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PIXE and XRF analysis of atmospheric aerosols from a site in the West area of Mexico City
Nuclear Instruments and Methods in Physics Research B xxx (2013) xxx–xxx
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PIXE and XRF analysis of atmospheric aerosols from a site in the West area of Mexico City R.V. Díaz a, J. López-Monroy a, J. Miranda a,b,⇑, A.A. Espinosa b a b
Instituto Nacional de Investigaciones Nucleares, Centro Nuclear ‘‘Nabor Carrillo’’, Autopista México-Toluca, Salazar, Edo. Mex., Mexico Instituto de Física, Universidad Nacional Autónoma de México, Apartado Postal 20-364, 01000 México, DF, Mexico
a r t i c l e
i n f o
Article history: Received 19 January 2013 Received in revised form 29 April 2013 Accepted 27 May 2013 Available online xxxx Keywords: Mexico City Aerosols Cuajimalpa PIXE XRF
a b s t r a c t Due to geographical factors, most of the Metropolitan Area of Mexico City features, on average, similar heights above the sea level, climate, wind speed and direction, with very uniform pollution degrees in most of the frequently studied sites. A site with different characteristics, Cuajimalpa de Morelos, was studied. It is located to the West of the urban area at 2760 m above sea level, in contrast to other sites (2240 m). Here, the wind is mostly directed towards the center of the city. Then, the site should not be affected by pollutants from the Northern/Northeastern industrial zones, so lower aerosol concentrations are expected. In this work, the elemental composition of coarse (PM10-2.5) and fine (PM2.5) fractions of atmospheric aerosol samples collected in Cuajimalpa is studied. The sampling period covered the colddry season in 2004–2005 (December 1st, 2004 to March 31, 2005), exposing polycarbonate filters with a Stacked Filter Unit of the Gent design along 24 h, every two days. The samples were analyzed with Particle Induced X-ray Emission (PIXE) and X-ray Fluorescence (XRF), to obtain elemental concentrations. The EPA code UNMIX was used to determine the number of possible influencing polluting sources, which were then identified through back-trajectory simulations with the HYSPLIT modeling software. Four sources (mostly related to soil) were found in the coarse fraction, while the fine fraction presented three main sources (fuel oil, industry and biomass burning). Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction The pollution by atmospheric aerosols in the Metropolitan Area of Mexico City (MAMC) is still showing aspects that require thorough studies [1]. With about 20 million inhabitants, this urban area has suffered a dramatic increase in the number of motor vehicles, in spite of the efforts to improve public transit systems. Moreover, because of geographical factors, most of the MAMC features, on average, very similar characteristics. These include height above the sea level, climate, wind speed and direction, which gives as a result very uniform pollution levels in most of the traditionally studied sites (North, Center, South and Northeast) [2–4]. Therefore, a site with different characteristics with respect to them, Cuajimalpa de Morelos, was selected for the present work. It is located to the West of the MAMC at 2760 m above sea level (a.s.l.), in contrast to other sites (2240 m a.s.l.); it is a sub-humid area with lush vegetation, strongly influenced by the forest of the ‘‘Desierto de los Leones’’ National Park. Here, the wind for most part of the day is ⇑ Corresponding author at: Instituto de Física, Universidad Nacional Autónoma de México, Apartado Postal 20-364, 01000 México, DF, Mexico. Tel.: +52 55 56225073; fax: +52 55 56161535. E-mail addresses: miranda@fisica.unam.mx, [email protected] (J. Miranda).
directed towards the center of the MAMC, joining flows that run from North to South. This prevents the site from receiving influence of pollutants generated either in the Northern industrial zone, Xalostoc or Naucalpan. Thus, it is expected that this area should present lower concentration of pollutants than the rest of the MAMC. Therefore, the present work is aimed to study the elemental composition of coarse (PM10-2.5) and fine (PM2.5) fractions of atmospheric aerosol samples collected in Cuajimalpa, using the elemental analysis methods Particle Induced X-ray Emission (PIXE) and X-ray Fluorescence (XRF). The resulting elemental contents are studied with the multivariate statistical analysis code UNMIX [5] to identify possible contribution factors, which are then related to emitting sources in the MAMC or the surroundings. Although it is highly desirable to include in this study a full characterization of the aerosols (organic and black carbon, nitrates and other compounds), in previous works [2,4] it has been demonstrated that the elemental analyses are powerful enough to identify certain polluting sources and quantify their impacts on the sites.
