Metal Content in Air Samples Collected in an Urban Zone in Tampico, Mexico: A First Survey

By Flores-Rangel, R M Rodriguez-Espinosa, P F; de Oca-Valero, J A Montes; Mugica-Alvarez, V; Ortiz-Romero-Vargas, M E; Navarrete- Lopez, M; Dorantes-Rosales, H J

ABSTRACT With the aim to know possible risks to the population, Cd, Co, Cr, Cu, Mn, Ni, Pb, and Tl were determined for the first time in airborne samples of particulate matter in an urban zone in Tampico, Mexico, during the winter of 2003. The 24-hour PM^sub 10^ samples were collected every 6 days on quartz-filters by using a high volume sampler and analyzed by Inductively Coupled Plasma Optical Emission Spectrometer. Standard reference material was used to verify metal recovery. The maximum PM^sub 10^ and lead concentrations were 12.05 and 0.040 [mu]g/m^sup 3^, respectively, not exceeding Mexican standard values. The greatest metal concentration was that of manganese with 0.90 [mu]g/m^sup 3^, followed by Cu and Ni with 0.17 and 0.012 [mu]g/m^sup 3^, respectively. Agglomerates, well-defined particles, and heavy metals (e.g., Mn and Cu) were found in PM^sub 10^ using Scanning Electron Microscopy and Energy Dispersive Spectroscopy. Meteorological conditions associated with the sampling period showed that Pb and Ni are being continuously emitted, and that Mn, Cu, and Co could come from one industry located to the WSW of the region. All of these concentrations do not constitute a potential risk to human health, although it is necessary to continue studying the high concentrations of Mn and Cu in longer sampling periods.

Key Words: PM^sub 10^, air pollution, Mexico, airborne lead, airborne manganese.

INTRODUCTION

Particulate matter in air includes a wide variety of solid and liquid particles, organic and inorganic materials; it is classified as particles less than (or equal) to 2.5 mum, usually called PM^sub 2.5^ or fine particles. Particles less than (or equal) to 10 mum are well known as PM^sub 10^, and total particulate matter (TSP) are those that are less than 100 mum (WHO 2000). Particles less than 0.1 mum are formed by nucleation processes, associated with diverse chemical reactions that take place in the atmosphere capable of creating new particles. The fine particles are mainly emitted by combustion processes and condensation and transformation of atmospheric gases (Moragues 1999), and are composed as follows: 31% organics, 14% elemental carbon, 29% secondary inorganic aerosols, 14% geological material, and less than 2.4 and 1.8% of non-crustal elements and sodium chloride salt, respectively. The coarse particles come from mechanical and anthropogenic processes, soil, and wind erosion, which are mainly composed of geological material (48%), organic carbon (23.1%), elemental carbon (8.4%), secondary inorganic aerosols (19%), non-crustal elements (2.3%), and salt 1.2% (Chow et al. 2002a).

Although the aerosols have been widely studied around the world (Lopes et al. 2006; Okuda et al. 2004; Chen et al. 2003), in Mexico this material has been principally studied in Mexico City (Chow et al. 2002b; Mugica et al. 2002; Diaz et al. 2002; Barfoot et al. 1984) and in some other cities like Monterrey (Aldape et al. 1999b), Hidalgo (Aldape et al. 1999a), San Luis Potosi (Aragon-Pina et al. 2002), and Colima (Miranda et al. 2004). The RAMA1 was established in 1986, but historical databases are available since 1990 (SIMAT 2007). In addition to RAMA monitoring, the air quality in Mexico has been studied under several comprehensive projects (EGCAMARY, IMADA, Azteca, MCMA-2003 and MILAGRO-2006) that allow explaining the nature and causes of particulate concentrations (Los Alamos 1994; Edgerton et al. 1999; Raga et al. 1999; Molina and Molina 2002). On the other hand, some other projects, such as Mexicali Imperial Valley and BRAVO have been conducted along the Mexico-U.S. Border (Chow et al. 2000, 2004).

