Science of the Total Environment 493 (2014) 694–700

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Determination of water-soluble and insoluble elements in PM2.5

by ICP-MS

M. Manousakas a, H. Papaefthymiou a,⁎, K. Eleftheriadis b, K. Katsanou c

a Department of Chemistry, University of Patras, 265 00 Rio-Patras, Greeceb Institute of Nuclear and Radiological Sciences, Energy Technology and Safety, Environmental Radioactivity Laboratory, N.C.S.R. “Demokritos”, 15310 Ag. Paraskevi, Athens, Greecec Laboratory of Hydrogeology, Department of Geology, University of Patras, 26500 Rio-Patras, Greece

H I G H L I G H T S

• Elemental analysis of PM2.5 samples from two Greek cities was performed by ICP-MS.• The study has given information on the dissolution behavior of 14 elements.• Concentration of Cd in Patras exceeds European Commission’s assessment threshold.

⁎ Corresponding author. Tel: +30 2610 997132.E-mail address: epap@chemistry.upatras.gr (H. Papaef

http://dx.doi.org/10.1016/j.scitotenv.2014.06.0430048-9697/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 27 March 2014Received in revised form 20 May 2014Accepted 11 June 2014Available online 1 July 2014

Editor: Pavlos Kassomenos

Keywords:PM2.5

Water soluble fractionPM compositionElemental analysisICP-MS

The elemental composition ofwater-soluble and acid-soluble fractions of PM2.5 samples from twodifferent Greekcities (Patras and Megalopolis) was investigated. Patras and Megalopolis represent different environments.Specifically, Patras is an urban environment with proximity to a large port, while Megalopolis is a small citylocated close to lignite power plants. Both cities can serve as a representative example of European cities withsimilar characteristics. The concentration of 14 elements (As, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, Fe, Sr, Ti, V and Zn)was determined in each fraction by ICP-MS. Microwave assisted digestion was used to digest the samplesusing a mixture of HNO3 and HF. For the determination of the water soluble fraction, water was chosen as thesimplest and most universal extraction solvent. For the validation of the extraction procedure, the recoverieswere tested on two certified reference materials (NIST SRM 1648 Urban Particulate Matter and NIST 1649aUrban Dust). Results showed that Zn has the highest total concentration (273 and 186 ng/m3) and Co the lowest(0.48 and 0.23 ng/m3) for Patras and Megalopolis samples, respectively. Nickel with 65% for Patras and Aswith 49% for Megalopolis displayed the highest solubility, whereas Fe (10%) and Ti (2%) the lowest ones,respectively.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Ambient airborne particulate matter (PM), is considered as animportant environmental pollutant. Airborne particulates are very com-plexmulti-componentmixtures generated through a variety of process-es and mechanisms and emitted from numerous sources (both naturaland anthropogenic). Both the concentration and composition of PMdepend on a number of different factors. It is known from a number ofstudies that PM levels have an adverse effect on human health (Ladenet al., 2000; Baldauf et al., 2001). However, recent epidemiologicalstudies indicated that the relationship between inhalation of PM andadverse health effects cannot be solely explained by the PM10 andPM2.5 mass concentration levels. Other physical, chemical or biological

thymiou).

properties play also an important role on the effects of particulate mat-ter on human health (Analitis et al., 2006; Mitsakou et al., 2007;Valavanidis and Fiotakis, 2008; Kassomenos et al., 2013). Characteriza-tion of chemical composition and physical properties of atmosphericaerosol in South East Europe have been conducted at a Regional back-ground level (Lazaridis et al., 2005; Eleftheriadis et al., 2006) and overlarge ormediumurban areas (Diapouli et al., 2011; Pikridas et al., 2013).

Several chemical components, including specific elements found inparticulate matter, have been implicated with a variety of cardio-respiratory illnesses associated with exposure to urban air pollution(Saldiva et al., 2002; Wellenius et al., 2002; Ntziachristos et al., 2007;Hays et al., 2011). The biological mechanism triggered by the toxicityof certain PM elemental constituents is not yet well defined. For thispurpose the elemental characterization of PM particles is of great im-portance. Information about the mass and total content of trace ele-ments in PM is necessary, but insufficient in order to evaluate overall

Table 1Instrumental parameters for microwave digestion program.

