Iron, manganese and copper emitted by cargo and

ARTICLE IN PRESS
Atmospheric Environment 41 (2007) 878–889
www.elsevier.com/locate/atmosenv
Iron, manganese and copper emitted by cargo and passenger
trains in Zürich (Switzerland): Size-segregated mass
concentrations in ambient air
Nicolas Bukowieckia,b,, Robert Gehrigb, Matthias Hillb, Peter Lienemannb,
Christoph N. Zwickyb, Brigitte Buchmannb, Ernest Weingartnera,
Urs Baltenspergera
a
Laboratory of Atmospheric Chemistry, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland
b
Empa, Materials Science and Technology, 8600 Dübendorf, Switzerland
Received 10 October 2005; received in revised form 24 July 2006; accepted 28 July 2006
Abstract
Particle emissions caused by railway traffic have hardly been investigated in the past, due to their obviously minor
influence on air quality compared to automotive traffic. In this study, emissions related to particle abrasion from wheels
and tracks were investigated next to a busy railway line in Zürich (Switzerland), where trains run nearly exclusively with
electrical locomotives. Hourly size-segregated aerosol samples (0.1–1, 1–2.5 and 2.5–10 mm) were collected with a rotating
drum impactor (RDI) and subsequently analyzed by synchrotron radiation X-ray fluorescence spectrometry (SR-XRF). In
this way, hourly elemental mass concentrations were obtained for chromium, manganese, iron and copper, which are the
elements most relevant for railway abrasion. Additionally, daily aerosol filters were collected at the same site as well as at a
background site for subsequent analysis by gravimetry and wavelength dispersive XRF (WD-XRF). Railway related
ambient air concentrations of iron and manganese were calculated for the coarse (2.5–10 mm) and fine (o2.5 mm) particle
fraction by means of a Mn/Fe ratio investigation. The comparison to train type and frequency data showed that 75% and
60% of the iron and manganese mass concentrations related to cargo and passenger trains, respectively, were found in the
coarse mode. The railway related iron mass concentration normalized by the train frequency ranges between 10 and
100 ng m3 h iron in 10 m distance to the tracks, depending on train type. It is estimated that the personal exposure next to
a busy railway line above ground is more than a magnitude lower than inside a subway station.
r 2006 Elsevier Ltd. All rights reserved.
Keywords: Railway; Aerosol; Emissions; Abrasion; Trace metals; Iron
1. Introduction
Corresponding author. Current address: Empa, Materials
Science and Technology, 8600 Dübendorf, Switzerland.
E-mail address: [email protected]
(N. Bukowiecki).
1352-2310/$ - see front matter r 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.atmosenv.2006.07.045
In the last decades it has been widely recognized
that particulate air pollution implies a broad variety
of adverse health effects. Imposed by the steadily
increased need for mobility in modern society,
automotive traffic has become one of the major
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N. Bukowiecki et al. / Atmospheric Environment 41 (2007) 878–889
anthropogenic emitters of particulate air pollution.
Conclusively, the investigation of aerosol and gas
phase emissions of automotive traffic has become
and is still an important issue in current atmospheric chemistry and physics. In many countries,
public transportation by railway systems is promoted especially in urban areas, to reduce the use of
individual vehicles. Compared to light duty vehicles,
trains have obviously negligible aerosol emissions
per passenger and km. As trains run nearly
exclusively with electrical locomotives in Switzerland, the only direct particulate emissions of railway
traffic occur by different forms of material abrasion,
i.e. from tracks, wheels, brakes and the overhead
traction line. Due to the less urgent need for
research compared to automotive traffic, railway
emissions and their contribution to ambient PM
(particulate matter) have hardly been investigated in
the past, despite the dense network of railway traffic
in many European countries. Most of the few peerreviewed studies related to railway traffic focus
either on in-train exposure to air pollutants or on
measurements in subway systems, since these are the
most obvious issues of public concern. In-train
exposure has e.g. been investigated in France
(particle number size distributions in smoker
coaches, Abadie et al., 2004), in the US (EC/OC
measurements in diesel locomotive cabs, Liukonen
et al., 2002) and Switzerland (particle bound
polycyclic aromatic hydrocarbons (PAHs) in passenger trains, Leutwyler et al., 2002). Particulate
pollution in subway systems has recently been
investigated in Helsinki (PM2.5, trace elements,
Aarnio et al., 2005), Stockholm (PM10, PM2.5
measurements, Johansson and Johansson, 2003),
Italy (Ripanucci et al., 2006), Hong Kong (PM10,
PM2.5 measurements, Chan et al., 2002), Tokyo
(trace elements, Furuya et al., 2001), Washington
DC (Birenzvige et al., 2003) and New York City
(Cr, Mn and Fe measurements in PM2.5, Chillrud
et al., 2004, 2005). The latter study showed that
frequent subway users were exposed to significantly
higher steel abrasion emissions than a control group
not using the subway. PM2.5 in the London
underground was investigated in recent studies
(Seaton et al., 2005, Pfeifer et al., 1999), where an
iron oxide contribution of 70% to the measured
PM2.5 concentrations was found. Karlsson et al.