2. Materials and methods The sampling period covered the cold-dry season in 2004–2005 (December 1st, 2004 to March 31, 2005), exposing polycarbonate
0168-583X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2013.05.095
Please cite this article in press as: R.V. Díaz et al., PIXE and XRF analysis of atmospheric aerosols from a site in the West area of Mexico City, Nucl. Instr. Meth. B (2013), http://dx.doi.org/10.1016/j.nimb.2013.05.095
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R.V. Díaz et al. / Nuclear Instruments and Methods in Physics Research B xxx (2013) xxx–xxx
filters with a Stacked Filter Unit (SFU) of the Gent design along 24 h (starting at 0:00 h), every two days. The filters were NucleporeÒ with 47 mm diameter and 8 lm pore size for the coarse fraction (PM10-2.5), and 47 mm in diameter and 0.4 lm pore size for the fine fraction (PM2.5). For the selection of the monitoring site the United States Environmental Protection Agency (EPA) [6] reference procedures were considered. The selected site corresponds to the Cuajimalpa RAMA station (latitude 19° 210 49.77500 N, longitude 99° 170 55.15600 W). The sampler was located 6 m above street level. The area is mostly residential, with moderate to heavy traffic during the mornings and evenings. Total gravimetric mass was measured by pre- and post-weighing the filters with a CahnÒ 33 electrobalance (0.1 lg resolution). Temperature during gravimetric mass measurements varied between 20 and 25 °C, while relative humidity was in the range 30–40%; filters were pre-conditioned during 24 h within similar temperature and relative humidity ranges. Elemental concentrations for PM10-2.5 were obtained only with XRF, while those of PM2.5 were determined as averages from PIXE and XRF results. PIXE analysis was carried out with a 2 MeV, 5 nA proton beam produced by the 2 MV Tandetron accelerator at the Instituto Nacional de Investigaciones Nucleares (ININ). An Ortec Ge(Li) X-ray detector (resolution 180 eV at 5.9 keV, located at an angle of 135° with respect to the incoming beam direction) collected the X-rays. XRF analysis was performed with the X-ray spectrometer developed at the Instituto de Física, UNAM (IFUNAM), for environmental applications [7]. An Oxford Instruments X-ray tube with Rh anode, as well as an Amptek Si-PIN X-ray detector (resolution 160 eV at 5.9 keV), were employed. The tube operated at 50 kV and a current of 250 lA, irradiating during 1200 s per spectrum. The detection systems’ efficiencies were measured using thin film standards (MicroMatter Co., Vancouver, Canada) in every case. A validation of the results obtained after a comparison with PIXE and XRF was presented in [7]. The resulting spectra were integrated with the GUPIXWincomputer code [8] for PIXE and the WinQXAS program [9] for XRF. Uncertainties in elemental concentrations were evaluated following the method described by Espinosa and Miranda [10]. 3. Results and discussion Table 1 presents the 24 h average elemental and gravimetric mass concentrations found in the coarse (PM10-2.5) and fine Table 1 24 h average elemental concentrations (lg m3) in coarse (PM10-2.5) and fine (PM2.5) fractions and PM10 of samples collected in Cuajimalpa.
a b c
Element
Na
Coarse fraction
Mass Al Si P S Cl K Ca Ti V Cr Mn Fe Ni Cu Zn Br Sr Pb
60 59 60 52 60 60 60 60 60 48 60 60 60 60 60 60 2 9 60
37.2 0.55 3.48 0.27 0.81 0.38 0.81 2.73 0.18 0.018 0.024 0.069 1.70 0.033 0.13 0.09 0.001 0.004 0.047
(2.4)b (0.15) (0.53) (0.06) (0.13) (0.05) (0.09) (0.26) (0.02) (0.004) (0.004) (0.007) (0.11) (0.004) (0.011) (0.009) (0.004) (0.007) (0.009)
Fine fraction
PM10
12 N.D.c N.D. N.D. 0.60 0.037 0.096 0.13 0.009 0.010 0.002 0.003 0.10 0.002 0.003 0.012 0.005 N.D. 0.009
48.8 – – – 1.41 0.42 0.91 2.85 0.19 0.028 0.026 0.071 1.80 0.035 0.14 0.10 0.006 – 0.056
(1.3)
(0.06) (0.004) (0.006) (0.009) (0.001) (0.001) (0.0001) (0.0002) (0.005) (0.0002) (0.0003) (0.001) (0.0003) (0.001)
Numbers of valid observations. Numbers between parenthesis represent the combined uncertainty. N.D.: not measured/detected.
(2.4)
(0.11) (0.02) (0.05) (0.19) (0.01) (0.003) (0.001) (0.003) (0.07) (0.001) (0.01) (0.01) (0.001) – (0.001)
(PM2.5) fractions, as well as total PM10, of the collected samples; sixteen and fourteen elements were found in the coarse and fine fraction, respectively. The number of valid observations was small only for Sr and Ba in the coarse fraction, while Al, Si and P showed a very low peak/background ratio in the PM2.5 spectra, so the quantification was not accurate neither with PIXE nor XRF. In order to set the present results in an adequate context, Table 2 displays data for representative elements (S, Fe, Zn and Pb) as measured in different studies [2,4,11–19] at the MAMC, with a time span from 1993 to 2009. Whenever needed, PM10 results were obtained after adding the coarse and fine fractions for each element. For the West area only PM2.5 data were found in previous papers. This comparison should be taken with precautions, because the cited works did not necessarily employ the same sampling devices or analytical techniques (which are mostly X-ray spectrometries). Moreover, the time period is rather long. In order to reach valid conclusions on this, it is necessary to rely on previous comparisons and evaluations of sampling devices [20] and aerosols analytical methods [21,22]. For the fine fraction it is apparent that S and Zn have much lower concentrations than those found in the other studies, a fact explained by the low influence of industrial sources, as well as the wind regime, already mentioned above. For this fraction, S is expected to be present in primary aerosols. However, Fe, mostly produced by soil-related sources, has very similar concentrations to those found in other areas and periods, in particular with more recent papers. Moreover, Pb, which should be associated to either traffic or industrial sources, has definitely decreased as a function of time, especially after the elimination of Pb in gasoline at the beginnings of the 1990 decade. It is not completely absent in the aerosols, though, possibly because of resuspended dust from the streets. However, the official standard in the MAMC for Pb atmospheric concentrations is fulfilled since 1993 [23]. Regarding PM10, the S