Use of PIXE analysis to study urban atmospheric aerosols from downtown Havana City

Aplicación del análisis PIXE al estudio de aerosoles atmosféricos urbanos en el centro de Ciudad de La Habana

Grizel Pérez Zayas1, , Ibrahin Piñera Hernández1 , Maridelin Ramos Aruca1, Rolando Guibert Gal 1 , Enrique Molina Esquivel2, Miriam Martínez Varona2, Ariadna Fernández Arocha2 , Francisca Aldape Ugald 3 , Javier Flores Maldonado3

1 Departamento de Física, Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear, La Habana, Cuba.
2 Laboratorio de Contaminantes Atmosféricos, Instituto Nacional de Higiene, Epidemiología y Microbiología, La Habana, Cuba
3 Laboratorio de Análisis PIXE, Instituto Nacional de Investigaciones Nucleares, México D.F., México. ]]> ipinera@ceaden.edu.cu

ABSTRACT

The present work shows the results of a first study aimed at determining the elemental composition in airborne particulate matter (in fine and coarse particle size fractions). It was collected at the Atmospheric
Monitoring Station in the Municipality of Centro Habana, using the Particle Induced Xray Emission (PIXE) technique. At present, there is no information available about elemental contents in airborne particulate matter from this region. For this study, we carried out a sampling campaign during five months (November 14, 2006 to April 19, 2007). The samples were collected every second day during 24 h under an air flux of 20 l/min. The air sampler used was a Gent Sampler equipped with a Stacked Filter Unit (SFU) system which allows the aerosol collection in both size fractions simultaneously. A total of 144 aerosol samples were collected (72 correspond to the fine mass particle and 72 to the coarse mass particle). For PIXE analysis, the samples were irradiated by 2.0 MeV energy protons from the 2 MV Tandetron Accelerator from the Laboratory of PIXE analysis at ININ, Mexico. A total of 14 elements (S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Br and Pb) were consistently detected in both of particle size fractions with minimum detection limits in the range of 1-10 ng/ . The quantitative results obtained from PIXE elemental analysis for mass of particles in both fractions have revealed important information that has been used in a first attempt to understand and characterize the atmospheric pollution in this area. A general discussion about these results is presented in this paper.

RESUMEN

El presente trabajo reporta los resultados de un primer estudio realizado para determinar la composición elemental en material particulado atmosférico (en tamaños de partícula fina y gruesa) colectado en la Estación de Monitoreo Ambiental del Instituto Nacional de Higiene y Epidemiología, del Municipio de Centro Habana, utilizando la técnica PIXE (emisión de rayos X inducida por partículas). Para este estudio se realizó una campaña de muestreo durante cinco meses, entre los meses de noviembre, 2006 y abril, 2007, con un período de colección de una toma cada segundo día y 24 horas de duración. Se utilizó un muestreador de aire del tipo Gent con posibilidades de colectar de forma simultánea el material particulado fino y grueso. Un total de 144 muestras de MPA se colectaron para este estudio (72 de fracción fina y 72 de fracción gruesa). Las muestras se irradiaron tras la incidencia de un haz de protones de 2,5 MeV. Un total de 14 elementos (S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Br y Pb) fueron consistentemente detectados en la mayoría de las muestras, con límites mínimos de detección entre 1-10 ng de partículas/ de aire. Los resultados del análisis PIXE para ambos tamaños de partícula revelaron una importante información que se ha utilizado en una primera intención para entender y caracterizar la contaminación atmosférica del área muestreada. Se presenta una discusión general de estos resultados en este trabajo.

Key words: particulates, air pollution monitoring, pixe analysis, urban areas

Introduction

Atmospheric pollution studies in more populated cities of the world have been an important issue to diverse authors in recent years, because of the relevant impact that pollution has on the health of the population [1-6]. One of the most important factors of air pollution problem in Havana City is the presence of airborne particles, which are responsible for effects on public health, especially when their dimensions are below 10 ìm, as they can penetrate deeply into the human respiratory tract, reason for which studying the properties of these kind of pollutants and its elemental characterization have become an important task to achieve.

