Study of elemental mass size distributions at Skukuza,South Africa, during the SAFARI 2000 dry season campaign

Willy Maenhaut a,*, Jaroslav Schwarz a, Jan Cafmeyer a, Harold J. Annegarn b

a Institute for Nuclear Sciences, Ghent University, Proeftuinstraat 86, B-9000 Gent, Belgiumb Atmosphere and Energy Research Group, University of Witwatersrand, WITS 2050, South Africa

Abstract

As part of the final dry season campaign of SAFARI 2000, a 12-stage small deposit area low pressure impactor

(SDI) was operated at Skukuza, in the Kruger National Park, South Africa, from 17 August until 19 September 2000.

Separate day and night samples were collected (64 in total), starting at about 7:00 and at about 18:00 local time, re-

spectively. The samples were analysed for 28 elements by PIXE. The total concentrations (summed over all 12 stages)

varied quite substantially during the campaign (up to a factor of 50), but no systematic day/night difference pattern was

observed. Also the size distributions were rather similar during day and night. S, K, Zn, As, Se, Br, Rb and Pb had most

of their mass in the submicrometre size range, with maximum typically at about 0.3 lm equivalent aerodynamic di-

ameter. Several of those elements are good indicators for biomass burning. Mass median aerodynamic diameters

(MMADs) were calculated for the various elements and compared with those obtained during SAFARI-92. During this

earlier campaign, which also took place in the dry season, 41 daily samples were taken at Skukuza with a PIXE In-

ternational cascade impactor (PCI). For the crustal and sea-salt elements, fairly similar MMADs were obtained in the

two campaigns. For the fine-mode elements, however, the MMADs were substantially lower during SAFARI 2000 than

during SAFARI-92. During this earlier campaign, the MMADs were most likely overestimated. Compared to the SDI,

the PCI is much less appropriate for studying the size distribution in the submicrometre size range. � 2002 Elsevier

Science B.V. All rights reserved.

PACS: 07.85.-m; 82.80.Ej; 92.60.Mt; 92.60.Sz

Keywords: PIXE; Multielement analysis; Atmospheric aerosols; Size distribution; Cascade impactor; South Africa

1. Introduction

The Southern African Regional Science Initia-tive (SAFARI 2000) is an international scienceinitiative aimed at developing a better understand-ing of the southern African earth–atmosphere–human system [1]. Particular emphasis is placed

upon biogenic, pyrogenic and anthropogenic emis-sions, their characterisation and quantification, theirtransport and transformations in the atmosphere,their influence on regional climate and meteoro-logy, their eventual deposition, and the effects ofthis deposition on ecosystems. The SAFARI ini-tiative includes continuous efforts as well as threeintense, episodic field campaigns, i.e. two dry-season campaigns in August–September (one in1999, the other in 2000) and one wet season cam-paign (February–March 2000). The tropospheric

Nuclear Instruments and Methods in Physics Research B 189 (2002) 254–258

www.elsevier.com/locate/nimb

*Corresponding author. Tel.: +32-9-264-6596; fax: +32-9-

264-6699.

E-mail address: willy.maenhaut@rug.ac.be (W. Maenhaut).

0168-583X/02/$ - see front matter � 2002 Elsevier Science B.V. All rights reserved.

PII: S0168 -583X(01)01053 -9

aerosol research team from Ghent University(RUG) participated in the field work of the finaldry season campaign. It deployed a wide variety ofaerosol collectors and also some in situ aerosolmeasurement devices at Skukuza, in the KrugerNational Park, South Africa. One of the aerosolcollectors was a 12-stage small deposit area lowpressure impactor (SDI) [2], and the samples col-lected with it were analysed for 28 elements byPIXE. Here we present and briefly discuss the re-sults of these SDI collections.

