composition of the suber tissue was established through ICP-MS analysis and thepresence of solid mineral particles included in this suber was established and described through SEM-EDX. Fractionation of the suber elements mixture was obtained after ashing at 550 �C through successivewater (C fraction) and HNO3 2 M (D fraction) extraction, leading to an insoluble residue mainly composedof the solid mineral particles (E fraction). The triplicated % weight of C, D and E were respectively34.4 � 2.7, 64.8� 2.7 and 0.8� 0.1% of the suber ashes weight. The main component of C was K, of D wasCa. Noticeable amounts of Mg were also observed in D. The E fraction, composed of insoluble particles,was mostly constituted of geogenic products, with elements such as Si, Al, K, Mg, representing primaryminerals. E also contained Ca3(PO4)2 and concentrated the main part of Pb and Fe. Moreover, The SEM-EDX analysis evidenced that this fraction also concentrated several types of fly ashes of industrial origin.The study of the distribution between C, D and E was analysed through ICP-MS with respect to theirorigin. The origin of the elements found in such bark was either geogenic (clay, micas, quartz.),anthropogenic or biogenic (for instance large amounts of solid Ca organic salts having a storage role). Asopposed to the E fraction, the C fraction, mainly composed of highly soluble Kþ is characteristic ofa biological pool of plant origin. In fraction D, the very high amount of Caþþ corresponds to two differentorigins: biological or acid soluble minerals such as calcite. Furthermore, the D fraction contains the mostpart of pollutants of anthropic origin such as Zn, Cu, Ni, Co, Cd. As a whole, the fractionation procedure ofthe suber samples allows to separate elements as a function of their origin but also gives valuableinformation on distribution and speciation of trace elements.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Today, the elemental composition of atmospheric gazes andparticles suspended in the air column leads to reproducible resultsdespite the inherent difficulties related to the analysis of lowsample yields (Freitas et al., 2004; Salma and Maenhaut, 2006;Geagea et al., 2008). Such analysis allows to determine themomentaneous composition of a given defined volume of air. But

le.fr (M. Catinon), Sophie.ujf-grenoble.fr (L. Spadini),ta@ujf-grenoble.fr (J. Asta),l@ujf-grenoble.fr (P. Ravanel).

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such techniques may be only uneasily correlated to effectivedeposition rates, as this process depends strongly on the contin-uous changes of the atmosphere particle composition as well as onlittle predictable eolian transfer of deposited particles etc. More-over, the prediction of the biological integration or, at least, stabi-lization of deposited particles inside the biological substratesremains to be further determined. As a whole, methods based onthe use of biological sensors (Schulz et al., 1999; Purvis et al., 2007)appear more appropriate to evaluate the effective amounts issuedfrom atmospheric deposition. Appropriate matrices were proposedfor the first time half a century ago. The analysis of barks (Ballachet al., 2002; Mandiwana et al., 2006), tree trunks wood (Nabaiset al., 1999; Bindler et al., 2004; Lageard et al., 2008), barkpockets (Satake et al., 1996; Bellis et al., 2002; Conkova andKubiznakova, 2008) or attic dusts (Cizdziel and Hodge, 2000; Tye

M. Catinon et al. / Atmospheric Environment 45 (2011) 1102e1109 1103

et al., 2006) yielded valuable information about atmosphericcontamination changes over long periods of time. Furthermore, inseveral cases, the specific hyperaccumulative properties of someplant species were used to show a soil contamination which mayoriginate from an atmospheric transfer (Baker and Brooks, 1989;Clemens et al., 2002).

Trunk and stems of trees appear to be most studied. Indeedthese substrates are particularly suitable as they are commonlyfound in very different ecosystems including urban areas and astheir lifespan is appropriate. The main drawback is that the accu-mulation process of particles and elements in bark remainsunknown.

Consequently it seems difficult to isolate anthropogenic, bio-logical and geogenic inputs without isotopical measures, thuslimiting the analysis to a simple biomonitoring use. However,calculation of the content of an “Anthropogenic AtmosphericElement Fraction” (AAEF) allows to overcome this limitation at leastpartly (Catinon et al., 2009a). This method, based on the referencecomposition of biological substrates and on the local geologicalbackground composition allows to determine the anthropogenicelements input.

Furthermore, a previous SEM-EDX (scanning electron micro-scope - energy dispersive X-ray) study on ash-tree stem cuttingsdemonstrated the occurrence of pure solid mineral particles whichwere only detected inside the suber tissue and not in the otherplant tissues. The suber material thus appears to irreversiblycumulate atmospheric particles over a long time scale (Catinonet al., 2009b). The lead isotopic signature studied in tree barksconfirms this long term accumulation process (Patrick and Farmer,2007).