PIXE is a nuclear analytical technique that has long been applied to the study of atmospheric aerosols and it continues as one of the most important areas of application [9-15]. The technique has certainly contributed to the understanding of source-receptor relationship for aerosol particles as well as to aerosol physics and chemistry. It is a method based on charged particle accelerators, that allows fast simultaneous analysis of all elements with atomic number larger than 13 (> Al), with a high sensitivity (around 1 ng of an element per 1 of air), nondestructive and extremely small targets can be effectively analyzed because of the well defined dimension of the incident proton beam on the samples. PIXE does not offer information on chemical states of elements and it is not applicable for lighter elements, such as C, N, H and O, which generally occupy more than 80% of the atmospheric aerosols, but as in this type of analysis the sample is not destroyed, PIXE aerosol analysis can be complemented with other techniques. Examples of theme are PESA (proton elastic scattering analysis) for measurement of hydrogen, PIGE (proton induced gamma ray emission) for detection of light elements, like B, Li, Na, Mg, Al, Si and P and RBS (rutherford backscattering) for measurement of C, N and O, allowing to obtain a complete reconstruction of the aerosol mass and to achieve a more comprehensive view of the characteristic elements of the aerosol.

Due to all these advantages in order to determine elemental concentrations of a variety of airborne pollution samples, the use of PIXE technique is recommended over other ones that can be used too for aerosol analyses.

In the present work, the quantitative results obtained from PIXE elemental analysis for fine and coarse particles mass collected at the Municipality of Centro Habana, Cuba, are presented.

Experimental measurements

The aerosol samples were collected during five months (from November 14, 2006 to April 19, 2007), every second day, during 24 h (00:00 h-24:00 h) each one at an average flow rate of 20 l/min. The sampler used in this study was a Gent Sampler with a Stacked Filter Unit (SFU-PM10\PM2,5) system [9, 16]. This sampling device allowed the particulate collection into two size fractions simultaneously: particles PM2,5 smaller than 2.5 µm (fine fraction) and particles PM2,5-10 with aerodynamic diameter between 2.5 µm and 10 µm (coarse fraction). The particles were collected on a 47 mm diameter Nucleopore polycarbonates filters; coarse mode on filters with 8.0 µm pore size and fine particles on filters with 0.4 µm pore size. A total de 144 aerosol samples were collected (72 correspond to the PM2,5 fine mass particle and 72 to the PM2,5-10 coarse mass particle).

The total amount of mass deposited of fine and coarse particles on the samples were obtained by gravimetric analysis. The filters were weighed before and after sampling in a Cahn 33 electronic microbalance with 0.1 µg sensitivity. Before weighing, all filters were stabilized during 48 h under constant relative humidity and temperature conditions (45% and 22 °C). An alpha source of was used to eliminate static charge on the samples during weighing. Measured fine particulate matter (FPM) and coarse particulate matter (CPM) mass concentrations (µg/) are shown in figure 1. Levels of PM2,5-10 up to 44.2 µg/ and PM2,5 values up to 22.4 µg/ were obtained.

Other analyt ical equipments operat ing simultaneously at this station provided complementary data of several variables such as and .

PIXE analysis. The experimental system of PIXE analysis consists of an accelerator, an ion beam, a vacuum irradiation chamber with a target holder included and a measurement system. ]]>

The aerosol samples, without any pre-treatment, were bombarded by 2.5 MeV protons from the 2 MV Van de Graff (Tandetron) accelerator, located at the Laboratory of PIXE Analysis of the ININ, Mexico. Ion beam currents in the system were 15 nA and total integrated charges were 6 µC. For the irradiation, the samples were placed in a sample holder with an angle of 45° with respect to the beam direction. The characteristics X-rays emitted from the samples were detected with an Ortec Si(Li) detector with an active area of 80 mm 2 and 200 eV resolution at 5.9 keV of the Mn K a line, coming from an 55 Fe source. The detector in the experimental arrangement was located outside the irradiation chamber at 90° angle with respect to the ion beam direction and is coupled to standard electronics and PC based multichannel analyser. Spectra were obtained in this experimental line using EG&G Ortec Maestro II computer code [17]. The precision and reproducibility of the PIXE system were verified on a regular basis, using spectroscopically pure thin films of known area density, deposited on Nucleopore filters (MicroMatter Co., Deer Harbor, WA, USA). Details of the experimental setup, instrumentation and analytical procedures used to perform the PIXE analysis at the ININ PIXE Laboratory are better explained in reference [18].

