Characterization of individual particles in the antwerp aerosol

0@-%6981/89S3.00+0.00 Pergamon Fves plc

AtmospherfcEndronnvnt Vol.23, No. 5, pp. 1139~1151. 1989. Printed in Great Britain.

CHARACTERIZATION OF INDIVIDUAL PARTICLES ANTWERP AEROSOL

IN THE

W. A. VAN BORM and F. C. ADAMS* University of Antwerp (UIA), Universiteitsplein 1, B-2610 Wilrijk, Belgium

and W. MAENHAUT Institute for Nuclear Sciences, Rijksuniversiteit Gent (RUG), Proeftuinstraat 86, B-9000 Gent, Belgium (First received 25 June 1988 and received for publication 21 October 1988)

Abstract-About 8ooOindividual particles (0.2-H pm) in 16 12-h air particulate matter samples, taken at an urban site near the city of Antwerp, Belgium, were analyzed by automated electron probe micro analysis (EPMA) for 26 elements and for morphology, including the projected particle diameter. Also, the samples were analyzed by particle induced X-ray analysis (PIXE) for total element analysis. The particles could be divided into six main classes: soil dust, auto exhausts, sulfates, sea salt particles, biological particles and miscellaneous anthropogenic emissions. Each class was split up into several particle types, of which the abundance (number %), the temporal variation, size distribution and chemical composition were determined. Evidence was found of several atmospheric reactions, e.g. secondary SO:formation by heterogeneous oxidation of SO*, SO:- condensation on existing fine and coarse particles and conversion of compounds as NaCl and PbBrCl into SO:-. Key word index: Electron probe micro analysis, EPMA, PIXE, air particulate matter, size distributions.

INTRODUCTION

Methods for the chemical and morphological analysis of individual particles have’ been shown to be important for the characterization and identification of individual atmospheric particles, originating from several natural and anthropogenic sources such as urban street dust (Linton et al., 1980), fly ash (Fisher and Natusch, 1979), soil dust (Gillette and Walker, 1975), automobile exhausts (Post and Buseck, 1985a), nonferrous industrial emissions (Van Borm et al., 1987a) and SO:- generation processes (Webber, 1986). These methods provide in addition to the chemical composition of the particulate matter valuable information concerning the particle size distribution, particle type, morphology and chemical composition on the individual particle level. Many studies of separate source materials have been reported but few comprehensive studies have been performed up to now for determining the individual particles arising from emission sources mixed within a complex urban airshed. Yet, valuable information can be obtained from the distribution of elements over distinct types of particles or from the observation of surface coatings by e.g. atmospheric SO:-. Also, gas to particle conversion of heavy metal fumes and SO, from anthropogenic emissions can be directly put to evidence. *Author to whom correspondence should be sent.

Using a combination of automated electron probe micro analysis (EPMA) of individual particles and particle induced X-ray emissions (PIXE) for bulk analysis, we investigatd the composition of particles, most of them in the PM 10 range ( < 10 pm), sampled in an urban area within metropolitan Antwerp, Belgium. Thus, the low detection limits of a PIXE analysis and the individual particle features of an EPMA analysis are combined. Results are reported of an intensive characterization of the major particle types within the urban aerosol by determining their composition, morphology, size distribution and abundance within 1 week of continuous sampling of air particulate matter on Nuclepore polycarbonate filters. Prior to this study, quantitative energy-dispersive X-ray analysis of individual aerosol particles was used by Post and Buseck (1985b), who conducted a comprehensive study of particulate material in the 0.1-30 pm size range in the aerosol of Phoenix, Arizona, by Van Borm and Adams (1987b) in a preliminary study of the Antwerp aerosol for particles in the size range 0.2-l 5 pm and by Mamane and No11(1985) and No11 et al. (1987) who analyzed large and giant particles (> 5 pm) at several rural and urban sites in the Eastern U.S. Previous analyses of the Antwerp aerosol, using total filter samples and neutron activation analysis (NAA) or emission spectrometry (ES) were reported by Demuynck and Dams (1975, 1981), Demuynck (1980), Kretzschmar et al. (1977, 1980), Navarre et al. (1981) and the Center against Air and

11139

1140

W. A.

VAN BORM et

Water Pollution (CLW) of the City of Antwerp (CLW, 1985). A complex mixture of several sources was qualitatively characterized on the basis of the trace element distribution using specific elements as tracers, including sea salt particles (with Na, Cl, I), soil dust resuspension (relying on Al, Si), liquid fuel combustion (K, V, I, also SO:-), coal combustion (Al, Cs, La, Ce, Th, SO:-), non-ferrous industries (Cu, Zn, As, Se, Cd, In, Sb, Pb, SO:-), automobile exhaust emissions (Br, Pb, NO,) and several thus far unspecified anthropogenie emissions (Cl, Ag, Hg). As will be shown in this paper, most of these sources can be recognized by analyzing their immissions, e.g. particles and condensates that make up the mass of the ambient air particulate matter.

SITE DESCRIPTION

The city of Antwerp is the second largest city in Belgium with a population of ca. 600,OfXlinhabitants (1986) within a metropolitan area of 210 km2. Because of its location along the river Scheldt, at the entrance of the estuary and 100 km away from the sea, it is a major seaport for the Western European hinterland. The city is located within a very dense traffic system; large, non-ferrous industries and chemical plants are located along the banks of the river Scheldt, mostly in the northwest and southwest of the downtown area. Antwerp is located within a few hundred kilometers of several other large industrial concentrations such as the German Ruhrgebiet, the Rotterdam industrial area and densely populated and heavily industrialised Belgian cities. All this gives rise to a complex and wind direction-dependent mixture of transported anthropogenic aerosols. Annual mean air pollutant concentrations recorded in the period April 1984March 1985 show an average concentration of ca. 67 pgrnM3 for SO1 and 18 pgrnw3 for TSP, while in summertime meanlevelsofca.45 pgrnm3 NO,,24 pgrn-‘NO and 2.5 pgrnm3 H,SO, were measured (CLW, 1985). All

al.

these concentration levels have decreased over the last 5 years. In view of all this, it was anticipated that SO:- of anthropogenic origin would make up a large fraction of the particles, and that SOi- coatings would be present on a variety of particle types.

