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Measurements of Size-Segregated
Emission Particles by a Sampling
System Based on the Cascade
Impactor
JANJA TURŠI ˇ
C,
†
IRENA GRGI ´
C,*
,†
AXEL BERNER,
‡
JAROSLAV ŠKANTAR,
§
AND IGOR ˇ
CUHALEV
§
Laboratory for Analytical Chemistry, National Institute of
Chemistry, Slovenia, Hajdrihova 19, SI-1000 Ljubljana, Slovenia,
Faculty of Physics, University of Vienna, Boltzmanngasse 5,
A-1090 Vienna, Austria, and Environmental Department,
Electroinstitute Milan Vidmar, Hajdrihova 2, SI-1000 Ljubljana,
Slovenia
Received May 10, 2007. Revised manuscript received October
22, 2007. Accepted November 09, 2007.
A special sampling system for measurements of size-
segregated particles directly at the source of emission was
designed and constructed. The central part of this system is a low-
pressure cascade impactor with 10 collection stages for the
size ranges between 15 nm and 16 µm. Its capability and suitability
was proven by sampling particles at the stack (100 °C) of a coal-
fired power station in Slovenia. These measurements showed
very reasonable results in comparison with a commercial cascade
impactor for PM10 and PM2.5 and with a plane device for
total suspended particulate matter (TSP). The best agreement
with the measurements made by a commercial impactor
was found for concentrations of TSP above 10 mg m-3, i.e.,
the average PM2.5/PM10 ratios obtained by a commercial impactor
and by our impactor were 0.78 and 0.80, respectively. Analysis
of selected elements in size-segregated emission particles
additionally confirmed the suitability of our system. The
measurements showed that the mass size distributions were
generally bimodal, with the most pronounced mass peak in the
1–2 µm size range. The first results of elemental mass size
distributions showed some distinctive differences in comparison
to the most common ambient anthropogenic sources (i.e.,
traffic emissions). For example, trace elements, like Pb, Cd,
As, and V, typically related to traffic emissions, are usually more
abundant in particles less than 1 µm in size, whereas in our
specific case they were found at about 2 µm. Thus, these mass
size distributions can be used as a signature of this source.
Simultaneous measurements of size-segregated particles at
the source and in the surrounding environment can therefore
significantly increase the sensitivity of the contribution of a
specific source to the actual ambient concentrations.
Introduction
One of the more pernicious problems in air quality is the
persistence of fine suspended particulate matter. Until
recently, the total mass concentration of airborne particulate
matter (TSP) was the only standard for particulates used for
air quality assessment. New developments in aerosol mea-
surement techniques enabled size-dependent analyses of
aerosol particles, thus better classifications and more com-
prehensive studies of the effect of ambient aerosols are
feasible. Exposure to elevated concentrations of respirable
ambient aerosol particles has been associated with various
health problems (1, 2). Particle size and shape are key factors
controlling the extent of particle penetration into the human
respiratory tract. In addition, the potential health effects
depend on many other factors, such as chemical and physical
characteristics of aerosols, the amount of toxic substances,
and their solubility in biological fluids, etc. (3, 4). As a result
of numerous studies on the health effects and epidemiological
investigations, the regulations have focused on controlling
PM10, which refers to the mass concentration of inhalable
particles with an aerodynamic diameter less than 10 µm and
PM2.5, which refers to the alveolar size fraction with a diameter
less than 2.5 µm(5, 6).
Among the combustion sources that have the most
significant relative contribution to air pollution are power
and industrial plants, with coal the most commonly used
fuel for commercial power generation due to its relatively
abundant reserves (7). The emission of particles is a complex
function of fuel type and quality, combustion technology,
type and size of facility, and control technology, etc. (8, 9).
Despite the progress in emission reduction technology, which
has greatly decreased the emission of TSP (e.g., particulate
controls in power stations have high efficiency rates),
considerable amounts of particles are released into the
environment. This fact is true particularly for the big power
stations with high coal consumption (10).
Therefore, information on the particulate matter emission
from combustion sources is important for a number of
reasons, such as examination of source status according to
regulations, generation of emissions inventories, prediction
of ambient air quality in the areas affected by the source as
well as source apportionment and exposure assessment for
the affected human population and ecological systems.