However, there is a need to generate more information about aerosol characterization in other important cities in Mexico, such as Tampico, due to the rapid growth of human settlements and industries observed in the region, many of them related to petrochemical activities. Tampico is one of the most important commercial, tourist, and industrial ports, located in the south of the Tamaulipas border state between the U.S. and Mexico, next to the Gulf of Mexico, due to the oil extraction activities as well as a very important refining industry that processes oil from several parts of Mexico. The aim of this article is to study the PM^sub 10^ and metal concentrations in an urban monitoring site in Tampico during a 3-month winter period in order to generate the scientific basis for implementing airborne metal regulations. The PM^sub 10^ was continuously collected in Tampico every 6 days according to the monitoring schedule established by the U.S. Environmental Protection Agency (USEPA) and to the Mexican regulations adopted in 1994 (NOM- 025-SSA1-1993). The monitoring station, operated by the government of Tampico since 2002, belongs to the government of Tamaulipas State as part of the state monitoring network (Red Estatal de Monitoreo Atmosferico, REMA).

METHODOLOGY

Sampling Site

This study was performed in the urban zone of Tampico, south of the state of Tamaulipas, Mexico. The population of Tampico city is 295,442 inhabitants, although there are 65,991 vehicles, mainly private automobiles and public transportation (INEGI 2000). Other relevant activities in Tampico are commerce and tourism. The industrial sector, with 23 petrochemical industries and 1 thermoelectric, is located at the north of this city. An oil refinery and a ferromanganese industry are located to the NE and W. The total PMi0 emissions (industries, mobiles, commerce, and services) generated 10,388 Mg in 1999 (SEMARNAT 2006). The monitoring site in Tampico (22[degrees]13’09” N, 97[degrees]51’29.5” W) located in an urban zone of the commercial port is shown in Figure 1. This site was established by federal environmental authorities (INE), because it is located in Tampico’s downtown 5 m above ground level and within the wake of nearby buildings and vegetation. The meteorological station is also located downtown, 200 m south from the sampling site. The particulate matter and metal concentration at this sampling site depend on dust suspension, vehicle emissions, industrial and port activities near the zone and are directly influenced by wind direction and precipitation.

Particulate Collection

PM^sub 10^ samples were collected during the 24 hours from midnight to midnight at a flow rate of 1.0-1.1 m^sup 3^/min (1470 m^sup 3^ for 24 hours) every 6 days by a high volume air sampler (Wedding and Associates), which was 5.0 m above ground level at the urban site. The high volume sampler is a device for sampling large volumes of an air atmosphere for collecting the contained particulate matter by filtration. It consists of a high-capacity blower, a filter to collect suspended particles, and a means for measuring the flow rate (USEPA 1999a). A modified Thermo inlet is used as the sampling inlet for the PM^sub 10^ beta gauge. The inlet employs an omnidirectional cyclone fractionator, which allows the particles to enter from any angle of approach. Particle removal is realized on the oiled surfaces of the inner collection tube (USEPA 1999b).

Monitoring was conducted in 20.3 x 25.4 cm Whatman quartz microfiber filters, which were stabilized in a controlled temperature and humidity room before and after sampling for gravimetric procedures. Meteorological parameters were measured at downtown Tampico by CNA.2

Gravimetric Procedures

The concentrations of PM^sub 10^ were obtained by the Secretaria de Obras Publicas Desarrollo Urbano y Ecologia in the Laboratorio Ambiental in Tamaulipas, according to Mexican Standard NOM-035-ECOL- 1993. The filters were stabilized 24 h before and after sampling in a stabilization room, with controlled temperature and humidity (25 +- 10[degrees]C and 50 +- 5 humidity). The PM10 concentration was calculated in fig/m* as the difference between the filter weight before and after sampling divided by the volume of sampled air (corrected to standard conditions of 25[degrees]C and 760 mm Hg).