EXPERIMENTAL

SECTION

Sampling

From 9 to 16 July 1986, 16 12-h air particulate samples were taken on top of a 15 m high building, located 5 km south of the city center on the edge of the suburban area (Fig. 1). Filters were changed at 8.00 a.m. and 8.00 p.m. The site is dominated by grassland and major highways are within 1 km distance. A large non-ferrous plant and a waste incinerator are located at 5 and 2 km, respectively. Particulate matter was collected on 47 mm diameter Nuclepore polycarbonate filters with a pore sire of 0.4 nm. The filters were mounted in a filter holder (Schleicher and Schuell GmbH. D-3354 Dame. F.R.G.). modified with a cutoff tube to prevent wind turbulence from reaching the filter surface, thus eliminating particle transport other than air entrainment. The set-up provides a theoretical particle cutoff radius of 60 pm. However, as the sampling procedure was anisokinetic, this cut-off diameter is only significant when sampling from still air, while in our study sampling was performed under conditions of varying wind speed. It was estimated from the theory of Durham and Lundgren (1980) that a significant loss of particles occurs at a particle diameter above 35 pm. This effect could be experimentally verified in the size distribution of all particles. We therefore only took into account particles < 15 pm projected diameter. The filter holder was operated with a Becker DT/VT 1.5 carbon vane rotary pump at a flow rate of 20 Cmin - ’ and a face velocity of 25.4 cm s- ‘. The flow rate was checked every 12 h with a rotameter; the total sampled volume was measured with a dry gas meter (Contigea SG6, Brussels, Belgium). To obtain a representative and reliable air particulate matter sample while preventing the filter from being overloaded for electron microscopical investigation the pump was turned on by an automated timer for only 15 min every hour. This procedure provided a sufllcient loading for &Kcient analysis with an average interparticle distance more than 4 times the mean particle diameter, thus reducing absorption and secondary fluorescence by adjacent particles to a minimum. Sampling errors in a similar experimental set-up were estimated by Geladi and Adams (1982): precision and accu-

Legend m

urban areas

w

industrial

m

woods

= -

highways roads sampling site

8

Fig. 1. Map of survey area.

areas

Characterization of particles in Antwerp aerosol racy ofthe gas meter was estimated as 0.3-1.4% and O&5%, respectively; loss of particulate matter in the filter handling process was estimated as l%, consistent with results obtained by Highsmith and Bond (1986). To prevent filter contamination from the pump, the latter was mounted in a closed cabinet, while the air exhaust itself was placed about 20m downwind of the filter holder. Samples were stored in PVC Petri dishes and transported to the lab where a circular section was cut from the center of the filter, glued on a plexiglass stub and vacuum-coated with a carbon layer of ca. 40 nm. All samples were analyzed within a few days after they were prepared for EPMA. Parallel with the set-up for collecting samples for EPMA, a similar sampling line was used to sample air particulate matter for gravimetric determination of total suspended particulate matter (TSP) and for the determination of the bulk aerosol composition by particle induced X-ray emission analysis (PIXE). In this set-up, each sample was taken during the whole 12 h period to ensure maximum mass loadings. Meteorological data were obtained from the Royal Meteorological Institute of Belgium (K.M.I., Tervuren). They were recorded at the city airport, 5 km northeast of the sampling site. The weather during the sampling period was usually dry and moderately warm, with occasional rain showers. Mean temperatures for the sampling period varied between 12.8”C at night and 21.4”C by day. The relative humidity ranged from 86.5% at night to 65.0% by day. The wind direction varied between west over north to eastsoutheast (Fig. 2) with a periodic mean wind speed of 3 m s- ‘. As the air masses came mostly from over the city, we have assumed that the samples are representative for the average Antwerp aerosol, which is a mixture of locally generated and transported aerosols. The authors are aware of the problems associated with sampling artefacts due to the reactions of particles on the filter with gaseous SO, and H2S04. A solution to circumvent these problems could have been the use of a denuder system or filter packs. However, only very recently it has been shown that both alternatives are also prone to a number of interferences (Sturges and Harrison, 1989)! It was therefore decided to use common sampling equipment, high flow rates and short sampling times to minimize the change on in situ reactions. Instrumentation

and analysis

The single particle analysis was done with a JeoiJXCA 733 Superprobe electron probe micro analyzer (Jeol Ltd, Tokyo, Japan) equipped with an energy-dispersive Xray detector, an annular backscattered electron detector and

Fig. 2. Frequency diagram of the 6-h mean wind directions at the sampling site at the University of Antwerp, for the vriod 9-16 July 1986. The sectors are a measure for the number of times the wind came from a particular direction.