Step Time (min) Power (W)

1 5 2502 1 03 5 4004 1 05 5 6506 1 07 5 2508 45 Ventilation

695M. Manousakas et al. / Science of the Total Environment 493 (2014) 694–700

pollution and hazard levels, because the effect of trace elements in theenvironment and humans depends on the association form in thesolid phase to which the elements are bound (Dos Santos et al., 2009).Toxicological studies have implicated the water-soluble fraction of themetal content as a possible harmful component of PM (Heal et al.,2005), and a number of epidemiological studies implicated it withacute respiratory illnesses and child asthma (Peel et al., 2005; Sinclairet al., 2010; Strickland et al., 2010). Potentially toxic elements of PMare As, Cd, Cr, Hg and Pb (1999/30/EC; 2004/107/EC). Redox activemetals play a crucial part in the generation of reactive oxygen species,and thus in PM adverse effects (Donaldson et al., 1997; Van Maanenet al., 1999). Costa and Dreher (1997) suggested that the dose ofwater-extractable, thus bioavailable transition metals and not the PMmass was the primary determinant of the acute inflammatory response.So, in order to perform a complete evaluation of the potential toxic ef-fects (thus the risk on human health) of the PM on an area we have todetermine both the elemental composition and water-soluble fractionof the elements present in PM.

One of the most effective methods for the determination of trace el-ements in PM samples is the microwave assisted digestion followed bychemical analysis by ICP-MS (Holmes et al., 1995; Yang et al., 2002). Anumber of different digestion parameters, such as the concentrationand volume of oxidizing agents and microwave oven settings can bevaried to achieve complete digestion. The most crucial step is to find asuitable mixture of acids for the digestion of the samples. Over theyears researchers have investigated different combinations of acids.Acids containing Cl and S are generally avoided because these elementsintroduce additional interference to the ICP-MS system (Yang et al.,2002). Mixture of HNO3–H2O2 is usually preferred, because both arestrong oxidizing agents and produceminimummatrix effects. However,HNO3 cannot fully digest silicon-containing compounds. For suchcomponents the use of HF is mandatory (Wang et al., 1996).

The use of HF is considered to be a necessary evil because if it is usedin high concentration the sample cannot then be used in the ICP-MS sys-tem, because HF solutions can etch the silica based sampler tube in theICP torch, causing signal drift and contamination (Karthikeyan et al.,2006b). To overcome this problem, H3BO3 is usually added to removethe excess HF. This may cause yet another problem to the analysis, asB species cause spectral interferences (Yang et al., 2002). Based on pub-lished reports it is possible to have satisfactory recoveries using a lowconcentration of HF, thereby avoiding the addition of H3BO3, but alonger digestion timemight be necessary (Pekney and Davidson, 2005).

As discussed above, in order to fully investigate the health risks de-rived from PM exposure, bioavailability of elements must be takeninto consideration. The most common extract used in the past years iswater, chosen as the simplest and most universal extraction solvent(Heal et al., 2005).

In this study, total and water-soluble elemental fractions of PM2.5

samples were determined in samples collected from two differentGreek cities: Patras, which is a typical urban city and Megalopolis,which is a small city with two lignite-fired power plants in operation lo-cated in its vicinity. Specifically, a one-step chemical leaching procedurewasused to determine thewater-soluble fraction of the 14measured el-ements (As, Cd, Co, Cr, Cu, FeMn,Ni, Pb, Rb, Sr, Ti, V, Zn), whereas for thetotal elemental concentration a digestion mixture of ΗΝΟ3-HF withoutH3BO3 addition was used (Kulkarni et al., 2007). In both fractions theelemental content was determined by ICP-MS.

