Magnetic particles in atmospheric particulate matter collected at sites ...

Magnetic particles in atmospheric particulate matter collected at sites with different level of air pollution EDUARD PETROVSKÝ1, RADEK ZBOŘIL2, TOMÁŠ MATYS GRYGAR3, BOHUMIL KOTLÍK4, JIŘÍ NOVÁK5, ALEŠ KAPIČKA1 AND HANA GRISON1 1 2 3 4 5

Institute of Geophysics AS CR v.v.i., Boční II/1401, 141 31 Praha 4, Czech Republic ([email protected]) Regional Centre of Advanced Technologies and Materials, Department ofPhysical Chemistry, Palacký University, Šlechtitelů 11, 783 71 Olomouc, Czech Republic Institute of Inorganic Chemistry AS CR v.v.i., 250 68 Řež, Czech Republic National Institute of Public Health, Šrobárova 48, 100 42 Praha 10, Czech Republic Czech Hydrometeorological Institute, Na Šabatce 17, 143 06 Praha 4, Czech Republic

Received: June 3, 2013; Revised: July 2, 2013; Accepted: August 2, 2013

ABSTRACT Magnetic measurements of deposited atmospehric dust can serve as an additional parameter in assessing environmental pollution. This method is based on the assumption that atmospherically deposited particles contain significant portion of ferrimagnetic iron oxides of anthropogenic origin, which can be easily detected. Aim of this paper is to identify clearly magnetic fraction of daily samples of particulate matter less than 10 m (PM10), routinely used for air quality assessment and monitoring. We used combination of thermomagnetic analyses and other physical and chemical methods, including scanning electron microscopy (SEM) and Mössbauer spectroscopy. Our results show that daily samples of PM10, collected at sites with different degree of atmospheric pollution, contain magnetite of spherical shape, which is presumably of industrial origin. Thus, magnetic methods can be applied directly to the same substances, which are used routinely in air quality assessment and monitoring. K e y w o r d s : magnetite; atmospheric dust; pollution; rock magnetism

1. INTRODUCTION Solid particles present in the atmosphere cause visible degradation and serious health problems. These aerosols enter the atmosphere by emissions (primary particles) and nucleation (secondary particles). Primary particles may be of natural or anthropogenic origin and may be emitted from point, mobile or areal sources. Natural emission processes include volcanic eruptions, soil-dust uplift, sea-spray uplift, natural biomass burning (wildfires) and biological material release. Major anthropogenic sources are fugitive dust emissions (dust from road paving, vehicles and building construction/demolition), fossilfuel combustion, anthropogenic biomass burning and industrial emissions.

Stud. Geophys. Geod., 57 (2013), 755770, DOI: 10.1007/s11200-013-0814-x © 2013 Inst. Geophys. AS CR, Prague

