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Atmospheric Environment 42 (2008) 8987–8996

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Atmospheric Environment

journal homepage: www.elsevier .com/locate/atmosenv

The speciation of iron in desert dust collected in Gran Canaria(Canary Islands): Combined chemical, magnetic and optical analysis

Francisco J. Lazaro a,*, Lucıa Gutierrez a, Vidal Barron b, Marıa D. Gelado c

a Departamento de Ciencia y Tecnologıa de Materiales y Fluidos, Universidad de Zaragoza, 50018 Zaragoza, Spainb Departamento de Ciencias y Recursos Agrıcolas y Forestales, Universidad de Cordoba, 14071 Cordoba, Spainc Departamento de Quımica, Facultad de Ciencias del Mar, Universidad de Las Palmas de Gran Canaria, 35017 Las Palmas, Spain

a r t i c l e i n f o

Article history:Received 28 July 2008Received in revised form23 September 2008Accepted 24 September 2008

Keywords:Mineral dustIron oxidesMagnetic susceptibilityDiffuse reflectance

* Corresponding author.E-mail addresses: franciscoj.lazaro@gmail.co

(F.J. Lazaro).

1352-2310/$ – see front matter � 2008 Elsevier Ltddoi:10.1016/j.atmosenv.2008.09.035

a b s t r a c t

Atmospheric dust collected on filters at a coastal site in Gran Canaria has been analysed bya combination of chemical, magnetic and optical methods with the aim of determining theiron speciation. The fraction of total iron as particulate (oxyhydr)oxides, determined by thecitrate–bicarbonate–dithionite method, was 0.39 � 0.11 (mean � s.d.); the fraction of(oxyhydr)oxide iron in ferrimagnetic form, through analysis of the saturation magnet-isation, was 0.053 � 0.038 (mean � s.d.); and the fraction of haematite iron with respect tothe iron in haematite þ goethite form, by diffuse reflectance spectroscopy measurements,was 0.47 � 0.12 (mean � s.d.). Consistent with these findings, low temperature in-phaseand out-of-phase AC susceptibility measurements reveal also the presence of para-magnetic iron, most likely in silicates with ionic substitution, and indicate that, whilemagnetite or haematite particles may be present in the dust, their particle size should bevery small, as the typical magnetic transitions characteristic of large crystals of theseoxides are practically impossible to detect. The comparison of the Fe/Al elemental ratioswith typical crustal values indicates that the great majority of captured dust iron hasa non-anthropogenic origin. Although no significant correlations have been found betweenthe analysed dust properties and the dust provenance, the obtained magnetic data cor-responding to the dust collected at this site may be useful, as a middle step, in futuremagnetic monitoring studies of the iron biogeochemical cycle.

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1. Introduction

The compositional and microstructural properties ofatmospheric dust play a very important role in the radiativebalance of the Earth’s surface (Sokolik and Toon, 1999) aswell as in the supply of key nutritional elements to marineorganisms (Duce and Tindale, 1991). In particular, the roleof iron, one of the elements in the composition of suchparticulate matter, is somehow singular because the pres-ence of its oxides has a substantial effect on the opticalproperties of the atmosphere in the visible range of the

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spectrum (Arimoto et al., 2002), and also because the ironconcentration in some ocean waters seems to be a limitingfactor for phytoplankton growth (Watson, 2001). Since ironis the most abundant transition metal in the composition ofthese mineral aerosols (Chester et al., 1993), and given theconspicuous magnetic properties of this element, a detailedmagnetic characterisation of these materials may supplycomplementary information about its chemical speciation.Through carefully designed magnetic experiments it hasbeen possible to differentiate iron in the form of (oxy-hydr)oxide nanoparticles and iron in molecular structureswith sparely distributed iron ions (paramagnetic iron); inparticular, magnetic monitoring of quantitative changes ofbiogenic and nonbiogenic particulate iron in animal tissueshas been carried out (Gutierrez et al., 2006; Lopez et al.,

F.J. Lazaro et al. / Atmospheric Environment 42 (2008) 8987–89968988

2007). These ideas gain additional interest as not onlysoluble/labile iron but also colloidal iron oxides seem to beincorporated to the organism of some phytoplanktonspecies (Nodwell and Price, 2001). A general overview ofthe biogeochemical iron cycle in this context has beengiven by Jickells et al. (2005).