2. Experimental

2.1. Study area and sampling procedure

The selected cities represent different urban/regional backgrounds,thus giving a clearer perspective for PMpollution inGreece.Megalopolisis a small city (~10,000 citizens) located in the vicinity of two lignite-fired power plants and opencast lignite mines. A major amount of fly

ash collected by the electrostatic filters of the power plants is stockpiledin open areas around the city until its deposition in exhausted lignitemines. In addition, fugitive dust from the mining processes, emissionsfrom the bucket-wheel excavators, vehicle traffic on unpaved roads, aswell as transportation and deposition of lignite and fly ash are sourcesof PM affecting the nearby area (Manousakas et al., 2013). Patras is amediumsized city (~150,000 citizens) located in northern Peloponnese.Patras is a coastal city, with a relatively large port, minimum industrialactivities and heavy traffic.

Several PM samples have been collected on a yearlongmeasurementcampaign, which took place in 2011, in order to get a statistically signif-icant dataset for each area. The sampling was performed every 3 daysfor Patras and with several 9 day long sampling campaigns throughthe year in Megalopolis. A total of 25 PM2.5 samples from each citywere selected to be analyzed for this study. Samples were selected tobetter represent the seasonal variation of PM2.5. Specifically, half of thesamples selected were collected during the cold season of the yearand the other half during the warm. The monitoring station for PM2.5

sampling in Megalopolis was within a self-contained isobox locatedabout 700 m from the city center. The sampling was performed usinga custom made air sampler, described elsewhere (Manousakas et al.,2013). For Patras, samplingwas performed in the city center. A low vol-ume sampler, PARTISOL-FRM model 2000 (Tanner and Parkhurst,2011), was installed in the roof of a public building and was operatedat 16.7 L/min. Sampling for both cities was on a 24 h basis. All PM2.5

samples were collected onto Teflon membrane filters (47 mm in diam-eter, 1 μm in pore size — Pall Corporation).

All filters used for sampling at the Megalopolis site were weighedbefore and after sampling to determine the collected PM2.5 mass usinga Sartorius PB211D microbalance (readability of 0.1 μg). Beforeweighing, thefilterswere equilibrated for 24 h inside a customdesignedchamber with automated controls designed to maintain environmentalconditions at a constant air temperature of 20 °C and constant RH of50%. To avoid static electricity interference the balance was equippedwith a 210Po static eliminator. Thefilterswere loaded into clean polysty-rene Petri dishes and transferred to the sampling site in MegalopolisCity. After sampling, the filters were kept in the same holders andreturned to the same place for post-weighing using the procedure de-scribed earlier. For Patras, a humidity desiccator was used to keep thehumidity of the samples and filters constant before weighing. Aftergravimetric analysis (microbalance readability of 0.1 μg) each samplewas transferred to an individual sample container (Petri dish) andstored at 4 °C until extraction and subsequent analysis.

2.2. Chemical analysis

The analyzed samples were divided into two sub-samples. One halfwas used for the determination of the total concentration of the ele-ments present and the other half for the determination of the water-soluble fraction. To determine the water-soluble fraction, sampleswere sonicated in 15 mL of ultra pure water at room temperature for30 min. After the extraction, the solution was filtered and acidified to1% HNO3 to prevent metal adsorption. To determine the total

Table 2Instrumental characteristics and settings for ICP-MS.

Nebulizer Meinhard

Spray chamber CyclonicNebulizer gas flow 0.82 L min−1

Lens voltage 6.25 VCones PtAuxiliary gas flow 1.2 L min−1

Plasma gas flow 15 L min−1

ICP RF power 1150 WAnalog stage voltage −1737Pulse stage voltage 1250Discriminator threshold 70AC rod offset −6Service DAC 1 60Quadrupole rod offset 0Scan mode Peak hopingDetector mode DualProcess spectral peak AverageProcess signal profile AverageAcq. dead time 52 nsDwell time 100 msSweeps/reading 20Replicates 4

Table 4Analytical recoveries of elements in the SRM 1649 (mix-ture 3.5 mL HNO3 + 0.5 mL HF + 2.5 mL H2O2)(n = 3).