contents measured in this work present comparatively much higher values than in the fine fraction, but very similar to those of the other studies. This is explained by the presence of soil-related secondary aerosols in the site. In the paper by Barrera et al. [4], it was demonstrated (using PIXE, microPIXE and Positive Matrix Factorization) that S-rich soil particles are present in most of the MAMC, produced by chemical reactions of SO2 with primary soil-derived particulate matter. As for Fe, also higher concentrations were found, which must be due to local soil/resuspended dust sources. Finally, Zn and Pb present values very similar to the other publications, showing that the concentrations of these elements reached a stationary level. This must be carefully considered by the authorities to propose a solution. Although the referred studies were carried along a wide time span, for different sites and using different analytical techniques, it is perceived that, in general, the elemental concentrations reached uniform values around the MAMC, both temporally and geographically, a fact already commented by other authors [1,3], although in the past both S and Pb reached much higher concentrations than in the present. The Environment Ministry of the local Government, has reported that the trends for PM10 and PM2.5 concentrations at the MAMC have steady values since the years 2000 and 2003, respectively [23]. Thus, it is not surprising to find similar trends for elemental concentrations. The causes of this behavior are still under investigation. A further step in the analysis is the use of the EPA computer code UNMIX, which allows the identification of possible influencing sources in the site, providing a simple receptor model [24]. The model is applied only to a small fraction of the whole aerosol mass (namely, the elemental concentrations determined with PIXE and XRF). Thus, it must be emphasized that UNMIX will give information only on the sources of these elements, not about the complete composition of the airborne particles. Nevertheless, this procedure has been applied successfully in other studies related to other
Please cite this article in press as: R.V. Díaz et al., PIXE and XRF analysis of atmospheric aerosols from a site in the West area of Mexico City, Nucl. Instr. Meth. B (2013), http://dx.doi.org/10.1016/j.nimb.2013.05.095
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R.V. Díaz et al. / Nuclear Instruments and Methods in Physics Research B xxx (2013) xxx–xxx Table 2 24 h average concentrations of PM10 and PM2.5 (lg m3) for representative elements at various dates and sites from the MAMC. Site/date/fraction This work/2004/PM2.5 South/1993/PM2.5 [11] West/1994/PM2.5 [12] South/1995/PM2.5 [13] West/1996/PM2.5 [14] South/2001/PM2.5 [15] South/2002/PM2.5 [2] West/2003/ PM2.5 [16] Center/2004/PM2.5 [17] This work/2004/PM10 South/1995/PM10 [13] Center/2002/PM10 [18] Center/2002/PM10 [2] South/2006/ PM10 [19] Center/2009/ PM10 [4] a b c
S a
0.60 (0.06) 3.77 (0.35) 2.6 (0.28) 1.30 (0.10) 4.11 (0.39) 0.287 (0.022) 0.530 (0.038) 1.54 (0.12) 1.60b 1.41 (0.11) 1.60 (0.12) 2.63b 1.24 (0.95) N.A.c 1.3 (0.59)
Fe
Zn
Pb
0.10 (0.05) 0.198 (0.043) 0.37 (0.03) 0.260 (0.20) 0.385 (0.051) 0.053 (0.005) 0.110 (0.012) 0.18 (0.014) 0.133b 1.80 (0.07) 0.66 (0.050) 1.46b 0.672 (0.075) 1.0 b 0.84 (0.34)
0.012 (0.01) 0.096 (0.008) 0.13 (0.02) 0.110 (0.010) 0.202 (0.023) 0.017 (0.003) 0.014 (0.001) 0.091 (0.007) 0.108b 0.10 (0.01) 0.15 (0.015) 0.2b 0.110 (0.010) 0.098 b 0.095 (0.047)
0.009 (0.001) 0.081 (0.006) 0.25 (0.05) 0.059 (0.005) 0.100 (0.010) 0.009 (0.001) 0.008 (0.001) 0.040 (0.003) 0.068b 0.056 (0.001) 0.081 (0.009) 0.08b 0.018 (0.002) 0.026 b 0.088 (0.058)
Numbers between parenthesis represent the combined uncertainty. No uncertainty was quoted. N.A.: not available.
small components of the total aerosol mass, such as volatile organic compounds [25] or polycyclic aromatic hydrocarbons [26]. Total gravimetric mass was considered as the reference total variable in UNMIX, so the source profiles are given in lg m3. The results of this procedure are found in Fig. 1, where the contributions of the identified emitting sources are shown. Four factors were identified in the coarse fraction (two types of soil/fugitive dust sources, identified as A and B, Soil secondary and Industry); these factors contribute with 1%, 11%, 16% and 1% to total gravimetric mass, respectively. In the fine fraction only three sources were seen (Fuel oil, Biomass burning and Traffic/industry); here, the contributions
Fig. 1. Source profiles for the (a) coarse fraction and (b) fine fraction.
to gravimetric mass are 2%, 1% and 4%, respectively. For PM10-2.5, the elements Cr, Ni, Cu and Pb do not appear in the model because UNMIX took them as weak elements, due to their low concentrations. It must be noticed that the contributions of the elements that can be measured with PIXE in the fine fraction are much lower than in the coarse fraction, which is explained by the high contents of organic carbon, elemental carbon and nitrates in the fine aerosols. The Soil A and B sources are identified mainly by the presence of the crustal elements Al, Si, K, Ca, Ti, Mn and Fe. The proportions of these elements, though, are different in each type of soil, in particular Ca, which is much lower in A than in B, while Ti is higher in Soil A. Investigations are under progress to characterize possible sources of soil-derived airborne particles. The industrial source is typified by the inclusion of P, S, Ti, V and Zn. Finally, the presence of the Soil Secondary factor is in full agreement with the findings of Barrera et al. [4]. It is remarkable that the profile for Soil Secondary is very similar to that of Soil B, except for the S contents. This strongly suggests that the Soil Secondary source could be attributed to a Soil B reacting chemically with SO2, as remarked in Ref. [4]. Furthermore, if the ratio of several contributing elements to the Si concentration in the Soil B profile is compared to the ratios for the Soil 2 source in the Center site from the latter study [4], a clear resemblance is perceived (Fig. 2). Thus, a common source
Fig. 2. Comparison of Soil B profile in the coarse fraction and Soil 2 in PM10 from the work by Barrera et al. [4].