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Air quality is, among others, evaluated using PM10 (defined as particulate matter which passes through a size-selective inlet with a 50% efficiency cut-off at 10 m aerodynamic diameter). Previously, this particulate pollution has been measured as total suspended particulates (TSP), but size, physical and chemical properties of the particles have proved more important in relation to human health effects than their total mass. It is beyond doubts that particulate air pollution represents serious threat to human health and lead to long term illnesses such as cancer and cardiovascular diseases and acute effects such as allergies. For example, Samet et al. (2000) conclude, that the levels of fine particulate matter in the air are associated with the risk of death from all causes and from cardiovascular and respiratory illnesses. Importance of the respirable fraction of particulate matter is underlined also by the finding, that these particles show significant correlation with several heavy metals. For example, Mugica et al. (2002) reported, that there is a higher proportion of nine metals (Cd, Cr, Cu, Mn, Ni, Pb, Ti, Fe) in the respirable fraction than in the particles with bigger diameter. Furthermore, scanning electronic microscopy enabled them to identify particles from natural soils and clays, from combustion and from industrial processes. In another case study, Protonotarios et al. (2002) deal with correlation between PM10 and heavy metals in a mining-industrial site in Greece. Using statistical correlation and the enrichment factor (EF), they conclude that some substances certainly originate from contaminated soils (Fe, Pb, Zn, Mn, and Cu), while As, Ni, Cd, and Cr are very much enriched with respect to contaminated soil, indicating another possible source attributed to the adjacent industrial plants. Numerous studies, dealing with the relationship between PM10 and heavy metals, rely on rather time consuming and expensive geochemical analyses of collected atmospheric dust. Therefore, additional methods, reflecting this relationship, are needed. Since about eighties, several studies used magnetic measurements as proxy for PM10 and/or heavy metal concentration. Experimental procedures developed in environmental rock magnetism are based on the assumption that atmospheric dust contains significant portion of ferrimagnetic minerals, namely Fe-oxides (Flanders, 1994, 1999; Kapička et al., 2001, 2003). For instance, Kapička et al. (2001, 2003) estimated that magnetic fraction represents about 10% of power-plant fly ashes. Therefore, measurements of concentration-dependent magnetic parameters, which are fast, robust, highly sensitive, non-destructive and relatively cost-effective, can be used as approximation of the industrial imissions (e.g., Strzyszcz et al., 1996). Furthermore, specific characteristics can help in discriminating the origin (lithogenic vs. anthropogenic, Fialová et al., 2005), as well as pollution source (Shu et al., 2001; Lecoanet et al., 2003). Oldfield et al. (1985) magnetically differentiated atmospheric dust. For aerosols, collected in the Mediterranean area, Chester at al. (1984) were able to identify ferrimagnetic particles derived from various origins, such as urban and industrial combustion, or soil erosion. Hunt (1986) used magnetic mineralogy and grain size parameters to delimit atmospheric dust populations with different origins that have been transported long distances. More recently, Xia et al. (2008) used magnetic properties to study urban dustfall affected by anthropogenic activities. Kim et al. (2007) applied magnetic methods in conjunction with geochemical and electron microscopic analyses to monitor the roadside dust in Seoul, Korea. Later on, Kim et al. (2009) proposed quantitative magnetic proxy suitable for the monitoring of spatial and temporal pollution patterns in urban areas using roadside dust samples. Composition and element solubility of magnetic and non-magnetic fly ash fractions were

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Magnetic particles in atmospheric particulate matter

studied by Kukier et al. (2003). Extensive characteristics on magnetic fraction of coalfired power-plant fly-ash particles, combining magnetic, chemical, and microscopic methods, were reported by Blaha et al. (2008). Several recent studies focused on particulate matter atmospherically deposited on different substances. For example, Matzka and Maher (2002), Moreno et al. (2003), Gautam et al. (2005), Zhang et al. (2006), Davila et al. (2006), McIntosh (2007), Maher et al. (2008), Sagnotti et al. (2009), Aguilar Reyes et al. (2012) and others studied magnetic properties of tree leaves in major cities, and observed magnetic spherules of presumably industrial origin. Moreover, significant correlation between concentration of magnetic particles and several heavy metals (e.g. Fe and Pb) was found (Matzka and Maher, 2002). Hanesch et al. (2003) compared spatial distribution of magnetic susceptibility, measured on tree leaves in an industrial area of Leoben (Austria), with that obtained on topsoil. Good agreement between the two distributions served as evidence that topsoils in the area are strongly affected by atmospherically deposited industrial particles. Urbat et al. (2004) reported on significant relationship (R2 = 0.9) between massspecific magnetic susceptibility and Fe content of pine needles. Lehndorff et al. (2006) studied accumulation histories of magnetic particles on pine needles as function of air quality. They concluded that accumulation histories for magnetic fraction within PM are systematic and reflect exposure to environmental pollutant load. More recently, Zhang et al. (2008) reported on atmosphericaly deposited anthropogenic magnetic particles in tree rings. Bućko et al. (2011) characterized in detail magnetic particles deposited on snow along roads. Moss bags and lichens, exposed to atmospheric deposition, were analyzed by magnetic methods in order to assess spatial distribution of atmospheric pollutants (Salo et al., 2012). Only a few recent studies evaluated magnetically directly PM10. For example, Muxworthy et al. (2003) found good agreement between daily variations of PM10 and concentration of magnetic particles in atmospheric samples collected at two different sites in Munich, Germany. Sagnotti et al. (2006) investigated magnetic properties of atmospheric particulate matter from automatic air sampler stations in Latium (Italy). Their results indicate that the magnetic fraction of PM10 is composed of a mixture of magnetitelike ferrimagnetic particles with a wide spectrum of grain sizes, related to a variety of natural and anthropogenic sources. Górka-Kostrubiec et al. (2012) studied particulate matter in relation to annual changes of metrological conditions on a series of filters collected during 1977, 1980, 1981 and 1985 in Warsaw. Despite this progress and numerous data on magnetic particles in atmospheric dust, collected on different carriers, there is still need for detailed description of magnetic fraction within PM10, which is used routinely as one of the air-quality indicators. In particular, the general assumption on the presence of magnetite/maghemite was not unambiguously proved yet. Such knowledge is important and should serve as basis for interpretation of magnetic studies of PM10 in terms of environmental stress. In this paper, we report on identification of ferrimagnetic iron oxides present in PM10 samples, collected using automated high-volume samplers at sites with different level of air pollution and used for routine air quality monitoring. Aim of this paper is to provide direct proof of the presence of magnetite in PM10 samples, collected routinely for air-quality assessment. More detailed magnetic characterization of PM10 samples, such as grain-size