For the elemental analysis the filters with the aerosol were cut in two pieces: 1/4 of it was analysed by PIXE; the remaining 3/4 was kept for other possible measurements.

The PIXE analysis revealed the presence of 14 elements in both particle size fractions, ranging from sulphur (S) to lead (Pb). Irradiation time was in the interval 10-15 min. Elemental minimum detection limits (MDL) were in the range of 0.1-10 ng/ for most of the detected elements. The precision of the elemental concentration measurements was typical in the range of 5-10%.

The evaluation of the PIXE spectra was performed with the softwares AXIL v3.0 and WinAxil v4.5.3 [19]. Two of the obtained PIXE spectra are shown in figure 2 for the airborne particulate matter PM2,5 and in figure 3 for PM2,5-10.

Results and Discussion ]]>

The basis of the present work is the elemental mass concentrations of 144 aerosol samples (fine and coarse) analysed by PIXE. The PIXE analysis permitted to identified and to quantify the elemental content of the airborne particulate matter (APM) deposited on the filter and revealed consistently the presence of the following 14 elements: S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Br and Pb; in both particle size fractions. A summary of the elemental concentrations for the PM2,5 and PM2,5-10 particles is presented in table 1.

The table allows an easy comparison of the results obtained for each particle size fraction. The information presented shows the MDL of the system for each element, the number of appearan-ces of each element above the MDL (n) and the mini-mum, maximum, mean and standard deviation values reached by each element. As can be seen from the values showed in the table 1, remarkable differences are observed between fine and coarse components.

The fine particle fraction shows significantly higher concentrations of S, than the coarse fraction and significantly lower values for K, Ca, Ti, while the coarse fraction has higher contents of Cl than the fine one and soil dust related elements, such as Ca, K, and Fe, dominate the elemental mass. Pb was found not in all samples above the MDL, but it was found in both the fine and coarse modes in significant quantities. Higher values of V, Cr, Ni, and Zn were observed too in the fine fraction, bromine concentration was found apparently uniform in both fractions.

Some elements (like S, V, Ni, Zn, Br and Pb) are considered element of anthropogenic origin and are present mainly in the fine fraction. Other elements (like K, Ca, Ti, Fe, and Cl) are considered element of natural origin and appeared mainly in the coarse fraction. These tendencies correspond to the chemical characteristics of atmospheric aerosols [20].

Sulphate particle is an important component of the aerosol in this area, mainly originating from gas to particle conversion from . S is considered an anthropogenic element, which is most likely present in the fine aerosol. Sulphur was present in all samples in concentrations ranging from 121 ng/ to 1712 ng/ of air sampled, however appeared significantly higher in the fine fraction of aerosol.

The elemental concentrations determined by the PIXE analysis were used to analyse the temporal variations of each element. The concentrations in the complete collection period are shown in figure 4 for S in fine mode and in figure 5 for Cl in coarse one.

In the temporal variation of the PM2,5 and PM2,5-10 mass concentrations showed in figure 1, the peaks of particulate concentrations coinciding with the periods of changes in the weather conditions, when dispersion of the contaminants is more difficult, i.e. entrance of a cold front with temperature decreasing and high pressure. High concentration for some elements, as the S in fine mode and Cl in coarse mode were also observed in these days (see figure 4 and figure 5).

Conclusions ]]>

The results obtained in this work represent a first effort to evaluate the elemental composition in the airborne particulate matter (fine and coarse) in an urban area, heavily populated, of the Havana City with the use of nuclear analytical techniques.

