Nuclear Instruments and Methods in Physics Research B 219–220 (2004) 166–170

www.elsevier.com/locate/nimb

Atmospheric aerosol characterisation by Ion BeamAnalysis techniques: recent improvements at the

Van de Graaff laboratory in Florence

Massimo Chiari a,*, Piero Del Carmine a, Franco Lucarelli a,Graziella Marcazzan b, Silvia Nava a, Leonardo Paperetti a, Paolo Prati c,

Gianluigi Valli b, Roberta Vecchi b, Alessandro Zucchiatti c

a Dipartimento di Fisica and INFN Sezione di Firenze, via Sansone 1, I-50019 Sesto Fiorentino, Italyb Istituto di Fisica Generale Applicata and INFN Sezione di Milano, via Celoria 16, I-20123 Milano, Italy

c Dipartimento di Fisica and INFN Sezione di Genova, via Dodecaneso 33, I-16146 Genova, Italy

Abstract

At the KN3000 Van de Graaff accelerator in Florence, PIXE analysis of aerosol samples is routinely performed to

measure the concentration of elements with Z > 10. In order to allow a complete reconstruction of the aerosol mass,

recently we implemented the light element detection by means of in-vacuum Particle Elastic Scattering Analysis (PESA)

with 3 MeV protons on particulate matter collected on Teflon filters. PESA with a surface barrier detector (SBD) at a

forward angle of 30� was used to estimate the H content; another SBD was placed at 150� backward-angle to determineC, N and O concentrations. Results concerning PM10, PM2:5 and PM1 aerosol fractions, collected on a daily basis over

some weeks in a small industrial town near Florence, will be shown.

� 2004 Elsevier B.V. All rights reserved.

PACS: 82.80.Yc; 89.60.)k; 82.70.Rr; 82.80.EjKeywords: Aerosol composition; PIXE; PIGE; PESA; Light element detection

1. Introduction

At the external beam facility of the 3 MV single-

ended Van de Graaff accelerator in Florence, PIXE

analysis of aerosol samples is routinely performed

[1] to measure the concentration of elements with

Z > 10. However, due to X-ray attenuation inside

* Corresponding author. Tel.: +39-55-4572273; fax: +39-55-

4572121.

E-mail address: chiari@fi.infn.it (M. Chiari).

0168-583X/$ - see front matter � 2004 Elsevier B.V. All rights reser

doi:10.1016/j.nimb.2004.01.047

the target and in the atmosphere, PIXE cannot

detect light elements, which are main constituents

of particulate matter. To obtain a complete ele-

mental composition reconstruction of the aerosol

mass, recently we implemented the detection of H,

C, N and O, by means of in-vacuum Particle Elastic

Scattering Analysis (PESA).As a first application, PESA analyses have been

performed on samples of PM10, PM2:5 and PM1

(aerosol particles with aerodinamic diameter less

than 10, 2.5 and 1 lm, respectively), collected on adaily basis in the industrial district of Montelupo

ved.

M. Chiari et al. / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 166–170 167

Fiorentino, a little town about 20 km west of

Florence, characterised by the presence of a large

number of ceramic and glass factories and by high

aerosol concentration levels. Before PESA mea-surements these samples were also weighted and

analysed by PIXE. This campaign is still in pro-

gress and we give here only few preliminary re-

sults, with the main aim to present the first results

obtained by PESA.

2. Experimental

For the in-vacuum PESA measurements we

used two fully depleted surface-barrier detectors of

300 lm thickness, 50 mm2 area and about 20 keV

energy resolution, arranged at forward and back-

ward angles in an IBM geometry. The detector

used to measure the H content was placed at a

scattering angle h ¼ 30� and collimated to 0.8mm2, while the other detector, used for the C, N

and O concentration determination, was placed at

h ¼ 150� and collimated to 15 mm2; both detectors

were 65 mm apart from the sample, which was

placed in a vertical plane orthogonal to the beam.

The resulting acceptance solid angles allowed to

roughly compensate for the different count rates at

different angles. The beam dimension on thesample was 2 mm · 2 mm. The beam current was

integrated, after the sample, by a Faraday cup,

kept at a positive voltage of about 70 V to avoid

secondary electrons escape.