Chemical Analysis

To determine metal content in PM^sub 10^ 2.5 cm x 20 cm, samples taken from the filter were digested with suprapure hydrochloric and nitric acid according to the microwave program established by Method IO 3.1 (USEPA 1999c). The solution obtained was filtered with Nylon 0.45 mu Millipore membranes and placed in sterile 15 ml polypropylene tubes. Metal concentrations in PM^sub 10^were determined by Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES) using Perkin Elmer model 5000 equipment at the Centro de Investigacion y Desarrollo Tecnologico of Penoles Co.

Data Validation

The data validation was performed by a chain of custody in order to maintain the internal and external consistency of the field and laboratory measurements to avoid common mistakes among the state, county, and academic institutions involved. This process was constituted by some steps like the identification of the sample with a specific code, dilution, and analysis of the samples, among others. A standard reference material (High Purity QGTMFM-A) was used not only to validate and verify the laboratory digestion program accuracy and efficiency, but also to calculate metal recoveries, % Rec, and traceability coefficients. The metal content in this material was extracted and quantified as previously explained. The metal recoveries were then calculated as the ratio between the concentration determined by the ICP-OES and the standard concentration according to Eq. (1). %Rec = SRM determined concentration/SRM standard concentration = 100 +- 15 (1) where SRM = Standard Reference Material.

The recovery of the metals should be between 85 and 115% if a small quantity of metal is presented in the sample (Franson 1998). As a part of the validation process, the traceability coefficient for each metal was calculated in order to correct the results of metal recoveries as the inverse of Eq. (1). A coefficient close to 1 indicates that the extraction was correctly obtained and that the concentrations determined by the analytical mediod are close to standard concentrations in the filter sample. In addition, intercomparison analyses were performed with the Environmental Laboratory of Applied Chemistry at Universidad Autonoma Metropolitana-Azcapotzalco using a Thermo Jarrel Ash ICP, which showed no significant differences.

Morphology

The shape, size, and elemental composition of individual atmospheric particles were established by die Jeol JSM-5300 SEM- EDS. The filter samples were coated in vacuum with an Au-Pd thin film in order to minimize charging effects during SEM observations and X-Ray microanalysis.

RESULTS AND DISCUSSION

A PM^sub 10^ and Meteorological Conditions

A PM^sub 10^ limit was established by the Mexican government in 1993 as 150 [mu]g/m^sup 3^, which was modified in year 2006 as 120 [mu]g/ms for a 24-h sampling period (NOM-025-SSA1-1993). PM^sub 10^ concentrations during October-December 2003 in Tampico are shown in Table 1. These were the first results obtained and reported for this city. They are less than the permissible limit and are not comparable to results for Mexico City. For example, Marquez et al. (2004), reported 734.8, 993, and 394.6 [mu]g/m^sup 3^ as maximum concentrations for Xalostoc, San Agustin, and Iztapalapa, respectively, in February and March 2003. Mugica et al. (2002) reported average concentrations of 172,89,86,74, and 46 [mu]g/m^sup 3^ for the respirable fraction (PM^sub 10^) measured over 3years at Xalostoc, Cerro de la Estrella, Tlalnepantla, Merced, and Pedregal sites, respectively. In this work, the maximum and average values presented were 12.05 [mu]g/m^sup 3^ and 4.4 [mu]g/m^sup 3^, respectively. These concentrations are quite lower in comparison with the reported in other studies, not exceeding the Mexican standard.

The data in Table 1 also present relevant meteorological conditions in Tampico related to the sampling period. During this period, winds from ESE predominated in the afternoons and some winds from W, NW, N, and NNE were also observed. Wind speed ranged from 1 to 4.5 ms^sup -1^. The highest wind velocity was detected with winds coming from the N and NNE. Some precipitation was observed during three sampling days in October and November.

The PM^sub 10^ concentrations did not correlate with wind speed, which means that higher or lower PM^sub 10^ concentrations could be obtained during different wind speed conditions. On the other hand, wind direction, which is also important to localize sources of pollution (Wawros et al. 2003), will be used to study the possible metal origin in Tampico.