The use of magnetic techniques to determine environ-mentally relevant parameters in geological materials isa well-established discipline, but the interpretation of themagnetic data is usually not obvious (Evans and Heller,2003). The magnetic susceptibility of rocks and sedimentsresults from ferrimagnetic, paramagnetic and antiferro-magnetic contributions that mostly correspond to ironcontaining oxides or oxyhydroxides. These materials,usually massive, very often involve particles or grains largerthan the magnetic single domain limit and whose physicalproximity does not allow to discard dipole–dipole inter-action effects. Due to the interplay between these micro-scopical magnetic entities, the magnetic properties of thewhole can hardly be interpreted as the sum of those of itsparts. In this way, the solution of the inverse problem, thatis, obtaining compositional information from the samplesmagnetic characterisation, is subject to limits. In thisrespect, the magnetic characterisation of atmospheric dustwithin the filter materials where it is usually collected, thatis, in a dilute state, is substantially free from the interpre-tation problems mentioned above.

Mineral aerosols of urban origin have been studied bymagnetic methods (Hunt et al., 1984; Muxworthy et al.,2003; Gomez-Paccard et al., 2004; Spassov et al., 2004;Sagnotti et al., 2006) and magnetic studies of wind-trans-ported desert dust (Chester et al., 1984; Oldfield et al.,1985; Hunt,1986) and ice-entrapped dust (Lanci et al., 2004)do also exist. As the anthropogenic contribution to themagnetism of mineral aerosols is mostly due to ferrimag-netic iron minerals, and not to antiferromagnetic and para-magnetic ones that characterise the contributions of naturalorigin, the magnetic signal of the former is usually greaterthan the latter. Therefore the experimental sensitivity, inaddition to the evaluation of the human impact, may be oneof the practical reasons for the more numerous magneticstudies on urban dust compared with those on natural one.

In this paper, a chemical, magnetic and optical study ofatmospheric dust collected on filters at a coastal site inGran Canaria (Canary Islands) has been carried out. Thework is focused on the combination of different experi-mental techniques with the aim of determining thedifferent fractions in which the iron element is present inthe dust. The work intends to put the basis for futurestudies, employing the same techniques, on either atmo-spheric dust of different provenances or on its degradationproducts under various biogeochemical conditions.

2. Materials and methods

2.1. Dust collection

The samples were collected at Taliarte (Gran Canaria)during the CLIMAAT and CLIMARCOST projects (2002–2007). Gran Canaria is one of the Eastern Canary Islands,located in the north-west of Africa, in front of the coast of

Morocco, being Taliarte (27�590N, 15�220W, at sea level) acoastal site with low urban background. The influence ofmeteorological conditions was examined by determinationof 5-days isentropic back trajectories (00:00 and 12:00 UTC)using the HYSPLIT-4 (HYbrid Single-Particle LagrangianIntegrated Trajectory) model (Draxler and Rolph, 2003)and the FNL meteorological data set produced by theU.S. National Center for Environmental Prediction (NCEP).

The dust was collected on acid-washed cellulose filters(Whatman 41) in a high-volume sampler (MCV, modelCAV-A/M). The collector was made of metal-free polymericmaterial in order to minimise trace metal contamination.Samplers are situated on the roof of the Instituto Canario deCiencias Marinas building at Taliarte. Collection was per-formed daily for 12 h (18–06 UTC) and the sampler was runat flow rates of 60 m3 h�1.

In order to get more complete information about thechemical, optical and magnetic properties of the atmo-spheric dust, the whole dust-loaded filters were alwayscharacterised, that is, no separation procedure betweendust and filter, likely being unwantedly selective, wasemployed.

2.2. Chemical analysis

From the whole project, 145 samples suffered some typeof chemical characterisation but only 19 were included inthis combined chemical, optical and magnetic study. Theselection criteria: (i) African origin, (ii) high particulateconcentration and (iii) similar total suspended particleamount values, were chosen to improve the analyticalsignals and to work with a unique metal calibration curvein the spectrophotometric determination. In what follows,in this paper, every result will make reference to this set of19 samples.

