PIXE-PIGE Analysis Of Aerosol Composition In Urban
Italian Environments
S. Nava*, A. D’Alessandro*, F. Lucarelli¶, P.A. Mandò¶, G. Marcazzan†, P. Prati*,
G. Valli†, R. Vecchi†, and A. Zucchiatti*
*
Dipartimento di Fisica and INFN, Via Dodecaneso 33, 16146 Genova, Italy
Dipartimento di Fisica and INFN, Via Sansone 1, 50019 Sesto Fiorentino, Firenze, Italy
†
Istituto di Fisica Generale Applicata, Via Celoria 16, 20133 Milano, Italy
¶
Abstract. Fine particulate has become one of the biggest concerns in Italian cities pollution; the study of its
composition is a powerful tool to evaluate the effects on health and identify pollution sources. PIXE is an established
technique for particulate analysis (being multi-elemental, sensitive, fast, non-destructive and requiring no sample
preparations) and has been extensively used, in combination with other IBA techniques, for particulate characterization
in Italian urban environments. Here we report the preliminary results on the analysis of the aerosol collected, by twostage continuous streaker samplers, in two Italian cities, Florence and Milan, during July 2001. Elemental concentrations
have been extracted in the fine and coarse fractions, with hourly resolution, by PIXE-PIGE analysis, performed at the 3
MeV external proton beam INFN facility at the University of Florence.
speed and reasonable costs) of IBA techniques (PIXE,
PIGE), make our approach quite unique. In particular,
the hourly resolution puts on evidence fast phenomena
and time correlations not visible with the standard 24hour sampling.
INTRODUCTION
An extensive investigation is in progress aiming at
the study of the air particulate composition in four
major Italian towns (Florence, Genoa, Milan, Naples).
The aim of our study is to characterize the particulate
matter in the four towns by the same methodological
approach and in the same periods. The aerosol has
been collected simultaneously in the four towns during
winter (January-February) and summer (July-August)
2001, by two-stage continuous streaker samplers [1],
which provide the separation of the inhalable
particulate matter in two fractions. The sampling sites
had been selected in order to be representative of
medium-heavy traffic urban zones; they all are close to
traffic roads, known to have similar annually average
concentration of CO (2.5–3 mg/m3). The
concentrations in air of about 20 elements have been
measured with hourly resolution by PIXE and PIGE
analyses at the 3 MeV external proton beam of the
INFN accelerator facility at the University of Florence.
The characteristics of our campaign (contemporary
continuous sampling in different towns, measurement
of hourly elemental concentrations), which was
possible thanks to the high sensitivity (and therefore
The results concerning the aerosol collected in the
winter period are reported elsewhere [2]; here we
present the preliminary results concerning the aerosol
collected during July 2001 in Florence and Milan.
SAMPLING AND ANALYSIS
Sampling
We used two-stage streaker samplers provided by
PIXE International Corporation. Briefly, they consist
of a pre-impactor that stops particles with aerodynamic
diameter (Dae) >10 µm, a thin Kapton® film that
collects by impaction particles with 2.5 µm<Dae<10
µm (coarse stage), and of a Nuclepore® filter that
stops all smaller particles (fine stage) [1]. The
CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan
© 2003 American Institute of Physics 0-7354-0149-7/03/$20.00
448
Na, Mg and Al) may be underestimate due to the onset
of self-absorption effects within the particle grains,
mainly in the coarse fraction. Corrective factors are
not easy to calculate theoretically because aerosol
samples
contain
particles
whose
elemental
composition and size distribution are a priori not
known and not uniform. In order to evaluate
experimentally these underestimations, in the last
years we often used PIGE simultaneously with PIXE
to measure the concentration of Sodium.
Kapton® impaction plate and the Nuclepore® filter
are paired on a cartridge, which slowly rotates
continuously, at an angular speed of about 45°/day. At
the end of a week of sampling, therefore, a circular
continuous deposit (‘streak’) is produced on each of
the two stages and can be analyzed ‘point by point’ in
order to reconstruct the hourly trends. Samplers, one
for each town, have been installed on the roof of
monitoring cabins of the municipal air quality network
(Florence – Gramsci, Milan – Zavattari), at about 3 m
above ground. The sampling lasted 18 days in
Florence (2-20 of July) and 10 days in Milan (2-12 of
July).