1141

a Tracer Northern TN 2000 automatic system (Tracer Northern, Middleton, Wisconsin) controlled by an LSI 1l/23 minicomputer. Working conditions were set at an electron energy of 25 keV, a beam current of InA and an X-ray spectrum acquisition time of 40 s per particle. A magnification of 2000 allowed the analysis of particles with projected diameters down to 0.2pm. In order to analyze as many particles as possible within a reasonable time span and to diminish any operator bias, the analysis was performed in the automated mode, using a software program, adapted in the laboratory from the commercially available Particle Recognition and Characterization (PRC) program (Tracer Northern). A full description is given by Van Borm and Adams (1989). Detection of the particles was carried out with an on-line image analysis program using the backscattered image because of the superior particle to background intensity contrast ratio thus achieved (Crewe and Lin, 1976).In this mode, the intensity is a function of the mean atomic mass (MAM) of both particle and substrate. For a fixed image intensity threshold setting, above which the image signal ofa particle is distinguished from the substrate signal, the minimum detectable particle diameter is a function of the particle MAM. Other errors in the particle diameter measurement were introduced because of inaccurate or incomplete localization of the particle. The latter situation is especially cumbersome for fly ash particles from an oil-fired power plant which consists of a C/SO:- matrix containing localized spots of heavy elements such as V and Ni. Such particles were often recognized by the image analysis system as a multiple set of smaller ‘particles’, containing only V, Ni or both. lndependent analysis of several particle size standards with different MAMs, indicated a positive shift of the minimum diameter by a factor of 3 between particles with a MAM of 207 (e.g. pure Pb-particles) and those with a MAM of 14 (e.g. ammonium sulfates). This problem was minimized by analyzing each field and sample at the same mean image intensity, using an image intensity threshold set as closely as possible to the average intensity of the substrate. Then only the fraction with the smallest particles suffers from the et&t described. In the size distributions of the different particle types shown further in this text, this fraction is depicted by a dashed line. Also, this type of size distribution was constructed from the combined number of particles in the 16 samples, belonging to a specific particle type. Total particle size distributions were recorded by analyzing particles in a sample only for their projected diameter, a procedure which is extremely fast (1 particle s-l), without discriminating against their composition. Semi-quantitative energy-dispersive analysis of individual particles was used to measure the various elements in the particles. Quantifying the information, contained in the Xray spectra includes various steps: spectrum deconvolution, peak identification and conversion of intensities into concentrations according to a rather elaborate calculation scheme. The spectrum deconvolution procedure used in this work was integrated in the automated analysis software and was done on-line using a first derivative procedure. Identification of the X-ray peaks was performed for each individual spectrum using an expert system which decides on the presence of an element in a particle taking into account information on energy and intensity of the total set of peaks obtained from the spectrum deconvolution procedure. Intensity corrections were made for peak overlap between peaks of different line families. In practice, this is especially useful for the Pb ,U. and S K, peak overlap (Janssens et al., 1987). Conversion of intensities into concentrations was accomplished using a standardless ZAF correction procedure for individual particles (Raeymaekers, 1986). This method is based on the method of Wernisch for bulk specimens (Wemisch, 1985). It was adapted for individual particle analysis by correcting the concentrations for particle size effects, on the basis of the measured projected diameter of a particle (the equivalent diameter) and assuming a spherical shape.

W. A. VAN BORMet al.

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The analysis can only be performed for elements with Z > 11 and provides no information on the lighter elements.. Unless otherwise stated, concentrations were calculated assuming the elements are present in their most common oxidation state. Also, for each particle the density was calculated as the sum of the densities of the most probable oxides of the elements, weighed by their concentration in the particle. For each sample, about %O particles were analyzed for 26 elements: Na. Me Al. Si. P. S. Cl. K. Ca. Ti. V. Cr. Mn. Fe, Ni, Cu, Zn, As; Se:Br,‘A& dd; S.& Sb, Bi a;d’Pb: The diameter of each particle was measured and its oxygen content and density were estimated as described above. A full description of the procedure is given by Van Borm and Adams (1989). The large amount of data resulting from this analysis was interpreted by multivariate cluster analysis, using the elemental concentrations of the individual particles to group these into clusters of particles with similar composition. Detailed descriptions of this method, together with some general information can be found in Van Borm and Adams (1989), Massart and Kaufman (1983) and Bernard et al. (1986). The particle-induced X-ray emission analysis was done at the University of Ghent, Belgium. All filter samples and 10 blank filters were analyzed for up to 22 elements (i.e. Mg, Si, P, S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, As, Se, Rb, Sr, MO, Ba and Pb) using a 2.4 MeV proton beam from a

cyclotron. Details of the experimental PIXE set-up and the analytical procedures have been given elsewhere(Maenhaut et al., 1981; Maenhaut and Vandenhaute, 1986; Vandecasteele et al., 1986).

RESULTS AND DISCUSSION

_m-

d Log0,

01

10

1

100 O,(pml

Fig. 3. Comparison of different number size distributions of particles sampled at an urban site during summer. dN is the number concentration of particles with diameters between D, and D,+dD. (d log D, = 0. l), (1) this work (samble n& 161.(21MCszaros (1981). (31Willeke et al. (1974), (4) sulfate size distribution, MCszaros (1981). N denotes the number concentration obtained in this work for particles in the size range 0.4-15 pm. ,I\,

,

,..,

Size distribution Figure 3 shows a typical size distribution of 10,000 particles, recorded on 17 July (sample no. 16), in comparison with several other size distributions cited in the literature and obtained at urban sites during summer (MCszaros, 1981; Willeke, 1974). Differences between our results and the others are especially striking for the particle diameters ~0.4 pm. This can be attributed to two general effects. First, and most importantly, there is an increasing deterioration of the detection ability of small particles below a few tenths of a pm in our detection system. Secondly, there is a decreasing collection efficiency of the filter as already reported in the literature (Heidam, 1981). It also appeared in our measurements that the number of SOi- particles ~0.4 pm is quite low compared to what could be expected in urban pollution aerosols (Miszaros, 1981). Considering that these particles constitute the most abundant category of the accumulation mode particles and that, on the other hand the total S concentration measured was not abnormally low, we concluded that the size distributions of our EPMA analysis are highly underestimated for SO:particles with diameters ~0.4 pm. However, our size distributions are considered to be fairly accurate for diameters between 0.5 and 15 pm as they are in quite good agreement with those depicted in Fig. 3. For the coarse particles our measurements showed a local maximum around 5 pm, as was also observed by other authors (Mbzaros, 1981; Willeke, 1974). The individ-