Formerly, emissions at the source have been monitored by
measuring TSP, e.g. using plane filter devices according to
the methods by the Environmental Protection Agency (EPA)
or VDI (11, 12). Whereas for the determination of mass size
distribution of particles at the source, measurements were
performed by cascade impactors or cascade cyclones (13, 14).
Measurements of emissions typically involve much more
complicated sampling approaches, such as source dilution
and isokinetic conditions (9, 15, 16). Recently, for more precise
measurements of PMx, a new PM10/PM2.5 cascade impactor
(GMU-impactor Johnas) for in-stack measurements was
developed (17, 18).
The objectives of this study were to design and construct
a special sampling system for measurements of size-
segregated particles in the size range from 15 nm to 16 µm
directly at the source of emission. To prove the feasibility
and capability of this system, it was used for size-segregated
sampling of particles at the stack of a coal-fired power station
in Slovenia, and the first results of mass and chemical size
distribution of emission particles are presented. In addition,
for checking the accuracy of our sampling system the
measurements were performed simultaneously with the
commercial impactor (GMU-impactor Johnas). Since size
segregation of some typical elements depends on the source,
an insight into their size distributions at the source and
comparison with the surrounding ambient concentrations
* Corresponding author phone: +386 1 476 0200; fax: +386 1 476
0300; e-mail: irena.grgic@ki.si.
†
National Institute of Chemistry, Slovenia.
‡
University of Vienna.
§
Electroinstitute Milan Vidmar.
Environ. Sci. Technol. 2008, 42, 878–883
878 9ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 3, 2008 10.1021/es071094g CCC: $40.75 2008 American Chemical Society
Published on Web 12/19/2007
and size distributions can significantly improve our knowl-
edge on the contribution of the individual source to the actual
ambient air pollution by particulate matter.
Experimental Section
Sampling Device. The measurements were carried out by a
specially constructed sampling system illustrated in Figure
1. The central part of our system is a Berner low-pressure
cascade impactor (HAUKE, LPI 25/0,015/2) with 10 collection
stages for the nominal size ranges expressed in Dae (aero-
dynamic equivalent diameter): 0.015–0.03 µm; 0.03–0.06 µm;
0.06–0.125 µm; 0.125–0.25 µm; 0.25–0.5 µm; 0.5–1.0 µm;
1.0–2.0 µm; 2.0–4.0 µm; 4.0–8.0 µm; 8.0–16.0 µm. The sampling
probe consists of two tubes of different diameters. The central
one is connected to the impactor inlet on one side, and on
the other side it is opened to the flue gas inside the stack.
The system for controlling and measuring the temperature
inside the impactor enables the impactor to be heated to the
temperature of the flue gas and also enables the desired
temperature to be maintained during the sampling. In the
case of undiluted sampling, the filter holder with a filter for
removing particles from ambient air, the device for delivering
constant volumetric air flow, and the compressor unit were
not used, and the inlet to the outer tube was blocked. The
nominal flow rate through this impactor is 25.8 L min-1.
Because of technical problems (i.e., small sampling port and
mechanical instability of the sampling probe because of the
rather bulky impactor), the impactor was positioned outside
the chimney stack, while a sampling probe was placed into
the chimney. Before the flue gas enters the stack it is washed
in a gas scrubber in order to remove a considerable part of
the SO2formed during the combustion of the coal. Therefore,
the flue gas contains a lot of water vapor at around 100 °C.
Thus, to prevent water condensation on the particles the
inside of the impactor was heated to the temperature of the
flue gas before sampling and kept constant during sampling.
As shown in Figure 1 the sampling probe was designed to
also allow dilution of the flue gas by compressed and filtered
ambient air. The flow rate of dilution air was controlled by
the device for delivering constant volumetric air flow.
According to selected conditions several dilution rates are
possible.
Emission Measurements. The measuring campaign was
performed in June 2005 and May 2006 at the stack of the
coal-fired power station in Slovenia. Sampling was conducted
from the 50 m platform on the stack using existing sample
ports, i.e., holes with an opening of 12 cm on the outer side
and 30 cm on the inner side of the stack and with a depth
of 50 cm. The sampling times were from 10 min to 1 h (without
dilution) and 5 h when dilution was introduced. The
impaction plates were covered with annular Tedlar foils, and
on top of these folded aluminum foils were placed. This
arrangement enabled the simultaneous determination of the
mass and chemical composition with one sampling only.