A total of 144 aerosol samples were collected (72 correspond to the fine mass fraction and 72 to the coarse mass fraction). PIXE analysis revealed the detection of 14 elements (S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Br and Pb) in both particle size fractions.

Toxic elements (like S, V, Ni, Zn, Br and Pb) were observed with higher concentrations in the PM2,5 fine mode and are mainly considered element of anthropogenic origin, aspect that should be considered in the impact and health studies that are carried out in this place. Those elements that are considered from natural origin (like K, Ca, Ti, Fe, and Cl) appeared mainly in the coarse fraction.

PIXE has showed that is a suitable technique for the analysis of atmospheric aerosol and particularly for the fast and routine analysis of aerosol particulates collected on filters. It is a technique highly sensitive and multielemental, no sample preparations is required, analyses can be automated and carried out in few minutes, the low detection limits allow particle fractions to be observed and the sample is not destroyed, allowing further characterization using other techniques in the same filter. By means of PIXE, it is possible to measure a sample of few micrograms in a few minutes reaching MDL’s in the order of ng/m 3 for most of the elements (Z>11).

These results obtained by the PIXE analysis have revealed important informat ion about elemental composition of airborne particulate matter PM2,5 and PM2,5-10 from sampled area, and especially about the inhalable fine fraction, from which didn’t exist any information. It is important to notice that with this technique was possible to measure significant elements in environmental and health related studies like S and Pb, which are difficult to detect and evaluate using other techniques.

Furthermore, the elemental database obtained from the PIXE analysis combined with gas measurements and statistical analysis will allow extracting a first idea of the pollution sources of the area under study, but much more work is required. It is necessary to get much more information, for example, if it is possible to sampling over a longer time (1 year), giving the possibility of study the elemental seasonal variations in a whole year, also to be able to correlate those profiles with meteorological varia-bles and with larger amounts of samples get better statistically results.Furthermore, the analysis can be complemented on the same samples with other physics and chemical analytical techniques, as the ion beam analysis techniques for detection of light elements, ion chromatography for cation ( , , , ) and anion (, , ) species, reflectance technique for black carbon concentration, and gas chromatography – mass spectrometer for organic compound characterization.

All this will allow obtaining a complete scheme about atmospheric pollution in the area, allowing to understand how it is affecting life of people in the cities densely populated and in order to establish air pollution control and reduction programs.

Acknowledgements

This work has been financially supported by the International Atomic Energy Agency (IAEA) under the Regional Project ARCAL RLA7011. We are deeply grateful to F. Aldape and J. Flores, from PIXE Laboratory at the ININ-Mexico, for their kind collaboration. We also appreciate the technical support and assistance provided by the technical staff of that Laboratory during gravimetric, irradiation and PIXE analysis of the samples.

