Synchrotron radiation total reflection X-ray fluorescence

Microchemical Journal 137 (2018) 78–84

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Synchrotron radiation total reflection X-ray fluorescence (SR-TXRF) and X-ray absorption near edge structure (XANES) of fractionated air particulates collected from Jeddah, Saudi Arabia Abdallah A. Shaltout a,b,⁎, Messaoud Harfouche c, Sameh I. Ahmed a,d, Mateusz Czyzycki e,f, Andreas G. Karydas f,g a

Physics Department, Faculty of Science, Taif University, 21974 Taif, P.O. Box 888, Saudi Arabia Spectroscopy Department, Physics Division, National Research Centre, El Behooth St., 12622 Dokki, Cairo, Egypt c Synchrotron-light for Experimental and Scientific Applications in the Middle East (SESAME), P.O. Box 7, Allan 19252, Jordan d Physics Department, Faculty of Science, Ain Shams University, Cairo, Egypt e AGH University of Science and Technology, Faculty of Physics and Applied Computer Science, Al. A. Mickiewicza 30, 30-059 Krakow, Poland f International Atomic Energy Agency, Department of Nuclear Sciences and Applications, Division of Physical and Chemical Sciences, Physics Section, Wagramer Str. 5, A-1400 Vienna, Austria g Institute of Nuclear and Particle Physics, NCSR “Demokritos”, 15310 Aghia Paraskevi, Athens, Greece b

a r t i c l e

i n f o

Article history: Received 15 August 2017 Received in revised form 30 September 2017 Accepted 1 October 2017 Available online 02 October 2017 Keywords: Fractionated atmospheric aerosol Synchrotron radiation total reflection X-ray fluorescence X-ray absorption near edge structure Jeddah - Saudi Arabia

a b s t r a c t Fractionated atmospheric aerosols with sizes ranged from 0.25 μm to N16 μm have been collected on silicon wafers using a 7-stage cascade impactor from the centre location of Jeddah city, Saudi Arabia during May 2015. Two fractionated sizes were selected in the present work, namely 0.5–0.25 μm (PM0.5–0.25) and 2–1 μm (PM2.0–1.0), because their mass concentrations were the most dominant. Fractionated atmospheric aerosols were examined under ultra-high vacuum environment with synchrotron radiation total reflection X-ray fluorescence (SRTXRF) and X-ray absorption near edge structure (XANES) spectroscopy techniques at the International Atomic Energy Agency (IAEA) experimental end station operating at Elettra Sincrotrone Trieste, Italy. The mass concentrations in PM2.0–1.0 were found greater than those within PM0.5–0.25 material and the mass concentration ratio of PM2.0–1.0/PM0.5–0.25 was reached to 2. The homogeneity in the spatial deposition of different elements in both PM0.5–0.25 and PM2.0–1.0 fractions was evaluated by the means of X-ray fluorescence (XRF) scanning. XANES showed us that Cr species exist mainly in the trivalent oxidation state, while for Mn the co-existence of both the divalent and trivalent oxidation states was determined. The results of the present study provide an improved understanding on the origin of Cr and Mn species in these two fractionated particulates and give an insight to their contribution to atmospheric and physicochemical processes. © 2017 Elsevier B.V. All rights reserved.

1. Introduction The air pollution originating from natural and anthropogenic sources represents a common concern worldwide and the air quality might change from place to another based on the intensification of these sources. The increase of air pollution might have a remarkable influence on human health and climate change [1]. The particulate matter (PM) was considered as the most important air pollutant [2–4]. The size of particulate matter has attracted a remarkable attention in bibliography since the fine particulates have an important impact on global climate due to their long life in the atmosphere and transport efficiencies [5–10]. Moreover, recent large scale epidemiological studies [3,11, 12] have clearly demonstrated a correlation of the increased concentration of fine particulates in the atmosphere with the appearance of ⁎ Corresponding author at: Spectroscopy Department, Physics Division, National Research Centre, El Behooth St., 12622 Dokki, Cairo, Egypt. E-mail address: [email protected] (A.A. Shaltout).

https://doi.org/10.1016/j.microc.2017.10.001 0026-265X/© 2017 Elsevier B.V. All rights reserved.