Please cite this article in press as: R.V. Díaz et al., PIXE and XRF analysis of atmospheric aerosols from a site in the West area of Mexico City, Nucl. Instr. Meth. B (2013), http://dx.doi.org/10.1016/j.nimb.2013.05.095
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R.V. Díaz et al. / Nuclear Instruments and Methods in Physics Research B xxx (2013) xxx–xxx
4. Conclusions The results obtained in this work showed in general lower elemental concentrations in the atmospheric aerosols collected in Cuajimalpa de Morelos than in other sites in the MAMC. In this regard, the initial hypothesis is correct. Nevertheless, with the receptor model obtained with UNMIX, it was possible to see that there is an influence of industrial areas (Soil Secondary factor), as well as biomass burning. A possible stabilization of elemental levels is noticed, suggesting that other regulations may be required to further decrease those values in the MAMC. References
Fig. 3. Representative back-trajectories obtained with HYSPLIT [21] and fire spots localized with the Web Fire Mapper [22], for the PM2.5 Biomass burning factor episode on December 29, 2004.
can be identified for both sites, regardless of the geographical and temporal differences. Regarding the factors in the fine fraction, the usual set of tracers S–V–Ni are found with the highest concentrations in the Fuel oil component. The elements found in the Biomass burning factor (S, Cl, K, Ca, Ti and Fe) are in complete agreement with those described as components of forest fire emissions in the US-EPA Speciate 4.3 database [27]. Finally, the most important source in this fraction (Traffic/industry) contains the elements S, Cl, V, Mn, Fe, Cu, Zn and Pb, which are undoubtedly anthropogenic contributions associated either to vehicle or industrial emissions. The inspection of the time series for all the factors identified with UNMIX signals several episodes for each source during the sampling period. The simulation model HYbrid Single-Particle Lagrangian Integrated Trajectory, better known as HYSPLIT [28] was then employed to calculate back-trajectories during the episodes spotted with UNMIX. A period of 24 h (24 trajectories per day, with 24 points per trajectory, corresponding each point to 1 h), and three different heights (500, 1000 and 1500 m, as recommended) above ground level were taken into account for the simulations, to overcome the low resolution limitations of the simulation software. Similar remarks about HYSPLIT results as those given by Barrera et al. [4]can be applied to the present work. An example of representative back-trajectories is given in Fig. 3, during an episode for the Biomass burning factor in PM2.5 on December 29, 2004. For this, it is also helpful to make use of the open access resource known as Web Fire Mapper (http://earthdata.nasa.gov/data/near-real-time-data/firms/), which is a part of MODIS41, 45 (moderate-resolution imaging spectro-radiometer) [29]. In this particular episode, the back-trajectories come mostly from the West, with a few contributions from Southwest and Northwest. There is a good agreement in the trajectories for all the heights used in the reproduction. Additionally, probable fire spots obtainedaround that date, together with the simulations by HYSPLIT, appear in the figure. It must be noted that the back-trajectories coincide with many forest fires, suggesting strongly that the winds transported the emitted particles towards the sampling site. In every case, the fire spots were only present during the episodes. Very similar inferences as in [4] are obtained here, with all the factors identified with UNMIX. For instance, Secondary soil corresponds to back-trajectories from fugitive dust sources crossing industrial areas and Soil episodes to direct back-trajectories from agricultural areas.
[1] L.T. Molina, S. Madronich, J.S. Gaffney, E. Apel, B. De Foy, J. Fast, R. Ferrare, S. Herndon, J.L. Jiménez, B. Lamb, A.R. Osornio-Vargas, P. Russell, J.J. Schauer, P.S. Stevens, M. Zavala, Atmos. Chem. Phys. 10 (2010) 8697. [2] J. Miranda, V.A. Barrera, A.A. Espinosa, O.S. Galindo, J. Meinguer, X-Ray Spectrom. 34 (2005) 315. [3] E. Vega, S. Eidels, H. Ruiz, D. López-Veneroni, G. Sosa, E. González, J. Gasca, V. Mora, E. Reyes, G. Sánchez-Reyna, R. Villaseñor, J.C. Chow, J.G. Watson, S.A. Edgerton, Aerosol and Air Quality Res. 10 (2010) 193. [4] V.A. Barrera, J. Miranda, A.A. Espinosa, J. Meinguer, J.N. Martínez, E. Cerón, J.R. Morales, P.A. Miranda, J.F. Dias, Int. J. Environ. Res. 6 (2012) 597. [5] W. Maenhaut, The Gent Stacked Filter Unit (SFU) Sampler for the Collection of Atmospheric Aerosols in Two Size Fractions: Description and Instructions for Installation and Use, IAEA Contract Number 6948 (IAEA, Vienna, May 1992). [6] United States Environmental Protection Agency, Quality Assurance Guidance Document 2.12, Monitoring PM2.5 in Ambient Air Using Designated Reference or Class I Equivalent Methods, USA-EPA, 1998. Available from:. Accessed December 11, 2012. [7] A.A. Espinosa, J. Reyes-Herrera, J. Miranda, F. Mercado, M.A. Veytia, M. Cuautle, J.I. Cruz, Instrum. Sci. Technol. 40 (2012) 603. [8] J.L. Campbell, N.I. Boyd, N. Grassi, P. Bonnick, J.A. Maxwell, Nucl. Instrum. Meth. B 268 (2010) 3356. [9] Manual for QXAS, International Atomic Energy Agency, Vienna, 2005. [10] A.A. Espinosa, J. Miranda, Rev. Mex. Fís. E 56 (2010) 134 (in Spanish). [11] J. Miranda, E. Andrade, A. López-Suárez, R. Ledesma, Atmos. Environ. 