Elements % Recovery

Ag 9 ± 11As 15 ± 8Ba 8 ± 7Cd 20 ± 9Co 10 ± 10Cr 1 ± 2Cu 1 ± 2Fe 4 ± 5Mn 555 ± 113Ni 11 ± 6Pb 13 ± 5Ga 3 ± 7Sr 10 ± 6V 93 ± 8Zn 1.0 ± 4

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concentration of the elements the second half of the filter was subjectedto digestion procedure in a Teflon vessel using amixture of HNO3 (3mL)and HF (0.3 mL). Concentrated HNO3 is usually the first choice becauseof its strong oxidizing potential. HF has the ability to digest silicon con-taining compounds. All digestionswere performed in amicrowave oven(Milestone MLS 1200). The steps of the digestion procedure are shownin Table 1. After the extraction procedure the samples were filtered anddiluted with ultrapure water to 50 mL. All reagents used for the diges-tion procedures were of analytical grade quality or better (HNO3-Suprapur 65%, Merck; HF-Suprapur 40%, Merck; H2O2-Pro-Analysis30% Merck) and all solutions were prepared using ultra pure water(18.2 MΩ) obtained from a MILLI-Q water purifier system.

All samples and standards were stored in falcon tubes (50 mL) andkept at 4 °C. All glassware were soaked in 10% HNO3 for at least 24 hand rinsed repeatedly with MILLI-Q water before use. Sample handlingand preparationwere carried out in a laminar flowhood to prevent con-tamination. The analyses were carried out using ICP-MS (Perkin Elmer,ELAN 6100). The sample introduction system consisted of a standardMeinhard nebulizer with a cyclonic spray chamber. All measurementswere performed using instrumental software. Instrumental parametersare listed in Table 2. Several sample and instrumental induced variationswere compensated using internal standardization, choosing 103Rh as in-ternal standard and found to be negligible. Reagent and filter blankswere also analyzed for background elemental content and appropriatecorrections were made.

Table 3Limits of detection for total and water-soluble fractions of the determined elements(n = 10).

Element Total fraction (ng/m3) Water-soluble fraction (ng/m3)

As 0.14 0.01Cd 0.25 0.10Co 0.06 0.03Cr 2.17 0.63Cu 1.21 0.36Fe 8.01 1.14Mn 2.13 0.34Ni 1.31 1.34Pb 0.58 0.17Rb 0.69 0.11Sr 1.65 0.52Ti 2.66 0.17V 0.77 0.04Zn 4.68 2.06

3. Results and discussion

3.1. Limits of detection and method validation

Limits of detection (LOD) for elements determined were calculatedbased on three times the standard deviation (3σ) of the blank values(n = 10), and are shown in Table 3.

To check the accuracy and precision of extraction protocol the SRMsNIST 1649a Urban Dust and NIST 1648 Urban Particulates were used. Alow sample mass (~10 mg) was accurately weighed in order to repro-duce as close as possible the real life conditions. Although the use of100mg of SRMs is recommended for the purpose of method validation,a lowermass has also been reported to have satisfactory reproducibilityof the analytical data (Karanasiou et al., 2005). Because the two stan-dards have different organic carbon fractions, different acid mixtureswere tested for their digestion. Two digestion mixtures were testedfor NIST 1649a, both containing H2O2. H2O2 is an oxidizing agent,which when added to HNO3 reduces the nitrous vapors and acceleratesthe digestion of the organic fraction by raising the temperature. A typi-cal mixture ratio is HNO3:H2O2= 4:1. Twomixtureswere tested for thedigestion of SRM 1649a: 4 mL HNO3 + 0.2 mL HF + 2 mL H2O2 and3.5 mL HNO3 + 0.5 mL HF+ 2.5 mL H2O2. The use of both mixtures re-sulted to very poor recoveries for every elementwith the exception of V.Results, shown in Tables 4 and 5, indicate that the usedmixtures of acidsare not suitable for the complete digestion of NIST SRM 1649a, probablydue to its high organic content. High organic content was not expectedto be present in the samples, because in urban sites of Greece water-soluble inorganic ions, mainly sulfate, dominate PM mass followed by

Table 5Analytical recoveries of elements in the SRM 1649 (mix-ture 4 mL HNO3 + 0.2 mL HF + 2 mL H2O2) (n = 3).