Emission measurements were also carried out with the
filter sampler for TSP (SICK, Gravimat SHC 502) and with the
cascade impactor for PM10 and PM2.5 (Paul Gothe Bochum,
GMU cascade impactor Johnas). Samples were taken in
parallel either by both the LPI and Johnas impactor or one
impactor and TSP sampler. Simultaneously, continuous
measurements of TSP were performed with a dust concen-
tration monitor (OMD41).
Mass and Chemical Analysis. Mass size distribution of
particles was determined gravimetrically using aluminum
foils, while for chemical analysis Tedlar foils were used (19).
On the basis of preliminary experiments done with certified
material (Reference Material 1648, Urban Particulate Matter)
and samples of TSP collected on quartz fiber filters at the
stack of the thermo-power plant, the extraction with con-
centrated HNO3was selected because of the lower detection
limits for some of the elements in comparison with other
extraction methods (i.e., HNO3/H2O2and HNO3/HF diges-
tion) and its simplicity. The applied extraction procedure
leads to the total recovery of Zn, As, Se, Cd, and Pb. For Mg,
V, Mn, and Ni, recoveries were in the range of 60 to 80%,
while for K and Co lower recoveries were obtained (ap-
proximately 50%). After extraction of aerosol deposits on
Tedlar foils as well as PM2.5,PM
10, and TSP samples in HNO3,
measurements of Mg, K, V, Mn, Co, Ni, Zn, Ga, As, Se, Sr, Mo,
Cd, and Pb were performed by inductively coupled plasma
mass spectrometry (ICP-MS, Agilent 7500ce system with
collision reaction cell). After extraction of deposits on
aluminum foils in Milli-Q water, SO42- was determined by
ion exchange chromatography, with a Dionex IonPac AS4A
separation column and a mixture of Na2CO3/NaHCO3as the
eluent (19).
Results and Discussion
Mass Concentration and Chemical Composition of Emis-
sion Particles. The results of particle mass concentrations
measured at the stack of the thermo-power plant in Slovenia
by different techniques are shown in Table 1. All the results
are expressed at normal conditions (273 K, 1013 mbar). During
the first sampling in 2005, measurements were performed in
parallel using one plane filter device (for TSP) and our
upgraded system with a 10-stage Berner impactor, while
measurements with the GMU-impactor Johnas were only
feasible immediately after sampling with our system. For
sampling during 2006, this technical problem was solved
allowing measurements with the GMU-impactor Johnas and
our impactor to be carried out simultaneously. Although the
energy production was practically the same during these
two sampling periods, there were differences regarding the
particle mass concentrations. Due to some improvements
in the operating technology and emission reduction tech-
nology, the concentrations were reduced by more than twice
in the second period. TSP concentrations in 2005 were about
25 mg m-3over the whole sampling period, while in 2006 the
concentrations were much lower. For particles collected by
the Berner impactor, the concentrations of PM10 and PM2.5
were determined graphically from the dependence of particle
cumulative mass on the aerodynamic diameter of particles.
From Table 1 it is evident that for concentrations of TSP
above 10 mg m-3our measurements are in good agreement
with those made by a commercial impactor, even though
they were performed in series. However, from Table 1 it is
obvious that particle concentrations vary for about 20%
during one working day. Measurements with the Johnas
FIGURE 1. Simplified scheme of the sampling system: 1, stack
wall; 2, sampling probe; 3, 10-stage low-pressure Berner impactor;
4, oven; 5, filter holder with filter for removing particles in ambient
air; 6, system for controlling and measuring the temperature inside
impactor; 7, vacuum pump; 8, device for delivering constant
volumetric air flow; 9, compressor unit.
VOL. 42, NO. 3, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9879
impactor showed the average ratio of 0.78 for PM2.5/PM10,
while with the Berner impactor the average ratio was 0.80.
Comparison of TSP obtained by Gravimat showed that the
third run (29.6.05: 16.15–16.25) deviated from the previous
two measurements. The difference of 35% may be attributed
to a potential larger emission of particles above 10 µm.