Metal Recoveries

The results presented in Table 2 show the metal recoveries obtained for each metal in the standard reference material. These results should be 100% +- 15%, which means that Co, Cr, Cu, Mn, Ni, and Pb were completely extracted from the sample and that Cd and Tl showed poor recoveries. Despite this fact, the data obtained were corrected with the traceability coefficient. More metal extractions from standard reference materials are highly recommended to study the digestion procedure and to improve Tl and Cd recoveries.

Result Validation

The traceability coefficients presented in Table 2 were used to correct the metal concentrations in each sample. As presented in Eq. (1), an optimal traceability coefficient should be close to one. Co and Mn were completely recovered and the concentrations of these metals in the PM^sub 10^ samples were corrected with factors of 0.992 and 0.98, respectively. Conversely, metals that were not completely recovered, namely Tl and Cd, showed values far from unity. Concentrations of these metals in PM^sub 10^ samples were also corrected with their corresponding factors.

Metal Content in PM^sub 10^

The Cd, Co, Cr, Cu, Mn, Ni, Pb, and Tl PM^sub 10^ concentrations in Tampico during a 3-month period in 2003 are shown in Figures 2a and 2b. In general, the concentration of metals measured for the first time in Tampico during the period studied are low in comparison with other urban areas, because during this time, PM^sub 10^ and metal dispersion prevailed in Tampico due to the flatness of the landscape of the coastal zone. Lead concentrations did not exceed the 1.5 mug/m^sup 3^ Mexican limit during this period of study as they were less than 0.04 mug/m^sup 3^, perhaps because the most important source of atmospheric lead (tetraethyl lead) has been eliminated from Mexican gasolines since 1996. Manganese showed high concentrations of 0.48, 0.24, and 0.89 mug/m^sup 3^ with winds coming from the WSW, suggesting that the source of manganese was a ferromanganese production industry, which is located in that direction. The results showed some trends in the concentration of heavy metals. Thus, Cd and Co presented low concentrations, except on one day where maximum concentrations were found (3.0 x 10^sup – 3^ and 6.0 x 10^sup -4^mug/m^sup 3^, respectively). Mn and Tl showed low concentrations, except at the beginning and at the end of the period studied. Pb and Ni showed high concentrations just in the middle of the period studied. Finally, Cr and Cu showed decreasing concentrations over the studied period. However, in order to confirm these suggestions, more time series analyses of wind direction and their correlation with metal concentrations will be performed.

The standard deviation as well as the mean, minimum, and maximum concentrations of PM^sub 10^ and metals in Tampico are shown in Table 3. The maximum and minimum concentrations of PM^sub 10^ were 12 and 4.4 [mu]g/m^sub 3^, with a standard deviation of 3.7 [mu]g/ m^sup 5^ among them. The maximum concentration of Pb was 0.04 [mu]g/ m^sup 3^. It means, as said before, that neither PM^sub 10^, nor Pb concentrations exceeded Mexican standards. High concentrations of metals were observed for Mn and Cu with maximum concentrations of 0.89 and 0.16 [mu]g/m^sup 3^. Otherwise, low concentrations of metals were showed by Co and Tl.

Table 4 was constructed from the data concentrations of metals in the 12 samples. It shows the correlation coefficients among metals: as stated before, a correlation coefficient near one suggests a relationship among metals and possible emission sources. The highest correlation was obtained for Cd-Co followed by Tl-Mn, Pb-Cd, and Pb- Co.

Although it is known that PM^sub 10^ and metals concentrations are related, possibly having the same sources, tins relationship was not clearly observed because of the low metal concentrations and the need for more sampling sites and longer sampling times. PM^sub 10^ and metal sources in Tampico may be within an industrial zone, an oil refinery, and a ferromanganese industry located to the N, NE, and WSW, respectively. Also vehicle emissions and dust could be the sources of particles and metals. Dominating winds in the urban zone of Tampico are from ESE with average wind speeds from 1 to 4 ms^sup – 1^ in the afternoons; the smallest concentrations of metals were found only in a day (on December 27), whose wind mainly came from the ESE, probably because there is not an important source of particles and metals in that direction.