(2005) suggest that the iron particles found in
subway systems are present mainly in form of
magnetite (Fe3O4). Engineering literature specifically related to railway abrasion processes is scarce
879
and mainly limited to studies investigating material
damage (see e.g. Grieve et al., 2001).
Switzerland has a very high train density, both in
terms of regions served and daily train frequencies.
There exist over 5000 km of regularly frequented
tracks. Trains run nearly exclusively with electrical
locomotives, emissions from the small fraction of
diesel locomotives are thus negligible (Gehrig et al.,
2002). Beside passenger trains, there is also a
significant portion of cargo train traffic. Whereas
railway lines do usually not run through densely
populated areas outside of urban areas, there are
many urban residential areas that are in close
vicinity to frequented railway lines. Taking these
facts into account and to complete the national PM
emission inventory, an extensive investigation of the
PM emissions caused by railway abrasion processes
and their contribution to ambient PM was performed in Zürich (Switzerland) in 2003/2004. The
mass contribution to overall ambient PM10 is
discussed in detail by Gehrig et al. (2006). This
paper investigates the coarse and fine mode particles
emitted by abrasion from railway tracks, wheels and
the overhead traction line, with focus on iron,
manganese and copper. It compares the measured
elemental concentrations to the results found in
subway studies, by normalization of the railway
related mass concentrations with the hourly train
frequencies.
2. Study design and data analysis
Ambient aerosol was characterized in ZürichJuchhof, a central industrial area of Zürich, employing a measuring site which was located in the
immediate vicinity (10 m) of a major railway line.
The selected railway stretch encounters the highest
average train frequencies of entire Switzerland.
Train frequency data were obtained from the Swiss
railway authorities and are shown in Fig. 1. There
were totally over 600 trains per weekday, consisting
of 75% passenger trains and 25% cargo trains.
During weekends, the total number of trains
dropped to less than 500, consisting entirely of
passenger trains. Cargo trains differ from passenger
trains mainly by their increased length and by older
locomotives and wagon types (most of them
equipped with cast iron brake types). Besides the
railway line, the sampling location was in close
distance of local industrial activity, local traffic and
was also influenced by the total urban air mass.
Wind measurements, taken at the inlet position,
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N. Bukowiecki et al. / Atmospheric Environment 41 (2007) 878–889
880
trains per hour
40
30
20
10
0
ng m-3
20
16
12
8
4
0
ng m-3
24
Cr
coarse
intermediate
submicron
Mn
coarse
18
12
6
intermediate
submicron
ng m-3
0
coarse
1500
1000
intermediate
500
0
80
ng m-3
Fe
2000
submicron
Cu
coarse
60
40
20
0
intermediate
submicron
00:00
03:00
06:00
09:00
12:00
15:00
18:00
21:00
24:00
time of day
Fig. 1. Top panel: diurnal train frequencies at Zürich-Juchhof (Switzerland) for cargo and passenger trains in winter 2003/2004. Due to
time table synchronization the frequencies were highly constant. Lower panels: average diurnal variation of the size-segregated mass
concentrations of chromium, manganese, iron and copper measured in ambient air at Zürich-Juchhof in winter 2003/2004 (47 days). Error
bars represent standard error of the mean. Although these elements are main components of abrasion particles emitted by the railway line
close by, the diurnal variations were dominated by, atmospheric dilution rather than by the frequencies of the trains running by.
showed that the location was not directly influenced
by the turbulence of passing trains.