For the lower mass concentrations, the comparison is not as
good as that for higher ones but it is still adequate. Our system
gave about 30% lower values for PM10 and about 20% lower
values for PM2.5 in comparison with the Johnas impactor.
This is reasonable because the relative measuring errors
increase with lower masses. For the Johnas impactor, the
measuring uncertainty of the whole procedure is (2mgm
-3,
which represents nearly 30% of the PM10 value in the second
period. Similar uncertainty has been determined also for the
Berner impactor. In summary, the measurements with our
system gave reasonable results. However, to overcome the
limitations due to the low concentrations, longer term
sampling is necessary.
The suitability of our sampling system was additionally
proven on the basis of the chemical composition of the
particles emitted from the stack of the power plant. The
comparison of total average concentrations of some selected
elements in particles collected by Berner impactor, Johnas
impactor, and TSP sampler is shown in Table 2. It can be
seen that the concentrations of elements determined in
particles of the 15 nm to 16 µm size range in PM10 and TSP
are generally in good agreement. The best agreement was
found for V, As, Se, Sr, and Ga. In addition, concentrations
for Mg, K, and Ni determined in particles sampled by the
Berner impactor compare well to those determined in TSP.
The values for Mg, K, Co, Mo, Cd, and Pb are a bit lower and
for Mn and Ni a bit higher in comparison to PM10. However,
these results can be considered adequate and satisfactory,
especially because of measuring uncertainties and some
fluctuations in chemical composition of particles during the
plant operation. The variations in chemical composition can
also be seen in deviations from the average concentrations
for most of the elements in particles collected at different
times by the 10-stage impactor. Estimated reproducibilities
ranged from 7% (for Pb) to 40% (for Cd). Some of these
elements (e.g., Mn, V, As, Se, and Ga) can certainly be used
as markers of emissions from this particular power plant
(7, 8).
Mass and Chemical Size Distribution. The results of mass
size distribution of particles collected by our sampling system
at the stack of the thermo-power plant during sampling in
2005 and 2006 are shown in Figure 2a. An uncertainty of 8%
was determined for the size ranges between 60 nm and 4 µm
and 21% for the sizes below 60 nm and above 4 µm. The
distributions were generally bimodal, with the most pro-
nounced mass peak in the 1–2 µm size range. The second
mass peak was observed in the 0.06–0.25 µm size range.
Recent literature results suggest that coal fly ash particle
TABLE 1.Comparison of Particle Mass Concentrations (in mg m-3) Measured at the Stack of the Coal-Fired Power Station
in Slovenia by Different Techniques: Dust Concentration Monitor (OMD41), TSP Sampler (SICK, Gravimat SHC 502),
Emission Cascade Impactor (GMU-Cascade Impactor Johnas), and Upgraded 10-Stage Cascade Impactor (Berner Type)
sample OMD41 TSP Gravimat TSP
Johnas
impactor PM2.5
Johnas
impactor PM10
Berner
impactor PM2.5
Berner
impactor PM10
29.6.05 25.7 25.7 19.7 26.2
13.10–13.40
29.6.05 27.7 21.3 16.8 21.5
15.05–15.20
29.6.05 28.0 20.5 11.7 13.5
16.15–16.25
30.6.05 21.7 9.5 12.2
13.40–14.20
30.6.05 20.1 8.5 10.9
14.30–15.10
16.5.06 7.2 5.3 6.6 4.0 4.6
12.25–12.55
16.5.06 7.3 6.3 7.8 5.2 5.4
14.35–15.05
16.5.06 6.8 5.7 7.3 5.1 5.3
16.25–16.55
TABLE 2.Comparison of Chemical Composition of Particles Collected in 2005 at the Stack of the Coal-Fired Power Station
by Berner Impactor, Johnas Impactor, and TSP Sampler
element
Berner impactor