.uai particle analysis indicated that this maximum can be attributed to sources of coarse and giant particles, such as resuspended soil dust and fly ash particles. Total jilter analysis

Table 1 lists the mean atmospheric concentrations of 16 samples and the associated standard deviations for the 22 elements analyzed. A comparison is made with data obtained by Maenhaut and Cafmeyer (1987) for the city of Ghent, which is located about 50 km to the west of Antwerp. The mean concentrations at Antwerp ranged from 2 ngme3 for Cr to 2.6 pugme3 for S, while the mean TSP-level corresponded to 18.1 pg m-3. Approximately 24% of the aerosol mass was covered by the elements determined, the balance presumably being water, oxygen, carbon (soot) organic material. On the average and excluding Se, the concentrations obtained for the Antwerp aerosol are about 10% lower than for Ghent, Demuynck and Dams (1981), on the other hand, found that the concentrations for Antwerp were between 15 and 780% higher than for Ghent, except for Mn (Antwerp/Ghent ratio = 0.8 l), which they attributed to the presence of iron and steel industry in the latter city. The same authors also noted that the largest differences were obtained for the elements As and Se, observations that are in agreement with our results. Discontinuous emissions from the non-ferrous industry are believed to be responsible for the higher

1143

Characterization of particles in Antwerp aerosol Table 1. Comparison of average atmospheric concentrations and associated standard deviations at urban sites near Antwerp and Ghent, Belgium

Element

Mg

Si

P S Cl K Ca Ti V Cr Mn Fe Ni

cu

Zn AS Se Rb Sr MO Ba Pb TSP

Antwerp* xfs

Concentration (ngmm3) crustal enrichment$ factor

440:270

38+32 [lS] 2600*1400 190*200 170&-92 300+ 190 21&15 13*6 2.1 f 1.6 9.6 f 7.0 3305 190 6.1 k2.1 7.5k4.4 41+20 8.9 k 6.0 [4] 5.7k5.1 [ll] 3.810.7 [6] 1116 [13] 110+50

0.26 5.4 1800 350 1.00 1.3 0.76 20 3.3 1.4 1.0 16 25 100

430 13,600 1.4 _ 4.0 1500

Ghentt xfs 130*60 [35] 48Ok520 46 f 62 [39] 2OOOk1500 630+900 27Ok 190 250 & 230 20+19 17* 14 3.2+ 1.1 [9] 15&14 2OOk 130 6.2 k 4.4 7.4k4.8 90+70 7.Ok5.8 [8] 1.4kO.6 [31] 3.4& 1.3 [14] 4.0 f 4.8 1481 3.4+ 1.0 [8] n.d. 170* 100

Antwerp/Ghent concentration ratio 0.92 0.83 1.3 0.30 0.63 1.2 1.05 0.76 0.66 0.64

1.6 1.0 1.0 0.46 1.3 4.1 0.95 _ 0.65

18100~5500

*Average based on 16 samples, except when number of samples used is indicated between square brackets. TMaenhaut and Cafmeyer (1987); sampling period 27 October-16 December 1986; sixe fraction <5 m diameter. Values based on 68 samples, unless exact number used is indicated between square brackets. $ Reference element: Fe (Mason, 1966). < : detection limit.

n.d.: not determined. concentrations at Antwerp. Se, which is believed to originate from the combustion of coal, gives rise to a background of 1 to 2 ngm-’ only. Therefore, it was suspected that Se has some specific sources in the Antwerp area. The crustal enrichment factors (E.F.s) for the elements in the Antwerp aerosol, calculated relative to the average crustal rock and using Fe as a reference element, are listed in Table 1. They show that several elements are highly enriched pointing to various noncrustal sources in the Antwerp aerosol. Single particle analysis

From the cluster analysis 30 different clusters were retained. For ease of discussion, the clusters are grouped under six general denominators (particle classes), labeled according to their presumed source, i.e. soil dust, auto exhausts, sulfates, sea salt particles, biological particles and miscellaneous particle types. Table 2 summarizes the results of cluster analysis. Column 1 ¬es the particle type, column 2 the abundance of the number of particles averaged over the 16 samples, column 3 and 4 the number mean diameter (NMD) and volume mean diameter (VMD) of the particle size distribution. The last column gives AE 23/5-P

the concentration of the main elements as obtained from the cluster analysis. In what follows the different particle classes will be discussed separately. Sulfir-rich particles. Many studies of SOi- containing aerosols, collected near urban sites have shown that particulate S is present in the sub-pm size fraction (0.01-0.5 q diameter), with a maximum number concentration around 0.04 pm diameter and a maximum mass concentration at a diameter of 0.2-0.5 pm (Kadowaki, 1976; Mtszaros, 1981; Okada, 1985). Sometimes a separate coarse mode is observed around 3.5 pm VMD, which is assumed to be composed of NaSO, resulting from the attack of marine NaCl aerosol by HzS04 in mixed marine/polluted air masses (Harrison and Pio, 1983). Often SOi- is present as a strong acid, indicating oxidation of the precursor SO, gas to H,SO, by homogeneous or heterogeneous, reactions. Possible sources of SOiare numerous. Often, anthropogenic emissions, e.g. arising from combustion of fossil fuels, constitute the main sources. Usually, urban aerosols also contain NH:, in a large enough concentration that partial or complete neutralization of the H,SO:- acid occurs due to the uptake of NHf by the H,S04 droplets with the formation of ammonium sulfates such as (NH,)#04,

W. A. VAN BORMet al.

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Table 2. Mean cluster analysis results for 16 samples of air particulate matter, sampled at an urban site in Antwerp, Belgium Abundance Particle

type

NMD (pm)

VMD (fim)

