Narrow Results

The contribution to the atmospheric pollution in Mexico City's conurbation that comes from the northern area has been considered significant for a long time. Therefore, it is important to determine its elemental composition as well as their correlation in sites along the suspected trajectories of airborne particulate matter. Samples were collected simultaneously in two sampling sites along the prevailing wind trajectory in northern Mexico City, and its elemental composition were determined by PIXE. The Sample collection was performed daily from 17 February 14 March 1997 in Xalostoc, an industrial zone in the State of Mexico, and Ticomán within Mexico City. Two samples a day were taken in two periods: 7-19 h and 19-7 h. The samplers used separated particles into two particle size fractions fractions, PM₂₅ and PM₁₅. Three main correlation group of elemental concentrations were determined: fuel combustion, earth crust and industrial origin. The sampling site within Mexico City, Ticomán, was found with an overall tendency of higher elemental concentrations, showing not only influence of local pollution sources, but also that of wind transported particles from neighboring industrial zones such as Xalostoc.

Samples of airborne particulate matter were collected in four sites along an east-west line from the Popocatépetl volcano after the eruption episode of June 30, 1997. The Popocatépetl volcano, with variable activity since it was known, is currently under low but continuous activity prolonged for almost one decade, with occasional moderate eruption episodes producing mainly fumes, ashes and volcanic dusts. The main objective of this study is to determine whether or not some elements have increased their presence in the atmosphere as a result of the volcanic activity, and also if some others, not usually found in urban aerosols, have appeared because of the same reason. In addition, the information obtained will be a source of scientific data for health risk assessment of the population exposed to volcanic emanations. The sample collection was performed on alternate days from July 10 to August 13 1997 in Puebla and Atlixco in Puebla State. Tlalpan within Mexico City, and Salazar in the State of Mexico. Two samples a day were taken in two periods: 7-19 h and 19-7 h. The samplers separated particles into two particle size fractions. PM₂₅ and PM₁₅. Elemental concentrations were determined by PIXE and the results obtained showed increased concentrations of mainly Ti and Fe in all sampling sites, thus indicating a long range transportation of volcanic dusts in both particle size fractions. Concentrations of Ti were found clearly above the average values of urban areas such as Mexico City, and although this element can be considered of low toxicity, the biological, metabolic and toxic effects on human beings are still under investigation.

Samples of fine airborne particulate matter (PM_2.0) have been collected at three sites located in the Mexico City Metropolitan Area (MCMA) during July, August, and September 2000. The elemental concentrations of these samples were determined by PIXE so that twelve elements (S, Cl, K, Ca, Ti, Mn, Fe, Ni, Cu, Zn, Br and Pb) were consistently detected. These results produced a database, which was statistically analysed; first by Principal Components so to identify the pollution sources, and, subsequently, by Absolute Principal Component Scores analysis in order to estimate the corresponding source apportionment contributions. Other elements such as V and Cr were also detected in around 20% of the samples and, therefore, they were not considered statistically significant. Thus, these elements, as well as some others found only occasionally and in the trace range, were left out of the discussion. All calculations were performed using a standard statistical package following the method developed by Thurston and Spengler. Four main pollution sources were identified and the source apportionments were found in good agreement with other estimations found in the literature. The results were obtained individually for the three sampling sites, but the complete results are presented for just one of the sites in order to simplify this presentation.

Fine particulate matter PM_2.5 samples have been regularly collected in the Mexico City Metropolitan Area commencing in the year 2003, when a new PM_2.5 sampling network was made fully operational. Samples have been collected during 24 h periods every third day using US EPA Federal Reference Method (FRM) samplers. In this study, the elemental composition of two sets of PM_2.5 samples collected at the "Merced" sampling site located in downtown Mexico City (19°25'28''N, 99°07'09''W) were determined by PIXE. Ten elements (S, Cl, K, Ca, Ti, Mn, Fe, Cu, Zn, and Pb) were detected in most of the samples. Some others appeared occasionally, but only those consistently detected were considered significant for the analysis. Other analytical equipments operating simultaneously at the site provided complementary data of several variables such as SO₂, NO₂, NOx and CO. An enhanced database was thus built adding these results to the database derived from previous PIXE analysis. Further statistical analysis of the database allowed identifying four principal sources in two seasons and permitted to estimate the corresponding source apportionment.

As part of a wide research project aimed to understand and eventually control atmospheric pollution in the Mexico City Metropolitan Area (MCMA), the Secretaría del Medio Ambiente del Gobierno del Distrito Federal (SMA-GDF) operates, since 2003, an automated sampling network of fine airborne particulate matter (FAPM). In this study, the samples collected simultaneously in three sampling sites during years 2003–2004, and those collected (simultaneously) in seven sites during the whole of 2005 and the first three months of 2006, have been analyzed using the PIXE technique. The results have provided ample databases of elemental concentrations (ng/m³) of 13 elements, consistently detected in the samples. As it is well known, Mexico City and its conurbation has been ranked as one of the most densely populated areas in the world, enduring high levels of several pollutants such as PM_2.5 fine particles (≤ 2.5 μm) and sulfates among others, which diminish air quality, visibility and endanger the health of the population. According to the emission inventories of the SMA-GDF, around eight thousand tons of SO₂ are emitted to the atmosphere in the MCMA every year. Despite continuous efforts of the local authorities aimed to reduce SO₂ emissions, S elemental concentrations determined in the PM_2.5 samples collected by the fine particle sampling network still show significant levels of this element. Since sulfur compounds, particularly sulfates, are among the most important pollutants affecting the atmosphere, it is compulsory to evaluate its presence and concentrations. As it is has been pointed out by several authors, the ratio of sulfate content to S elemental content in FAPM, such as that determined by PIXE, is fairly constant (approximately three). Using this result, sulfate concentrations were calculated, and these were found in very good agreement with previous experimental results.