The dust-loaded filters were transferred to individualzip-lock plastic bags immediately after collection andhandled under a laminar flow clean bench housed ina trace-metal-clean laboratory. One-quarter portions(about 101 cm2) of the exposed filters were used to deter-mine, among other elements, total Fe and Al; this lastelement was used as a dust concentration indicator (Pros-pero et al., 1987; Arimoto, 2001). Filter portions weredigested with a strong acid (6 ml HNO3/1 ml HF) to destroythe aluminosilicate matrix of some mineral components ofthe aerosol. The residue was dissolved in 0.5 M HNO3 andtransferred quantitatively to volumetric flasks. Analysis fortotal metal was made by Graphite Furnace AtomicAbsorption Spectroscopy (GFAAS) and Flame AtomicAbsorption Spectroscopy (FAAS). The accuracy of themethods was evaluated on the basis of analyses of a Inter-national Standard Reference Materials MESS-3 sediment.

Additionally, filter pieces of about 4 cm2 were used toanalyse the citrate/bicarbonate/dithionite-extractable iron(Fed), using the method of Mehra and Jackson (1960) exceptthat 1.2 g of dithionite was added to 50 mL of the citrate/bicarbonate solution, and that the suspension was stirred at40 �C for about 1 h before shaking it in a reciprocatingshaker at 25 �C for 16 h. The so-determined Fed is a measureof the Fe content in free (nonassociated to silicate minerals)Fe (oxyhydr)oxides. Iron in solution was eventually

F.J. Lazaro et al. / Atmospheric Environment 42 (2008) 8987–8996 8989

determined with the o-phenanthroline method (Olson andEllis, 1982) using a wavelength of 508 nm.

2.3. Diffuse reflectance spectroscopy

The reflectance (R) spectra of the filters were measuredwith an UV-Vis-NIR spectrophotometer Cary 5000, equip-ped with a diffuse reflectance attachment (an integratingsphere of 110 mm). Filters, backed with a blank of poly-tetrafluoroethylene (PTFE) to insure complete opacity, wereplaced in the 16 mm sample window of the instrument, andscanned at intervals of 0.5 nm from 380 to 710 nm since themost significative absorption bands for iron oxides occurwithin the visible range (Sherman and Waite, 1985;Scheinost et al., 1998). The second derivative of theKubelka–Munk function, f(R) ¼ (1 � R)2/(2R), was calcu-lated using a cubic spline procedure based on segments of30 data points. The relative intensity of the w425 nm andw535 nm bands in the second derivative spectrum, AHm/(AGt þ AHm), was used to estimate the relative concentra-tions of goethite (Gt) and haematite (Hm) (Scheinost et al.,1998), the calibration curve to determine Hm/(Hm þ Gt)being constructed from samples of soils from the Medi-terranean region (Torrent et al., 2007). Eventually, reflec-tance curves were also converted to tristimulus valuesaccording to the CIE colour-matching spectral energydistribution weighted for CIE Source C (Wyszecki and Stiles,1982) and then to the Munsell hue (H), value (V) andchroma (C) using the method given by the Munsellconversion program (http://wallkillcolor.com).

2.4. Magnetic characterisation

The magnetic characterisation was carried out ina Quantum Design MPMS-5S SQUID magnetometer with

Table 1Sample reference numbers, collection dates, provenance, and selected chemical,The provenances are SA (Sahara), SL (Sahel) and SA/SL (mixed). The iron enrichmethe saturation remanence, saturation magnetisation and coercive field, respectivefree iron, ferrimagnetic oxides, antiferromagnetic oxides, haematite and goethite,also indicated.

Chemical Magnetic

Sample Collect.date

Prov. EFFe Fed/Fetotal Msat

(A m2 kg�1 Fetotal)Fe(F

112 18-2-04 SA 0.9 0.32 2.03 0.210 5-3-04 SA 1.3 0.42 1.19 0.174 24-8-04 SA 1.1 0.26 1.33 0.134 26-8-04 SA/SL 1.3 0.39 1.78 0.135 29-8-04 SA 1.4 0.33 2.74 0.138 18-9-04 SA/SL 1.3 0.39 5.09 0.1148 31-12-04 SA 1.5 0.27 1.45 0.211 16-1-05 SA 2.0 0.46 0.91 0.212 18-1-05 SA/SL 1.3 0.35 1.84 0.254 11-2-05 SA 1.7 0.38 0.93 0.158 15-2-05 SA 1.0 0.42 1.66 0.160 8-3-05 SA 0.9 0.47 8.58 0.1257 6-5-05 SL 2.9 0.56 8.86 0.1164 7-6-05 SL 0.7 0.60 7.03 0.183 17-8-05 SA/SL 0.5 0.63 5.56 0.187 3-9-05 SA 1.0 0.28 2.22 0.188 6-9-05 SA 1.5 0.30 3.69 0.190 28-9-05 SA 1.4 0.33 2.33 0.191 30-9-05 SA/SL 1.4 0.31 1.50 0.