2. Experimental

The RUG samplers and devices were set up atSkukuza airport (25�010S, 31�350E, 260 m asl),which was also used by us during the SouthernAfrica Fire – Atmosphere Research Initiative(SAFARI-92) in September–October 1992 [3,4].The SDI cascade impactor deployed at the site is amulti-jet device with jets clustered closely together,so that the diameter of the aerosol deposition arearemains smaller than 8 mm. This is a very ad-vantageous feature when PIXE is used for theanalysis. The SDI operates at a flow rate of 11 l permin and the experimental cut-points (d50-values)for its 12 stages (numbered 12–1) are 8.50, 4.08,2.68, 1.66, 1.06, 0.796, 0.591, 0.343, 0.231, 0.153,0.086 and 0.045 lm equivalent aerodynamic di-ameter (EAD). An inlet with a cut-point of 15 lmEAD was placed upstream of the device. ThinKimfol polycarbonate films (1.5 lm thick) wereused as collection surfaces. SDI samples weretaken from 17 August until 19 September 2000.Separate day and night samples were collected(64 in total), starting at about 7:00 and at about18:00 local time, respectively. The samples wereanalysed for up to 28 elements by PIXE using a 2.4MeV proton beam. Details on the PIXE proce-dures are given in [2] and in the references citedtherein.

One of the RUG in situ aerosol measurementdevices set up at the Skukuza site was a Rupprechtand Patashnick tapered element oscillating mi-crobalance (TEOM, model 1400A). By using thisdevice, PM2.5 aerosol mass concentrations with 5min time resolution were obtained.

3. Results and discussion

3.1. Time trends and day/night concentration differ-ences

The total concentrations (summed over all 12SDI stages) of the measured elements varied quitesubstantially during the campaign, up to a factorof 50. The sums over stages 9–1, representing theconcentrations in the <2.68 lm EAD size rangeare shown in Fig. 1 for sulphate (obtained as 3times S), K and Si, together with the concentra-tions of the PM2.5 aerosol mass, as derived fromthe TEOM and averaged over the SDI collectiontimes. Fine S at the Skukuza site originates mostlikely predominantly from industrial emissionson the Mpumalanga (formerly called Transvaal)Highveld [4]. Fine K is an indicator for biomassburning (pyrogenic) emissions, and fine Si origi-nates from soil dust. The PM2.5 aerosol mass isbest correlated with fine K, suggesting that bio-mass burning is the major contributor to it. Thiswill be further examined by detailed analysis ofour additional SAFARI 2000 data sets and mul-tivariate receptor modelling. From Fig. 1 it alsoappears that there is no systematic day/night dif-ference pattern for PM2.5 and the depicted ele-ments. The ratio of the concentration during thenight to that during the preceding daytime was

Fig. 1. Time trends of the PM2.5 aerosol mass concentration

and of fine sulphate, fine K and fine Si at Skukuza during

SAFARI 2000. The concentrations for the elements were de-

rived from the SDI samples and are for the <2.68 lm EAD size

range. Separate 12-h SDI samples were collected during day-

time and night-time. Date labels are given in the abscissa every

other day for samples taken during daytime.

W. Maenhaut et al. / Nucl. Instr. and Meth. in Phys. Res. B 189 (2002) 254–258 255

calculated for each of the 32 night/day pairs, andthe median (and interquartile ranges) of theseratios were 1.11 (0.87–1.37) for PM2.5, 0.99 (0.65–1.20) for fine S, 1.17 (0.90–1.75) for fine K, and0.93 (0.56–1.64) for fine Si.

3.2. Average size distributions, day/night differ-ences, mass median aerodynamic diameters, andcomparison with SAFARI-92

Elemental mass size distributions providevaluable information on the source processes forthe particulate trace elements and on the trans-formation processes during atmospheric transport.From the SDI data, average elemental mass sizedistributions were calculated for the daytime sam-ples, the night-time samples, and for both samplesets combined. And on the basis of these averagesize distributions, average mass median aerody-namic diameters (MMADs) were subsequentlyderived for each of the three sample sets. Detailson the calculation of MMADs from cascadeimpactor data can be found in [5] and in a text-book by Hinds [6]. Fig. 2 shows the average dis-tributions for seven elements in the combined day/night sample set. These elements have essentially aunimodal size distribution, over 90% of their massoccurs in supermicrometre particles, and theirMMADs are between 4 and 5 lm EAD (Table 1).They are clearly mainly attributable to dispersionprocesses, i.e. soil dust dispersal. Fig. 3 shows theaverage size distributions for eight other elements