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sensitive data and low-temperature measurements down to 10 K, is beyond the scope of this paper and will be reported elsewhere.

2. SAMPLES AND METHODS We studied samples of PM10, collected at five sampling sites located within the Czech Republic and are involved in permanent monitoring of air quality, performed by the National Institute of Public Health and Czech Hydrometeorological Institute. The sites are classified as industrial (Bartovice near Ostrava, BART), traffic (Legerova street, Prague downtown, LEG), city (Prague, campus of the National Institute of Public Health, SZU), city background (Prague, campus of the Czech Hydrometeorological Institute, LIB) and regional background (Košetice near Pelhřimov, KOS, Dvorská et al., 2008). Basic characteristics of the sites are listed in Table 1. Samples of PM10 were collected using Grasseby-Andersen high-volume sampler (flow: 1218 m3/h, volume: 250400 m3/24 h, Fig. 1) on Whatman quartz microfiber filter during 1 week of a summer 2006 campaign and 1 week of a spring, autumn and winter 2010 campaigns. Each sample represents 24 hours of sampling, starting between 7 and 8 a.m. every day. Climatic conditions during the sampling days were practically the same for all the sites. Thus, the effect of climate on composition of the PM10 in terms of iron oxides can be neglected and environmental stress (imissions) should dominate. Actually, it is of great interest to assess the effect of climatic conditions, period of year, traffic, etc., on magnetic properties of PM10 (and finer PM2.5 and PM1 fractions). Such study requires much longer and consistent sampling campaign. At present, we have samples available and some of these effects will be discussed in manuscripts under preparation. Samples did not undergo any special treatment. It was impossible to detach the deposited particles from the filter, any mechanical treatment results in mixture of filter material and dust of unknown contributions. Therefore, the samples were measured in Table 1. Basic characteristics of sampling sites. MeanY - 2007 annual average PM10 concentration, MedianD - 2007 median of PM10 daily average concentration (CHMI, 2007). Site Bartovice BART Legerova LEG State Inst. Publ. Health SZU Czech Hydromet. Inst. LIB Košetice KOS

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Classification Industrial Traffic Urban Urban Background Regional Background

Location 49°4825N 18°2020E 50°421N 14°2548E 50°433N 14°2825E 50°027N 14°2656E 49°3422N 15°449E

MeanY

MedianD

[g/m3]

[g/m3]

65.4

58.6

46.2

42.0

28.7

26.0

26.1

22.8

18.3

15.8



Fig. 1.

Grasseby-Andersen high-volume sampler at the urban site in Prague.

their pristine state, only smaller specimens, fitting the instrument sample holders (stripes of 1  10 cm for vibrating-sample magnetometer and about 0.5  2 cm for kappabridge KLY4S), were cut using stainless scissors. Morphology of the collected particles was analyzed using SEM PHILIPS XL 30 CP. Speciation of iron oxides was performed by Mössbauer spectroscopy at room temperatue (300 K) and 25 K. The transmission 57Fe Mössbauer spectra of 512 channels were collected using a Mössbauer spectrometer at constant acceleration mode with a 57Co(Rh) source. Basic measurements were carried out at room temperature (RT). Low-temperature Mössbauer spectra were recorded at 25 K using a cryomagnetic Oxford Instruments system. The isomer shift values were referred to alpha-Fe at RT. Voltammetry of microparticles (Scholz et al., 2005) was performed in regime optimized for the identification of Fe oxides (Grygar et al., 2002; Čapek et al., 2005). Briefly, the measurements were performed by linear sweep at scan rate 3 mV/s in acetic acid-sodium acetate buffer (1:1, 0.2 M total acetate). The peak potentials were compared with those reported by Grygar et al. (2002) and Čapek et al. (2005). Thermomagnetic analyses were performed on the basis of temperature dependence of magnetic susceptibility, measured from liquid nitrogen temperature (80 K) to about 1000 K using KLY-4S/CS-3 kappabridge (AGICO Brno, Czech Republic, Hrouda, 1994). These measurements were carried out on pristine samples in ambient atmosphere; heating rate was about 8.5 K/min. The data were normalized with respect to the room-temperature value. Background data of furnace and blank quartz filter of the same size as the specimens were subtracted. Isothermal remanent acquisition (IRM) curves were measured using an EV9 vibrating magnetometer (DSM Magnetics; ADE Corporation, Lowell, MA, USA) with the maximum magnetic field of 2 T. Individual contributions were estimated