harmful and adverse effects on human population. The characterization of fractionated atmospheric aerosols is important because the chemical composition and the quantities of trace elements may vary with the sources of particles as well as with the size of atmospheric aerosols collected [13]. Therefore, the concentration of different pollutants may have different distributions. The study of PM2.5 fraction in particular has taken a lot of attention in the literature [14–17]. In the Middle East (e.g. Greater Cairo - Egypt, Taif and Makkah - Saudi Arabia), atmospheric aerosol samples have been studied extensively [18–25]. Jeddah city represents the most important city in the Saudi Arabia next after the capital of the kingdom, Riyadh. The human population in Jeddah increases annually and it has reached currently N3.4 million inhabitants. Recently, the city started remarkable diverse activities in the field of construction and industry such as oil refinery, various industrial activities, plants desalinization, vehicle fuels and power plants. However, these developments as well as other natural sources, such as desert storms, and earth crust corrosion cause environmental degradation and, consequently, the air quality has worsened. It was recognized

A.A. Shaltout et al. / Microchemical Journal 137 (2018) 78–84

that the residential and urban areas of Jeddah suffer from different anthropogenic contributors, even if the most of human-made activities are located outside the residential areas [26]. In addition, the authors investigated the elemental composition as well as the source apportionments of PM2.5 and PM10 in Jeddah. An early study, that had been carried out on Jeddah urban dust, confirmed that the concentration of atmospheric aerosols collected in Jeddah was higher than the air quality standard [27] while Pb concentration in Jeddah urban dust is still permissible. Another study aimed at the quantitative determination of lead in ambient air collected from different six urban sites in Jeddah [28] using atomic absorption spectrometry. The authors found a correlation between Pb concentration and traffic density although the vehicle fuel used in the whole kingdom is unleaded while Pb concentration ranged from 0.19 μg/m3 to 1.27 μg/m3 which is close to a maximum allowance level. For the first time, the concentrations of the natural radioactivity of 40K, 232Th, 238U and Raeq as well as an associated internal inhalation dose emitted to the public were evaluated in the PM2.5 particles collected from Jeddah [29]. It was found that the mass concentration of PM2.5 aerosol is greater twice than the maximum allowance level and the inhalation dose ranges from 15.03 to 58.87 nSv/year. Simultaneous gravimetric measurement of PM10, PM2.5 and PM1.0 fractions and the elemental composition of PM2.5 in Jeddah during March 2012, including dust storm and non-dust storm periods, was conducted [30]. The elemental concentration during dust storm periods carried much more coarse and fine particles and was higher than its corresponding concentration during the non-dust storm period. In this paper the authors reported that the percentage of crustal elements (Na, Mg, Si, K, Ca, Ti, Cr, Mn, Fe, Rb and Sr) represented 44.6% and 67.5% from the total element concentrations in non-dust and dust storm samples, respectively. In the present work, a fractionated particulate matter was collected on Si wafers using a cascade impactor of seven different stages, ranging from particle size of ≥16 μm to 0.5–0.25 μm. Two fractions with highest mass concentration, PM0.5–0.25 and PM2.0–1.0, were selected for a highlysensitive analysis by SR-TXRF to determine the element composition and by XANES to investigate oxidation states of Cr and Mn. XANES was performed because redox reactions are of great importance in controlling the oxidation state and thus, the mobility and the toxicity of many elements [31]. For this work the synchrotron radiation was used because it improves the analytical capability of XRF spectrometry in terms of elemental sensitivity and absolute detection limits. The high degree of linear polarization, monochromaticity and energy tunability combined with a high flux make the synchrotron radiation based probes a great experimental method to perform a compositional and chemical speciation analysis at trace-level [32]. Usually, the fractionated air particulate matter samples are deposited onto polycarbonate filters. However, for this research fractionated aerosols were deposited on Si wafers with a diameter of 24 mm. Silicon reflectors are ideal substrates to deposit PM samples and for performing SR-XRF and XANES analysis in a total reflection geometry. In this case, PM deposited on reflectors is doubly excited by the extended footprint of the exciting and reflected X-ray beam, whereas the XRF signal to background ratio is significantly enhanced due to the very shallow penetration of the exciting beam in the sample carrier and reflection at symmetrical angle away from the detector aperture [33,34]. 2. Experimental setup 2.1. Sampling The fractionated atmospheric aerosols were collected from Sharafyia district close to the centre of Jeddah city. Geographical coordinates of the sampling site are 39°11′ E and 21°30′ N. This location is characterized by a high traffic density (1.4 million vehicles run every day inside the city). The city is located on the Red Sea coast and the sampling location was also close to the coast. Anthropogenic activities are relatively