1.08 0.80 0.53 0.12 1.26

6.93 5.22 1.83 3.30 6.22

Al(8.8). Si(29.5), S(2.2), K(2.1), Ca(l.9), Fe(5.4) Al(4.4). Si(l2.2). S(6.6). Caf8.6). Fef21.8). Zn(1.4) Ali2.5j; Si(2.0),‘Ti(Sl.$, Fe(l.bj ’ A1(39.6), Si(l.8), S(4.3), Ca(2.4), Fe(1.6), Zn(l,2)

15.4

0.33

1.50

S(7.0), Ca(2.7), Cu(3.0), Zn(1.4), As(l.2), Pq57.6)

3.6

0.49

2.11

Cl(1.9), Br(23.7), Pq65.4)

0.68 0.25 2.00

5.52 0.56 4.50

Si(l.7), P(l.5), S(21.0), Cl(l.O), Ca(29.0), Fe(l.0) S(24.0)* S(24.0)*

0.58

2.70

Na(l5.9), S(3.9), Cl(47.9), K(5.9)

0.94 1.63

8.55 7.94

Si(2.3), P(21.1), S(9.7), C1(1.3),K(7.1), Ca(6.8) Al(2.4). Si(4.0), P(1.2), S(14.5), Cl(2.6), K(26.4), Ca(7.6) Si(2.7), Fe(61.7) Si(3.6), K(l.3), Fe(2.4), Cu(lO.4), Zn(7.Q Sq6.4), Pq44.7) Si(l.O), K(8.8), Ca(6.01, Fe(2.7), Zn(22.0), Sn(l.6), Pq36) Si(4.4), V(1.2), Fe(l.6), Ni(67.6). Si(5.9), Si(ll.6), V(24.1), Ni(ll.2) Si(l.O), P(3.6), Ti(l7.9), Zn(42.9), Pq3.6) At(6.1), P(3.6). S(l.4). Ca(l.6), Cr(l2.0), Mn(2.1), Fe(23.6), Zn( 1.4) A1(5.7),Si(9.6), S(lO.9), Ba(36.8) S(4.9), Zn(l2.8), Sn(50.9), Pq3.7) S(l.4), C&1.6),Cu(l.6). Br(4.5), Ag(63.0), Pq12.7) S(l6.4), C&4.2),Cd(41.4) A&1.6),Si(1.3), S(2.0), Mn(36.1), Fe(28.6) S(lO.5), Fe(8.6), Cu(44.8), Pb(1.2) Si(2.5), Cu(23.9), Zn(2.9), As(44.5), Sq1.7) Na(1.4), Zn(l.4), Sq79.4) S(l.l), SeJ70.5)

(number

Soil dust

30.0

Si-rich Fe-rich Ti-rich Al-rich Mg-rich

20.5 5.2 1.6 1.5 1.2

Auto exhausts

19.0

PbSO, PbSO,.PbO PbSO,.(NH,),SO, PbBrCl Sulfates

18.9

CaSO, Fine sulfur-rich Coarse sulfur-rich

11.6 4.8 2.3

Sea salt particles

3.2

NaCI, KC1 Biological

particles

P/S rich K/S rich

3.2

%)

1.8 0.5

Miscellaneous

26.6 11.1 2.8 1.2 0.1 1.0 1.0 0.7

0.52 0.31 0.54 0.77 0.67 0.39 0.45

4.49 1.09 1.46 2.69 2.34 1.41 2.38

0.5 0.3 0.3 0.2 0.2 0.2 0.2 0.1 0.1

0.53 0.34 0.20 0.62 0.45 0.52 0.25 0.33 0.46

1.36 0.79 0.25 1.04 0.52 1.54 0.53 0.38 0.59

ZnO(Cr, Fe) oxides BaSO, SnO, Ag/CI rich CdSO,-CdCI,

(Fe, Mn) oxides cuso, As,% SbO, SeO,

(weight

%) of the main elements

Mg(l6), A1(9.2),Si(24), Fe(2.5)

2.3

Fe-oxides (Pb, Cu, Zn, Sb) oxides (Zn, Pb) oxides Oil fly ash (Ni-rich)

Oil flv ash (V-rich)

Concentration

* Calculated as (NH,)$O,. Number Mean Diameter. VMD: Volume Mean Diameter. NMD:

(NH4)HS04 and (NH&H(SO,), (Charlson et al., 1978; Tomasi et al., 1975). Energy-dispersive X-ray analysis provides no means of distinction between the different ammonium sulfate compounds or H,SO,. However, several authors have reported on discriminating between the SOi- and H,SO, particles on the basis of morphology, H,SO, droplets being domeshaped particles which are often surrounded by concentric rings of smaller droplets (Grass and Ayers, 1979). These dome-shaped particles are also electron translucent and quite unstable upon irradiation by an electron beam. On the other hand, pure (NH&SO4 particles appear as wart-shaped, with few or no tings of smaller droplets and are stable upon irradiation (Webber, 1986). Partially neutralized H2S0, droplets

have less droplet rings and larger diameters than completely neutralized particles and can exist as two phase compounds, consisting of a flat central ammonium sulfate crystal covered by a layer of H,SO,. Also, it has been shown that SOZ- particles often contain small nonsoluble nuclei like soot C on which condensation of the acid droplets takes place eventually foRowed by heterogeneous oxidation of the gaseous SO,, catalyzed by the metal ions present (Be&e and Gravenhorst, 1978). In this study, as a first approximation, SOi- particles were identif%zl by their high elemental S cona%tration. Furthermore, microscopical observations of all sampies showed no presence of droplet ring systems, bading us to the mnclusion that all of the SOi-