Hourly aerosol samples were collected in ZürichJuchhof in winter 2003/2004 (47 days) in three size
ranges (2.5–10, 1–2.5 and 0.1–1 mm), deploying a
rotating drum impactor (RDI). The instrument inlet
was located 3.5 m above ground level, 1 m above the
roof of the measuring container. The collected RDI
samples were analyzed at the Hamburger Synchrotronstrahlungslabor (HASYLAB/DESY Hamburg,
Germany, beamline L) using synchrotron X-ray
fluorescence spectrometry (SR-XRF). Within the
available spectral energy range, 10 trace elements
were detected in each of the three particle size
ranges (S, Cl, Ca, Cr, Mn, Fe, Cu, Zn, Br, Pb). The
instrumental detection limit was below 50 pg m3
for most of these elements, with a concentration
uncertainty of 10% on average. A detailed description of the RDI sampling and the SR-XRF analysis
is given elsewhere (Bukowiecki et al., 2005).
Additionally, daily PM10 HiVol (high volume)
filter samples were collected at the same site and
analyzed both gravimetrically and with laboratory
based WD-XRF (wavelength dispersive X-ray
fluorescence spectrometry, Gehrig et al., 2006).
These measurements were used for the validation
of the RDI-SR-XRF data and showed a good
agreement between the two methods (Bukowiecki
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N. Bukowiecki et al. / Atmospheric Environment 41 (2007) 878–889
et al., 2005). Further daily HiVol-WDXRF measurements were also performed at Zürich-Kasernenhof, an urban background site. This site is
located in a courtyard park in the downtown area of
Zürich (in approx. 5 km distance of Zürich-Juchhof)
and has been extensively characterized by a number
of previous air quality studies (Fisseha et al., 2006;
Gehrig et al., 2004; Hueglin, 2000; Hueglin et al.,
2005; Szidat et al., 2004). Additionally, it has
served as long-term air pollution monitoring site
of the Swiss monitoring network NABEL since 20
years (BUWAL, 2003). It is not directly influenced
by any fresh contributions of close pollution
sources, with exception of episodes with increased
anthropogenic activity (e.g. social events). This was
not the case during the time period considered in
this study.
Gehrig et al. (2006) showed a one-year average
mass balance for the railway related mass concentrations by orthogonal distance measurements at
the same sampling site. They have found iron,
manganese and copper to be the main railway
contributors. Railroad steel abrasion (Cr, Mn, Fe)
and abrasion from the overhead traction line (Cu)
were mentioned to be the two main source processes
involved in railway abrasion. Table 1 lists the
average concentrations for chromium, manganese,
iron and copper measured by SR-XRF and WDXRF at the railway site Zürich-Juchhof. From all
detected elements related to railway abrasion, iron
clearly dominated the mass concentrations, followed by minor amounts of copper and manganese
and very low amounts of chromium. Fig. 1 shows
average size-resolved diurnal variations for iron,
881
manganese, copper and chromium. For all four
elements the RDI-SR-XRF measurements at the
railway site showed highest concentrations in the
largest size fraction. The diurnal patterns are
strongly influenced by atmospheric dilution of the
ground-near mixing layer in the afternoon. Thus,
there is no obvious correlation to the train
frequencies.
Based on limited resources, hourly trace element
mass concentration measurements were only performed at the railway site. Furthermore, neither at
the railway nor the background site hourly PM10
data were collected. Thus, the identification of
hourly railway abrasion emissions is not straightforward and requires a special approach as described in
this manuscript. Due to the same reason it was not
possible to calculate trace levels expressed in ppm
(mg g1). An alternative possibility to extract railway related sources from the measured elemental
ambient concentrations is the use of source apportionment techniques. This is currently done by
Sunder Raman et al. (2006) using positive matrix
factorization (PMF) adapted to size-segregated
data. However, the limited number of input parameters that can be used (10 trace elements) as well
as the missing mass balance makes a meaningful
PMF more difficult.
The following nomenclature is used throughout
the entire paper for the individual particle size
fractions within PM10. Submicron: 0.1–1 mm (bottom impactor stage), intermediate: 1–2.5 mm (intermediate impactor stage) and coarse: 2.5–10 mm (top
impactor stage). In agreement with widely used
terminology, fine is defined as the sum of submicron
Table 1
Mass concentrations of railway related trace elements measured in Zürich (Switzerland) next to a busy railway line (10 m) and at an urban
background site in the time period 25.11.2003–31.01.2004 (47 days)
Sampling site
Railway site
Railway site Background Difference ¼ railway contribution
Aerosol collection RDI
XRF analysis
SR-XRF
Time resolution
Hourly
Size range
3
Cr (ng m )
Mn (ng m3)
Fe (ng m3)
Cu (ng m3)
HiVol
WD-XRF
Daily
HiVol
WD-XRF
Daily
HiVol
WD-XRF
Daily
Coarse
Intermediate Submicron o10 mm o10 mm
o10 mm
o10 mm
11.0
18.4
1497
61.5
4.78
6.51
495
25.2
3.6
9.95
748
26.0
4.3
12.0
1250
63.1
0.864
1.42
89.4
4.54
16.6
26.3
2081
91.2
7.9
21.5
1998
89.1
RDI: rotating drum impactor; size ranges: coarse (2.5–10 mm), intermediate (1–2.5 mm), submicron (0.1–1 mm). SR-XRF: energy dispersive
synchrotron-XRF, WD-XRF: wavelength dispersive laboratory XRF, HiVol: high volume sampler.