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near to the sample site such as oil refinery, desalinization plants, power plants and different industrial factories. A cascade impactor was used to collect the fractionated air particulate matter in seven different stages (PIXE Cascade impactor, Model I-1, PIXE International Corp., Tallahassee, USA). The stages were arranged in a descending order of their cut-off size. The individual cascade impactor stages work by impinging a jet of aerosols onto a collection plate. Each stage of impactor is designed to retain all aerosols above a certain cut-off aerodynamic diameter. Each stage represents a specific aerodynamic diameter, whereas the particulate matter was separated in up to seven diameter ranges, namely: 0.5–0.25, 1.0–0.5, 2–1, 4–2, 8–4, 16–8, ≥16 μm. The sampling setup consisting of cascade impactor, critical orifice and pump was mounted 30 m above the ground. A critical orifice was placed between the pump and the impactor to restrict the airflow to 1 L min−1, which is recommended by its manufacturer. Samples were collected on Si wafers over a period of 48 h in May 2015. Silicon wafers prior to their use in the cascade impactor were heated on a hotplate at a temperature of 60 °C. In order to catch up effectively and separate all the fractionated particles the wafers were coated with few mg of vaseline. Silicon wafers used as a sample carrier are an excellent alternative to polycarbonate filters in terms of their sampling efficiency. Once the PM material had been collected on Si wafers, a 10 μL solution with 10 μg/g of Ga was added to the aerosol and was used as an internal standard for the TXRF quantification. It should be emphasized that, intensities of Ga-K lines remain constant through the vertical linear scan measurements which confirms its uniform deposition. The diameter of PM material collected on Si wafers was 1.2 mm and 0.5 mm for the fractions PM2.0–1.0 and PM0.5–0.25, respectively. 2.2. Synchrotron radiation TXRF and XANES In the present work SR-TXRF and XANES experiments were performed at Elettra Sincrotrone Trieste, Italy at X-ray fluorescence beamline [33], based on a Si(111) double crystal monochromator with its resolving power of 1.4 × 10−4 that brings a photon flux of 108– 109 s−1 in the energy range between 3.65 and 14.5 keV. The experimental station was an advanced X-ray spectrometry instrument of the International Atomic Energy Agency [33,35] equipped with a multi-axis motorized sample manipulator (four linear and three rotational stages) which provides different degrees of freedom to align the sample surface relative to the incident beam. An ultra-thin window (UTW) silicon drift detector (SDD, Bruker Nano GmbH, XFlash 5030) with a 30 mm2 nominal crystal area, a crystal thickness of 450 μm and an energy resolution of 131 eV at Mn-Kα was used for the detection of elements characteristic X-rays. The vacuum environment, the UTW SDD and the possibility to tune the incident beam energy down to 3.65 keV allow the efficient detection even of light elements (Z b 14) [36]. 3. Results and discussion 3.1. Mass concentration of the analyzed fractionated samples Two fractionated particulates samples were selected from the set of the cascade impactor, namely: PM2.0–1.0 and PM0.5–0.25. The size of particles emitted from coal and oil fired-power plants and those from engine related motor vehicles is usually b1 μm [37]. Also, the average diameter of particles from the catalyst-equipped motor vehicles, burning unleaded petrol, is also b1 μm. Furthermore, the exhausts emitted from freshly produced petrol engines exhibit a size distribution mostly within the range of 0.5–0.25 μm. Consequently, the aerosol particles with the aerodynamic diameters b1 μm are dominant from anthropogenic sources with a well-mixed origins [38]. The two selected samples represent the highest mass concentration when comparing with other stages of the cascade and more investigation and characterization for these two fractions is important.