Higher Co, Cu, and Mn concentrations were found during days when winds came from the WSW where there the ferromanganese production industry is located. Ni and Pb showed the same trend and higher concentrations were observed during periods of calm, suggesting that the source (located near the sampling zone) was mainly attributed to vehicles, as mentioned by Mugica et al. (2002). On the other hand Co, Cu, and Mn were not found during very low wind speeds, as they were collected during WSW-wind conditions. There was not a specific source for Tl, Cd, and Cr that could be correlated with meteorological parameters. Lower concentrations of these metals are associated with intermittent emissions from industries to the north of the sampling zone.

SEM Observations Contain

As mentioned by Umbria et al. (2004), the morphology and chemical composition of the particles are important factors that need to be investigated in order to determine the origin of particles. The use of SEM-EDS, in addition to ICP techniques, complements the analytic information and allows knowing the way the metals are contained in the particle as well as the presence of toxic elements not determined by ICP in this work. SEM investigations were performed to provide information on the shape, size, and other enclosed elements of some individual and agglomerate atmospheric particles. According to the morphology of aerosols, most of the irregularly shaped particles are classified as natural, and tiiose having spherical shape are associated with anthropogenic activities, mainly combustion sources (Wawros et al 2003).

The results obtained show significant differences in the morphology and chemical composition of the particles collected in an urban zone in Tampico during a 3-month period. Most of the particles were agglomerates consisting of smaller particles enriched by heavy metals such as Fe, Zn, Cu, Al, Tl (which present toxicity and represent a potential risk for population), and other elements like S and C, from combustion sources. Na, K, Mg, and Ca, characteristic of dust particles were also observed; the latter elements reveal the presence of natural aluminosilicates, feldspars and clays, as reported by Sobanska et al. (2003). Shown in Figures 3a and 3b are two examples of secondary electron image micrographs of the particles analyzed together with their corresponding X-ray spectra shown in Figures 3c and 3d. The micrographs obtained indicate the presence of spherical and irregularly shaped particles.

Large agglomerates of irregularly shaped particles (see Figure 3b), were classified as natural according to their composition (C, Zn, Al, O, Ca, K) (Figure 3d). On the other hand, a spherical and porous particle containing V, Mn, and Cu probably coming from an incinerator are shown in Figure 3a (Chow et al. 1996).

Comparison of Metal Concentrations

Table 5 shows a comparison of PM^sub 10^ and metal concentrations in some cities in the US, Mexico, South America and Europe. The highest PM^sub 10^ concentrations were observed in Mexicali (Chow et al 2001), Milan (Marcazzan et al. 2001) and Chilian (Celis et al. 2004) with 130, 110 and 86.2 [mu]g/m^sup 3^, respectively. The lowest PM^sub 10^ concentrations were observed in Tampico (This work) and Londrina (Lopes et al 2006). Although PM^sub 10^ concentrations in Chilian and Hong Kong are similar, the highest metal concentrations were observed in Hong Kong (Ho et al 2003). Chow et al (2001) reported higher concentrations of PM^sub 10^ and metals in the Mexican sampling zone than in the US. This table also shows that the metal concentrations reported in this work are low in comparison to other urban zones in Mexico and the world, except for Mn and Cu whose mean concentrations were 0.14 and 0.05 [mu]g/m^sup 3^, respectively. Low concentrations of metals in Tampico are due to the city’s flat topography, since this is a coastal zone, this situation allows the metals to dissipate quickly and be airborne with low concentrations.