an AC susceptibility option. For these measurements,9.0 � 0.1 cm2 portions of the dust-loaded filters were cutwith a ceramic blade, to avoid possible metal contamina-tion, and directly placed into gelatine capsules. As the dustload represented always a very small fraction of the totalweight of the filter, it was decided not to include this datumin further quantitative analyses. In fact its determination bysubtracting the weight of the clean filter to the weight ofthe dust-loaded one would yield unacceptable interpreta-tion errors.

Field dependent magnetisation measurements havebeen done by using a built-in superconducting magnet of5 T. The AC measurements were always performed ata frequency of 1 Hz with an AC amplitude ofm0hac ¼ 0.45 mT and in the temperature range 5–300 K.Portions of clean filter of the same type and with the samearea as used for the collected dust were characterised bytemperature-dependent AC susceptibility in the sametemperature range.

In order to assure the absence of any magnetic contri-bution coming from adsorbed oxygen in the samples,repeated runs above and below its boiling temperaturewere used in the measurements of each sample (Gregory,1978).

3. Results

3.1. Chemical analyses and dust provenance

In Table 1, a list of the analysed samples with collectiondates and results of the chemical analyses is presented. Thedifferent provenances of the dust have been classified as:Sahara (SA), those air masses crossing the Sahara desert,establishing as limits for this region (20�N–36�N, 18�W–50�E); Sahel (SL), the air masses crossing the (0�N–20�N,

magnetic and optical data for the dust-loaded filters included in the study.nt factor is indicated by EFFe ¼ (Fe/Al)sample/(Fe/Al)crust. Mr, Msat and Hc arely. Fed, Feferri, Feaf, FeHm, FeGt are the elemental iron contents in the form ofrespectively. The Munsell parameters H (hue), V (value) and C (chroma) are

Optical

ferri/eferri þ Feaf)

Mr/Msat m0Hc

(mT)H V C FeHm/

(FeHm þ FeGt)

045 0.17 11.1 9.8YR 8.4 2 0.41016 0.15 9.4 9.2YR 7.1 3 0.61035 0.16 11.8 0.0Y 7.8 1.7 0.55031 0.18 14.2 0.2Y 8.6 1.3 0.52062 0.19 13.3 0.5Y 9 1.1 0.5201 0.13 13.5 1.4Y 8.9 0.7 0.35

037 0.14 12.3 0.2Y 8.5 1.2 0.63008 0.17 11.4 8.4YR 7 4.4 0.34036 0.16 11.8 0.9Y 8.2 1.4 0.26012 0.17 10.8 8.4YR 8 3.9 0.45025 0.12 11.1 9.6YR 7.7 1.7 0.6445 0.13 11.8 1.1Y 8.3 0.7 0.5424 0.12 13.0 1.4Y 8.5 0.6 0.61

089 0.13 – 1.6Y 9 0.8 0.29066 0.14 13.0 1.2Y 9.2 0.8 0.31058 0.16 – 0.5Y 9 0.9 0.39094 0.15 – 1.6Y 9.4 0.5 0.49051 0.18 15.1 0.5Y 8.5 1.3 0.61032 0.16 – 0.7Y 8.8 1.1 0.51

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18�W–50�E) sector; and Sahara–Sahel (SA/SL), for thosecrossing both regions. To illustrate the geographical originof the dusts, the calculated back trajectories for represen-tative dates corresponding to the SA, SL and SA/SL groupsare shown in Fig. 1.

Fig. 1. Back trajectories (a, b and c), calculated with the HYSPLIT model, forrepresentative dust collection dates corresponding to the groups SA, SL andSA/SL, respectively. The specific trajectories belong to the following dates:(a) 8-3-05, 3-9-05 and 28-9-05; (b) 6-5-05 and 7-6-05; (c) 26-8-04, 18-9-04and 30-9-05. For each date, the trajectories at heights 750, 1500, 2000, 2500,3000 and 3500 m have been indicated.