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using the numerical decomposition of Kruiver et al. (2001). Since we were not interested in concentration-related data, but in identifying relative contribution of minerals with different coercivities, there is no need for mass or area normalization. All the IRM data are volume-normalized using nominal volume of 1 cm3, which is set in the measurement protocol of the vibrating sample magnetometer. This procedure is routine in case of samples of irregular or unknown volume.

3. RESULTS 3.1. Morphology Figure 2 presents a set of typical SEM images, showing typical spherical particles of presumably industrial origin. The spherical shape is typical for particles originated from fast cooling of melts, or as a result of combustion of fossil fuel (e.g., Flanders, 1994, 1999). Obviously, spherical particles were identified in samples coming from all the 5 stations. Although we did not perform any image analyzes aimed at shape and count determination, it was easy to observe subjectively the relative differences in the number of iron-rich spherules; their abundance reflected the corresponding level of air pollution and they were identified even at the regional background KOS station, although rather scarcely. Energy dispersive spectrometry (EDAX) of spherules present in PM10 samples collected at the BART (industrial) and LEG (traffic) sites (with the most abundant spherical particles) revealed dominance of C, O, Al and Si (Table 2). Although Si data are most probably biased by fibers of the filters, high portion of Al suggests presence of alumina-silicates. In both locations, Fe was convincingly detected. Its relatively low portion can be explained by the fact that the analyses are relevant to the surface of the particles. Therefore, surface composition can mask the interior, where iron oxides can be much more abundant. 3.2. Mössbauer spectroscopy Mössbauer spectroscopy was carried out on samples coming from all the five sites. However, data acquired on the urban and urban background samples were, despite long integration time, too noisy and did not yield reliable and unambiguous results. Data obtained on the industrial, traffic and regional background samples are summarized in Table 3. In all the three cases, presence of Fe3+ was proved. Fe2+ was identified in the KOS sample. The BART sample showed clear sextet at both room temperature and 25 K (Fig. 3a,b), suggesting presence of coarse-grained multidomain ferrimagnets, most probably maghemite/magnetite, and/or antiferromagnetic hematite. The hyperfine field (BHF) of the phase magnetically ordered at room temperature is above 50 T that could indicate a contribution of magnetically ordered ferric oxides (stoichiometric hematite has BHF of 51.75 T, maghemite 50.0 T, and magnetite 49 and 46 T, Cornell and Schwertmann, 2003). Although Fe2+ was not proved in this sample, magnetite cannot be excluded. Spectrum of the KOS sample (Fig. 3c) is best fitted by doublet, suggesting prevalence of paramagnetic form of iron. However, sextet could be seen in the spectrum as well, but is rather noisy and not that reliable. Thus, ferrimagnets may be present, but in very minute concentrations.

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Fig. 2. SEM images of PM10, showing typical spherules, which are supposed to be magnetic of presumably industrial origin. a) and b) industrial site, c) and d) traffic site, e) and f) urban site, g) and h) urban background, i) and j) regional background site.


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E. Petrovský et al. Table 2. Energy dispersive spectrometry (EDAX) results of typical spherules identified in two PM10 collected at the industrial (BART) and traffic (LEG) sites. Site

Element C O Mg Al Si Fe C O Al Si Fe K Ca Ti

BART

LEG

Weight% 19.07 36.98 2.28 7.85 17.46 16.36 21.95 45.90 4.65 25.94 1.56

Atomic% 22.38 46.27 0.53 0.84 28.75 1.23 19.84 52.76 17.54 2.26 2.41 2.90 2.29

30.55 44.46 1.80 5.59 11.96 5.64 31.40 42.29 2.96 15.87 0.48

31.82 49.41 0.37 0.53 17.49 0.38 28.37 56.64 11.17 0.70 1.06 1.24 0.82

Table 3. Results of Mössbauer spectroscopy of PM10 samples. Tmeas - measurement temperature,  - isomer shift, EQ - quadrupole splitting, Q - quadrupole shift, BHF - hyperfine field,  - halfwidth of resonant lines, RA - relative area. Sample