Elements % Recovery

Ag 4 ± 6As 1 ± 5Ba 8 ± 5Cd 354 ± 95Co 10 ± 8Cr 1 ± 8Cu 3 ± 6Fe 6 ± 8Mn 446 ± 101Ni 8 ± 9Pb 13 ± 6Ga 3 ± 7Sr 11 ± 15V 116 ± 10Zn 1 ± 5

Table 6Analytical recoveries of elements in the SRM 1648a (mixture: 3 mL HNO3 + 0.3 mL HF)(n = 10) and their water-soluble concentrations.

Elements % Recovery % W.S. fraction

As 109 ± 6 11Cd 87 ± 10 25Co 96 ± 9 8Cr 50 ± 10 1Cu 96 ± 4 8Fe 99 ± 8 1Mn 112 ± 5 12Ni 113 ± 11 18Pb 112 ± 5 1Rb 67 ± 17 7Sr 113 ± 9 12Ti 71 ± 15 1V 111 ± 7 4Zn 95 ± 12 30

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mineral dust components and carbonaceousmatter (Siskos et al., 2001;Gerasopoulos et al., 2006; Karanasiou et al., 2009; Theodosi et al., 2011).

Thus, due to the fact that the examined samples were not expectedto have high organic content, theirmatrixwas expected to bemore sim-ilar to the SRM1648. In addition, SRM1648 is more suitable for methodvalidation, when performing elemental analysis, because it containscertified values for a large number of trace elements. SRM 1649a ismainly intended for use in evaluating analytical methods for the deter-mination of organic compounds such as PAHs, nitro-PAHs and PCB.

The samemixture used for the digestion of the samples (3mL HNO3

and 0.3 mL HF) was also used for the digestion of NIST SRM 1648. Yanget al. (2002) reported that 0.5mL of HF should be sufficient for the com-plete recovery from silicate matrix samples. Karthikeyan et al. (2006b),who have tested different concentrations of HF in the range of 0.1–0.5,suggested that there is no significant improvement in recovery beyondthe addition of 0.2 mL HF. As shown in Table 6, the recoveries of 11 ele-ments (As, Cd, Co, Cu, Fe, Mn, Ni, Pb, Sr, V, Zn) ranged from 87 to 113%and of 3 elements (Cr, Rb, Ti) from 50 to 71%. Problems regarding Crcharacterization in NIST 1648 have been reported in literature and ithas been hypothesized that the presence of high soot content inhibitsdissolution of Cr (Pekney and Davidson, 2005). The poor recoveries forTi and Rb may be explained by their high concentration in blanks,which created a high level of noise in their analysis.

The filters used to collect the samples are not free of metals. In orderto take this contribution into account, blank filters were treated likesamples and the concentrationsmeasuredwere subtracted from samplemeasurements.

SRM 1648 was also tested for water-soluble trace element content,in spite of the absence of data regarding its water-soluble elemental

Table 7Total mass concentration and concentration range of elements for the cities of Patras and Meg

Elements Average concentration (ng/m3)

Patras M

As 1.21Cd 12.2Co 0.48Cr 6.19Cu 7.28Fe 124 8Mn 4.73Ni 7.60Pb 7.13Rb 1.73Sr 6.26Ti 87.8 2V 3.86Zn 273 18PM2.5 concentration (μg/m3) 17.4 2

fraction. Results are also shown in Table 6. In SRM 1648 the water-solu-ble fractionwas found to be lower than 5% for Al, Cr, Fe and Pb, from 5 to20% for As, Co, Cu, Mn, Ni, Rb, Sr and V and from 20 to 30% for Cd and Zn.Karthikeyan et al. (2006b) have reported higher water-soluble fractionvalues for this SRM, but they have used amicrowave assisted extractionprocedure.

3.2. Total metal concentrations

A summary of the total PM2.5 mass and their elemental concentra-tions for the cities under study is given in Table 7.