in the combined day/night sample set. These ele-ments have a bimodal or in some cases a trimodalsize distribution, most of their mass (with the ex-ception of As) is in the submicrometre size range,the major fine mode peaks at around 0.3 lm EAD,and the MMADs of the elements are generallybetween 0.3 and 0.7 lm EAD (Table 1). The finemodes point to high-temperature sources and/orgas-to-particle conversion processes for the eightelements. The likely sources are biomass burning,fossil fuel (coal) combustion and/or industrial ac-tivities. Other bimodal elements, but with most of

Fig. 2. Average mass size distributions for seven elements in

the combined day/night SDI sample set of Skukuza during

SAFARI 2000 (N ¼ 64).

Table 1

Average elemental mass median aerodynamic diameters

(MMADs, in lm EAD), as derived from the SDI daytime

(N ¼ 34) and night-time (N ¼ 34) samples collected at Skukuza

during SAFARI 2000

Element SAFARI 2000 SAFARI-92

Daytime Night

Crustal elements

Al 4.4 4.2 5.6

Si 5.0 4.6 6.1

P 4.4 4.4 6.4

Ca 4.7 4.7 5.1

Ti 5.0 4.6 6.3

V 4.3 3.5 5.2

Cr 5.7 4.6 5.8

Mn 4.7 4.6 6.0

Fe 5.0 4.7 6.1

Ni 4.9 4.3 5.5

Ga 4.1 3.9 5.3

Sr 4.2 4.3 4.8

Zr 4.0 3.9 6.3

Sea-salt elements

Na 2.5 2.1 3.5

Mg 3.2 3.3 3.6

Cl 3.8 3.6 4.0

Other elements

S 0.46 0.52 0.75

K 0.41 0.39 3.2

Cu 3.1 2.0 5.0

Zn 0.52 0.46 3.8

As 1.06 1.05

Se 0.75 0.63 1.32

Br 0.39 0.40 0.96

Rb 0.63 0.54 5.0

Pb 0.33 0.34 0.74

Comparison with MMADs from 24-h PIXE cascade impactor

samples (N ¼ 41) collected at the same site during SAFARI-92

[3].

256 W. Maenhaut et al. / Nucl. Instr. and Meth. in Phys. Res. B 189 (2002) 254–258

their mass in the supermicrometre size range, wereCu, V and Ni. The coarse mode for V and Nirepresents the mineral dust contribution for theseelements, and their fine mode most likely thecontribution from residual oil combustion.

The average size distributions of the separateday and night sample subsets were generally rathersimilar to each other. For the soil dust elements ofFig. 2 there was somewhat more mass present inthe coarsest particles (stage 12) during day thanduring night. A similar day/night difference forthese elements was also seen in a study in Israel [7].For the bimodal elements, i.e. the ones shown inFig. 3 plus Cu, V and Ni, the mass contribution ofthe fine modes was somewhat more importantduring night than during day. Furthermore, therewas a tendency for the peak diameters of the finemodes for certain elements (in particular for S) toshift to a slightly larger value during night, pre-sumably as a consequence of particle growth underthe higher relative humidity. A similar phenome-non was also observed in the study in Israel [7].

The MMADs obtained for the separate day andnight SDI sample subsets are compared with eachother in Table 1. The elements are separated inthree groups in this table, according to their prob-able major source and/or similarity in MMAD.For the crustal elements, the average MMADs arearound 4.5 lm EAD and the average (nightMMAD/day MMAD) ratio is 0:93� 0:07 (N ¼13). The sea-salt elements have somewhat lowerMMADs than the crustal elements, but the night/