Depending on the severity of the event and otherfactors, the filters collected a variable amount of solidmaterial, a part of which corresponded to deposition ofmarine spray, as revealed from occasionally high Na and Mgcontents. Therefore, only the dust-relevant data are pre-sented in the table, namely: (i) the ratio EFFe¼ (Fe/Al)sample/(Fe/Al)crust that represents the iron enrichment factor of thedust relative to the average crustal composition (Taylor andMcLennan, 1995) and (ii) the Fed/Fetotal ratio that gives thefraction of the total elemental iron that is not incorporatedinto silicate minerals, usually called free iron. Analysis ofpotentially magnetogenic transition metals indicates thatiron is always the most abundant element. In decreasingorder, the concentrations of titanium, manganese andcopper were about one order magnitude less than iron,being the rest in a much lower concentration.

3.2. Diffuse reflectance spectroscopy

The experimentally observed reflectance spectra ofselected filters are shown in Fig. 2a. The second derivativeof the Kubelka–Munk function as a function of the wave-length is shown in Fig. 2b for the same selected samples inorder to show the characteristic minima at w425 nm andw535 nm, typically assigned to goethite and haematite,respectively (Kosmas et al., 1984). An analogous procedureexamining the first derivative of the reflectance spectra wasdone by Arimoto et al. (2002) for identification of ironoxides in aerosols collected in filters. Munsell parameters,calculated from the reflectance curves, oscillated froma light grey colour 1.6Y 9.4/0.5 for filter no. 188 with a scantpowder load, to reddish yellow 8.4YR 7/4.4 for filter no.211 with the highest content of iron oxides.

3.3. Magnetic characterisation

3.3.1. AC susceptibilityThe temperature-dependent (c0 and c00) AC magnetic

susceptibility data for the whole set of studied samples arethe huge amount of information that cannot be shown withenough clarity in a single plot. Instead, the results of onlyfive representative samples, together with those of theclean filter, are shown in Fig. 3(a) and (b).

As it has been explained before, and in order to avoidinterpretational errors, it was decided to work with themagnetic results for the whole dust-loaded filters withoutconsideration of the dust mass. As the amplitude of theexciting AC field has been the same for all the samples, themagnetic moment (m0 and m00 for the in-phase and out-of-phase components, respectively) of the whole set (filter,collected dust and gelatine sample-holder capsule) is theexperimental quantity that will be shown in the figures. Inany case either the magnetic moments (m0 and m00) or themass susceptibilities (c0 and c00), related with the former bym0 ¼ c0mhac and m00 ¼ c00mhac being m the sample mass, willbe used throughout the text. The susceptibilities, given thelow value of hac, will have the consideration of the so-calledinitial or low-field susceptibility.

Under the exciting field hac, the clean filter (filter celluloseplus capsule gelatine) acquires a temperature independentin-phase magnetic moment of�1.7� 0.05� 10�10 A m2 and

Fig. 2. Diffuse reflectance (R) curves (a) and second derivative off(R) ¼ (1 � R)2/(2R) (b) for selected filters. Sample reference numbers foreach curve are shown aside in the figure.

Fig. 3. Temperature dependence of the AC susceptibility for selected filters.In the plots the susceptibility is expressed as (a) in-phase magnetic momentand (b) out-of-phase magnetic moment measured under an AC field of0.45 mT at 1 Hz. Sample reference numbers for each curve are shown aside.

F.J. Lazaro et al. / Atmospheric Environment 42 (2008) 8987–8996 8991

an out-of-phase component indistinguishable from zerowithin the accuracy of the experiments, consistent witha pure diamagnetic behaviour.

The susceptibility of all the dust-loaded filters, froma qualitative point of view, is composed of (i) an m0(T) lowtemperature tail with no counterpart in m00(T) consistentwith a paramagnetic contribution and (ii) a progressiveincrease of m0(T), with associated increase of m00(T) up to, atleast, the highest experimental temperature, consistentwith the presence of magnetic particles with a distributionof superparamagnetic blocking temperatures wide enoughas to be extended well above and below room temperature.In some of the samples an extremely weak m0(T) step mayhardly be visible in the vicinity of 120 K, most probably dueto the presence, in a trace concentration, of not too smallmagnetite particles (whose Verwey transition is expectedat that temperature).

3.3.2. Field dependent magnetisationIn this section, for the same reasons mentioned above,

the magnetisation will be expressed as magnetic momentper sample.

Magnetisation as a function of increasing external fieldup to m0H ¼ 5 T, and at various temperatures, has beenmeasured for selected samples. The results for sample no.211 are shown in Fig. 4a. The most salient feature of theseM(T) curves is a Brillouin-like behaviour that enormouslyincreases the magnetisation for decreasing temperature,absolutely in agreement with the important paramagneticcontribution observed in the AC susceptibility.