Tmeas [K]

KOS

300

LEG

300 300

BART 25



Q

[mm/s]

EQ [mm/s]

[mm/s]

BHF [T]

 [mm/s]

0.35 0.99 0.19 0.24 1.12 0.25 1.09 0.22 1.32 0.43 0.51

0.88 2.09 – 0 1.14 – – 0 1.88 – –

– – – – – 0.21 0.35 – – 0.48 0.77

– – – – – 50.7 47.9 – – 53.0 50.6

0.60 0.45 0.93 1.14 0.45 0.78 0.48 1.14 0.73 0.91 0.79

RA [%] 87.4 12.6 100.0 19.4 5.8 57.5 17.3 20.1 5.0 53.6 21.3

Valence Fe3+ Fe2+ Fe3+

Fe3+ Fe3+

3.3. Voltammetry Results of voltammetry of the industrial (BART) and traffic (LEG) PM10 samples are shown in Fig. 4. As regards the other three samples, the curves did not express any typical features, most probably due to the concentration of iron oxides below the sensitivity limit of the method. Voltammetric peak C1, observed on the LEG curve, is in the potential region typical for magnetite. On the BART curve, the C1 peak is missing, while the C2

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Fig. 3. Mössbauer spectra of two PM10 samples. a) Industrial sample measured at room temperature, b) industrial sample measured at 25 K, c) regional background sample measured at room temperature.

peak is well developed. This may be assigned to coarse crystalline hematite (in micrometer-size range) or maghemite, less likely to magnetite. The sensitivity of the method is higher for hematite than for maghemite or magnetite. Therefore, the BART curve may reflect the presence of either coarse hematite, or coarse magnetite/maghemite, or all of them. The Fe-valence estimates from Mössbauer spectroscopy (Table 3) indicate that ferric oxides (hematite or maghemite) must be prevailing rather the magnetite (stoichiometric magnetite has 33% of Fe in divalent form).


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Fig. 4. Voltammograms of the industrial (BART) and traffic (LEG) samples. The arrow indicates the start and direction of the measurement, C1 and C2 are voltammetric peaks of Fe oxides.

Fig. 5. Thermomagnetic curves of PM10 samples, normalized with respect to the roomtemperature value. a) Industrial site, b) traffic site. Black line – heating, grey line – cooling.

3.4. Thermomagnetic analyses Low-temperature curve of the industrial BART sample (Fig. 5a) shows well pronounced maximum at about 130 K (grey dashed line in Fig. 5a), which is very close to the Verwey temperature for bulk magnetite (120 K). There is another change in shape at about 100 K, which is similar to that observed by Mang et al. (2013) at about 95 K and interpreted as the effect of significant number of vacancies or surface oxidation of small

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magnetite grains. The high-temperature heating curve is typical for magnetite. The Curie point is estimated using the test for linearity of inverse susceptibility (Petrovský and Kapička, 2006). Our estimate yields the Curie point of 840 K (grey dashed line in Fig. 5a), which corresponds to that of magnetite (848 K). The traffic sample is much more noisy, but the same main features can be recognized (Fig. 5b). All the other samples contained such small amount of ferrimagnetic iron oxides that temperature dependence of magnetic susceptibility did not show any features which could be interpreted as Verwey or Curie temperature of magnetite. 3.5. Analyses of IRM acquisition curves Curves of acquisition of isothermal remanent magnetization (IRM) were analyzed using numerical decomposition described by Kruiver et al. (2001). Due to the minute amount of iron oxides in the measured samples, resulting in rather noisy IRM acquisition curves, only those samples from each site that show enough smooth pattern were used here. These are shown in Fig. 6 and the corresponding representative values of saturation IRM (SIRM), coercivity of acquisition H1/2 (field at which 1/2 of SIRM is acquired) and dispersion parameter DP are listed in Table 4. No detailed analyses in terms of comparing the sites or assessing different climatic conditions and effects were done. For that purpose, more samples have to be analyzed. In this study, we focus on the evidence of strongly magnetic iron oxides. Anyway, the data indicate certain differences. For example, SIRM values are in the order of environmental classification of the sites in terms of air pollution (industrial - traffic - urban - urban background - regional background). This suggests that concentration of magnetite may correspond to PM concentration. H1/2 values are between 500 and 750 Oe (i.e. about 40000 and 60000 A/m), which are slightly higher, but still of the same order as those reported by Bućko et al. (2011) (about 400470 Oe, i.e. about 3200037400 A/m). Bućko et al. (2011) analyzed particles originating presumably from road traffic and deposited on snow near the roads. Therefore, his particles should be coarser and this difference in grain sizes may be responsible for their softer IRM acquisition. Similar values (480 Oe, i.e. about 38200 A/m) were reported for polluted tree leaves in Kathmandu by Gautam et al. (2005). Blaha et al. (2008) studied power-plant fly-ash samples and reposrted on H1/2 values between 710 Oe (56500 A/m) and 810 Oe (64500 A/m) and attributed them to magnetite. Unlike Blaha et al. (2008) or Gautam et al. (2005), we did not identify unambiguously hematite; however, we can’t exclude it.