1145

Characterization of particles in Antwerp aerosol was present as ammonium sulfates. Sulfate number distributions were ma& solely on the basis of individual particle analysis and electron microscopy. Figure 4a shows the size distributions of all particles of all samples in which number S was detected as the main element. In each sample we found two size modes present: fine sub-pm and coarse particles. The modes were separated by fitting a lognormal function to the observed size distributions. In Fig. 4a, the coarse mode is shown hatched. The NMD and the VMD of the fine mode (0.25 pm and 0.56 Frn, respectively) indicate that this size class originates from gas-to-particle condensation and from coagulation of nuclei < 0.1 pm. The diameters obtained agree closely with those obtained by Post and Buseck (1985b). Note that we restricted our analysis to particles >0.2 pm diameter and hence excluded the nuclei which contain most of the sub-pm SO:-. The coarse mode is characterized by a NMD of 2.0 m. Microscopical observations of these
N

Ibf

the X-ray spectra showed a high background due to Brehmsstrahlung from light elements, probably H, C, N and 0, we identified these particles as biological particles, possibly pollen, with S species, probably SOi- associated. Mamane and Not1 (1985) observed a similar phenom~on in particles sampIed at a rural site. They postulated that large particles could act as SOi- carriers. In this case, SOi- is accumulated on the surface of the particles either by interception of the individual fine S containing particles or as a uniform SOi- layer due to gas-particle conversion, with the humid surface of biological particfes providing an attractive substrate for adsorption of SO, upon which oxidation occurs. Hence, the amount of SOi- present on these particles is certainly not insignificant, especially in the urban SOi--rich aerosol. Sulfate was also found to be associated with many other particles in lower concentrations, presumably as ammonium sulfate or as H,SO, coatings. It was observed that about 60% of all aerosol particles contain S in a detectable concentration (the detection limit for S was about 1%). In view of these observations, it seemed interesting to calculate the internal mixing ratio Mi for a11 Scon~ning particles (including the coarse sulfates), assuming all S was present as SO:-. This ratio was defined by Jaenicke (1978) as:

with M,,, the number mixing ratio, as the ratio of S cont~ning particles to the total number of particles in a given, narrow, size range and M,, the volume mixing ratio, as the ratio of the volume of the sulfur containing particles to the total volume of the aerosol. The internal mixing ratio is a quantitative measure for the mixing state of the S aerosol. For the coarse SOi- the volume of the SO:- layer was estimated according to Mamane and No11 (1985). We found that for al1 samples the volume mixing ratio has a constant value of 0.21 rtO.01 over the size range 0.2-4 pm, and decreases at higher diameters (Table 3). This indicates that a large volume of nonSOi- compounds is present in the aerosol. As will be shown later, these are predominantly heavy metal particles and soil dust. On the other hand, the number mixing ratio increases with the diameter Table 3. Number mixing ratio (M,), volume mixing ratio (IV,,) and internal mixing ratio (M,) for sulfur containing particles in 5 size fractions in the Antwerp urban aerosol (all samples)

Fig. 4. Number size dist~bution of sulfate part&s for a11sampies analyzed: (a) particles with S detected as the major element. The fine mode is identified as (NH,)zSO,-particles; the coarse mode is made up of biological particlesthat act as sulfate carriers. (b) CaSO,-particles.

Fraction (pm)

M”

Mv

M,

CO.5 OS-l.0 1.0-2.0 2.0-4.0 > 4.0

0.58 0.65 0.69 0.81 0.76

0.48 0.56 0.61 0.76 0.73

Mean

0.698

0.20 0.21 0.20 0.22 0.13 0.192

0.628

W. A. VANBow et al.

1146

due to the fact that a considerable part of the S is present as coatings of larger particles, while non-S containing particles from anthropogenic emissions are present in the fine mode. In this case, the value of the volume mixing ratio is such that it strongly dominates the internal mixing ratio and so it shows the same tendency. Hence, we found the S aerosol to be more an externally mixed than internally mixed aerosol, though the mixture state is more shifted to the internally mixed aerosol for particles between 2 and 4 pm. In the literature there is evidence on the state of mixture of S measured as (NH&SO,. Jaenicke (1978) assumed a perfect external mixture for the fine particles ( < 0.5 pm). He observed a decreasing trend with particle size for M, and for M,. Our data show a decrease for M, but not for M, for particles > 0.5 pm. The fact that the large particles seem to be internally mixed is due to the presence of the ‘coarse sulfates’. Apart from these findings, it was found that 35% of all S-rich particles contained a small but detectable concentration of elements other than S. Table 4 lists the range of concentrations (weight %) for these elements within a particle, calculated relative to the assumed SOi- concentration. The elements found agree with those reported by Charlson et al. (1978). No particular trend towards larger concentrations for smaller particles was found. This may well agree with the assumption that a heterogeneous oxidation of SO2 is occurring in particles < 0.1 pm upon which coagulation takes place. This type of oxidation is thought to be the most important SOi- formation mechanism in urban airsheds in which the concentration of metal containing nuclei as soot C is high enough (Beilke and Gravenhorst, 1978). The element we found associated with S with highest concentrations was Na. Post and Buseck (198Sb) also observed this phenomenon and attributed the occurrence of Na above a desert land 1000 km from the west coast of the U.S. to coal burning power plants. However, we feel that in our case we must attribute the Na to marine sea salt, which reacts with the H,SO, aerosol to form Na,SO,. This phenomTable 4. Elements, co-existing with sulfur-rich particles in the 0.2-l 5 Mm diameter size range Element Na Mg Al Si P K Ca V Mn Fe Ni cu Zn