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N. Bukowiecki et al. / Atmospheric Environment 41 (2007) 878–889
and intermediate and represents the complete size
range below 2.5 mm.
3. Calculating hourly railway related elemental mass
concentrations of iron and manganese
The hourly measured mass concentrations of the
elements relevant for railway abrasion (Cr, Mn, Fe,
Cu) cannot be directly linked to individual trains,
since they also include the urban background
contribution. Additionally, the temporal evolution
of the elemental concentrations is strongly influenced by the atmospheric dilution in the groundnear mixing layer, both on a daily and hourly base
(Gehrig et al., 2006; Bukowiecki et al., 2005, see
Fig. 1). In this section we show that by an analysis
of elemental concentration ratios, the hourly
elemental background concentrations can be obtained without direct hourly background measurements. Basically, the railroad steel related Mn
and Fe mass concentrations can be derived from
the total elemental mass concentrations using
Eqs. (3.1)–(3.5), under the condition that the
Mn/Fe mass ratios of the railroad source (rrailway)
and the background (rbackground) are constant:
Mnbackground =Febackground ¼ rbackground ,
(3.1)
Mnrailway =Ferailway ¼ rrailway ,
(3.2)
Fetotal ¼ Febackground þ Ferailway ,
(3.3)
Mntotal ¼ Mnbackground þMnrailway .
(3.4)
Solving this equation system (four equation, four
unknowns) yields
Ferailway ¼
Mntotal rbackground Fetotal
.
rrailway ð1 rbackground =rrailway Þ
(3.5)
To check the applicability of the measured data for
the above model, the hourly and daily Mn/Fe ratios
were analyzed in more detail. Ratio analysis has
been suggested to trace back to possible sources of
trace elements in ambient air (Chillrud et al., 2004).
A good linear fit between the mass concentrations
for a pair of elements indicates that the two
elements originate from a dominant source with
constant composition. The slope represents the
elemental ratio of this source. Since both axes infer
uncertainties, an orthogonal regression model is
applied to get correct slope values (Brown, 1982).
Table 2 lists the main regression parameters
(Correlation coefficient r2, slope and intercept)
obtained by linear fitting of the iron and manganese
mass concentrations both for the hourly RDI-SRXRF and daily HiVol-WD-XRF measurements.
Additionally, average elemental ratios are shown
for the railway and the background site, representing the arithmetic mean of the HiVol-WD-XRF
measurements over the entire 47-day sampling
period. The basic correlation between the elements
is given by the large influence of meteorology
(Gehrig et al., 2006) For the daily values it is seen
that only the Mn vs. Fe fit results in a correlation
coefficient well above 0.8. Not surprisingly, the 47day average Mn/Fe ratio (0.011) agrees well with
the slope of the daily linear fit (0.010). For Cu/Fe
and Ca/Fe, there is neither a good value agreement
nor a high correlation coefficient (o0.8), giving a
first indication that the emissions of these two
elements are not dominated by the same source
process. To take into account the rapid temporal
dynamics of anthropogenic pollution sources, it
makes more sense to use the hourly mass concentration values in the different size fractions for a
refined ratio analysis. Cr/Mn and Mn/Fe slopes
show a high degree of correlation for all stages (r2Cr/
2
Mn40.85, rMn/Fe40.97). For coarse mode iron and
manganese, where most of the railway abrasion
particles are expected, the r2 value for the linear fit is
maximal (0.996). Fig. 2 shows the average diurnal
variation for the Mn/Fe ratio (coarse and submicron mode). The submicron mode Mn/Fe ratio
shows a slight diurnal pattern, which oscillates
between the reference ratio for the crustal background during nighttime and a very general
literature value for gasoline fuel during rush hour
time (Falbe and Regitz, 2006; Vouk and Piver,
1983). This observation seems reasonable, since
fresh urban submicron aerosols mainly originate
from combustion sources. The most important fact
for our analysis however is that the coarse mode
Mn/Fe ratio (0.0126) shows no diurnal pattern and
lies between the ratio for railroad steel (0.008,
ThyssenKrupp GfT Gleistechnik GmbH, 2006) and
the urban background (0.013, Table 2). It means
that the measured Mn and Fe mass concentrations
in this size range are dominated by railroad steel
abrasion as a source with constant composition and
by an equally constant background. Thus, the
boundary conditions to use Eq. (3.5) are met.