4. CONCLUSIONS Samples of atmospheric PM10, collected at 5 sites with different air-pollution level and used for standard air quality monitoring, were analyzed from the point of view of presence and basic characteristics of magnetic fraction, which is assumed to be present in form of spherules rich in iron oxides and have thus potential for magnetic monitoring of PM10. Our results provide evidence that such spherules are rather abundant at industrial and traffic sites, and, although rare, are found also at a regional background site. Iron oxides were found also in particles of irregular shape, which can be of other sources than combustion of fossil fuel. Relatively low portion of iron revealed using EDAX elemental analyses can be attributed to masking effect of the aluminia-silicate surface.


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Fig. 6. Representative examples of linear acquisition plots of isothermal remanent magnetization of PM10 samples. a) Industrial site, b) traffic site, c) urban site, d) urban background site, and e) regional background site. Based on numerical decomposition of Kruiver et al. (2001). Table 4. Results of numerical decomposition of isothermal remanence acquisition curves (after Kruiver et al., 2001) shown in Fig. 6. SIRM - saturation remanent magnetization, H1/2 - field after which half of SIRM is acquired, DP - dispersion parameter. Sample

Component

Contribution

SIRM [A/m]

H1/2 [A/m]

DP [A/m]

BART LEG SZU LIB KOS

1 1 1 1 1

100% 100% 100% 100% 100%

4.22  103 2.83  104 2.22  104 5.00  105 2.90  105

39884.2 57645.9 46855.2 52576.9 39884.2

0.34 0.32 0.36 0.32 0.34

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Mössbauer spectroscopy proved the presence of Fe3+ in the industrial, traffic and regional background samples. In addition, Fe2+ was detected in the regional background sample. The spectra of the industrial sample, measured at room temperature and 25 K, show the same features and are best fitted by sextet, suggesting presence of coarse-grained multidomain magnetically ordered ferric oxides (hematite and/or maghemite). Voltammetry clearly confirmed the presence of iron oxides in the industrial PM10 samples, suggesting coarse hematite/magnetite/maghemite in the former and magnetite in the latter sample. In the other samples these particles were not determined, most probably due to their low concentration, which was beyond the sensitivity limit of the method. Thermomagnetic analyses and decomposition of the isothermal remanence acquisition curves confirmed the presence of coarse-grained multidomain magnetite, in particular in the sample from industrial and traffic sites. To conclude, samples of PM10, collected at five sites with different environmental stress, contain spherical particles typically of anthropogenic origin. The presence of iron oxides in form of coarse-grained multidomain magnetite in PM10 from industrial site was confirmed by thermomagnetic analyses and decomposition of isothermal remanent magnetization acquisition curves. Although it was not unambiguously proved in samples from other sites, there are indicators suggesting its presence here as well. Our data provide a reliable basis for the interpretation of results of highly-sensitive magnetic measurements of atmospheric PM10 and for air-quality monitoring purposes, especially at industrial, traffic and urban sites. Acknowledgements: This study was carried out with the support from the Czech Science Foundation through grant #P210/10/0554. We thank Dr. J. Tuček for his assistance with the Mössbauer spectroscopy measurements. We are grateful to Marcos A.E. Chaparro, an anonymous reviewer and Associate Editor Agnes Kontny for their helpful comments on the manuscript.

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