This suber, in which mineral particles can be detected by SEM-EDX, is a typical protector tissue of tree stems or trunks in dicoty-ledonae or gymnospermae. Young stems have only a primarystructure with the epidermis representing the external cell layer.After one to two months, this epidermis dies and is replaced by thesuber cells produced by the suberophellodermic layer. The subercells are strictly contiguous and their walls are impregnated witha lipophilic polymer named suberin. These cells die rapidly,constituting therefore an apoplastic, lipophilic external layer witha noticeable mechanical resistance. After several years, the diam-eter increase of the stems allows cracks to occur in the suber layer,inducing the formation of new suber cells at a deeper level (Fig. 1).

Fig. 1. Scheme showing the location and structure of the

In order to reinforce the AAEF calculation approach, it wastherefore attractive to try to separate these suber-included particlesfrom the matrix, to obtain them in large amount and in a purifiedstate, to study their chemical compositions, to evaluate theirquantitative importance as compared to the whole elementscontent of the suber samples and to try to determine their origin.

Consequently, the purpose of the present report was to find anexperimental procedure allowing this separation and to evaluatewhether the purified particles fraction was representative of thesolids initially included inside the bark suber of ash-trees.

2. Materials and methods

2.1. Material and sampling sites

The chosen species for suber analysis was the ash tree (Fraxinusexcelsior L.) as this tree is very common in the alpine range where itcan grow at various altitudes from 200 to 1800 m. It is also commoninside towns. Its suber remains smooth and without cracks for over20 years thus avoiding artefactual trapping. Four year-old stemswere studied. In this case, the suber tissue is constituted of 3 to 4cell layers. The suber of young ash tree stems (generally 10 g dryweight) was isolated using the separation of tissues methoddescribed by Catinon et al. (2008). After washing the stem surfacewith distilled water and gently brushing, the stems were dried andcarefully scraped with a scalpel in order to peel the brown suberlayer without taking any of the green parenchyma underneath.

The sampling site (45�1103400N; 5�4604200E) was the botanicalgarden of the LECA Laboratory on the University Campus (Univer-sity Joseph Fourier, Saint Martin d’Hères, Isère, France) wherenumerous Fraxinus excelsior aged from 1 to 40 years were availablefor study. This site is located 800 m away from the Grenoble-Chambéry A41 highway with a 50 000 vehicles-per-day traffic and4 km away from the center of the town, at an altitude of 200 m, inthe Isère valley surrounded by mountains reaching 1500 m formost of them, and some over 2000 m.

2.2. Conceiving a purification procedure for the solidparticles (Fig. 2)

As shown in Fig. 2, a five-steps fractionation procedure wasconceived for isolating the suber included particles: step 1: suber

suber layer in dicotyledonous trees such as ash-tree.

ANALYSIS

ICP-MS SEM-EDX

H2O

C

HNO3

(pH 2.5)

D

Résidual solid

E

Incineration 550°CSample

A

Microscopic observation

Dry Weight Mass per unit surface

Ashes

B

Fractionation

Fig. 2. Scheme showing the different steps of the fractionation procedure.

M. Catinon et al. / Atmospheric Environment 45 (2011) 1102e11091104

isolation from stems, giving “fraction A”; step 2: suber incinerationat 550 �C giving “fraction B”; step 3: isolation of the water-solublecomponents of the ashes, giving “fraction C”; step 4: extraction ofthe elements which were dissolved through the addition of 2 MHNO3 until pH 2.5 was reached giving “fraction D”; step 5: isolationthrough centrifugation of the insoluble residue giving “fraction E”.

Under a practical point of view, the fractionation was carriedout as follows: one gram of each sample was heated at 550 �Covernight. The ashes were weighed and the ashes/dry weight(DW) ratios calculated. The organic matter contents correspondedto dry weight minus ashes weight. One hundred mg of ashes weresuspended in 10 ml of distilled water, agitated with a magneticstirrer for 10 min. The mixture was then centrifuged at 5000 rpmfor 5 min. The remaining solution was measured for pH, dried at80 �C and weighed (so-called “fraction H2O” or “C fraction”). Thepellet was suspended in 10 ml of distilled water, the mixture wasagitated and the pH was measured. Nitric acid 2 M was progres-sively added and pH continuously measured. The addition of 2 MHNO3 was stopped when pH 2.5 was reached (15 min., roomtemperature). The suspension was then centrifuged 5 min at5000 rpm. The supernatant was dried in an oven at 80 �C and thenweighed (“HNO3” or “D fraction”). The pellet was washed with10 ml of distilled water to remove dissolved acid, and thencentrifuged 5 min at 5000 rpm. The pellet was recovered, dried inan oven at 80 �C and then weighed (“residual solid fraction” or “Efraction”). The C, D, E fractions were kept in closed flasks to avoidrehydration.