Before applying the equation to the hourly RDISR-XRF data it was validated using the daily HiVolWD-XRF measurements. For the latter, the arithmetic mean Mn/Fe ratios were 0.009 for the railway
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883
Table 2
Elemental mass ratios measured in Zürich (Switzerland) next to a busy railway line (10 m) and at an urban background site in the time
period 25.11.2003–31.01.2004 (47 days)
WD-XRF
RDI-SR-XRF
Mn/Fe
Mn/Fe
Mn/Fe
Cr/Mn
Cr/Mn
Cr/Mn
Cu/Fe
Cu/Fe
Cu/Fe
Mn/Fe
Cr/Mn
Cu/Fe
Ca/Fe
Mn/Fe
Cr/Mn
Cu/Fe
Ca/Fe
Mn/Fe
Cr/Mn
Cu/Fe
Ca/Fe
Mn/Fe
Cr/Mn
Cu/Fe
Ca/Fe
Size range
Time interval
r2
Elemental ratio
Intercept (ng m3)
o10 mm
47 day—average
o10 mm
Daily
—
—
—
—
—
—
—
—
—
0.853
0.76
0.73
0.773
0.011 (railway site)a
0.013 (background)a
0.009 (difference, railway only)a,c
0.37 (railway site)a
0.30 (background)a
0.42 (difference, railway only)a,c
0.045 (railway site)a
0.035 (background)a
0.050 (difference, railway only)a,c
0.010b
0.321b
0.035b
0.154b
—
—
—
—
—
—
—
—
—
1.64
0.965
19.4
46.2
Coarse
Hourly
Intermediate
Hourly
Submicron
Hourly
0.996
0.853
0.514
0.396
0.98
0.95
0.706
0.125
0.978
0.943
0.701
0.446
0.0126b
0.65b
—
—
0.0123b
0.889b
0.1b
—
0.0121b
0.757b
0.087b
—
0.812
0.671
—
—
0.354
0.865
18.9
—
0.264
0.151
2.09
—
The mass ratios are determined based on a linear fit (orthogonal regression) of the elemental concentrations. r2 denotes the correlation
coefficient.
a
Arithmetic mean over entire campaign.
b
Slope of orthogonal linear fit.
c
Elemental ratio of the background subtracted concentrations, which are assumed to be fully railway related.
0.020
Mn/Fe crustal (Falbe, 1996)
0.018
Mn/Fe submicron
Elemental Ratio
0.016
Mn/Fe gasoline
(Vouk et al., 1983)
0.014
Mn/Fe background (this study)
0.012
Mn/Fe coarse
0.010
Mn/Fe railroad steel
(ThyssenKrupp GmbH,2006)
0.008
0
3
6
9
12
15
18
21
24
27
303
3
36
Time of day
Fig. 2. Average diurnal variation (47 days) of elemental mass concentration ratios, in comparison to known reference values for sources
that were expected to influence the sampling site.
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884
contribution (railway site–background) and 0.013 for
the background site, during the 47-day measuring
period. The respective values for the annual mean are
0.010 (railway contribution) and 0.0145 (background). Using Eq. (3.5) with the 47-day average
ratios, the railway related mass concentration is
calculated to be 1.2 mg m3 for iron, which is 60%
of the total average iron concentration (2.0 mg m3).
This average value over 47 days agrees well with the
annual average contribution of 66% (Gehrig et al.,
2006). Thus, the average contribution is considered to
be in the same range also for the daily average of the
hourly mass concentrations. As Eq. (3.5) is very
sensitive to tiny changes of the input parameters, Mn/
Fe ratios of 0.011 (rrailway) and 0.015 (rbackground) were
used for the hourly data to obtain the average 60%
railway contribution, to correct for the minor
systematic difference between the RDI-SR-XRF
and HiVol-WD-XRF data sets (see Table 1). Having
validated the use of Eq. (3.5) with the above steps,
hourly background and railway related mass concentrations were calculated for iron and manganese.
They are used in Section 4 for further calculations.
Fig. 3 shows the average diurnal variations of the iron
mass concentrations for the railway and background
contributions. A significant diurnal pattern is observed for the railway related fraction, which is still
strongly influenced by atmospheric dilution effects.