CONCLUSIONS

Metal contents in particulate matter less than 10 [mu]m were measured for the first time in Tampico during a 3-month period. Eight metals were determined: Cd, Co, Cr, Cu, Mn, Ni, Pb, and Tl in 12-Whatman quartz filters using ICP-OES. The maximum, minimum, and average PM^sub 10^ concentrations were 12.05,1.52 and 4.4 [mu]g/ m^sup 3^, respectively, whereas Pb presented a maximum concentration of 0.018 fig/m3; neither PM^sub 10^ nor Pb concentrations exceeded the air quality standards established in Mexico. The most abundant metal was Mn with an average of 0.154 [mu]g/m^sup 3^. The analyses of meteorological conditions helped finding the possible main sources of anthropogenic particles, such as Mn, which could be associated to one ferromanganese production industry located at the WSW, as well as Cu and Co. Ni and Pb presented high concentrations during very low wind speed conditions, so tiiey can be related with vehicles. Scanning Electronic Microscopy analysis confirms the presence of C, Fe, Zn, Al, O, Ca, and K in natural particles that may be associated to aluminosilicates, feldspars, and clays, as well as the presence of antrophogenic particles containing Cu, V, Mn and other heavy metals such as Pb. More detailed EDX-SEM analyses could determine the presence of other toxic metals and the possible relationships among them.

Not only PM^sub 10^ concentrations, but also metal concentrations are low in comparison with other industrial or urban places. This means that in spite of Tampico being an important port where many minerals arrive from other countries, that is close to many petrochemical and chemical industries, the meteorological conditions are favorable for dispersion of pollutants. Therefore, the presence of particles and metals do not represent a risk for the population. Nevertheless, the industrial activity in tins city is growing so fast that it is necessary to study further the PM^sub 10^ and metals close to the source of industrial emissions during all day long, taking day and night samples, or samples in 4 periods along the day in order to have a better understanding of the behavior of contaminants through the day and determine present and future risk to the health of the inhabitants.

ACKNOWLEDGMENTS

This work was supported by the Consejo General de Posgrado e Investigacion of the Instituto Politecnico Nacional under the project No. CGPI-20050979. The authors thank the Direccion de Medio Ambiente del Municipio de Tampico, Tamaulipas; the Direccion de Recursos Naturales y Medio Ambiente de la Secretaria de Obras Publicas Desarrollo Urbano y Ecologia in Tamaulipas for providing filters and PM^sub 10^ data; and the Centro de Investigacion y Desarrollo Tecnologico of PENOLES Inc., for the facilities and course provided. Thanks are also due to Comision Nacional del Agua and Servicios a la Navegacion en el Espacio Aereo Mexicano for providing meteorological data. VMA is also indebted to SNI for the distinction of her membership and the stipend received.

1 National Automatic Network for Atmospheric Monitoring.

2 Comision Nacional del Agua.

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R. M. Flores-Rangel,1 P. F. Rodriguez-Espinosa,1 J. A. Montes de Oca-Valero,1 V. Mugica-Alvarez,2 M. E. Ortiz-Romero-Vargas,2 M. Navarrete-Lopez,3 and H. J. Dorantes-Rosales4

1 Centro de Investigacion en Ciencia Aplicada y Tecnologia Avanzada del Instituto Politecnico Nacional, Altamira, Tamaulipas, Mexico; 2Universidad Autonoma Metropolitana Unidad Azcapotzalco, Reynosa, Mexico; 3 Laboratorio Central de Investigacion de la Escuela Nacional de Ciencias Biologicas del Instituto Politecnico Nacional, Prol. de Carpio y Plan de Ayala, Mexico; 4 ESIQIE-IPN Departamento de metalurgia, Lindavista, Mexico

Received 15 December 2006; revised manuscript accepted 17 March 2007.

Address correspondence to R. M. Flores-Rangel, Centro de Investigacion en Ciencia Aplicada y Tecnologia Avanzada del Instituto Politecnico Nacional, KM. 14.5 Carretera Tampico-Puerto Industrial. Altamira, Tamaulipas, Mexico. E-mail: rmflores@ipn.mx

Copyright Taylor & Francis Ltd. Nov/Dec 2007

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