2.3. SEM-EDX studies

The composition and structure of suber samples, ashes and C, Dand E fractions was studied using a ZEISS SUPRA 55 VP scanningelectron microscope (SEM) with a 3rd generation GEMINI fieldemission column allowing a spatial image resolution down to1.0 nm (Service du MEB, UFR 928, Paris VI University). This device

was coupled to an energy dispersive X-ray (EDX) microanalysissystem (SAHARA Silicon Drift Detector with Spirit Software of PGT)allowing high counting rates.

Carbon-coated samples were analysed in secondary electron[SE] back scattered electron [BSE]) X-ray point and X-ray elementalmapping modes using an accelerating voltage of 15 kV at a 7 mmworking distance.

2.4. ICP-MS studies

For ICP-MS analysis, 200 mg of sample (suber, ashes, C, D, Efractions) were digested in PTFE closed flasks on a digestionheating block (Digiprep, SCP Science) as previously described(Catinon et al., 2008). HNO3, HF and HClO4 were added succes-sively to these materials to obtain their complete digestion. The Cand D fractions were brought to 50 mL with 0.5 M HNO3. Theelemental contents were analysed by inductively coupled plasmamass spectrometry ICP-MS (XIICCTseries, Thermo ElectronCorporation, France). ICP-QMS spectrometer was calibrated usingstandard solutions and checked for with certified river water (SRM1640, National Institute for Science and Technology, Gaithersburg,USA) routinely during analysis, at least once per day of analysisand once for every 20 samples at the most. Instrumental drifts andplasma fluctuations were corrected using internal standards (Re,Rh, and In (SPEX, SCP Science, France) (1 mg L�1)) for all the metalsstudied, and Ge (10 mg L�1) for major elements including Ca, Al,and Mg. To minimise interferences, analysis with the Collision CellTechnology (CCT) introducing a supplementary gas mixture of H2

(7%) and He (93%) was applied for Fe, Mn, and trace elements (Cd,Cr, Cu, Ni, Pb, Zn). The data quality was controlled with referencematerials (lichen-336 and sediment-SL1, both from the Interna-tional Atomic Energy Agency, Vienna). The values obtained withthis procedure agreed well with the certified values. The confi-dence intervals can be found in Revel and Ayrault (2000) andAyrault et al. (2001, 2007).

M. Catinon et al. / Atmospheric Environment 45 (2011) 1102e1109 1105

3. Results

3.1. SEM-EDX observation in 4-year-old stems cuttings

The presence of mineral particles dispersed inside suber cellswas already demonstrated in Catinon et al. (2009b). Their SEM-EDXanalysis showed that their composition was similar to that of theatmospheric deposited particles on the stem surface (Catinon et al.,2009b). On the contrary in the living tissues underneath (paren-chyma, collenchyma, phloem.) the same mineral particles couldnot be observed. Instead, a single intracellular organo-mineralcomponent was abundantly represented under a crystallized forminside the sieve tube and parenchyma cells of ash-tree (Fig. 3).These crystals associate calciumwith organic matter which may beoxalic acid as is often the case in the plant kingdom. Their abun-dance changes with seasons and these particles are probably a typeof storage of organic matter under a solid state, avoiding osmoticeffect. This component represents an important part of the bio-logical calcium observed in the ash-tree living tissues.

3.2. SEM-EDX study of the incinerated suber

Fig. 4 (B fraction) shows the EDX spectrum of the incineratedsuber tissue. The highest concentrations are reached for Ca, K andO. Obviously most of Ca and K are combined with oxygen to formoxides. Al and Si are expected to be mostly of geogenic origin. Ca, K,Mg, P, S are dominantly of biological origin and minoritarily ofgeogenic origin. These elements are clearly visible and quantifiedby SEM-EDX. As awhole, it is possible to conclude that the elementscomposition of the suber tissue in ash-tree results from 3 origins:first, the organomineral components of the tree cells (named“biogenic”). Secondly, the mineral components of the earth uppercrust (named “geogenic”) and third, the elements involved inindustrial activities (named “anthropogenic”). Among this complexelements mixture, a process was investigated, that would allow toobtain large amounts of purified solid particles included inside thesuber. Obtaining these amounts leads to study their composition,their origin and their quantitative importance when compared tothe whole elements content in the suber samples.