30 (1996) 3471. [12] R. Paredes-Gutiérrez, A. López-Suárez, J. Miranda, E. Andrade, J.A. González, Rev. Int. Contam. Amb. 13 (1997) 81. [13] J. Miranda, I. Crespo, M.A. Morales, Environ. Sci. Poll. Res. 7 (2000) 14. [14] J. Miranda, A. López-Suárez, R. Paredes-Gutiérrez, S. González, O.G. De Lucio, E. Andrade, J.R. Morales, M.J. Ávila-Sobarzo, Nucl. Instrum. Meth. B 136–138 (1998) 970. [15] J. Miranda, V.A. Barrera, A.A. Espinosa, O.S. Galindo, A. Núñez-Orosco, R.C. Montesinos, A. Leal-Castro, J. Meinguer, Nucl. Instrum. Meth. B 219–220 (2004) 157. [16] F. Aldape, J. Flores, Int. J. PIXE 14 (2004) 147. [17] F. Aldape, J. Flores, J. Flores, A. Retama, O. Rivera-Hernández, Int. J. PIXE 15 (2005) 263. [18] E. Vega, E. Reyes, H. Ruiz, J. García, G. Sánchez, G. Martínez-Villa, U. González, J.C. Chow, J.G. Watson, J. Air Waste Manage. Assoc. 54 (2004) 786. [19] X. Querol, J. Pey, M.C. Minguillón, N. Pérez, A. Alastuey, M. Viana, T. Moreno, R.M. Bernabé, S. Blanco, B. Cárdenas, E. Vega, G. Sosa, S. Escalona, H. Ruiz, B. Artíñano, Atmos. Chem. Phys. 8 (2008) 111. [20] W.C. Hinds, Aerosol Technology, second ed., Wiley Interscience, New York, 1999. [21] K.R. Spurny (Ed.), Analytical Chemistry of Aerosols, Lewis Publishers, Boca Raton, 1999. [22] M. Creatchman, S. Landsberger (Eds.), Elemental Analysis of Airborne Particles, Gordon and Breach Science Publishers, New York, 1999. [23] Annual Report on Air Quality in Mexico City 2011, Secretaría del Medio Ambiente, Gobierno del Distrito Federal, Mexico, City, 2012 (in Spanish). [24] G. Norris, R. Vedantham, R. Duvall, R.C. Henry, EPA Unmix 6.0 Fundamentals & User Guide, Report EPA/600/R-07/089, United States Environmental Protection Agency, Washington, 2007. [25] H. Jorquera, B. Rappenglück, Atmos. Environ. 38 (2004) 4243. [26] M.A. Khairy, R. Lohmann, Chemosphere 91 (2013) 895. [27] H. Simon, L. Beck, P.V. Bhave, F. Divita, Y. Hsu, D. Luecken, J.D. Mobley, G.A. Pouliot, A. Reff, G. Sarwar, M. Strum, Atmos. Poll. Res. 1 (2010) 196. [28] R.R. Draxler, G.D. Rolph, HYSPLIT (HYbrid Single-Particle Lagrangian Integrated Trajectory) Model access via NOAA ARL READY Website (http:// ready.arl.noaa.gov/HYSPLIT.php). (NOAA Air Resources Laboratory, Silver Spring, MD, 2010); G.D. Rolph, Real-time Environmental Applications and Display sYstem (READY) Website (http://ready.arl.noaa.gov). (NOAA Air Resources Laboratory, Silver Spring, MD, 2010). [29] C.O. Justice, L. Giglio, S. Korontzy, J. Owen, Remote Sensing Environ. 83 (2002) 244.
Please cite this article in press as: R.V. Díaz et al., PIXE and XRF analysis of atmospheric aerosols from a site in the West area of Mexico City, Nucl. Instr. Meth. B (2013), http://dx.doi.org/10.1016/j.nimb.2013.05.095
Contents lists available at SciVerse ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
PIXE and XRF analysis of atmospheric aerosols from a site in the West area of Mexico City R.V. Díaz a, J. López-Monroy a, J. Miranda a,b,⇑, A.A. Espinosa b a b
Instituto Nacional de Investigaciones Nucleares, Centro Nuclear ‘‘Nabor Carrillo’’, Autopista México-Toluca, Salazar, Edo. Mex., Mexico Instituto de Física, Universidad Nacional Autónoma de México, Apartado Postal 20-364, 01000 México, DF, Mexico
a r t i c l e
i n f o
Article history: Received 19 January 2013 Received in revised form 29 April 2013 Accepted 27 May 2013 Available online xxxx Keywords: Mexico City Aerosols Cuajimalpa PIXE XRF
a b s t r a c t Due to geographical factors, most of the Metropolitan Area of Mexico City features, on average, similar heights above the sea level, climate, wind speed and direction, with very uniform pollution degrees in most of the frequently studied sites. A site with different characteristics, Cuajimalpa de Morelos, was studied. It is located to the West of the urban area at 2760 m above sea level, in contrast to other sites (2240 m). Here, the wind is mostly directed towards the center of the city. Then, the site should not be affected by pollutants from the Northern/Northeastern industrial zones, so lower aerosol concentrations are expected. In this work, the elemental composition of coarse (PM10-2.5) and fine (PM2.5) fractions of atmospheric aerosol samples collected in Cuajimalpa is studied. The sampling period covered the colddry season in 2004–2005 (December 1st, 2004 to March 31, 2005), exposing polycarbonate filters with a Stacked Filter Unit of the Gent design along 24 h, every two days. The samples were analyzed with Particle Induced X-ray Emission (PIXE) and X-ray Fluorescence (XRF), to obtain elemental concentrations. The EPA code UNMIX was used to determine the number of possible influencing polluting sources, which were then identified through back-trajectory simulations with the HYSPLIT modeling software. Four sources (mostly related to soil) were found in the coarse fraction, while the fine fraction presented three main sources (fuel oil, industry and biomass burning). Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction The pollution by atmospheric aerosols in the Metropolitan Area of Mexico City (MAMC) is still showing aspects that require thorough studies [1]. With about 20 million inhabitants, this urban area has suffered a dramatic increase in the number of motor vehicles, in spite of the efforts to improve public transit systems. Moreover, because of geographical factors, most of the MAMC features, on average, very similar characteristics. These include height above the sea level, climate, wind speed and direction, which gives as a result very uniform pollution levels in most of the traditionally studied sites (North, Center, South and Northeast) [2–4]. Therefore, a site with different characteristics with respect to them, Cuajimalpa de Morelos, was selected for the present work. It is located to the West of the MAMC at 2760 m above sea level (a.s.l.), in contrast to other sites (2240 m a.s.l.); it is a sub-humid area with lush vegetation, strongly influenced by the forest of the ‘‘Desierto de los Leones’’ National Park. Here, the wind for most part of the day is ⇑ Corresponding author at: Instituto de Física, Universidad Nacional Autónoma de México, Apartado Postal 20-364, 01000 México, DF, Mexico. Tel.: +52 55 56225073; fax: +52 55 56161535. E-mail addresses: miranda@fisica.unam.mx, [email protected] (J. Miranda).