PIGE, based on the detection of gamma rays from
(p,γ), (p, p’γ) and (p, αγ) reactions, doesn’t present the
problem of self-absorption. Nevertheless, due to the
stronger energy dependence of PIGE cross-sections,
more attention to the beam energy loss in the target has
to be paid. For thin samples the yield is integrated over
an energy interval which is in general unknown; to
override this problem it is necessary to work at a beam
energy such that the cross-sections remain sufficiently
constant though the proton energy loss in the sample,
being at the same time high enough to provide good
sensitivity [5, 6]. In case of aerosol samples the
average thickness may be even much smaller than the
single grain size (in the coarse stage the thickness of
the bigger grains can reach 1 mg/cm2, corresponding
to an energy loss for 3 MeV protons of about 80 keV).
We calculated the PIGE yield of the reaction 23Na (p,
p’γ) 23Na (Eγ = 441 keV) using a reference NaCl thin
(46 µm/cm2) standard supplied by Micromatter Inc. As
can be seen in Figure 1, the yield shows a good
‘plateaux’ from 2930 to 3000 keV (fluctuations from
the mean value less than 10%); although it is a
minimum zone, the values are high enough to ensure
good sensitivity (MDL = 150 ng/m3).
PIXE-PIGE Analysis
PIXE (Particle Induced X-ray Emission) - PIGE
(Particle Induced γ-ray Emission) analysis was
performed with 3 MeV protons at the Van de Graaff
accelerator of INFN, in the Physics Department of the
University of Florence [3]. The beam was extracted in
air by an Upilex window of nominally 6,4 µm
thickness.
Two Si(Li) detectors, optimized
respectively for low and medium-high X ray energies,
collected the X-rays, while a HPGe detector collected
the γ rays. A Helium gas flow into the volume in front
of the smallest detector reduced the Ar peak in the
PIXE spectra and minimized the attenuation of the
lower X rays. The beam (12 nA maximum, 10 nA
average) moved along the streak in steps
corresponding to 1 hour of aerosol sampling (each step
taking 5 minutes of beam time) and was collected on a
graphite Faraday cup behind the target. PIXE spectra
were fitted using the GUPIX software package [4] and
elemental concentrations obtained via a calibration
curve from a set of thin standards of known areal
density. Minimum Detection Limits (MDLs), at 3σ
level, ranged between 0.9 and 49 ng/m3 for the coarse
stage and between 1.9-122 ng/m3 for the fine one
(being different the deposition area on Kapton foils
and Nuclepore filters). The following elements were
detected: Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Cr,
Mn, Fe, Ni, Cu, Zn, Se, Br, Sr, Zr and Pb. The
uncertainty on the detected concentrations was usually
around 5% mainly coming from the calibration
uncertainties. The errors are obviously higher when
concentrations
approach
MDLs.
Since
the
concentrations are obtained by a direct comparison
with thin standards, the accuracy of the quantitative
analysis depends on the thickness of the samples. Both
coarse and fine aerosol samples can be considered thin
for the matrix effects related to the beam energy loss
(because of the slowly varying PIXE cross sections),
but the concentrations of the lightest elements (like
100
Na - 441 keV
75
50
25
0
2650
2750
2850
2950
3050
3150
3250
Proton Beam Energy (keV)
FIGURE 1. Experimental yield for the reaction 23Na (p, p’γ)
23
Na, measured in 10-20 keV steps; the values are in counts
per µC of integrated charge and per µg/cm2 of nominal areal
density of Na. For each point the integrated charge ranged
from 1 to 20 µC, keeping the statistic error under 10 %. The
energy scale refers to the beam nominal energy (in-vacuo
energy ± 30 keV).
449
Between the detected elements, fine Sulphur gets
the highest values. This element is present in the fine
particulate mainly as sulphate [7]; sulphate particles
can be emitted directly from fossil fuel combustion
processes, but mainly they are produced by oxidation
(in the atmosphere) of SO2, also emitted from fossil
fuel combustion processes. The persistency time in
atmosphere of these fine particles is quite long, and
Sulphur concentration time pattern is characterized by
a very high, slowly varying background (typical of
secondary aerosols), which is highly influenced by
meteorological conditions (Figure 4).
RESULTS
In Figure 2 we report the preliminary results of the
Sodium measurements, relative to the Florence coarse
fraction aerosol: as can be seen, the correlation
between the PIXE and PIGE measured values is high
(R2 = 0.97) and the X-ray attenuation resulted about
40 %.