For the choice of an organic decomposition giving the B fraction,it was previously demonstrated that careful progressive heating at550 �C did not change the elemental composition of an organo-

Fig. 3. Storage of Ca organic derivative in parenchyma cells in a 40-year-old ash-treestem cutting.

mineral mixture (Harju et al., 2002; Saarela et al., 2005; Catinonet al., 2008). It was therefore chosen to eliminate the organicmatter which reaches 94.4� 0.5% of the suber dry weight in thestudied material. During step 3, the major components dissolved inwater were K2O and Na2O, reaching 1.9� 0.2% of the suber dryweight, as shown in the EDX spectrum of fraction C (Fig. 4, C frac-tion and Fig. 5). For moderate acidic solubilization, H2SO4 wasavoided, due to limited solubility of CaSO4 and other artefacts.HNO3 was chosen to be added until pH 2.5. Under these conditions,it solubilized CaO and MgO which were easily observed on the EDXspectra (Fig. 4, D fraction). The weight of the elements in this Dfraction reached 3.6� 0.1% of the suber weight and Ca alone rep-resented 75% of this D fraction. In the end, the residual solid fractionE reached 0.045� 0.003% of the initial suber weight and wasexclusively constituted of particles. This corresponded to a purifi-cation factor higher than 2000 (Fig. 5).

3.3. SEM-EDX analysis of the E fraction

The E fraction was exclusively composed of individual particles.Its SEM-EDX global spectrum was mainly composed of Si (55%), Al(12.3%), P (11.1%), Ca (9.3%), Fe (4.7%), K (3.3%), Mg (2%) and Na(1.7%) (Fig. 4). This spectrum was definitely different from that ofthe dry suber or of its ashes. It shows the presence of a large amountof silica, and of Al, K, Na, Fe and Mg. These mixtures of elements arecharacteristic of geogenic particles. In order to see whether thisfraction was stable in the presence of concentrated HNO3, it wassuspended in 10 M HNO3 for one week at room temperature. Afterthis strong acidic treatment, Ca and P were solubilized. Thissuggests the presence in E of a calcium phosphate phase, insolublein the D fraction (pH 2.5) and slowly soluble in 10 M HNO3. Thepresence of calcium phosphate particles was effectively observed asindividual particles recrystallized in A and B fractions (prior to theacidic treatment). All the other elements constituting the fractionremained in the pellet after 10 M HNO3 treatment confirming therefractory behaviour of these particles constituting “E” as showneither through SEM-EDX or ICP-MS analysis. The strong resistanceof these particles and also their elemental composition stronglysuggests that they are for the main part geogenic.

Microscopic observation of the “E” fraction showed that 70% ofthe particles had a diameter smaller than 3 mm (Catinon et al.,2009b). Concurrently, the composition of no less than a hundreddifferent particles was established through SEM-EDX. Severalclasses of compounds can be distinguished: a) commonly presentpure minerals (e.g. silica) b) heterogeneously distributed minerals(e.g. Zircon) c) mixed or multielemental particles (e.g. feldspars,micas.) d) fly ashes of anthropogenic origin (Fig. 6).

The main biological elements were previously discarded infractions C or D (K, Ca, soluble Mg.) (Fig. 4, C and D fractions). Themain geogenic elements of the E fraction were silicic acid (quartz),silicates, magnesium, aluminum, iron, manganese. typicallytracing the presence of primary or secondary minerals. In moredetails, silica, mostly found as pure phenocrystals, reveals thepresence of Quartz. The high stability of pure calcium phosphatephases suggests a non-biological origin. Some other elements werewell-represented in some E preparations, not in others. It was, forinstance, the case for fluorides or Zr, Cu, Zn. Some spots representan aggregate of mixed particles having different compositions andgiving complex spectra (Mn, Mg, Al, Si, P, K, Ca, Fe. or Zn, Mg, Al, Si,P, Cl, K, Ca, Ti, Mn, Fe.). In most of the preparations, particles witha spherical shape were found. They were identified as fly asheswhich are known to be mainly emitted by high temperatureindustrial activities (1100 �C or more) and to be therefore especiallyinteresting for ecotoxicological studies (Lecoanet et al., 2001;Iordanidis et al., 2008; Zhang et al., 2009). In the studied material,

Fig. 4. Global X-ray emission spectrum of suber ashes (fraction B) and of the C, D and E fractions.