The hysteresis loop within the range�1 T< m0H< 1 T, atT¼ 300 K, is shown in Fig. 4b for the same sample, togetherwith a magnified plot of the central part of the same loop inFig. 4c. Salient features of these curves are: (i) a fractionof the sample magnetisation is already saturated at

F.J. Lazaro et al. / Atmospheric Environment 42 (2008) 8987–89968992

m0H z 0.5 T, (ii) the loop hardly becomes closed atm0H z 1 T, and (iii) the loop has a somehow wasp-waistedappearance. This behaviour, qualitatively, has beenobserved for all the magnetically characterised samplesincluded in the study.

Saturation magnetisation Msat, obtained by linearextrapolation of the high field (0.75 T < m0H < 1 T) M(T)data, saturation remanence Mr (after saturation at 1 T) andcoercive field m0Hc have also been experimentally deter-mined for all the samples (see data in Table 1).

4. Discussion

The dust samples present, in all cases, iron enrichmentfactors well below 10, what indicates that the dust have notbeen appreciably enriched with anthropogenic iron(Chester et al., 1993) from the point of view of elementalconcentrations. The determined free iron fraction, Fed/Fetotal, 0.39 � 0.11 (mean � s.d.), is of the same order asfound by Lafon et al. (2004) for other desert aerosols.

Among the dust-loaded filters, those with lowerreflectance values in the 389–450 nm range, where char-acteristic absorption bands for iron oxides occur (Shermanand Waite, 1985), actually correspond to higher amounts offree iron oxides. The calculated Munsell parameters allowfor a quantitative approach; in particular, the ratio betweenchroma and value was highly correlated with Fed (Fig. 5)indicating a more saturated and darker colours for filterscontaining a higher amount of iron oxides. From the pointof view of the colour parameters, the differences foundamong filters are just a manifestation of the dust load ineach filter and seem not to be significatively associated toany other qualitative property of them. The data points inFig. 5 have been fitted with a Langmuir-type function C/V¼(C/V)max/((aFed)�1 þ1) yielding parameters (C/V)max ¼ 0.78and a ¼ 1.1 dm2/mg with r2 ¼ 0.93. This behaviour, thatwould be more visible with filters with a higher dustcontent, indicates a saturation of the C/V value in the limitof high content of iron oxides as usually observed in soils(Barron and Torrent, 1986).

The ratios of iron in the form of haematite to iron in theforms of haematite and goethite, determined from thereflectance data, gave a rather wide interval of values,between 0.26 and 0.63, mean 0.47 (� 0.12 s.d.) (Table 1)although apparently no correlation with the dust sourceseems to exist.

From the measured susceptibilities (Fig. 3a and b) of thedust-loaded filters, after subtraction of the omnipresentdiamagnetic contribution of filter and capsule, we mayobtain the following compositional information: (i) theparamagnetic contribution most likely corresponds to sili-cate minerals with ionic substitution of Fe and, to a muchlesser extent, other magnetogenic metals, and (ii) the c0(T)and c00(T) high temperature increase revealing the presenceof magnetic particles is consistent with ferrimagnetic(magnetite, maghaemite) or antiferromagnetic (haematite,

Fig. 4. Field dependence of the magnetic moment of a representativesample (211, see Table 1). (a) Magnetic moment measured in ascending fieldat three different temperatures. (b) Hysteresis loop at 300 K. (b) Centraldetail of the same hysteresis loop.

Fig. 5. Chroma (C)/value (V) ratio as a function of the chemically determinedfree iron (Fed), here expressed per filter surface, for the whole set of dustsamples. The solid line corresponds to a fit with a Langmuir function whoseparameters are explained in the text.

F.J. Lazaro et al. / Atmospheric Environment 42 (2008) 8987–8996 8993

goethite, ferrihydrite, etc.) iron (oxyhydr)oxide particles. Atthis point we should recall that for small magnetic particlesthe mere existence of a nonzero c00, indicative of magneticrelaxation, is concomitant with the variation of c0 with theAC frequency (Garcıa-Palacios and Lazaro, 1998). Therefore,at a given temperature and within the noninteractingregime as we assume for this in-filter dispersed mineralaerosol, a nonzero c00 is indicative of the progressiveblocking of magnetic particles or, in other words, thetransition between the so-called superparamagnetic and(blocked) single domain state.