3
PIGE Sodium (µg/m )
4
3
6
S
W
2
4
y = 1.65 x - 0.03
R2 = 0.97
1
0
0
1
2
2
3
3
PIXE Sodium (µg/m )
FIGURE 2. Linear regression between the Sodium
concentrations obtained by PIXE and PIGE measurements,
relative to Florence coarse fraction aerosol.
0
7/2/01 7/3/01 7/4/01 7/5/01 7/6/01 7/7/01 7/8/01 7/9/01 7/10/01
0.00 0.00
0.00 0.00
0.00 0.00 0.00
0.00 0.00
From the PIXE spectra, concentration time series
have been obtained, both in the fine and coarse stage
covering 12 days in Florence (2-14 July) and a week
(2-10 July) in Milan. The bar diagram of Figure 3
shows the average elemental concentrations of the
elements which exceeded their MDLs more than the
10% of the cases.
FIGURE 4. Fine stage Milan Sulphur concentration (µg/m3,
continuous) and wind speed (m/s, dashed), measured in the
same site.
Pb and Br are highly correlated (Florence R = 0.82,
Milan R = 0.81); these elements are mainly produced
in urban Italian environments by leaded fuel
combustion. Because of the increasing use of unleaded
fuels, the concentration of these elements drastically
dropped since the beginning of the 90’s, but the high
sensitivity of PIXE analysis makes it possible to still
detect them in short measurement times. Figure 5
shows the Pb and Br concentration trends in Florence:
the patterns exhibit correlated daily variations, with
peaks in the morning traffic rush hours (around 8:30
a.m.). Only on Sunday (8th of July) morning the traffic
peak is absent (otherwise it can be noted a peak during
the Saturday-Sunday night). In Florence the Br/Pb
ratio is 0.24, similar to those reported in literature for
the traffic source profile in urban areas [8] and to the
values found in our previous works [2, 3, 9, 10]; in
Milan a bit lower value (about 0.21) could confirm the
presence of different sources of Pb [11].
10000
Florence fine stage
Milan fine stage
Florence coarse stage
Milan coarse stage
1000
100
10
1
Na M g Al
Si
S
Cl
K
Ca Ti Cr M n Fe Ni Cu Zn Br
Sr
Pb
FIGURE 3. Mean concentrations (ng/m3) obtained by PIXE
measurements. The values are calculated considering only
the cases when element is above its MDL.
450
3
0.16
0.05
Pb
Br
0.04
0.12
0.03
0.08
0.02
0.04
0.01
0.00
7/2/01 7/3/01
0.00
0.00
7/4/01 7/5/01 7/6/01 7/7/01
0.00
0.00
0.00
0.00
Br concentration (µg/m3)
Pb concentration ( µg/m )
0.20
0.00
7/8/01 7/9/01 7/10/01 7/11/01 7/12/01 7/13/01 7/14/01
0.00
0.00
0.00
0.00
0.00
0.00
0.00
FIGURE 5. Fine stage Lead and Bromine concentrations in Florence; the grid lines correspond to the midnights.
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CONCLUSIONS
4. Maxwell, J. A., Teesdale, W. J., and Campbell, J. L.,
Nucl. Inst. and Meth. B95, 407 (1995).
The first results obtained by the analysis of time
series give indication of urban pollution processes that
are quite understood from previous works. The
analysis of the data relative to Genoa and Naples, as
well as a statistical study, including standard and
Absolute Principal Component Factor Analysis, is in
progress.
5. Calastrini, F., Del Carmine, P., Lucarelli, F., Mandò, P.
A., Prati, P., and Zucchiatti, A., Nucl. Inst. and Meth.
B136, 975 (1998).
6. Boni, C., Caridi, A., Cereda, E., and Marcazzan, G.,
Nucl. Inst. and Meth. B47, 1338 (1990).
7. Cahill, T. A., Aerosol Collection and Compositional
Analysis for Improve, NPS Annual Report (July 1994June 1995), Department of Physics, University of
California, Davis, 1995, pp.1-46.
ACKNOWLEDGEMENTS
We thank the local authorities of Florence, Genoa,
Milan and Naples for the permission to use structures
and data, and for their valuable assistance.
8. Thurston, G. D., and Spengler, J. D., Atmosph. Envir. 19
n.1, 9 (1985).
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Zucchiatti, A., Nucl. Inst. and Meth. B161, 819 (2000).
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Prati, P., and Zucchiatti, A., Nucl. Inst. and Meth. B150,
450 (1999).
REFERENCES
1. PIXE International Corp., P.O. Box 7744, Tallahassee,
FL 32316, USA.
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