The one-way ANOVA test was applied to the data set to search forsignificant differences in the examined parameters (mass and elementalconcentrations) between the two cities. Results showed that the differ-ences in mean PMmass and concentrations of Cd, Cr, Cu, Fe, Mn, Ni andPb were not significant (P N 0.05), while those of As, Co, Rb, Sr, Ti, V andZnwere significantly higher (P b 0.05 at the 0.05 level) in Patras than inMegalopolis samples. Titanium, Co, Rb, Sr andZn are related to vehiculartraffic (Taylor et al., 2007), V to oil burning and As to fuel combustion(Sánchez-Rodas et al., 2007). Titanium and Zn originate from tire andbreak ware (Lighty et al., 2013). Traffic is much heavier in Patras, so el-ements which are related to traffic are expected to have higher concen-trations in this case. Vanadium is a tracer of oil combustion (Li et al.,2004), and is mainly associated with diesel vehicle emissions and do-mestic heating. Vanadium and Ni also originate from ship emissions(Viana et al., 2009). High concentrations of V and Ni are an indicationof the effect of the port in the atmosphere of the area of Patras.

Similar studies conducted in Athens for PM2 fraction (Karanasiouet al., 2009) reported higher metal concentration for Mn, Cr, Cu, Feand Al and lower for Cd. The European Commission in a directive relat-ing to toxic metals has set assessment thresholds for Pb (1999/30/EC,1999), As, Cd and Ni (2004/107/EC, 2004). Themean annual concentra-tion proposed for As is 6 ng/m3, for Ni is 20 ng/m3 and for Cd is 5 ng/m3,all measured in PM10 fraction (2004/107/EC, 2004). Taking into accountthat these elements aremainly found in fine particles, wemay concludethat As and Ni concentrations are lower than the threshold, but Cd con-centration exceeds it for Patras. Results for PM2.5 metal concentrationsfor both cities under studywere quite comparable with results from an-other Greek city Thessaloniki (Saffari et al., 2013). This fact is an indica-tion that despite the unique sources that may exist in each area, mainPM2.5 sources are common throughout Greece (traffic, domesticheating, secondary formation, etc.). Patras results are also consistentwith the results of a study conducted in Birmingham (Harrison andYin, 2000), with the exception of Ti which has a higher concentrationand Cu and Pd which have lower concentrations for Patras, althoughsamples in Birmingham were roadside rather than urban background.

alopolis (n = 25).

Concentration range (ng/m3)

egalopolis Patras Megalopolis

0.60 0.24–3.08 0.31–1.013.08 1.15–136 0.25–12.90.23 0.20–1.15 0.10–0.625.64 2.55–18.3 2.64–14.54.02 1.37–84.0 1.36–12.67.0 25.1–673 30.3–2173.30 2.21–21.8 2.20–7.805.03 1.56–8.44 1.40–18.98.06 2.19–23.2 1.82–29.31.02 0.80–3.95 0.90–1.792.58 3.18–16.8 1.17–5.158.8 5.98–220 3.52–5.00.88 0.92–11.1 0.74–1.526 41.7–517 33.20–7073.0 8.12–34.6 6.44–45.5

Table 8Average concentration and concentration range of water-soluble fraction of themeasuredelements for the cities of Patras and Megalopolis.

Elements Average concentration(ng/m3)

Concentration range(ng/m3)

Patras Megalopolis Patras Megalopolis

As 0.24 0.25 0.01–1.26 0.06–1.26Cd 4.98 1.50 0.13–18.4 0.10–8.21Co 0.11 0.04 0.03–0.61 0.03–0.17Cr 1.25 0.79 0.63–8.94 0.63–5.66Cu 2.67 0.94 0.36–9.49 0.36–1.83Fe 11.9 8.51 1.43–94.0 1.14–32.7Mn 1.25 1.28 0.36–6.79 0.34–3.23Ni 4.92 2.06 1.57–24.1 1.34–4.32Pb 3.16 2.90 0.27–18.1 0.19–9.17Rb 0.33 0.23 0.12–1.81 0.11–0.48Sr 1.82 0.73 0.56–8.80 0.60–1.53Ti 14.7 0.54 0.19–95.2 0.17–3.74V 1.33 0.25 0.12–3.08 0.04–0.61Zn 163 51.7 21.6–403 14.9–128

698 M. Manousakas et al. / Science of the Total Environment 493 (2014) 694–700

This might be considered as an indication that Patras' atmosphere ishighly affected by road traffic.