Magnetite and maghaemite have a much larger massspecific magnetic susceptibility than the antiferromagneticiron oxides (Evans and Heller, 2003). This tells us that a tinyamount of iron in the form of these oxides may havea much larger contribution to the total magnetic momentthan larger amounts of iron in antiferromagnetic oxideform. In other words, the nearly imperceptible c0(T) step ataround 120 K, already mentioned above, may represent thequalitative presence of rather large particle size magnetite,although in an extremely small amount. Small particle sizemagnetite, of the order of few tens of nanometers orsmaller, may not show a clear Verwey susceptibility stepand therefore it may not be detectable in this way (Lopezet al., 2007). The c0(T) curves do not show either thepresence of a Morin transition (typical of the weak ferro-magnetic to antiferromagnetic transition of haematite inthe bulk state at around 265 K) suggesting that haematiteshould also be present in the form of very small particles. Itis of common experience that these magnetic transitions,typical of these oxides in the bulk state, are progressivelysmeared out as the particle size decreases in the lowernanometric range. The temperature dependence of thedynamical susceptibility of the dust samples is in any case

much simpler than in the case of soils of aeolian origin(Williamson et al., 2004). It is true that our measurementshave been extended to lower temperatures, what allow fora better detection of paramagnetic fractions, but ourresults, compared to those, do not show remains of phasetransition anomalies (as the bulk 50 K and 120 K transitionsof magnetite) most likely due to the smaller particle size ofthese oxides in the dust.

Although qualitatively there is no significative differ-ence between the AC susceptibility of the different samples(quantitative differences seem only to depend on the dustloadings of the filters), it has been accomplished a deeperanalysis based on assuming that (i) the contribution to thein-phase susceptibility at 300 K of paramagnetic iron isnegligible in comparison with that of particulate ironoxides and (ii) that the contribution to the in-phasesusceptibility at 5 K of particles is negligible in comparisonwith that of paramagnetic iron. Under these assumptionswe decided to plot some relevant quantities as a function ofthe samples elemental iron concentration (Fig. 6).

In Fig. 6a it is shown the very good linear correlationobtained in the case of the in-phase susceptibility at 5 K,meaning that the fraction of total iron in silicates is sampleindependent. In Fig. 6b, the in-phase magnetic suscepti-bility at 300 K, that represents a rough measure of the ironoxide particulate content, seems also well correlated withthe total iron content, although the scattering of theexperimental points is greater and the linear regression fityields a positive intercept, that contrasts with the negli-gible one obtained for the c0(5 K) data. The out-of-phasesusceptibility at 300 K (see Fig. 6c), to which paramagneticfractions do not contribute and indicative of the particulatefraction that undergoes superparamagnetic relaxation inthe vicinity of such temperature, appears also linearlycorrelated to the iron content and, here also, with a positiveintercept.

The experiments of field dependent magnetisation toa large extent corroborate the previously discussed find-ings. The curves shown in Fig. 4a agree with the existenceof an important paramagnetic contribution that becomesobviously remarkable for decreasing temperature as Bril-louin-law paramagnetism predicts.

The results shown in Fig. 4b and c reveal the existence ofa magnetic fraction that saturates below 0.5 T. This fractionlikely corresponds to a ferrimagnetic iron oxide asmagnetite because the coercivity does not surpass 15 mT asusual for this iron oxide (Evans and Heller, 2003). Thehysteresis cycles, however, close nearly at 0.7 T and showa slightly wasp-waisted shape indicating that magneticallyharder oxides are present, in agreement with the opticalexpectations for a goethite and haematite content.

In order to see whether magnetisation data offer addi-tional information with respect to the AC susceptibility, inFig. 6d and e, a temptative scaling of the saturation mag-netisation Msat and the saturation remanence Mr data, at300 K, with the elemental iron content is intended. As it canbe seen, both parameters reasonably scale with the ironcontent and, here again, with a positive intercept.

In Fig. 6f the ferrimagnetic fraction of iron oxides asa function of the elemental iron content in the sample isshown. Such fraction has been calculated by using the Msat

Fig. 6. AC susceptibility, after subtraction of the addenda diamagnetism, at different temperatures (a,b,c), saturation and remanent magnetisation (d,e), andmagnetically derived ferrimagnetic fraction (f) as a function of the total elemental iron content per filter surface. The dashed lines correspond to linear regressionfits of the data. For more details see text.