An extensive investigation, aimed to study the fine fraction of airborne particulate matter (FAPM) in the Mexico City Metropolitan Area (MCMA), started in 2003, after the FAPM sampling network of the Environmental Secretariat of the Federal District Government (Secretaría del Medio Ambiente del Gobierno del Distrito Federal, SMA-GDF) was put into operation. Formerly, several research institutions in Mexico, in collaboration with the local government, made continuous efforts during the last two decades, in order to improve air quality in the MCMA. Samples have been collected using US-EPA designated samplers, following the same agency protocols. In this study, 922 samples were analyzed by PIXE in order to determine the elemental concentrations (ng/m³) of the FAPM. The samples were collected every three days by the Atmospheric Sampling Network (Red Automática de Monitoreo Atmosférico, RAMA) of the SMA-GDF, starting by the end of 2003 and along 2004 on three sites, and during the whole 2005 to the beginning of 2006 in seven stations, taking samples every six days during this last stage. The sample collection has continued up to date at the latter pace. The results obtained have allowed determining the sulfur (S) spatial distribution over the area, showing a reasonable uniformity in space, but well marked time variability. The S spatial distribution has also showed similar patterns between equivalent seasons from year to year, although not a strict periodicity. Compared to other elements found consistently in the samples, S is mainly distributed over the area by a large amount of mobile (mainly transportation) sources, fairly distributed over it, while other elements come from fixed sources located at industrial or urban areas. Comparisons were also made among the sampling sites, and among the three climatic seasons (drycold, dry-hot and rain) from year to year. Correlations between temporal variations of pairs of sampling sites were also calculated for S and other elements, in order to support these conclusions.

Elemental analysis of airborne particulates collected in a site near downtown Mexico City have been performed. The samples were taken during one week of every season in 1988, Twenty three elements, from sodium to lead, were detected by Proton Induced X-ray Emission (PIXE). In addition, soot contents was determined by laser Integrating Plate Method (LIPM). Concentrations of these elements show hourly variations during the day, with maximum values in the period from 6 to 12 A.M. An overall tendency toward maximum elemental concentrations is observed in the winter season, especially for primary anthropogenic pollutants, as was anticipated based upon the frequency of thermal inversions.

As the second part of a wide project, we have carried out a study of airborne particulates in Mexico City, collected during two time periods (from 7:00 to 11:00, and from 11:00 to 15:00 Hrs.), one day of every week between March 1988 and February 1990. Elemental concentrations were determined by Proton Induced X-ray Emission (PIXE) for elements Na to Pb, while soot content was measured by means of Laser Integrating Plate Method (LIPM). As expected, the concentrations show an increase during the months October to March, due to frequent thermal inversions and dry weather. Moreover, the pollutant levels are higher during earlier hours, while the soil and anthropogenic origin of the elements can be better determined for the mid-day period, as seen from the grouping in the respective dendograms.

Airborne lead has been considered for many years one of the main pollutants adversely affecting the health of human beings. Moreover, this problem becomes remarkably important in large urban areas such as Mexico City. In order to assess the influence of atmospheric airborne lead in a children population, a biological blood sampling was carried out from September 1992 to June 1993 taking 698 samples in children with ages ranging from a few weeks to thirteen years old. Lead contents in whole blood were determined using anode stripping voltammetry as analytical technique. At the same time, aerosol lead contents were determined by PIXE from samples taken twice a week (two samples per day) in a neighbour area. In 58 % of the samples, lead contents in blood was found over the maximum permissible level established by the Center for Disease Control (CDC) of the U.S.A. The biological sampling was correlated to levels of airborne lead as well as children age and date of sampling. General results of these comparisons are presented.

Particulate matter (PM-3.9 and PM-15.8) samples were collected in the three zones at the Northeast, Southwest and Southeast suburbs of Mexico City, from July to August 1998, for one week for each sampling site. The concentrations of several elements in the PM-3.9 and PM-15.8 samples were determined by Particle Induced X-ray Emission (PIXE). In the PM-3.9 samples, 21 elements were determined for each zone, and Na, Mg, Al, Si, S, K, Ca, Ti, Fe, Cu, Zn and Pb are found to be the major elemental components. On the other hand, 22 elements including P were determined on the PM-15.8 samples, and the dominant elements were the same as in the PM-3.9. Factor analysis is applied to the 28 variables (14 elements for each PM-3.9 and PM-15.8 groups) and for 21 samples (seven days for three zones) in order to identify possible sources of the particles. The result of factor analysis allows to identify five major sources, being soil the major contributor.

We compare four methods, all based on Bayesian regressions, to predict response spectra at instrumented sites, located at the hill and lakebed zones in Mexico City, that have recorded several recent earthquakes. The regressions are built as functions of magnitude and closest distance to the rupture area of the earthquake, using more than 20 subduction earthquakes (thrust-faulting mechanism) recorded since 1964. The four methods are: (1) a regression on observed response spectra; (2) a regression on observed Fourier amplitude spectra and use of random vibration theory to estimate the response spectra, (3) a regression on observed response spectra at the reference hill-zone station multiplied by response spectral ratios; and (4) a regression on Fourier amplitude spectra at the reference hill zone, multiplied by Fourier spectral ratios and use of the random vibration theory to estimate response spectra.