M. Catinon et al. / Atmospheric Environment 45 (2011) 1102e11091106

they had generally a size between 5 and 15 mm (Fig. 6). They havequite specific chemical compositions (Fig. 6). Iron fly ashes areprobably the most common industrial contaminants, issued frommetallurgic estates. In such structures, Fe can be mixed with otherelements such as Cr, Zn, Mn, V. Furthermore, themineral additivesused in smelting processes can also give fly ashes at high temper-atures, for instance with pure silicium, sillimanite or mullite.

3.4. ICP-MS study of the C, D and E fractions and crossingwith the SEM-EDX analysis

Several elements are present in the suber ashes or after frac-tionation, at a sufficient concentration to be detected by SEM-EDX,

D (3,63 %)

C (1.92 %)

E (0.045 %)

B (5.6 %)

OM (94.4 %)

A

Fig. 5. Scheme illustrating the quantitative importance of each of the A, B, C, D and Efractions; OM¼Organic Matter, BþOM¼ A¼ suber sample, CþDþ E¼ B¼ ashesweight, C¼H2O fraction, D¼HNO3 fraction, E¼ residual fraction.

at least when focusing on specific crystals. This is, for instance, thecase of Ca, Fe, K, Mg, Na, Ti and Znwhich can be seen as well-formedpeaks in SEM-EDX spectra. The ICP-MS analysis, calibrated inconcentration, allows to access the effective concentration of theseelements in the studied matrices A, B, C, D or E. The lowestconcentration value corresponding to a peak in SEM-EDX observedin this work was 5100 mg kg�1 for K (C fraction), 15 900 mg kg�1 forFe (Fraction E), 6400 mg kg�1 for K (D fraction), 5100 mg kg�1 forMg (B fraction), 19 000 mg kg�1 for Mn (E fraction), 7000 mg kg�1

for Na (C fraction), 1300 mg kg�1 for Ti (E fraction). Other elementswere not detected as peaks in SEM-EDX spectra. It was for instancethe case of Pb, Cu or Cr. The concentration of these three elementswas 1900, 1420 and 95 mg kg�1 respectively in the E fraction asshown by ICP-MS. It seems therefore that the order of magnitude ofthe detection limit as peaks in the SEM-EDX spectra of the studiedmatrices was between 1000 and 5000 mg kg�1.

Table 1 and Fig. 7 showhoweach of the 17 elements quantified byICP-MS in one kg suber distributed between the C, D and E fractions.Table 2 shows the concentration of the same elements in the Efraction. The lowest concentration was for Cd. However it reached20 mg kg�1, a concentrationwhich was 444 times higher than in thesuber initial sample. The highest concentration was for Fe(51600 mgkg�1) and for Mn (62100 mg kg�1), showing that theseelements represent from 5 to 6% of the dry weight of the E fraction.

3.5. Elemental partition between the C, D and E fractions

ICP-MS analysis of the C versus D versus E fractions shows themain characteristics of the fractionation carried out on the suberashes (Fig. 7 and Tables 1 and 2). K and Na accumulate massively inthe C fraction after H2O solubilization. For instance, 86.6% of thetotal K were found in C, where it represented 40% of its total mass.In the D fraction, CaþMg represented 86% of the D weight and 98%of the (CaþMg) suber content. Moreover, Co, Ni, Cu, Zn and Ba alsoclearly concentrated in this fraction. The E fraction contained the

Fig. 6. SEM pictures and EDX spectra of three types of fly-ashes found in the E fraction and which are anthropogenic.

M. Catinon et al. / Atmospheric Environment 45 (2011) 1102e1109 1107

most part of Pb, Mn, Fe, Al, accumulating respectively 95, 78, 80 and47% of the total amount in the sample (Fig. 7 and Table 2). Cr wasmore uniformly distributed with 54, 13 and 33% in C, D and Erespectively, probably involving a different Cr speciation. Agdistributed almost equally between D and E and Cd was at 60% in Dand 40% in E (Fig. 7 and Table 2). Overall, it appeared that elementsof geogenic origin accumulate preferentially in the E fraction,whereas elements of biological origin associate more specifically inthe C and D fractions.

4. Discussion

In the ash-tree which has been chosen for this study, the subertissue is constituted of 3 to 4 cell layers in up to 5 years- aged trees.

In 40-year-old trees, the suber is much thicker, reaching 6 to 8 mm.How can atmospheric particles integrate the suber is a question ofmajor importance.

The positions of the particles inside the suber cuttings suggestsa physical deposition into a net constituted by the suberin impreg-nated cell walls, with some snags and holes corresponding to lenti-cels smaller than 50 mm. The sizes of the observed particles agreewith an integration process inwhich the first step would be sieving.The oldest structures are outside andnew suber cell layers are addedon the internal face of this dead tissue during tree growth. The rate ofaccess of these particles into the inner suber layers of older treetrunks has become a main question, currently unanswered.