References

[1] NEL A. Air Pollution-Related Illness: Effects of Particles. Science. 2005; 308(5723): 804-806.
[2] O'NEIL MS, LOOMIS D, TORRES-MEZA V, et. al. Estimating particle exposure in the Mexico City metropolitan area. Journal of Exposure Analysis and Environmental Epidemiology. 2002; 12(2): 145-156.
[3] ALDAPE F, FLORES MJ, FLORES AJ, et. al. Elemental Composition and Source Identification of PM 2,5 particles collected in downtown Mexico City. International Journal of PIXE. 2005; 15(3-4): 263-270.
[4] ARTAXO P, GERAB F, RABELLO MLC. Elemental composition of aerosol particles from two atmospheric monitoring stations in the Amazon Basin. Nucl. Instr. Meth. B. 1993; 75(1-4): 277-281.
[5] ALVES LC, REIS MA, FREITAS MC. Air particulate matter characterization of a rural area in Portugal. Nucl. Instr. Meth. B. 1998; 136(1-4): 941-947.
[6] BRAGA CF, TEIXEIRA EC, MEIRA L, et. al. Elemental composition of PM 10 and PM 2.5 in urban environment in South Brazil. Atmospheric Environment. 2005; 39(10): 1801-1815.
[7] JOHANSSON SAE, CAMPBELL JL. PIXE: a novel technique for elemental analysis. Chichester: John Wiley & Sons, 1988.
[8] MAENHAUT W, MALMQVIST KG. Particle-Induced X-ray Emission Analysis. In: Handbook of X-Ray Spectrometry. Second Edition. New York: 2002. p 719-809.
[9] MAENHAUT W, FRANÇOIS F, CAFMEYER J. 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 Report NAHRES-19. In: Applied Research on Air Pollution using Nuclear-Related Analytical Techniques. Vienna: IAEA, 1994. p. 249-263.
[10]ALDAPE F. Uso de los Aceleradores en la Búsqueda de Soluciones a la Problemática Ambiental: Trascendencia Social. In: Experiencia Mexicana en Aceleradores de Partículas. Serie: CIENCIA Y TECNOLOGÍA EN LA HISTORIA DE MEXICO. Editorial Siglo XXI, 2004. Primera Edición. p. 203.
[11]ALDAPE F, FLORES MJ, DÍAZ RV, et. al. Upgrading of the PIXE system at ININ (Mexico) and report on elemental composition of atmospheric aerosols from 1990 in the ZMCM. Nucl. Instr. Meth. B. 1996; 109: 459-464.
[12]ALDAPE F, FLORES MJ, GARCÍA R, et. al. PIXE analysis of atmospheric aerosols from a simultaneous three site sampling during the autumn of 1993 in Mexico City. Nucl. Instr. Meth. B. 1996; 109: 502-505.
[13]FLORES MJ, ALDAPE F. PIXE study of airborne particulate matter in northern Mexico City. International Journal of PIXE. 2001; 11(1-2): 61-67.
[14]MIRANDA J, LÓPEZ SUÁREZ A , PAREDES GUTIÉRREZ R, et. al. A study of atmospheric aerosols from five sites in Mexico city using PIXE. Nucl. Instr. Meth. B. 1998; 136(1-4): 970-974.
[15]FLORES MJ, ALDAPE F, DÍAZ RV, et. al. PIXE analysis of airborne particulate matter from Xalostoc, Mexico: winter to summer comparison. Nucl. Instr. Meth. B. 1999; 150(1-4): 445-449.
[16]HOPKE PK, XIE Y, RAUNEMAA T, et. al. Characterization of the Gent Stacked Filter Unit PM 10 Sampler. Aerosol Science and Technology. 1997; 27(6): 726-735.
[17]EG&G ORTEC Maestro II Emulation Software [software informático]. V.1.10. Oak Ridge, 1990.
[18]FLORES MJ, ALDAPE F, DÍAZ RV, et. al. Set-up and improvements of the PIXE facility at ININ, Mexico. Nucl. Instr. Meth. B. 1993; 75(1-4): 116-119.
[19]AXIL. Quantitative X-ray Analysis System. [software informático]. V.3.3 and WinAxil V.4.5.3. Developed under the auspices of the International Atomic Energy Agency (IAEA). Vienna, Austria.
[20] KASAHARA M. Characterization of atmospheric aerosols and aerosol studies applying PIXE analysis. In: Analytical Chemistry of Aerosols. CRC Press LLC, 2001. p. 145-171.
[21]WEAST RC. CRC Handbook of Chemistry and Physics : A Ready-Reference Book of Chemical and Physical Data. 57th ed. Cleveland: CRC Press, 1977. p. 203.
[22]THURSTON GD, SPENGLER JD. A quantitative assessment of source contributions to inhalable particulate matter pollution in metropolitan Boston. Atmospheric Environment. 1985; 19(1): 9-25.
[23]GREENBERG RR, GORDON GE, ZOLLER WH, et. al. Composition of particles from the Nicosia Municipal Incinerator. Environ Science and Technol. 1978; 12(12): 1329-1332.

Recibido: 14 de octubre de 2009
Aceptado: 12 de noviembre de 2009