directed towards the center of the MAMC, joining flows that run from North to South. This prevents the site from receiving influence of pollutants generated either in the Northern industrial zone, Xalostoc or Naucalpan. Thus, it is expected that this area should present lower concentration of pollutants than the rest of the MAMC. Therefore, the present work is aimed to study the elemental composition of coarse (PM10-2.5) and fine (PM2.5) fractions of atmospheric aerosol samples collected in Cuajimalpa, using the elemental analysis methods Particle Induced X-ray Emission (PIXE) and X-ray Fluorescence (XRF). The resulting elemental contents are studied with the multivariate statistical analysis code UNMIX [5] to identify possible contribution factors, which are then related to emitting sources in the MAMC or the surroundings. Although it is highly desirable to include in this study a full characterization of the aerosols (organic and black carbon, nitrates and other compounds), in previous works [2,4] it has been demonstrated that the elemental analyses are powerful enough to identify certain polluting sources and quantify their impacts on the sites.
2. Materials and methods The sampling period covered the cold-dry season in 2004–2005 (December 1st, 2004 to March 31, 2005), exposing polycarbonate
0168-583X/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nimb.2013.05.095
Please cite this article in press as: R.V. Díaz et al., PIXE and XRF analysis of atmospheric aerosols from a site in the West area of Mexico City, Nucl. Instr. Meth. B (2013), http://dx.doi.org/10.1016/j.nimb.2013.05.095
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filters with a Stacked Filter Unit (SFU) of the Gent design along 24 h (starting at 0:00 h), every two days. The filters were NucleporeÒ with 47 mm diameter and 8 lm pore size for the coarse fraction (PM10-2.5), and 47 mm in diameter and 0.4 lm pore size for the fine fraction (PM2.5). For the selection of the monitoring site the United States Environmental Protection Agency (EPA) [6] reference procedures were considered. The selected site corresponds to the Cuajimalpa RAMA station (latitude 19° 210 49.77500 N, longitude 99° 170 55.15600 W). The sampler was located 6 m above street level. The area is mostly residential, with moderate to heavy traffic during the mornings and evenings. Total gravimetric mass was measured by pre- and post-weighing the filters with a CahnÒ 33 electrobalance (0.1 lg resolution). Temperature during gravimetric mass measurements varied between 20 and 25 °C, while relative humidity was in the range 30–40%; filters were pre-conditioned during 24 h within similar temperature and relative humidity ranges. Elemental concentrations for PM10-2.5 were obtained only with XRF, while those of PM2.5 were determined as averages from PIXE and XRF results. PIXE analysis was carried out with a 2 MeV, 5 nA proton beam produced by the 2 MV Tandetron accelerator at the Instituto Nacional de Investigaciones Nucleares (ININ). An Ortec Ge(Li) X-ray detector (resolution 180 eV at 5.9 keV, located at an angle of 135° with respect to the incoming beam direction) collected the X-rays. XRF analysis was performed with the X-ray spectrometer developed at the Instituto de Física, UNAM (IFUNAM), for environmental applications [7]. An Oxford Instruments X-ray tube with Rh anode, as well as an Amptek Si-PIN X-ray detector (resolution 160 eV at 5.9 keV), were employed. The tube operated at 50 kV and a current of 250 lA, irradiating during 1200 s per spectrum. The detection systems’ efficiencies were measured using thin film standards (MicroMatter Co., Vancouver, Canada) in every case. A validation of the results obtained after a comparison with PIXE and XRF was presented in [7]. The resulting spectra were integrated with the GUPIXWincomputer code [8] for PIXE and the WinQXAS program [9] for XRF. Uncertainties in elemental concentrations were evaluated following the method described by Espinosa and Miranda [10]. 3. Results and discussion Table 1 presents the 24 h average elemental and gravimetric mass concentrations found in the coarse (PM10-2.5) and fine Table 1 24 h average elemental concentrations (lg m3) in coarse (PM10-2.5) and fine (PM2.5) fractions and PM10 of samples collected in Cuajimalpa.
a b c
Element
Na
Coarse fraction
Mass Al Si P S Cl K Ca Ti V Cr Mn Fe Ni Cu Zn Br Sr Pb
60 59 60 52 60 60 60 60 60 48 60 60 60 60 60 60 2 9 60
37.2 0.55 3.48 0.27 0.81 0.38 0.81 2.73 0.18 0.018 0.024 0.069 1.70 0.033 0.13 0.09 0.001 0.004 0.047
(2.4)b (0.15) (0.53) (0.06) (0.13) (0.05) (0.09) (0.26) (0.02) (0.004) (0.004) (0.007) (0.11) (0.004) (0.011) (0.009) (0.004) (0.007) (0.009)
Fine fraction
PM10
12 N.D.c N.D. N.D. 0.60 0.037 0.096 0.13 0.009 0.010 0.002 0.003 0.10 0.002 0.003 0.012 0.005 N.D. 0.009
48.8 – – – 1.41 0.42 0.91 2.85 0.19 0.028 0.026 0.071 1.80 0.035 0.14 0.10 0.006 – 0.056
(1.3)
(0.06) (0.004) (0.006) (0.009) (0.001) (0.001) (0.0001) (0.0002) (0.005) (0.0002) (0.0003) (0.001) (0.0003) (0.001)
Numbers of valid observations. Numbers between parenthesis represent the combined uncertainty. N.D.: not measured/detected.