The background concentration is however found to
be more constant.
Finally, the suggested procedure for background
subtraction is less suitable for chromium and not
applicable to copper and calcium. Gehrig et al.
1800
Railway related iron
Background iron
1600
ng m-3
1400
1200
1000
800
600
0
3
6
9
12
15
18
21
(2006) suggested that chromium and copper are
minor contributors for railway abrasion, but with
different source processes (steel and overhead
traction line abrasion, respectively). They also
stated that significant calcium emissions by resuspension were not observed. The calculated Cr/Mn
fits show linearity, but by far not as distinct as the
Mn/Fe fits (Table 2). This points to the presence of
other sources besides railroad steel and did not
allow for a proper use of Eq. (3.5). The Ca/Fe or
Cu/Fe correlations finally are very poor. While
calcium has manifold sources in urban air and was
not considered for this analysis, we went further
into the hypothesis of copper being abraded from
the overhead traction line. Fig. 4 shows that the
measured Cu/Fe ratio for the coarse mode shows a
distinct diurnal variation, which obviously tracks
the fraction of passenger trains per total train
frequency very well. This means that passenger
trains emitted more copper per unit iron. This
supports the observation that cargo trains are
longer, heavier and older and therefore abrade
more iron per current collector unit. Since the
hourly copper background could not be calculated
with Eqs. (3.1)–(3.5) due to the changing elemental
ratio, the quantitative calculation of the directly
railway related copper emissions was only possible
on a daily average base. The HiVol-WD-XRF
measurements showed that during Sundays (with
negligible cargo train traffic) the background
corrected Cu/Fe ratio was 0.0970.01 and dropped
down to 0.0370.01 during weekdays with mixed
cargo and passenger train traffic. The plausibility of
the hypothesis that the daily railway related copper
mass concentration of 63 ng m3 (Table 1) is mainly
caused by abrasion from the overhead traction line
was checked by looking at railway material and
performance statistics (Chrétien, 2005). Reported
values of 25 t per annum overhead line weight loss
and 180 millions train-kilometers result in a
copper abrasion of 140 mg Cu per train-kilometer.
This compares favorably with a value of 84 mg
Cu per train-kilometer calculated from the measured ambient copper mass concentrations and a
typical dilution factor for road-near aerosols
(40 000 m3 h1, see Gehrig et al., 2004).
24
time of day
Fig. 3. Average diurnal variation (47 days) of the directly railway
related iron mass concentration within PM10, as calculated in
this study. Additionally, calculated concentrations for the background are shown.
4. Size fractionation of railway related iron and
manganese
The calculated iron and manganese background
concentrations represent the concentrations found in
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885
0.060
0.9
0.055
Cu/Fe ratio
0.050
0.7
0.045
0.6
0.040
0.5
0.035
0.4
0.3
0.030
Cu/Fe coarse
0.2
Passenger train fraction
0.025
Passenger train fraction
0.8
0.1
0.020
0
3
6
12
9
15
18
21
24
Time of day
Fig. 4. Average diurnal variation (47 days) of the coarse mode Cu/Fe ratio and the passenger train fraction of the total train traffic
frequency.
the total PM10 fraction. To calculate railway related
elemental mass concentrations for the coarse and fine
fraction separately, the size fractionation of the
background aerosol has to be known for the
considered elements. This has not been measured
directly in this study, but previous studies performed
at the background site have shown that for iron
roughly 80% of the mass was found in the coarse
mode and 20% in the fine mode (Hueglin, 2000;
Hueglin et al., 2005). This appears reasonable, since
coarse mode iron is a common part of mineral dust.
Similarly, a coarse mode fraction of roughly twothirds for manganese was found in the background
aerosol. For neither of the elements reliable information of the contributions of the submicron background fraction was available from previous studies.
Thus, railway related mass concentrations for the
coarse and fine mode were calculated according to
Fecoarse;railway ¼ Fecoarse;total 0:8Febackground ,
(4.1)
Fefine;railway ¼ Fefine;total 0:2Febackground .
(4.2)
Febackground was obtained using Eqs. (3.3) and (3.5).
To link the evaluated railway related iron mass
concentrations to train traffic characteristics, hourly
train frequencies were used for the railway line next
to our measuring site. The hourly frequencies are
split into passenger and cargo trains (see Fig. 1). Due
to the high degree of timetable synchronization in
Swiss railway traffic, both cargo and passenger train
frequencies were highly constant. On weekdays the
cargo train fraction was around one-third, decreasing
to 20% on Saturdays and close to zero on Sundays.