(15 nm to 16 µm)
Johnas
impactor PM10 Gravimat TSP
Mg [mg g-1] 6.5 (1.5 8.4 (0.2 6.4 (0.5
K [mg g-1] 5.7 (1.6 7.6 (0.9 6.1 (0.8
V [mg g-1] 0.16 (0.04 0.17 (0.01 0.14 (0.01
Mn [mg g-1] 0.46 (0.13 0.33 (0.01 0.34 (0.06
Zn [mg g-1] 0.42 (0.27 0.46 (0.05 0.52 (0.13
As [mg g-1] 0.16 (0.04 0.16 (0.01 015 (0.02
Se [mg g-1] 0.24 (0.05 0.23 (0.01 0.20 (0.03
Sr [mg g-1] 0.15 (0.03 0.16 (0.01 015 (0.01
Ni [µgg
-1]44(12 25 (440(15
Ga [µgg
-1]30(18 27 (328(5
Mo [µgg
-1]65(14 76 (11 74 (9
Pb [µgg
-1]46(456(45 59 (16
Co [µgg
-1] 4.1 (1.0 6.1 (0.8 5.4 (0.4
Cd [µgg
-1] 1.9 (0.8 2.7 (0.7 2.9 (0.4
880 9ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 3, 2008
formation is described as a trimodal particle size distribution
that includes a submicrometer region at about 0.08 µm, a
fine region centered at approximately 2.0 µm, and a bulk or
supermicrometer region for particles at about 5 µm and
greater (20, 21). Since the stack gases were cleaned prior to
emission (gas scrubber and filters) supermicrometer particles
were considerably reduced, thus making the third mode less
evident. The comparison between the prolonged sampling
where dilution with clean air was introduced and sampling
with no dilution is presented in Figure 2b. It is evident that
in the case of dilution the mass size distribution was rather
unimodal. On the basis of the chemical composition of the
particles it was established that the reason for this discrepancy
lies in the losses of SO42-(as H2SO4). The largest losses (nearly
30 times) were determined for sizes below 0.125 µm. For the
0.125–0.50 µm size range the concentration of SO42-was about
400 µgm
-3when no dilution was introduced, while with
dilution the concentration was only about 50 µgm
-3. For
sizes above 0.5 µm the measured concentrations with dilution
were 3 times lower. But on the other hand, no losses of
elements in emission particles collected with dilution were
observed (e.g., the differences for Ga, Se, Pb, and V in all size
ranges were in the range of 20%, which can be attributed to
measurement uncertainty of the whole procedure). This
means that sampling with dilution is suitable for the mass
size distribution of important elements. Dilution however
reduces the formation of small particles of sulfuric acid. In
addition, insight into the molar ratios between SO42-, Ca,
and Mg (using sampling with and without dilution), showed
that the excess of SO42-in comparison to the sum of Ca and
Mg was most probably evaporated as H2SO4. When flue gases
are released from the stack it is believed that evaporation
and/or recondensation of H2SO4takes place, which changes
its distribution over different particle size ranges compared
to the situation in the stack.
Characteristic mass size distributions (for four different
concentration ranges) for some chemical species determined
in emission particles collected using the system without
dilution are presented in Figure 3. It is evident that SO42-
exhibits a typical bimodal size distribution, with modes at
FIGURE 2. Mass size distribution of particles collected at the
coal-fired power plant during sampling in 2005 and 2006 (a) and
comparison between sampling with and without dilution (b).
Concentrations are expressed at normal conditions (273 K, 1013
mbar).
FIGURE 3. Mass size distributions of SO42-(a), Mg and K (b), Zn, V, As, and Se (c), and Pb, Cd, and Ga (d) in particles emitted from
the thermo-plant.
VOL. 42, NO. 3, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9881
around 0.25 µm and 2 µm. For Mg and K (Figure 3b), V and
As (Figure 3b), as well as for Pb and Cd (Figure 3d), a unimodal
size distribution with a mode at around 2 µm was determined.