day ratios are similar. As to the other (mainly fine-mode) elements, the night/day ratios are mostlybelow 1, which is a reflection of the fact that thefine mode became more important during night. Incontrast, for S, the night/day MMAD ratio is 1.13,as a consequence of the upward shift in mode di-ameters during night. Table 1 also contains theaverage MMADs that were derived from 41 24-hcollections with a PIXE International cascadeimpactor (PCI) at the Skukuza site during SA-FARI-92 in September–October 1992 [3]. For thecrustal and sea-salt elements, the MMADs of thecurrent study are around 20% lower than thoseobtained during SAFARI-92. Although part ofthis difference may be due to real differences in themass size distributions between 1992 and 2000,most of the deviation is probably caused by dif-ferences between the SDI and PCI devices. In theabove-mentioned study in Israel [7], parallel col-lections were done with SDIs and PCIs; the crustalelements exhibited overall very similar size distri-butions with the two devices, but compared to thePCI, those for the SDI were shifted to diametersthat were 15% lower. This difference was attrib-uted to the fact that the actual cut-point values forthe PCI deviate somewhat from the nominal val-ues [7]. For the mainly fine-mode elements, theMMADs in the current study are substantiallylower than in SAFARI-92, and for the typicalpyrogenic elements K, Zn and Rb, the currentMMADs are only 10–15% of the 1992 values.During the earlier campaign, the MMADs of thesefine mode elements were most likely overesti-mated. From the study in Israel [7] and also froma study in Rome [8] where SDI and PCI sampleswere collected in parallel, it was concluded that thePCI is much less appropriate than the SDI forstudying the size distribution in the submicrometresize range. This becomes also quite evident whencomparing the size distributions in Fig. 3 withthose of fine-mode and bimodal elements of SA-FARI-92 as presented in [3].

Acknowledgements

This research was done within the frameworkof the SAFARI 2000 Southern African Regional

Fig. 3. Average mass size distributions for eight elements in

the combined day/night SDI sample set of Skukuza during

SAFARI 2000 (N ¼ 64).

W. Maenhaut et al. / Nucl. Instr. and Meth. in Phys. Res. B 189 (2002) 254–258 257

Science Initiative. We gratefully acknowledge thefinancial support by the Belgian Federal Office forScientific, Technical and Cultural Affairs (OSTC),the Ministry of the Flemish Community (Bilateralco-operation project BIL97/73), the ‘‘Fonds voorWetenschappelijk Onderzoek – Vlaanderen’’, andthe National Research Foundation, South Africa.Thanks are also due to Ineke Neefs for technicalassistance.

References

[1] R.J. Swap, H.J. Annegarn (Eds.), Southern African Regional

Science Initiative: Safari 2000: Science Plan, November 1999

(website: http://safari.gecp.virginia.edu/).

[2] W. Maenhaut, R. Hillamo, T. M€aakel€aa, J.L. Jaffrezo, M.H.

Bergin, C.I. Davidson, Nucl. Instr. and Meth. B 109/110

(1996) 482.

[3] I. Salma, W. Maenhaut, J. Cafmeyer, H.J. Annegarn, M.O.

Andreae, Nucl. Instr. and Meth. B 85 (1994) 849.

[4] W. Maenhaut, I. Salma, J. Cafmeyer, H.J. Annegarn, M.O.

Andreae, J. Geophys. Res. 101 (1996) 23631.

[5] W. Maenhaut, H. Raemdonck, A. Selen, R. Van Grieken,

J.W. Winchester, J. Geophys. Res. 88 (1983) 5353.

[6] W.C. Hinds, Aerosol Technology, Properties, Behavior, and

Measurement of Airborne Particles, Second edition, Wiley,

New York, 1999.

[7] W. Maenhaut, J. Ptasinski, J. Cafmeyer, Nucl. Instr. and

Meth. B 150 (1999) 422.

[8] W. Maenhaut, F. Kohl, R.E. Hillamo, I. Allegrini, in: P.M.

Borrell, P. Borrell (Eds.), Proceedings of EUROTRAC

Symposium’98, Vol. 1, WIT Press, Southampton, UK, 1999,

p. 490.

258 W. Maenhaut et al. / Nucl. Instr. and Meth. in Phys. Res. B 189 (2002) 254–258