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Fig. 1 illustrates the variations of the mean mass concentration expressed in μg/m3 with its standard deviation versus different stages of the cascade impactor (ranging from N 16 μm to 0.5–0.25 μm) during the spring 2015. As shown, the highest mass concentration was found in the fractionated particulates PM2.0–1.0 and PM0.5–0.25 with average concentrations of 65 ± 34 and 32 ± 21 μg/m3, respectively. The average mass concentrations of other stages were comparable with lower standard deviations and their mass concentrations ranged from 17 μg/m3 to 30 μg/m3. One could recognize that the error bars for the stages N 16, 2– 1, 0.5–0.25 μm was relatively high during the sampling time. The large standard deviation might be an indication to the remarkable weather variation during the spring. Atmospheric aerosol particles PM2.0–1.0 and PM0.5–0.25 were predominant even if their standard deviations were high. The mass percentage of stages PM2.0–1.0 and PM0.5–0.25 were 31% and 15%, respectively. The percentage of mass concentrations of other stages was in the range from 8% to 14%. For this reason the present work is focused on the atmospheric aerosols collected in the stages PM2.0–1.0 and PM0.5–0.25. 3.2. Quantitative elemental analysis of fractionated air particulates In order to understand the high mass concentration at selected PM2.0–1.0 and PM0.5–0.25 samples, the synchrotron based techniques were employed to perform the elemental analysis (SR-TXRF) and the chemical speciation of Cr and Mn trace impurities (XANES). Typical SR-TXRF PM2.0–1.0 and PM0.5–0.25 spectra are shown in Fig. 2 representing the sum of a series of linear scans across the whole area of the PM samples excited by the 13 keV incident monochromatic beam. XRF spectra were proceeded with QXAS/AXIL [39] and the PyMca [40] software. The following elements were quantified: Na, Mg, Al, S, Cl, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, As, Se, Br and Pb. Due to the spectral interference between Pb-Lα (10.553 keV) and As-Kα (10.532 keV) lines, As was quantified through the As-Kβ (11.727 keV) line. Silicon was not quantified since a great portion of Si-Kα intensity originates just from the Si wafer itself. Table 1 reports the determined average elemental concentrations. In general, the concentrations of most detected elements in the PM2.0–1.0 fraction were found greater than those in the PM0.5–0.25 particulates. This trend can be clearly explained by the fact that the mass concentration of the PM2.0–1.0 fractionated particulate is greater than the PM0.5–0.25 fractionated particulate as described earlier. The concentrations of Na, Zn, As and Se were approximately twice more in the PM2.0–1.0 fraction. For other elements, including V, K, Cr, Ni, Cu and Cl the concentrations in the PM2.0–1.0 fraction were higher from 4 up to 10 times than the corresponding quantities in the PM0.5–0.25. For the elements: Mg, Al, Ca, Ti, Mn, Fe, Cu, the concentrations in the PM2.0–1.0

fraction become even N10 times higher. An interesting result was found for S and Sc. In the case of Sc there was no change in the concentration even when the particle size decreases, as reported in Table 1. This could be an indication that Sc is distributed homogenously in the different fractionated air particulates. On the other hand, the mass concentration of S was found in the finest particulates that had the aerodynamic diameter equal to or b0.5 μm. This result was in the agreement with other findings [41] where S was more representative in the particle size from 0.5 to 0.06 μm. Fig. 3 illustrates the variation of normalized elemental PM2.0–1.0 and PM0.5–0.25 concentrations over vertical linear scans. It was carried out with a step size of 0.1 mm and counting time per step equal to 120 s. A small number of steps was obtained due to the small area of the samples, ~ 1.2 mm for PM2.0–1.0 and ~0.5 mm for PM0.5–0.25. Eleven and only four spots were probed from PM2.0–1.0 and PM0.5–0.25 particulates, respectively. In the case of PM2.0–1.0 samples, except Na, Mg, Sc, Se and Br, the mass concentrations were localized at the centre of Si wafer. However, most of elements in the PM0.5–0.25 particulate had approximately a homogenous distribution over the linear scan except Cr and Br. In general, the quantified elements originated from two main sources, the natural and anthropogenic one. The elements Ca, Fe, K, Mn and Ti are mainly associated with soil appeared in coarse, fine and ultrafine fractions at each stage. It has been reported that Ca is produced by the crustal erosion or suspension of the soil [42]. The elements Cu, Zn, Ti, As, Ni and V originated from fuel combustion as well as from other sources, while the elements As, Cl, Cr, S and Se came from coal powered stations. The presence of Cl, K, Cu, Zn and Br in the fine fraction is typical to road vehicular exhaust emissions [14,43,44]. Manganese is used as an additive in fuel to raise the octane level so that Mn particles could be emitted from diesel burning vehicles [45]. Zinc particles could be produced from the tyre wear, while the association between Cu and Zn characterizes brake lining particles [46] and the steel mill emissions. The element K could be derived from different sources and S was present in the form of oxides in the atmosphere in the industrial area [47]. The elements S and K could be produced from biomass burning and their presence in the fine fraction suggested a contribution of biomass burning [14]. In the case of Cl, it is mainly taken from sea spray [48]. The association of Ca and S with the metallic elements suggests an industrial source such as coal burning or steel mill industrial sources. The presence of Ni and V in this study could be the indicator of industrial activities, including burning of lubricating oils, metal works welding and vehicle repair [14,43]. Elements like Cr, Mn, Zn and Cu might be released into the atmosphere by a metallurgical industry. Many of the trace elements such as Fe, Cu and Zn could be related with traffic pollution in the form of fine and coarse particles as well [49–51]. 3.3. XANES of Cr and Mn