The SEM-EDX study carried out here established two mainpoints: a) insofar as a large amount of suber can be easily isolated,

Table 1Distribution of 18 elements in the fractions C, D and E in mg for 1 kg dry weightsuber; �SD, n¼ 3; C¼H2O fraction, D¼HNO3 fraction, E¼ residual fraction.

C D E

K 7800� 400 1200� 60 16.6� 0.8Ca 114� 11 6900� 700 13.1� 1.3Fe 7.4� 1.1 3.0� 0.4 41.3� 6.2Al 6.6� 0.9 18.3� 2.4 22.1� 2.9Mn 4.5� 0.7 9.2� 1.4 49.7� 7.5Mg 4.4� 0.1 1010� 30 13.3� 0.4Cr 0.13� 0.01 0.04� 0.01 0.08� 0.01Zn 0.09� 0.01 24.9� 2.5 2.6� 0.3Ba 0.09� 0.01 21.1� 1.3 0.74� 0.04Cu 0.05� 0.01 19.5� 1.0 1.1� 0.1Ti <0.05 1.9� 0.2 3.4� 3.4As 0.025� 0.003 0.09� 0.01 0.018� 0.002Ag 0.014� 0.003 0.7� 0.1 0.6� 0.1Sb 0.007� 0.001 0.088� 0.018 0.035� 0.007Ni 0.005� 0.001 0.54� 0.03 0.086� 0.004Pb 0.004� 0.001 0.076� 0.004 1.5� 0.1Co 0.002� 0.001 0.36� 0.02 0.08� 0.01Cd 0.001� 0.001 0.028� 0.006 0.016� 0.003

Table 2Element concentration (mg kg�1) in the E residualfraction; �SD, n¼ 3.

E

Mn 62100� 9300Fe 51600� 7700Al 27600� 3600K 20700� 1000Mg 16600� 500Ca 16400� 1600Ti 4200� 400Zn 3200� 300Pb 1900� 100Cu 1420� 70Ba 920� 55Ag 800� 160Ni 107� 5Cr 95� 5Co 94� 5Sb 44� 9As 22� 3Cd 20� 4

M. Catinon et al. / Atmospheric Environment 45 (2011) 1102e11091108

this tissue appears as the best one to study the accumulation ofelements of atmospheric origin in tree trunks, b) persistent mineralparticles can be found in this tissue. These particles are insoluble inwater and protected from rain. They may remain unchanged foryears because they are located inside a lipophilic apoplastic space.The direct SEM-EDX analysis on suber cuttings allows the rapididentification of a large number of particles in the cuttings.However, for the elements reaching only a low average concen-tration in the cuttings, the EDX signal is too low for their detection.The order of magnitude for this limiting concentration is about1000 to 5000 mg kg�1.The EDX analysis here requires to becompleted by ICP-MS. This is the case for elements playing a crucialrole in the anthropogenic contamination of the atmosphere, such asAs, Cd, Co, Cr, Ni, Pb, Sb, Sn. The full analysis of the samples istherefore the result of the combination of three complementaryapproaches: fractionation, MEB-EDX and ICP-MS analyses.

The three main advantages of the fractionation process givingaccess on the one hand, to very soluble and on the other, torefractory fractions, must be underlined. 1) As compared to theinitial matrix, the concentration of each element is much higher inspecific fractions (free K and Na in “C”; Co, Cr, Cu, Ni, Zn. in “D” Ag,Mn, Pb in “E”) allowing much more accurate results and especiallyimproving the measurement for traces such as Cd, Sb, V. 2)Fractionation gives access to speciation. For instance, free K (whichhas mainly a biological origin) concentrates in C fraction, while K

0%

20%

40%

60%

80%

100%

K Cr As Mg Ca Ba Cu Zn Ni Co Sb Cd Al Ag Mn Fe Pb

C D E

Fig. 7. Elemental distribution of 17 elements between the C, D and E fractions (%) inashes tree suber. C¼H2O fraction, D¼HNO3 fraction, E¼ residual fraction.