Concentration range (weight %) 7-20 0.8-5.6 S-20 0.6-10 4-10 3-14 0.7-14 2-l 0.8-3.5 0.7-6 3-10 1.6-7 3.6-8

enon was also seen in the sea salts that we grouped in the cluster of ‘sea salt’ particles. Calcium sulfate. An easily identified group of particles consists of CaSO,. Most are formed of aggregates of smaller fragments with several crystalline forms. The average S/Ca ratio for this particle type is equal to 0.72, which is close to the theoretical value of 0.80, considering the fact that some of the particles were present as Ca,(PO,), or as CaCl,. Contrary to the study of No11 et al. (1987) who found considerable amounts of CaCO, (limestone), originating from soil dust and road wear, we detected no CaCO, in our samples, indicating that CaCO, particles, emitted from limestone or cement were converted into secondary CaSO, upon reaction with SO,. Primary CaSO,particles are originating from the weathering of limestone buildings, covered by a crust of gypsum (Leysen et al., 1987). The number particle size distribution of the CaSO,-particles is depicted in Fig. 4b. Auto exhaust particles. The principal source of particulate Pb in an urban atmosphere is the combustion of leaded gasoline. As the Pb is added to the petrol together with ethylene dihalide (Br, Cl) compounds that act as scavengers, the emissions are normally characterized by Br, Cl and Pb, and the Br/Pb ratio is often used as a tracer for this type of emissions. Extensive work has been devoted to the characterization of the automotive exhaust particles, both of the freshly emitted particles and of particles aged in the atmosphere. Nevertheless, the composition of the car exhaust particles is not yet completely elucidated. Post and Buseck (1985a) analyzed about 300 lead halide particles and found the major species to be Br/Cl/Pb containing compounds identified tentatively as s(2PbBrCl.NH,CI. They could not find particles without Cl or with S. Biggins and Harrison (1980) and Harrison and Sturges (1983) identified a number of PbjS containing compounds using X-ray diffraction with PbS0,.(NH,),S04 to be the most important species. In the present study, we quantitatively analyzed 1,368 Pb particles and found the major Pb species to be PbBrCl, PbSO,, and two compounds which we tentatively assumed to be PbSO,.PbO and PbSO.,.(NH,),SO,. It has been shown that the latter two compounds are the most important reaction products resulting from the reaction of atmospheric H,SO, or (NH&SO4 with Pb halides (Biggins and Harrison, 1979). Together, all particles account for about 19% by number of the air particulate matter. The Pb M, and S K, peak overlap in the X-ray spectra was accurately resolved and the intensity ratios obtained for S/Pb and Br/Pb in the particles were compared with the corresponding ratios in standard particles of PbSO, and PbO,. The tentative identification of the latter two compounds was made on the basis of the S/Pb ratio. Note that both types of particles were found to exist in such a way that, except for sample 5, the Pb-sulfates were always present with a larger abundance than the Pb-halides (the ratio of

1147

Characterization of particles in Antwerp aerosol

P&halides over Pb-sulfates was 0.27 on the average). This supports the hypothesis that the emitted Pbhalides are readily converted to Pb-sulfates due to the reaction with atmospheric SO, or H,SO* or (NH&SO, upon loss of Br as HBr. This loss and the large variations in atmospheric Br levels has often been reported (Sturges and Harrison, 1986). We never observed any particles with a Br concentration higher than the analytical detection limit (0.1%) and we can therefore conclude that the voiatile Br is certainly not condensating on existing particles at concentrations above 0.1%. In connection with this, we observed that the Pb sulfate compounds size distribution (Fig. 5a) is very similar to the fine S-rich size distribution. This may point to a very similar formation m~hanism in which an interaction with the SO:- aerosol is of importance. We also observed that the maximum of the number size distribution of the PbBrCl-particles (Fig. Sb) occurred at a larger diameter than for the Pbsulfate compounds size distribution. A plausible explanation for this is that the fine PbBrCl-particles are more easily converted to Pb-sulfates than the coarse ones. Sea salt particles. A minor component of the airborne particulates consisted of sea salt particles, identified by their high Na, IC and Cl concentration and by their distinctly cubic shape. The average number abundance was a few per cent, except for sample 5 (7.7%) and sample 12 (26.2%), which seemed strongly influenced by maritime air masses. The presence of S in these particles, is probably due to the reaction of gas phase SO, with the (Na, K) Cl

I

upon loss of Cl and formation of Na,SO, (Harrison and Pio, 1983). The mean S concentration in the sea salt particles averaged about 4%. The number concentration of sea salt particles was highest in the fine fraction (Fig. SC),but most of the mass was present in the coarse fraction (VMD=2.7 pm), as is usually observed in bulk analysis of impactor samples. Biologicalparticles. Biological particles containing H, C, N and 0, orbiting from pollen, spores, plant and leave fragments and insects were also present. Most of these were visually identified in the secondary electron image mode but could not be detected with the automated analysis mode of our instrument as their backscattered image signal fell generally under the image threshold setting. Nevertheless, some biological particles were detected because they contained .elements with a higher atomic mass, e.g. K, S and P. The S-rich particles were already commented on. As S was readily detected, the abundance of these S containing biological particles was probably accurately measured. The other clusters are mentioned in the cluster analysis but together fail to represent the actual abundance of biological particles, which should be several times higher. As biological particles are often > 15 pm, they were excluded from the analysis. Soil derived particles. The resuspension of soil material by the action of the wind is the main source of soil derived particles in the air. Recent estimates of the amount of mineral aerosol generated in the particle size range capable of long-range transport are of the order of 100-1000 Mt a-’ (Schlitz and Rahn, 1982), despite the fact that the fraction of the soil which can

r

iJ)

N

(b)

30

L311

20

10 :_j

I :

o,

L

01

D, lpmk

1

10

II, lpm)

(d)

N

rh

40

20 r. 0i 01

* -:

~ I

10

4 Dp (pm1

Fig.5. Number size dis~bution ofz {a) Pb-sulfate particks, (b) PbB~i-p~ci~

1

10

D,tpml

(c) Na~-~~icl~

(d) Fe-

1

1148

W. A.

VAN

become airborne represents only a minor fraction of the top soil mass. In the Antwerp aerosol, soil derived particles make up nearly 30% of all particles analyzed. They are divided into five clusters, according to the element that is the most abundant, Si, Fe, Al, Ti or Mg. Their composition is similar to that of clay minerals as kaolinite and montmorillonite, feldspars and pure quartz. All particles are present as individual particles and aggregates. We did not try to identify each type of mineral particle, as this would be beyond the scope of this survey. Instead, we were interested in the number size distribution of these clusters, depicted in Fig. 6. Although crustal material is thought to be composed of mostly coarse particles, we found a significant fraction in the sub-pm size fraction but their respective

01

N

1

10

BORMet al.