Fig. 1. Mass concentrations of fractionated air particulates versus stages of the cascade impactor during the spring 2015.

XANES spectra were collected at Cr and Mn K-edges (5989 and 6539 eV, respectively) for the two fractionated PM2.0–1.0 and PM0.5–0.25 particulates in a fluorescence mode. To decrease the noise, three absorption scans were measured, properly aligned and averaged for each sample. The background was subtracted and the absorbance was properly normalized for each of the absorption spectrum using ATHENA software [52]. The absorption edge positions of the samples were taken at half of the edge step and were found shifted to 6004 eV and 6547 eV for Cr and Mn K-edges, respectively. In case of Cr, reference XANES spectra were obtained at K-edge by measuring the absorption of the model compounds; Cr metal, Cr(III)2O3, [Fe(III),Cr(III)](OH)3, Mn(II)Cr(III)2O4, Cr(VI)O3, Cr(VI)O4. It worth mention that, Cr(VI)O4 is an oxoanion of chromium in which the oxidation state of Cr is +6 and is considered as a strong oxidizing agent. For the reference Mn K-edge XANES spectra, the measured model compounds were Mn metal, Mn(II)O, Mn(II)SO4, Mn(II,III)3O4, Mn(III)2O3, Mn(IV)O2. All the model compounds were measured in transmission mode at the XAFS beamline of Elettra.


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Fig. 2. Typical SR-TXRF spectra of fractionated air particulate matter, PM2.0–1.0 and PM0.5–0.25 collected in Jeddah city, Saudi Arabia.

Normalized Cr and Mn K-edges XANES spectra were a subject to Linear Combination Fitting (LCF) using different experimental model compounds as shown in Fig. 4. The LCF were greatly affected by the low quality of the obtained XANES spectra caused by the fluctuating density of the particulate matter within each filter. In addition, the majority of the elements are not distributed humongous especially for the PM2.0– 1.0 fraction which was also observed in the XRF linear scanning. Table 2 reports the determined percentage abundances of Cr(III), Cr(VI), Mn(II) and Mn(III) species obtained for the PM2.0–1.0 and PM0.5–0.25 samples. The average values given in Table 2 are reasonable as seen from the resulting error bars. From the LCF results it can be observed

Table 1 Quantitative elemental analysis of twenty elements determined in the fractionated particulate matter PM2.0–1.0 and PM0.5–0.25 (average ± standard deviation). bLOD = below limit of detection. Element and its spectral line

Na-Kα Mg-Kα Al-Kα S-Kα Cl-Kα K-Kα Ca-Kα Sc-Kα Ti-Kα V-Kα Cr-Kα Mn-Kα Fe-Kα Ni-Kα Cu-Kα Zn-Kα As-Kβ Se-Kα Br-Kα Pb-Lα

Concentration, ng/m3 PM0.5–0.25

PM2.0–1.0

12.9 ± 3.3 0.9 ± 0.3 12.6 ± 5.8 40.4 ± 10.7 0.6 ± 0.3 9.3 ± 1.3 21.0 ± 4.3 22.8 ± 5.2 3.1 ± 1.0 0.2 ± 0.1 0.1 ± 0.0 0.9 ± 0.1 47.0 ± 11.1 0.2 ± 0.0 0.8 ± 0.1 2.1 ± 0.2 0.2 ± 0.0 0.02 ± 0.0 bLOD bLOD