originating from complex geological silicates is concentrated in Efraction. Big crystals of insoluble calcium phosphates can beobserved in the E fraction. Although Ca3(PO4)2 was theoreticallysoluble at pH 2.5, this suggests that the time required for theirdissolution was higher than that corresponding to our experi-mental conditions. This salt was fully solubilized when suspendedin 10 M HNO3 for several days. Hexavalent Cr which is soluble inH2O is found in C fraction. 3) Fly ashes issued from industrialactivities concentrate in E fraction. The detection, structure analysisand quantification of these fly ashes is much improved by frac-tionation as compared to the analysis of the bulk suber or its ashesin sites with a low atmospheric content for such particles orsubmitted to only sporadic emissions. The dense structure of themetallic fly-ashes allows them to resist to themild acidic treatment.In this case, SEM-EDX study of the E fraction mostly allows toevaluate their contribution to the whole elemental composition ofa sample. Moreover, the repetitive analysis of the fly ashes by SEM-EDX carried out on the E fraction, allows also to access the origin ofsome elements such as Si and Al. In the case of siliceous fly ashes,geogenic components such as clay are transformed throughanthropogenic heating processes into sillimanite or mullite whichcan be identified from their EDX spectra. They can be thereforequantified separately from natural silicates (Fig. 7).

The location of the main part of Pb in E, observed here, as shownby ICP-MS measurements in 4-year-old Fraxinus suber, is an inter-esting feature. It occurs concurrently with Fe and Mn accumula-tions. Elemental Pb, PbO, PbCO3 and numerous other salts wouldconcentrate in the HNO3 soluble D fraction. The accumulation of Pbin E shown here suggests that Pb was trapped inside mineralogicalcarrier phases such as clays or even alloys particles resisting the pH2.5 HNO3 treatment.

As a whole, this method combining fractionation and twoanalytical techniques, carried out on one suber sample, has givena sufficient panel of results to hope for a much larger use in thefuture. Moreover, as the E fraction seems fairly representative of theoriginal particles which were observed in the suber, the chemicaland microscopic study of E is likely to give very accurate resultsconcerning geogenic and anthropogenic particles over long periodsof time.

Acknowledgements

We are grateful to Anne Laure Tissut for her fruitful suggestionsabout the English form. This work has beenmade possible thanks to

M. Catinon et al. / Atmospheric Environment 45 (2011) 1102e1109 1109

funding from the project “Suivi de l’évolution de la pollutionatmosphérique cumulée d’un secteur de la vallée du Rhône, en avaldu couloir de la chimie” supported by the Rhône-Alpes RegionalCouncil. This is the LSCE contribution number 4177.

References

Ayrault, S., Bonhomme, P., Carrot, F., Amblard, G., Sciarretta, M.D., Galsomiès, L.,2001. Multianalysis of trace elements in mosses with inductively coupledplasma-mass spectrometry. Biological Trace Element Research 79, 177e184.

Ayrault, S., Clochiatti, R., Carrot, F., Daudin, L., Bennett, J.P., 2007. Factors to considerfor trace element deposition biomonitoring surveys with lichen transplants.Science of the Total Environment 372, 717e727.

Baker, A.J.M., Brooks, R.R., 1989. Terrestrial higher plants which hyperaccumulatemetallic elements e a review of their distribution, ecology and phytochemistry.Biorecovery 1, 81e126.

Ballach, H.-J., Wittig, R., Wulff, S., 2002. Twenty-five years of biomonitoring lead inthe Frankfurt/Main area. Environmental Science and Pollution Research 9,136e142.

Bellis, D.J., Satake, K., Nodal, M., Nishimura, N., McLeod, C.W., 2002. Evaluation ofthe historical records of lead pollution in the annual growth rings and barkpockets of a 250-year-old Quercus crispula in Nikko, Japan. Science of the TotalEnvironment 295, 91e100.

Bindler, R., Renberg, I., Klaminder, J., Emteryd, O., 2004. Tree rings as Pb pollutionarchives? A comparison of 206P/207Pb isotope ratios in pine and other envi-ronmental media. Science of the Total Environment 319, 173e183.

Catinon, M., Ayrault, S., Daudin, L., Sevin, L., Asta, J., Tissut, M., Ravanel, P., 2008.Atmospheric inorganic contaminants and their distribution inside stem tissuesof Fraxinus excelsior L. Atmospheric Environment 42, 1223e1238.

Catinon, M., Ayrault, S., Clocchiatti, R., Boudouma, O., Asta, J., Tissut, M., Ravanel, P.,2009a. The anthropogenic atmospheric elements fraction: a new interpretationof elemental deposits on tree barks. Atmospheric Environment 43, 1124e1130.

Catinon, M., Ayrault, S., Boudouma, O., Asta, J., Tissut, M., Ravanel, P., 2009b. Theinclusion of atmospheric particles into the bark suber of ash trees. Chemo-sphere 77, 1313e1320.