Since mass resolution improves at higher beam

energies, we selected the working energy in the

highest proton energy range available at our

accelerator. To obtain quantitative results by di-rect comparison with thin standards, we worked at

an energy of 3000 keV, where the proton elastic

scattering cross sections on H, C, N and O are

quite constant (better than 10%) over the range of

energy loss of the protons in aerosol samples

(6 100 keV) [2–5]. The calibration was achieved

using a thin Upilex-S foil (7.5 lm) containing

known areal density of H, C, N and O; the energyloss for 3 MeV protons in this foil is 120 keV.

Preliminary test measurements were carried out

to identify the best aerosol collecting filter for

these analyses. Both Cellulose (acetate, nitrate and

mixed ester) and Nuclepore (policarbonate) filters

resulted not appropriate, since they contain high

concentrations of H, C, O and N; moreover, after

few minutes, at a beam current of only 2 nA, weobserved a progressive degradation of Cellulose

filters induced by proton irradiation (a thickness

reduction of 10% after a dose of 1 lC). Obviously,glass and quartz fiber filters were not tested since

they are not suitable for PIXE. We found Teflon

membranes to be the only substratum appropriate

for PESA analysis, among all the filters commonly

used in aerosol sampling. Beam resistance tests at2, 5 and 8 nA gave good results: no significative

modification of the filter was observed up to an

integrated charge of about 10 lC.

3. Aerosol sampling

The particulate matter samples were collected on47 mm diameter (CF2)n Teflon filters by three sin-

gle-mode (PM10, PM2:5 or PM1) sequential particle

samplers: one PARTISOL 2025 (flow rate 1 m3/h),

and two IND PNS15D (flow rate 2.3 m3/h). The

samplers were located 4 m above ground level, on

the roof of an air quality monitoring station. The

sampling campaign lasted from September 2002 to

July 2003; during 17 subsequent days we put adifferent inlet on each instrument to measure the

granulometric fraction ratios (i.e. PM1/PM2:5 and

PM2:5/PM10). The PM10, PM2:5 and PM1 mass

concentrations were obtained on preconditioned

filters using a microbalance (Mettler UMT5).

4. Aerosol sample analysis

A total of 51 samples, collected in Montelupo

Fiorentino during the 17 days of simultaneous

sampling of the three fractions, were analysed by

PESA. Typical beam current was 5 nA and the

measurements lasted until a charge of about 3 lCwas integrated. Forward and backward scattering

spectra of a �blank� and of a �loaded� Teflon filterare shown in Fig. 1. Hydrogen concentration can

be immediately obtained from forward scattering

spectra, since the flat background can be easily

subtracted.

0

400

800

1200

2000 2200 2400 2600 2800 3000E (keV)

Cou

nts/

µC

loadedblank

x 20

θ = 30°

1H

Other Elements

0

100

200

300

1700 1900 2100 2300 2500 2700

E (keV)

Cou

nts/

µC

loadedblank

= 150°12C

19F

16O

19F(p, )16O

19F(p,p'

14N

θ )19Fγ

α

Fig. 1. Proton scattering spectra on �blank� and �loaded� Teflonfilters, at h ¼ 30� (above) and h ¼ 150� (below). The acquisitiontime was 8 min, at a beam current of 5 nA. The areal density of

aerosol deposit (PM10) was �120 lg/cm2.

Table 1

Comparison between the thickness of the black carbon stan-

dards and the measured values of C concentration

Standard thickness (lg/cm2) Measured thickness (lg/cm2)

20± 1 22.7± 2.5

99± 5 98± 5

183± 10 184± 6

The quoted errors are only statistical.

168 M. Chiari et al. / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 166–170

Concerning backscattering analysis, the Teflon

filter produces C and F elastic peaks with long

trailing edges (understandable in terms of

remarkable surface roughness of the filter) and

several F inelastic peaks; among the latters, the

peak produced by the (p,p0c) reaction can interferewith the N elastic peak. The O concentration can

be easily deduced subtracting the continuousbackground, since the �blank� spectrum does not

show large discontinuities in the energy region

corresponding to the O peak. On the other hand, it

is more complicated to identify the background

contributions to be subtracted from the C and N

peak total areas in the �loaded� spectra (i.e. in Fig.1 from 1900 to 2200 keV and from 2200 to 2300

keV, respectively). Moreover, the Teflon thicknessis not constant – up to 20% variations –, so the

contributions to be subtracted are not the same for

all the measurement points. These background

contributions can be anyway determined since,

assuming a constant C/F ratio in Teflon, they are

proportional to the �local� filter thickness and to itsF content, which can be measured by Particle In-