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values of Table 1, considering that Fed/Fetotal is the fractionof total iron not associated to silicates and that Msat is about120 A m2 kg�1 Fe and 1 A m2 kg�1 Fe, as rough estimates forferrimagnetic and antiferromagnetic iron oxidesrespectively.

Our interpretation of the positive intercepts found in thescalings of c0(300 K), c00(300 K), Msat and Mr with respect tothe elemental iron content, but not in the scaling of c0(5 K),and also in agreement with the result depicted in Fig. 6f, isthat a particulate fraction of the dust with a relatively highspecific magnetisation, very likely ferrimagnetic, but

involving a small concentration of elemental iron seems tobe present in a similar concentration in all the filtersamples. Given that the most probable iron compoundsresponsible for this phenomenon are magnetite ormaghaemite, and as their concentration seems not to berelated with the intensity of the dust event we think theymay be of anthropogenic origin. We should emphasise herethat the contribution of these ferrimagnetic iron oxides tothe total magnetic moment of the samples, due to theirhigh specific magnetisation, is not proportional to theelemental iron content in such oxides. Therefore, the

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higher relative content of ferrimagnetic oxides at low dustconcentrations (Fig. 6f) does not contradict the generalnonanthropogenic character obtained from the ironenrichment factor value, which is a parameter based onlyon elemental contents.

5. Conclusions

This work presents a combined chemical, optical andmagnetic analysis of desert dust collected in Gran Canaria.Although some works exist on the use of magnetism tomonitor iron oxides in urban aerosols, this work is inscri-bed among the very few ones dedicated to aerosols ofnatural origin. This is so, probably, because the former aredominated by iron in the most magnetically efficient oxidestate (i.e. ferrimagnetic) while the latter, mainly antiferro-magnetic oxides, have a much lower contribution, per ironatom, to the total susceptibility.

The mineral aerosol loadings of the analysed filters werevariable, mostly due to the different intensities of theatmospheric dust events. However, in spite of this quanti-tative difference among the studied filter samples, the jointchemical, optical and magnetic results, at the sensitivitylevel of these techniques, agree in that the capturedmineral matter does not show significant compositionaldifferences.

The low temperature AC susceptibility experimentshave given account of, in addition to the free iron fraction,a very important paramagnetic fraction (between 37% and74% from chemical analysis) of iron very likely in silicates,as indicated by the presence of chlorite and illite evidencedby X-ray diffraction (data not shown).

The practical absence of bulk-type magnetic anomalies(e.g., Verwey step) in the susceptibility indicates that thecontent of magnetite in the form of large particles should benegligible. Maghaemite and small particle magnetite mayindeed exist although the fraction of total iron in the form offerrimagnetic oxides never exceeds 7% (result obtained fromdata in Table 1). Still it has been evidenced the relativehigher sensitivity of magnetic methods, compared to theelemental analyses, in the detection of small iron amounts inthe form of ferrimagnetic iron oxides.

The presence of antiferromagnetic iron oxides, ashaematite or goethite, is confirmed from the optical spectraas well as from the slightly wasp-waisted hysteresis loops.In this aspect, again, no significative differences seem toexist among samples.

Given the environmental relevance of the form ofpresence of iron in atmospheric dust as well as theireventual biogeochemical role, this work confirms thepowerful synergy of combining chemical, optical andmagnetic analyses. The data presented here illustrate theproperties of materials originated at desert soils but alsoconstitute a good starting point for future studies on theoceanic transformation of iron species as well as the uptakeof this crucial element by living organisms.

Acknowledgements

The authors gratefully acknowledge the NOAA AirResources Laboratory (ARL) for the provision of the

HYSPLIT transport and dispersion model and/or READYwebsite (http://www.arl.noaa.gov/ready.html) used in thispublication. This work was supported by Gobierno deAragon through the funds assigned to group B28, by theEuropean FEDER funds (INTERREG IIIB, CLIMAAT MAC/2.3/A3, CLIMAAT II MAC 03/2.3/A5 and CLIMARCOST 05/MAC/2.3/A1 projects) and partially by project AGL2006-10927-C03-02. Lucıa Gutierrez thanks Gobierno de Aragon andFondo Social Europeo for grant B091/2005. Usefulcomments from J. Torrent and B.A. Maher are alsoacknowledged.

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