NMD and VMD show that the sub-pm part contributes little to the total mass. The presence of numerous sub-pm soil particles is quite important in view of the fact that they may act as condensation nuclei for SOi- formation. Indeed, Si, Al, Fe and Mg were also observed in the S-rich particles (Table 4). No discrimination was made between soil particles and coal fly ash particles, which have a similar chemical composition. This implies that the soil contribution is somewhat biased by the contribution of coal fly ash particles. Miscellaneous particles. A wealth of particles is present with a composition that does not lead immediately to a unique source identification. We labeled this group ‘miscellaneous’, although most probably a

01

OpIumI

1

10

OPluml (d)

1

N 10 -

(e) N 20

0

01

1

x)

Cl,Iprn)

Fig. 6. Number size distribution of crustal partidesz (a) Ferich soil particles, (b) Si-rich soil particles, (c) Al-rich soil particles, (d) Mg-rich soil particles, (e) Ti-rich soil particles.

1149

Characterization of particles in Antwerp aerosol

number of them can be attributed to anthropogenic emissions. For instance, the oxides of metals as Zn, Cu, Sn, Cr, Fe and Pb are originating from various abrasion processes of metal objects ((Fe, Cr)-oxides), from the emissions of incinerators ((ZnO), (Zn, Pb)oxides) lead-producing industry ((Zn, Pb)-oxide< (Pb, Cu, Zn, Sb)-oxides) and from oil burning (S, V, Ni). In connection with a possible Cu contamination by the pump, we looked specifically for abrasion particles with a high Cu concentration. The only particles with high Cu concentration were CuSO,-particles, showing high concentrations of Cu, Fe and S. The presence of the latter element excludes the possibility that these particles would originate from the abrasion of the internal parts of the pump. Otherwise severe reactions between SO,, present in the air drawn through the filter during sampling, and particles already deposited on the filter would have occurred. However, as was already mentioned, sampling time and particle loading was kept to a minimum to minimize this kind of problem. Hence, the recorded Cu concentration in the Antwerp air particulate matter should be due to a mixture of probably diverse anthropogenic emissions. Several particles were found with an X-ray spectrum consistent with metallic Se or SeO,, CdSO, or CdS, CdCl, and metallic As or As,O,. Their abundance in the urban aerosol was too low to be of any danger to public health. In accordance with this, the total filter analysis also showed that elements as Se and As were present in low concentrations. The somewhat high Se-concentration corroborates the results of the study of Jiang et al. (1983). These authors analyzed ambient air, river and lake water samples at several sites in Belgium for organoselenium and total Se and observed the highest levels of airborne Se near our sampling site. They suggested that these high Se levels were due to the emissions of a Se-producing factory at about 3 km of the sampling site. None of the Se-containing particles observed by us contained S in detectable concentrations.

The most abundant particle group in the ‘miscellaneous’ class consists of pure Fe-oxides, accounting for 17.7% of all particles analyzed. Their number size distribution is depicted in Fig. 5d. By manual morphological analysis of these particles, we could divide them into two distinct groups: one with spherical particles with no distinct features and thus characteristic for formation at high temperatures in e.g. smelters or incinerators. The other one containing particles, with sharp edges, also occurring as aggregates. Both types occurred in all samples, but with our analysis procedure, we could not give a reasonable estimate of the abundance of both spherical and non-spherical particle types in all samples. We are inclined to attribute the latter type to naturally occurring oxyironhydroxides (ferrite, haematite, goethite, magnetite), coming from soil resuspension. However, in an urban atmosphere iron oxides have still other sources such as oxidized iron (rust) and coal fly ash, making it quite difficult to apportion these particles to specific source categories. The occurrence of iron-oxides in urban air particulate matter in such high abundances has, until now, not been reported. We could not observe any correlations between e.g. weather conditions and the abundance of Fe-rich particles or between the abundance of soil particles and Fe-oxide particles. This suggests a complex mixture of Fe-oxide particles from different sources, but not distinguishable by our EPMA-procedure. Temporal variation of the different particle types. As the samples were taken on a 12 h basis, it was possible to obtain an idea of the variation in the abundance of the different particle types. With the total abundances set equal to 100% this variation is depicted in Fig. 7 as a stacked time series. Wind directions and speed are noted on top. Most of the particle types make up a rather constant partition of the total number of particles, both day and night. This is probably due to the fact that the meteorological situation was rather stable for the period under study; only the wind direction varied somewhat. The appearance of a specific type of

others Fe-oxides Biological Marine (NH&S04 Ca SO4 Auto Pb -Halides Auto Pb -S soii 1

2 1 Th

3 1

4 Fr

5 6 1 +ia

7 8 9 10 I .%I 1 MD

11 12 13 14 I Tue I Wed

15 16 ITh 1

Fig. 7. Abundance (number %) of particle types over the sampling period with a 12 h resolution. Wind directions and speeds are noted on top. For a detailed explanation see text.

1150

W. A. VAN

particle is readily seen for the marine particles which peak in samples 5 and 12. No general difference could be observed between day and night samples. Apparently, the effect of for instance the generation of the auto exhaust particles during daytime does not markedly diminish

at night, when most of the traffic ceased.

Acknowledgements-We are indebted to H. Nullens for the implementation of the X-ray software. One of us (W.M.) acknowledges the financial support from the Belgian “Nationaal Fonds voor Wetenschappelijk Onderzoek” and the “Interuniversitair Instituut voor Kemwetenschappen”. The financial assistance of GEBEG, Brussels is appreciated.

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