duced c-ray Emission (PIGE) analysis, simulta-neously with PESA, using a HPGe detector (at

h ¼ 90�) and exploiting the 19F(p,p0c)19F reaction

(Ec ¼ 110 keV). Analysing several blank filters, we

found a good linear correlation (v2 ¼ 1:3–1.5) be-tween the area of the 110 keV fluorine c peak andthe background contributions in the C and N re-

gion to be subtracted to the total peak areas in the

�loaded� spectra. Note that the 110 keV c-yield [6]integrated over the proton energy loss in the Tef-

lon filter (about 50–60 keV) does not differ more

than 15% between �blank� and the most �loaded�filters (proton energy loss in the overlaying aerosol

deposit about 100 keV).

This method was successfully checked measur-

ing the C content of black carbon (BC) standards

prepared by the Istituto di Fisica Generale Ap-plicata di Milano, depositing a powder of BC with

known areal density on Teflon filters (Table 1).

Typical minimum detection limits (MDLs) were

0.3 lg/cm2 for H, 4 lg/cm2 for C and 2 lg/cm2 for

N and O. The H concentration could be measured

to ±5%, while for the other elements the error

ranged from ±5% to ±25%, with the bigger errors

affecting the N concentration; an additional 5%systematic error has to be considered due to

uncertainties in the Upilex foil thickness.

PIXE analysis [1] was previously performed on

the same aerosol samples (detailed results will be

presented in a forthcoming article). As expected

the fluorine content of Teflon filters gave rise to a

strong Compton c-ray background, which in-

creased PIXE MDLs for medium–high Z aerosolelements. At a proton beam energy on the target of

2.85 MeV the high c-ray rate produced unsus-

tainable levels of pile-up in the Si(Li) detectors and

PM1

4%2%

1% 4%

6%

1%CONS

M. Chiari et al. / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 166–170 169

saturation of the preamplifiers. So, to decrease the

c-ray production cross sections [6,7] and maintainthe detector pile-up rate within acceptable levels

(5%), PIXE measurements were performed atproton energy of 2.2 MeV on the target.

20% 52%

10%HKNaSiother PIXE

PM2.5

3%

1%

47%

6%1%

4%

12%

4%

1%CONSHSiNaK

5. Results and conclusions

The coupling of PIXE and PESA allowed the

�mass closure�: the sum of the concentrations of all

the elements detected by the two techniques re-sulted equal to the gravimetric mass within 20%

for all the analysed samples and better than 10% in

the 85% of the cases (Fig. 2).

The percentage attribution (averaged over all the

samples) of the gravimetric PM10, PM2:5 and PM1

mass to the main elements detected by PIXE and

PESA is shown in Fig. 3: C gives themost important

020406080

100120

PIXE % PESA %

PM 1

020406080

100120

PM 2.5

020406080

100120

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

PM 10

Fig. 2. Sum of the concentrations of all the elements detected

by PIXE and PESA as a percentage of the gravimetric mass

during the 17 days of simultaneous sampling of the three

granulometric fractions.

21%other PIXEunexplained

PM102%

5%

14%

9%

3%

2%

41%

3%

4%

17%

CONCaSiSClHother PIXEunexplained

Fig. 3. Percentage contributions of the eight most abundant

elements detected by PIXE and PESA to the total gravimetric

mass. �Other PIXE� is the sum of the other elements detected by

PIXE. The values are averaged over the 17 days of simulta-

neous sampling of the three granulometric fractions.

contribution to the total mass in all the fractions(about 40% in PM10 and 50% in PM2:5 and PM1).

The PM2:5 and also PM1 fraction resulted a

substantial part of PM10; excluding three days in

which there was an anomalous increase in PM10

concentrations respect to PM2:5 and PM1, the

PM2:5 mass was in average 62% (with a maximum

of 75%) of PM10 mass and the PM1 mass 46% (with

a maximum of 58%) of the PM10 mass. Al, Si, Ca,Fe and other typical crustal elements, and also Zr

170 M. Chiari et al. / Nucl. Instr. and Meth. in Phys. Res. B 219–220 (2004) 166–170

(used in the manufacturing of tiles) resulted more

concentrated in the PM10 fraction, while elements

like C, As (related to emissions from artistic glass

manufactures) and S were mainly present, withsimilar levels, in the PM2:5 and PM1 fractions.

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