Cizdziel, J.V., Hodge, V.F., 2000. Attics as archives for house infiltrating pollutants:trace elements and pesticides in attic dust and soil from southern Nevada andUtah. Microchemical Journal 64, 85e92.

Clemens, S., Palmgren, M.G., Krämer, U., 2002. A long way ahead: understandingand engineering plant metal accumulation. Trends in Plant Science 7, 309e315.

Conkova, M., Kubiznakova, J., 2008. Lead isotope ratios in tree bark pockets: anindicator of past air pollution in the Czech Republic. Science of the TotalEnvironment 404, 440e445.

Freitas, M.C., Pacheco, A.M.G., Ventura, M.G., 2004. Anthropogenic sources ofPM2.5’s arsenic, lead, mercury and nickel in northern metropolitan Lisbon,Portugal. Nuclear Instruments and Methods in Physics Research Section B219e220, 153e156.

Geagea, M.L., Stille, P., Gauthier-Lafaye, F., Millet, M., 2008. Tracing of industrialaerosol sources in an urban environment using Pb, Sr and Nd Isotopes. Envi-ronmental Science and Technology 42, 692e698.

Harju, L., Saarela, K.E., Rajander, J., Lill, J.O., Lindroos, A., Heselius, S.J., 2002. Envi-ronmental monitoring of trace elements in bark of Scots pine by thick-targetPIXE. Nuclear Instruments and Methods 189, 163e167.

Iordanidis, A., Buckman, J., Triantafyllou, A.G., Asvesta, A., 2008. ESEM-EDX char-acterisation of airborne particles from an industrialised area of northernGreece. Environmental Geochemistry and Health 30, 391e405.

Lageard, J.G.A., Howell, J.A., Rothwell, J.J., Drew, I.B., 2008. The utility of Pinus syl-vestris L. in dendrochemical investigations: pollution impact of lead mining andsmelting in Darley Dale, Derbyshire, UK. Environmental Pollution 153,284e294.

Lecoanet, H., Leveque, F., Arnbrosi, J.P., 2001. Magnetic properties of salt-marsh soilscontaminated by iron industry emissions (southeast France). Journal of AppliedGeophysics 48, 67e81.

Mandiwana, K.L., Resane, T., Panichev, N., Ngobeni, P., 2006. The application of treebark as bio-indicator for the assessment of Cr (VI) in air pollution. Journal ofHazardous Materials 137, 1241e1245.

Nabais, C., Freitas, H., Hagemeyer, J., 1999. Dendroanalysis: a tool for biomonitoringenvironmental pollution? Science of the Total Environment 232, 33e37.

Purvis, O.W., Chimonides, P.D.J., Jeffries, T.E., Jones, G.C., Rusu, A.M., Read, H., 2007.Multi-element composition of historical lichen collections and bark samples,indicators of changing atmospheric conditions. Atmospheric Environment 41,72e80.

Patrick, G.J., Farmer, J.G., 2007. A lead isotopic assessment of tree bark as a bio-monitor of contemporary atmospheric lead. Science of the Total Environment288, 343e356.

Revel, G., Ayrault, S., 2000. Comparative use of INAA and ICP-MS methods forenvironmental studies. Journal of Radioanalytical and Nuclear Chemistry 244(1), 73e80.

Saarela, K.E., Harju, L., Rajander, J., Lill, J.O., Heselius, S.J., Lindroos, A., Mattson, K.,2005. Elemental analyses of pine bark and wood in an environmental study.Science of the Total Environment 342, 231e241.

Salma, I., Maenhaut, W., 2006. Changes in elemental composition and mass ofatmospheric aerosol pollution between 1996 and 2002 in a Central Europeancity. Environmental Pollution 143, 479e488.

Satake, K., Tanaka, A., Kimura, K., 1996. Accumulation of lead in tree trunk barkpockets as pollution time capsulus. Science of the Total Environment 181,25e30.

Schulz, H., Popp, P., Huhn, G., Stärk, H.J., Schüürmann, G., 1999. Bioimonitoring ofairborne inorganic and organic pollutants by means of pine tree barks. I.Temporal and spatial variations. Science of the Total Environment 232, 49e58.

Tye, A.M., Hodgkinson, E.S., Rawlins, B.G., 2006. Microscopic and chemical studies ofmetal particulates in tree bark and attic dust: evidence for historical atmo-spheric smelter emissions, Humberside, UK. Journal of Environmental Moni-toring 8, 904e912.

Zhang, C.X., Huang, B.C., Liu, Q.S., 2009. Magnetic properties of different pollutionreceptors around steel plants and their environmental significance. ChineseJournal of Geophysics-Chinese Edition 52, 2826e2839.