For Zn a unimodal size distribution was also found, but with
a peak near 1 µm. For Se and Ga two modes were identified,
one near 0.25 µm and other at around 2 µm. The mass size
distributions of chemical species provide important infor-
mation, since some elements have a characteristic mode in
a limited size range and can be used as markers for the
identification of a particular source (i.e., source signature)
and further for tracing the source contribution at locations
of interest, at various distances from the source. Results from
the Johnas impactor only give distributions between the
coarse and fine mode with a division at 2.5 µm. So, for the
components which have a peak maximum below 2 µm (see
Figure 3), the important information is lost. For example, in
the case of Zn (Figure 3c), for which measurements with our
system gave a maximum mass concentration at about 1 µm,
results from the Johnas impactor only gave limited informa-
tion, as 75% of Zn is present in the range below 2.5 µm. We
also want to point out that we are aware of the different
techniques applied in flue gas analysis, especially the
implication of cascade impactors for the measurement of
dust in flowing gases (22). However, the ANDERSEN M III
impactor has some limitations, i.e., particles of diameter lower
than 0.41 µm are collected on a back-up filter; therefore,
information on size-segregated composition below this size
is lost. Thus, in our case we would not have seen the ultrafine
mode of the combustion aerosols and the behavior of the
sulfate aerosol under dilution. In addition, the Berner LPI
impactor facilitates a split foil technique allowing the
simultaneous collection of aerosols on aluminum foil for
gravimetric analysis and on chemically inert Tedlar foil for
chemical analysis. This therefore removes any uncertainties
introduced by subsequent sampling by different instruments
at different conditions of the flue gas.
In the past few years, many studies on the size distribution
of elements in ambient aerosol particles from very diverse
environments have been done. For example, on the basis of
characteristic size distributions of trace metals in atmospheric
aerosols at background sites in England three main behavioral
types were identified: metals (Cd, Sn, Pb, and Se) whose mass
resisted mainly within the accumulation mode, metals (Ni,
Zn, Cu, Co, Mn, and Hg) that were distributed between fine,
intermediate, and coarse modes, and metals (Fe, Sr and Ba)
that were mainly found within coarse particles (23). For four
typical urban sites in Budapest, Hungary, it was shown that
typical coarse-mode elements (e.g., Na, Mg, Al, Si, Ca, Ti, Fe,
Ga, Sr, Mo, and Ba) exhibited unimodal size distributions,
while elements typically related to high-temperature or
anthropogenic sources (e.g., S, K, V, Cr, Mn, Ni, Cu, Zn, Ge,
As, Se, and Pb) either had a unimodal mass size distribution
with most of their mass in the fine size fraction or showed
a clear bimodal size distribution (24). For the two sites within
the Los Angeles Basin, crustal metals (e.g., Al, Si, K, Ca, Fe,
and Ti) were predominantly present in the supermicrometer
particles (25). Potentially toxic metals, which were mainly
partitioned in the submicrometer particles, have been traced
to vehicular emissions (Pb, Sn, and Ba) and to emissions
from power plants and oil refineries (Ni and Cr). Anyhow,
all these results showed that the measured size distributions
of elements in ambient aerosol particles are the result of a
combination of different processes including local anthro-
pogenic and natural sources as well as long-range transport
and resuspension and depend on meteorological conditions,
of which wind speed and direction are the most important.
It is evident that our results on elemental mass size
distributions showed some distinctive differences with results
obtained for atmospheric aerosol particles (23–25). For
example, trace elements, like Pb, Cd, As, and V, typically
related to other high-temperature sources, especially to traffic
emissions, are usually more abundant in the particle size
range below 1 µm, whereas in our specific case these elements
were found above 1 µm. So, to establish the influence of a
specific emission source to the actual ambient aerosol
concentrations, elemental mass size distributions can cer-
tainly be a very useful tool. This is particularly true in areas
relatively close to the source.
In general, for a good understanding of the contribution
from different emission sources adequate source fingerprints
through direct measurements of size-segregated emission
particles are required as well as measurements of size-
segregated ambient aerosol concentrations. Ambient air is
a complex mixture of pollutants emitted from numerous
diverse sources and undergoes continuous changes, but
source signatures may show narrower distributions of some
key components. By use of this approach, the contribution
of a particular source can be more precisely estimated. In
this work we showed that size-segregated sampling followed
by elemental analysis can be used to define a source signature.
Simultaneous measurements of size-segregated particles at
the source and in the surrounding environment can therefore
significantly improve the insight into the contribution of a
specific source to the actual ambient concentrations.
Acknowledgments
This work was supported by the Slovenian Research Agency
(Contracts L1-6100-0104 and P1-0034-0104) and Slovenia’s
thermo-power plant. We thank Dr. M. Kovacˇevicˇ and Dr. B.
Budicˇ from the National Institute of Chemistry, Slovenia, for
performing ICP-MS analysis.
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