27.7 ± 14.6 9.3 ± 5.2 105.8 ± 30.2 23.3 ± 5.6 2.3 ± 1.5 72.3 ± 37.2 354.8 ± 125.5 23.0 ± 3.3 41.4 ± 25.7 1.2 ± 0.9 0.9 ± 0.7 9.5 ± 4.9 597.4 ± 394.0 0.8 ± 0.5 7.1 ± 4.3 6.0 ± 3.4 0.3 ± 0.1 0.1 ± 0.0 0.02 ± 0.0 27.7 ± 14.6

that Cr occurs mainly in the trivalent oxidation state in the forms of Cr oxides and Cr hydroxides, such as Cr2O3, [Fe,Cr](OH)3, and MnCr2O4. The hexavalent Cr is also present in minute fractions less than one tenth of the total Cr content. The divalent form of Cr can only be observed in the form of a combined Cr redox state in compound such as Cr3O4 which can be written as (Cr(II)1/3 Cr(III)2/3)O4 and was not detected in the present fractionated samples. Therefore, the trivalent form of Cr that usually occurs in natural compounds dominates the other possible oxidation states. Similarly, Mn in the samples occurs mainly in its divalent oxidation state and evolves in structural form close to Mn(SO)4− like compounds. Nevertheless, the trivalent oxidation state is present in a minor content presenting a combination of both divalent and trivalent oxidation states as in the case of Mn3O2 −4 like compounds, where Mn(III) represents only one third. The fact that the Cr(VI) and Mn(III) species occur in natural compounds together with their respective reduced forms, Cr(III) and Mn(II), respectively, could be an indication that these elements start having a transition where an oxidation/reduction process is ongoing due to severe weathering processes. 4. Conclusions Due to the environmental hazards and the missing information of fractionated atmospheric aerosols apparent in Jeddah, Saudi Arabia, twenty elements in two fractionated aerosols (PM2.0–1.0 and PM0.5– 0.25) were quantified using the IAEA X-ray spectrometer under ultrahigh vacuum environment. As a result from the SR-TXRF linear scans, it can be observed that the elements contained in fractionated PM2.0– 1.0 particulates are mainly concentrated in the centre of the Si wafer with the exception of Na and Sc. On the other side, the fractionated PM0.5–0.25 particulates showed a homogenous distribution for the most of the detected elements. Although, the Cr trivalent oxidation state is dominant, the hexavalent state provides also a minor contribution with an abundance ranging from 7% to 9% in both fractionated PM samples. The presence of trivalent Cr and divalent Mn forms indicates a non-mobility and non-toxicity for the respective chemical species. The roads and dry climate of Saudi Arabia do not encourage

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Fig. 3. Elemental distribution over vertical linear scans on PM0.5–0.25 and PM2.0–1.0 particulates. Data points represent the mean values. The concentration was normalized to the quantity of Ga.

many changes in the oxidation state of the existing forms in the soil. Therefore, the origin of Cr and Mn species in the samples studied seems to be local and has not been exposed to severe challenging processes that usually accompany the long transportation of particulates.

Nevertheless, very minor changes in the oxidation state of Cr and Mn were observed which can be interpreted as a starting point for the influence of weathering and surrounding conditions, such as pressure, wind, inter particulate collision, rain, etc. The present results provided

Fig. 4. Linear combination fitting of the experimental XANES data collected on the PM2.0–1.0 sample around Cr (left) and Mn (right) K-edges respectively. XANES spectra of Cr and Mn compounds, acquired at the XAFS beamline at Elettra were also plotted in the same figures.

A.A. Shaltout et al. / Microchemical Journal 137 (2018) 78–84 Table 2 The percentage abundances of Cr(III), Cr(VI), Mn(II) and Mn(III)species obtained from the linear combination fitting in PM2.0–1.0 and PM0.5–0.25 samples. Impactor stage size, μm

% Cr(III)

% Cr(VI)

% Mn(II)

% Mn(III)

PM2.0–1.0 PM0.5–0.25

91 ± 7 93 ± 5

9±5 7±3

90 ± 5 87 ± 6

10 ± 5 13 ± 6

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