Pearson correlation coefficients were examined to investigate anystrong correlation of metal concentrationswith PMmass. No significantcorrelationwas found between the PM2.5mass and the element concen-trations except for As and Rb in the case of Megalopolis and Pb and V forPatras, which presented moderate, but significant correlations withmass at the 0.05 level. Arsenic is known to be amarker element for fossilfuel burning (Morawska and Zhang, 2002), thus giving an indicationthat the lignite-fired power plant located in the area of Megalopolis af-fects PMconcentration levels. Vanadiumand Pb as tracers of oil burning,as discussed before, indicate that diesel vehicular emission, domesticheating and possibly ship emissions are major sources of PM in Patras.

3.3. Water-soluble fraction

Table 8 shows the concentrations of thewater-soluble fraction of thedetermined elements in Patras and Megalopolis samples, respectively.

Fig. 1. Element partitioning between water-soluble an

To investigate if the differences inmean concentrations of thewater-soluble fraction of elements between the two cities were significant, theone-way ANOVA test was performed. Results revealed that the water-soluble fractions of Cd, Co, Cu, Ni, Sr, V and Zn were significantly higher(P b 0.05) for Patras' PM samples compared to those fromMegalopolis.Figs. 1 and 2 show the proportion of water-soluble elements to totalfraction for each element for the studied cities.

Water-soluble proportions varied considerably with element and itcan be used as an indication of the chemical form of the analyzed ele-ments. In general, low solubility suggests that only a minor part ofthese elements is forming soluble salts. The water-soluble proportionof Fe was ≤10% for Patras samples, whereas that of Megalopolis wasalso ≤10% for both Fe and Ti. These elements are crustal elements,thus they are retained in the crystalline network strongly makingthem less soluble and others weakly bound. Since the percentage of Fein the water-soluble fraction is very low, it could be suggested that Feis not bound to sulfate, nitrate, or oxalate and is probably bound toalumino-silicate matrix substituting Al and Si. Determination of water-soluble Fe levels is important because it generates OH• radicals fromH2O2 in which all human cells are exposed to some levels(Karthikeyan et al., 2006a). A substantial percentage (≥30%) of Cd, V,Zn, Cu, Pb and Ni was water-soluble for Patras and of As, Cd, Mn, Niand Pd for Megalopolis samples. Copper and Zn are generally reportedas metals with low environmental mobility (Dos Santos et al., 2009).The fact that Cu has 37% solubility and Zn has 60% solubility in Patrassamples leads to the conclusion that they are present in the form of sol-uble salts, such as CuSO4 and ZnSO4. Results are consistent with a studyconducted at the greater Thessaloniki area (Greece) (Voutsa andSamara, 2002), in industrial and urban sites. Data from this studyshow that urban atmospheric particles appear to contain higher per-centages of labile chemical forms of metals (particularly of Cd, Cu, Niand Zn) than particles in industrial areas. The elements with the highestwater-soluble fraction were Ni (65%) for Patras and As (49%) for Mega-lopolis samples, both representing the major sources of air pollution inthese areas (oil burning/ship emissions and lignite burning). Fig. 3 pre-sents the comparison of the water soluble fraction of the measured ele-ments for the two cities.

In a similar study conducted in Edinburgh (Heal et al., 2005), themost water-soluble elements were V, Zn, As and Cd with Mn, Cu and

d residual fraction in PM2.5 collected from Patras.

Fig. 2. Element partitioning between water-soluble and residual fraction in PM2.5 collected from Megalopolis.

699M. Manousakas et al. / Science of the Total Environment 493 (2014) 694–700

Pb beingmoderately soluble and Cr andNi almost insoluble.When com-paring our resultswith a similar study for 14 Chinese cities (Cheng et al.,2012),we can see that the concentration of thewater-soluble fraction ofevery element is much lower in our case for both cities with the excep-tion of Zn, which has a higher concentration.