In Fig. 5 the railway relevant coarse mode iron mass
fraction is plotted against the cargo train fraction of
the hourly train frequencies. Plotting the size fraction
ratios instead of individual size fractions eliminates
atmospheric dilution effects. The plot shows that the
coarse mode contribution for iron is increasing from
60% for passenger trains (cargo train fraction ¼ 0;
number of hourly values: 97; all day times) to 75%
for cargo trains (cargo train fraction ¼ 1; number of
hourly values: 73; mainly at nighttime). These two
values are significantly different (one-way ANOVA,
0.05 level). Due to the constant Mn/Fe ratio of 0.011
(Section 3) the same size fractionation applies for
railway related manganese. This difference is likely
attributable to the differences of the two train types
that run by our measuring site during this study.
Generally, cargo trains were usually older and
longer, equipped with solid iron wheels and brakes,
and had a higher average weight than passenger
trains. However, detailed information on train
weight and the number of axes was not available
for a more detailed interpretation. Since most trains
passed the sampling site with constant speed, the
observed size fractionation of the railway related iron
and manganese mass concentrations was mainly
caused by track and wheel abrasion, and not by
brake pad abrasion. The result is not sensitive to
elemental background ratios that are slightly different than 0.8 and 0.2 (Eqs. (4.1), (4.2)).
ARTICLE IN PRESS
N. Bukowiecki et al. / Atmospheric Environment 41 (2007) 878–889
time of day
886
21
18
15
12
9
6
3
0
Fecoarse / FetotalPM10 (railway related)
0.9
0.8
0.7
0.6
0.5
0.4
hourly fraction of cargo trains
Fig. 5. Railway related coarse mode iron mass fraction, split by the cargo train fraction at Zürich-Juchhof in winter 2003 (lower panel,
time period 25.11.2003–31.01.2004, 47 days). Boxes represent standard error of the mean, while crosses indicate the minima and maxima
values, respectively. The top panel shows the distribution of the individual cargo train fractions over the day (the cases of occurrence found
in the hourly data set are shown).
5. Concentrations of Fe and Mn normalized by train
frequency
A number of studies dealing with trace metal
investigation in subway stations have stated that
there is a lack of a possibility to compare the
exposure of people to airborne trace elements in
different traffic systems like subway, overground
railway, tramways, etc. (Chillrud et al., 2004; Aarnio
et al., 2005). To enable such a comparison, we
suggest to calculate the elemental mass concentrations normalized by the train frequency (cnorm,train):
cnorm;train ðng m3 hÞ ¼
cZ ðng m3 Þ
,
f train ðh1 Þ
where cZ is the railway related mass concentration of
element Z and ftrain the train frequency per unit time.
Table 3 lists the respective values for our sampling
site and compares them to values calculated from
data found in the literature. For the fine particle
fraction (PM2.5) the normalized iron mass concentration is two orders of magnitude lower at our site
above ground (7–26 ng m3 h) than in the subway
(2000–10 000 ng m3 h). For manganese the findings
are similar. The values increase two to five times for
the coarse particle fraction, as shown in this study.
The mass calculated for TSP (total suspended
particulate matter) for the Tokyo subway is roughly
four times higher than the fine mode values obtained
for the other subway studies. Finally, the multiplication of the normalized mass concentration
(cnorm,train) with the human respiratory volume
(typically 0.6 m h1 for an adult person in resting
condition, see Hollmann and Prinz, 1997) delivers the
approximate elemental mass inhaled per train
Place
Helsinki Rautatientori
Helsinki Sörnäinen
NYC
London
Roma
Tokyo
a
Passenger
Cargo
Passenger
Cargo
Subway
Subway
Subway
Subway
Subway
Subway
Train
frequencya
(h1)
30
10
30
10
10
10
10
10
10
10
Particle size
range
PM10
PM2.5
PM2.5
PM2.5
PM2.5
PM2.5
PM10
SPMd
Railway related mass
concentrationb (ng m3)
Mn/Fe ratio
Mass concentration
normalized by the train
frequencyc (ng m3 h)
Fe
Mn
Fe
Mn
553
1080
208
260
20 000
28 000
26 000
—
32 000
100 000
6.1
12
2.3
2.9
230
300
240
780
500
—
18
108
6.9
26
2000
2800
2600
0.20
1.2
0.08
0.29
23
30
24
78
50
3200
10 000
0.011
0.011
0.011
0.011
0.012
0.011
0.009
—
0.016
—
Reference
This study
Aarnio et al. (2005)
Aarnio et al. (2005)
Chillrud et al. (2004)
Pfeifer et al. (1999)
Ripanucci et al. (2006)
Furuya et al. (2001)
Assumptions: for the subway studies a general average train frequency of 10 h1 is assumed, corresponding to a train every 6 min.