(2.4)
(0.11) (0.02) (0.05) (0.19) (0.01) (0.003) (0.001) (0.003) (0.07) (0.001) (0.01) (0.01) (0.001) – (0.001)
(PM2.5) fractions, as well as total PM10, of the collected samples; sixteen and fourteen elements were found in the coarse and fine fraction, respectively. The number of valid observations was small only for Sr and Ba in the coarse fraction, while Al, Si and P showed a very low peak/background ratio in the PM2.5 spectra, so the quantification was not accurate neither with PIXE nor XRF. In order to set the present results in an adequate context, Table 2 displays data for representative elements (S, Fe, Zn and Pb) as measured in different studies [2,4,11–19] at the MAMC, with a time span from 1993 to 2009. Whenever needed, PM10 results were obtained after adding the coarse and fine fractions for each element. For the West area only PM2.5 data were found in previous papers. This comparison should be taken with precautions, because the cited works did not necessarily employ the same sampling devices or analytical techniques (which are mostly X-ray spectrometries). Moreover, the time period is rather long. In order to reach valid conclusions on this, it is necessary to rely on previous comparisons and evaluations of sampling devices [20] and aerosols analytical methods [21,22]. For the fine fraction it is apparent that S and Zn have much lower concentrations than those found in the other studies, a fact explained by the low influence of industrial sources, as well as the wind regime, already mentioned above. For this fraction, S is expected to be present in primary aerosols. However, Fe, mostly produced by soil-related sources, has very similar concentrations to those found in other areas and periods, in particular with more recent papers. Moreover, Pb, which should be associated to either traffic or industrial sources, has definitely decreased as a function of time, especially after the elimination of Pb in gasoline at the beginnings of the 1990 decade. It is not completely absent in the aerosols, though, possibly because of resuspended dust from the streets. However, the official standard in the MAMC for Pb atmospheric concentrations is fulfilled since 1993 [23]. Regarding PM10, the S contents measured in this work present comparatively much higher values than in the fine fraction, but very similar to those of the other studies. This is explained by the presence of soil-related secondary aerosols in the site. In the paper by Barrera et al. [4], it was demonstrated (using PIXE, microPIXE and Positive Matrix Factorization) that S-rich soil particles are present in most of the MAMC, produced by chemical reactions of SO2 with primary soil-derived particulate matter. As for Fe, also higher concentrations were found, which must be due to local soil/resuspended dust sources. Finally, Zn and Pb present values very similar to the other publications, showing that the concentrations of these elements reached a stationary level. This must be carefully considered by the authorities to propose a solution. Although the referred studies were carried along a wide time span, for different sites and using different analytical techniques, it is perceived that, in general, the elemental concentrations reached uniform values around the MAMC, both temporally and geographically, a fact already commented by other authors [1,3], although in the past both S and Pb reached much higher concentrations than in the present. The Environment Ministry of the local Government, has reported that the trends for PM10 and PM2.5 concentrations at the MAMC have steady values since the years 2000 and 2003, respectively [23]. Thus, it is not surprising to find similar trends for elemental concentrations. The causes of this behavior are still under investigation. A further step in the analysis is the use of the EPA computer code UNMIX, which allows the identification of possible influencing sources in the site, providing a simple receptor model [24]. The model is applied only to a small fraction of the whole aerosol mass (namely, the elemental concentrations determined with PIXE and XRF). Thus, it must be emphasized that UNMIX will give information only on the sources of these elements, not about the complete composition of the airborne particles. Nevertheless, this procedure has been applied successfully in other studies related to other
Please cite this article in press as: R.V. Díaz et al., PIXE and XRF analysis of atmospheric aerosols from a site in the West area of Mexico City, Nucl. Instr. Meth. B (2013), http://dx.doi.org/10.1016/j.nimb.2013.05.095
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R.V. Díaz et al. / Nuclear Instruments and Methods in Physics Research B xxx (2013) xxx–xxx Table 2 24 h average concentrations of PM10 and PM2.5 (lg m3) for representative elements at various dates and sites from the MAMC. Site/date/fraction This work/2004/PM2.5 South/1993/PM2.5 [11] West/1994/PM2.5 [12] South/1995/PM2.5 [13] West/1996/PM2.5 [14] South/2001/PM2.5 [15] South/2002/PM2.5 [2] West/2003/ PM2.5 [16] Center/2004/PM2.5 [17] This work/2004/PM10 South/1995/PM10 [13] Center/2002/PM10 [18] Center/2002/PM10 [2] South/2006/ PM10 [19] Center/2009/ PM10 [4] a b c
S a
0.60 (0.06) 3.77 (0.35) 2.6 (0.28) 1.30 (0.10) 4.11 (0.39) 0.287 (0.022) 0.530 (0.038) 1.54 (0.12) 1.60b 1.41 (0.11) 1.60 (0.12) 2.63b 1.24 (0.95) N.A.c 1.3 (0.59)
Fe
Zn
Pb
0.10 (0.05) 0.198 (0.043) 0.37 (0.03) 0.260 (0.20) 0.385 (0.051) 0.053 (0.005) 0.110 (0.012) 0.18 (0.014) 0.133b 1.80 (0.07) 0.66 (0.050) 1.46b 0.672 (0.075) 1.0 b 0.84 (0.34)
0.012 (0.01) 0.096 (0.008) 0.13 (0.02) 0.110 (0.010) 0.202 (0.023) 0.017 (0.003) 0.014 (0.001) 0.091 (0.007) 0.108b 0.10 (0.01) 0.15 (0.015) 0.2b 0.110 (0.010) 0.098 b 0.095 (0.047)
0.009 (0.001) 0.081 (0.006) 0.25 (0.05) 0.059 (0.005) 0.100 (0.010) 0.009 (0.001) 0.008 (0.001) 0.040 (0.003) 0.068b 0.056 (0.001) 0.081 (0.009) 0.08b 0.018 (0.002) 0.026 b 0.088 (0.058)
Numbers between parenthesis represent the combined uncertainty. No uncertainty was quoted. N.A.: not available.