In the study of Strickland et al. the concentration of water-solublemetals and in particular of Cr, Cu, Fe, Mn, Ni and V was examined in re-lation to emergency department visits for pediatric asthma (Stricklandet al., 2010). The authors suggested that if the concentration of theaforementioned water-soluble metals exceeded 12 ng/m3, emergencyvisits due to child asthma showed a statistically significant increase.This limit was exceeded for both Megalopolis (13.83 ng/m3) and espe-cially Patras (23.32 ng/m3) samples. This fact suggests that there is apossible health risk due to thewater-soluble metal concentration levelsin the studied areas.

3.4. Contribution of trace metals to particle mass

Table 9 summarizes the proportion of PM2.5 constituted by the 14measured elements. The mean proportion for Patras was found to be3.2% and forMegalopolis 2.1%. The difference between the two cities re-garding the proportion of the elementsmeasured for the total fraction isconsidered significant (P= 0.010) and shows that Patras samples havea higher proportion of metals.

The mean proportion for the water-soluble fraction of the metals is0.2% and 0.9% for Megalopolis and Patras, accordingly. The differencebetween the two cities is considered significant (P = 0.007) which isan indication that human activities affect more PM levels in Patras,

0%

10%

20%

30%

40%

50%

60%

70%

As Cd Co Cr Cu Fe Mn Ni

Fig. 3. Comparison between water-soluble percentages in

since metals in anthropogenic particles tend to be more labile thanmetal bound within crustal material. In general, as mentioned before,a higher proportion ofwater-solublemetal is indicative of anthropogen-ic rather than crustal sources. Metal in anthropogenic particles consistsof metal-dominated or hot-vapor condensation particles or metals thathave condensed onto the surface of other particles, and thus tend to bemore labile thanmetal boundwithin crustalmaterial (Heal et al., 2005).

4. Conclusions

A microwave assisted digestion procedure and a single step extrac-tion procedure coupled with ICP-MSwere used for the characterizationof water soluble and residual components of PM2.5 samples collected intwo Greek cities, Patras and Megalopolis.

Two standards NIST 1649a and NIST 1648were tested for procedurevalidation, but satisfactory recoveries were received only for NIST 1648.The study has given information on the dissolution behavior, thus thepotential bioavailability, of 14 elements (As, Cd, Co, Cr, Cu, Fe, Mn, Ni,Pb, Rb, Sr, Ti, V and Zn). Water-soluble fraction for Cd, Co, Cu, Ni, Sr, Vand Zn was higher for Patras leading us to the conclusion that Patras,as an urban environment, is subjected to higher pollution. The elementswith the highest water-soluble fraction were Ni (65%) for Patras and As(49%) for Megalopolis samples, both representing the major sources ofair pollution in these areas (oil burning and lignite burning).

Concentration of Cd in Patras exceeds the European Commission'sassessment threshold. Pearson correlation coefficients showed that Asand Rb in the case of Megalopolis and Pb and V in the case of Patraspresent a moderate but significant correlation with PM2.5 mass. This

Pb Rb Sr Ti V Zn

P

M

PM2.5 samples from Patras (P) and Megalopolis (M).

Table 9Total and water-soluble elemental percentages of the total PM2.5 mass.

Megalopolis Patras

% Total % Water-soluble % Total % Water-soluble

Mean 2.1 0.2 3.2 0.9Min 0.6 0.1 0.9 0.1Max 7.5 1.0 9.5 3.4

700 M. Manousakas et al. / Science of the Total Environment 493 (2014) 694–700

fact is an indication that lignite burning related to the power plant in thecase of Megalopolis and oil burning related to domestic heating, shipemissions and traffic in the case of Patras are the major PM2.5 sourcesin the area.

Source apportionment techniques should be used as a next step, toidentify and quantify PM source contribution in the aforementionedareas.

Acknowledgments

This work was supported by the K. Karatheodoris Program GrantD.165 from the Research Committee of the University of Patras andthe Municipality of Megalopolis.

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