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 ARTICLE IN PRESS 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, ARTICLE IN PRESS 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 ARTICLE IN PRESS 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. ARTICLE IN PRESS 882 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 ARTICLE IN PRESS N. Bukowiecki et al. / Atmospheric Environment 41 (2007) 878–889 883 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. ARTICLE IN PRESS N. Bukowiecki et al. / Atmospheric Environment 41 (2007) 878–889 884 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 ARTICLE IN PRESS N. Bukowiecki et al. / Atmospheric Environment 41 (2007) 878–889 885 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). 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