small components of the total aerosol mass, such as volatile organic compounds [25] or polycyclic aromatic hydrocarbons [26]. Total gravimetric mass was considered as the reference total variable in UNMIX, so the source profiles are given in lg m3. The results of this procedure are found in Fig. 1, where the contributions of the identified emitting sources are shown. Four factors were identified in the coarse fraction (two types of soil/fugitive dust sources, identified as A and B, Soil secondary and Industry); these factors contribute with 1%, 11%, 16% and 1% to total gravimetric mass, respectively. In the fine fraction only three sources were seen (Fuel oil, Biomass burning and Traffic/industry); here, the contributions
Fig. 1. Source profiles for the (a) coarse fraction and (b) fine fraction.
to gravimetric mass are 2%, 1% and 4%, respectively. For PM10-2.5, the elements Cr, Ni, Cu and Pb do not appear in the model because UNMIX took them as weak elements, due to their low concentrations. It must be noticed that the contributions of the elements that can be measured with PIXE in the fine fraction are much lower than in the coarse fraction, which is explained by the high contents of organic carbon, elemental carbon and nitrates in the fine aerosols. The Soil A and B sources are identified mainly by the presence of the crustal elements Al, Si, K, Ca, Ti, Mn and Fe. The proportions of these elements, though, are different in each type of soil, in particular Ca, which is much lower in A than in B, while Ti is higher in Soil A. Investigations are under progress to characterize possible sources of soil-derived airborne particles. The industrial source is typified by the inclusion of P, S, Ti, V and Zn. Finally, the presence of the Soil Secondary factor is in full agreement with the findings of Barrera et al. [4]. It is remarkable that the profile for Soil Secondary is very similar to that of Soil B, except for the S contents. This strongly suggests that the Soil Secondary source could be attributed to a Soil B reacting chemically with SO2, as remarked in Ref. [4]. Furthermore, if the ratio of several contributing elements to the Si concentration in the Soil B profile is compared to the ratios for the Soil 2 source in the Center site from the latter study [4], a clear resemblance is perceived (Fig. 2). Thus, a common source
Fig. 2. Comparison of Soil B profile in the coarse fraction and Soil 2 in PM10 from the work by Barrera et al. [4].
Please cite this article in press as: R.V. Díaz et al., PIXE and XRF analysis of atmospheric aerosols from a site in the West area of Mexico City, Nucl. Instr. Meth. B (2013), http://dx.doi.org/10.1016/j.nimb.2013.05.095
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R.V. Díaz et al. / Nuclear Instruments and Methods in Physics Research B xxx (2013) xxx–xxx
4. Conclusions The results obtained in this work showed in general lower elemental concentrations in the atmospheric aerosols collected in Cuajimalpa de Morelos than in other sites in the MAMC. In this regard, the initial hypothesis is correct. Nevertheless, with the receptor model obtained with UNMIX, it was possible to see that there is an influence of industrial areas (Soil Secondary factor), as well as biomass burning. A possible stabilization of elemental levels is noticed, suggesting that other regulations may be required to further decrease those values in the MAMC. References
Fig. 3. Representative back-trajectories obtained with HYSPLIT [21] and fire spots localized with the Web Fire Mapper [22], for the PM2.5 Biomass burning factor episode on December 29, 2004.
can be identified for both sites, regardless of the geographical and temporal differences. Regarding the factors in the fine fraction, the usual set of tracers S–V–Ni are found with the highest concentrations in the Fuel oil component. The elements found in the Biomass burning factor (S, Cl, K, Ca, Ti and Fe) are in complete agreement with those described as components of forest fire emissions in the US-EPA Speciate 4.3 database [27]. Finally, the most important source in this fraction (Traffic/industry) contains the elements S, Cl, V, Mn, Fe, Cu, Zn and Pb, which are undoubtedly anthropogenic contributions associated either to vehicle or industrial emissions. The inspection of the time series for all the factors identified with UNMIX signals several episodes for each source during the sampling period. The simulation model HYbrid Single-Particle Lagrangian Integrated Trajectory, better known as HYSPLIT [28] was then employed to calculate back-trajectories during the episodes spotted with UNMIX. A period of 24 h (24 trajectories per day, with 24 points per trajectory, corresponding each point to 1 h), and three different heights (500, 1000 and 1500 m, as recommended) above ground level were taken into account for the simulations, to overcome the low resolution limitations of the simulation software. Similar remarks about HYSPLIT results as those given by Barrera et al. [4]can be applied to the present work. An example of representative back-trajectories is given in Fig. 3, during an episode for the Biomass burning factor in PM2.5 on December 29, 2004. For this, it is also helpful to make use of the open access resource known as Web Fire Mapper (http://earthdata.nasa.gov/data/near-real-time-data/firms/), which is a part of MODIS41, 45 (moderate-resolution imaging spectro-radiometer) [29]. In this particular episode, the back-trajectories come mostly from the West, with a few contributions from Southwest and Northwest. There is a good agreement in the trajectories for all the heights used in the reproduction. Additionally, probable fire spots obtainedaround that date, together with the simulations by HYSPLIT, appear in the figure. It must be noted that the back-trajectories coincide with many forest fires, suggesting strongly that the winds transported the emitted particles towards the sampling site. In every case, the fire spots were only present during the episodes. Very similar inferences as in [4] are obtained here, with all the factors identified with UNMIX. For instance, Secondary soil corresponds to back-trajectories from fugitive dust sources crossing industrial areas and Soil episodes to direct back-trajectories from agricultural areas.
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Please cite this article in press as: R.V. Díaz et al., PIXE and XRF analysis of atmospheric aerosols from a site in the West area of Mexico City, Nucl. Instr. Meth. B (2013), http://dx.doi.org/10.1016/j.nimb.2013.05.095