Train related mass concentrations (background corrected average values for net cargo and passenger traffic, according to Sections 3 and 4). Passenger and cargo trains were
separated by looking at periods with cargo train fractions of zero (passenger trains only) and one (cargo trains only). The values for other studies represent approximate average values
calculated from the data in the respective articles.
c
According to Eq. (5.1).
d
Suspended particulate matter.
b
ARTICLE IN PRESS
Zürich
Train type
N. Bukowiecki et al. / Atmospheric Environment 41 (2007) 878–889
Table 3
Absolute (railway related) and normalized (per train frequency) iron and manganese mass concentrations in 10-m distance from the tracks, compared to data from various subway
stations
887
ARTICLE IN PRESS
888
N. Bukowiecki et al. / Atmospheric Environment 41 (2007) 878–889
passing by the sampling location. For a passenger
train at our site above ground (18 ng m3 h for the
iron coarse mode) the inhaled iron mass would thus
be roughly 10 ng, at a distance of 10 m to the railway
tracks.
6. Discussion and conclusions
The calculations performed in this study represent a relatively simple, straightforward way to
elucidate the emissions resulting from railway
abrasion processes. Hourly time resolution for the
measurement of elemental mass concentrations has
been shown to be crucial for the linkage to
anthropogenic source activities, which are highly
dynamic throughout the day. The list of elements
abraded by railway traffic includes mainly manganese and iron originating from steel abrasion, as
well as copper abraded from the overhead traction
line. For the latter, the separate emissions for the
coarse and fine mode could not be calculated in the
described way. As a result, the calculation of copper
exposure per train was not possible. No exposure
calculations were made for chromium either, since
the source apportionment for chromium remained
ambiguous in the ratio analysis. The results of this
paper show clearly that more than half of the iron
and manganese particles emitted by railway traffic
through wheel and track abrasion are found in the
coarse mode range (2.5–10 mm), and that on average
particles are larger for cargo trains than for
passenger trains. Since trains usually were running
with high speed at this site, contributions from
brake abrasion are not taken into account in the
results presented here. However, in most cases
passenger trains do not use the brake pads for
normal braking, only for emergency stops and the
last meters before stopping at a station. Thus, the
results presented here are sufficiently representative
for exposure estimation in urban areas that are
strongly influenced by railway traffic. As described
by Gehrig et al. (2006), the average mass concentration of all abrasion particles was roughly
1–2 mg m3, which only results in a minor contribution to total ambient PM10 (average during the
considered time period: 31.3 mg m3).
Comparison of the results with literature shows
that at our sampling location (10 m distance of a
busy over ground railway line) the railway related
elemental mass concentration normalized by the
train frequency is estimated to be more than a
magnitude lower compared to the results calculated
for the results from several subway studies. Potential health effects of the abraded particles are still
unclear, although there are no studies pointing to
drastic effects. A relatively low toxicity of PM2.5
containing high fractions of iron oxide measured in
the London underground is reported by Seaton
et al. (2005). Karlsson et al. (2005), however, state
that the abraded iron particles consist mainly of
magnetite (Fe3O4) and exhibit significant oxidative
stress in human lung cells. Although there are
presumably negligible health effects at the concentration levels discussed here, a number of additional
studies have found clear adverse health effects for
increased levels of trace metals like chromium, iron
and manganese (Gorell et al., 1997; Kadiiska et al.,
1997). As shown in this study, the coarse and fine
mode abrasion particles differ significantly in mass
contribution. Since particle size significantly matters
for the toxicological fate inside the human airways,
the findings presented here can contribute to a more
refined assessment of health effects induced by steel
abrasion in public transportation.
Acknowledgments
We gratefully acknowledge the opportunity
to perform our measurements at HASYLAB
(Hamburg, Germany). We also thank the Swiss
Federal Railways (SBB) for providing the train
statistics data for Zürich-Juchhof. Financial support of the project was granted by the Swiss Agency
for Environment, Forest and Landscape, as well as
by the Swiss State Secretariat for Education and
Research SER within the framework of the EC FP6
project ACCENT (Atmospheric Composition
Change: An European Network).
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