A. Báez

Carbonyl levels in indoor and outdoor air in Mexico City and Xalapa, Mexico.

Carbonyl compounds in air were measured at two houses, three museums, and two offices. All sites lacked air-conditioning systems. Although indoor and outdoor air was measured simultaneously at each site, the sites themselves were sampled in different dates. Mean concentrations were higher in indoor air. Outdoor means concentrations of acetone were the highest in all sites, ranging from 12 to 60 microg m(-3). In general, formaldehyde and acetaldehyde had similar mean concentrations, ranging from 4 to 32 and 6 to 28 microg m(-3), respectively. Formaldehyde and acetone mean indoor concentrations were the highest, ranging from 11 to 97 and 17 to 89 microg m(-3), respectively, followed by acetaldehyde with 5 to 47 microg m(-3). Formaldehyde and acetaldehyde had the highest mean concentration in the offices where there were smokers. Propionaldehyde and butyraldehyde concentrations did not show definite differences between indoor and outdoor air. In general, the highest outdoor and indoor hourly concentrations were observed from 10:00 to 15:00 h. Mean indoor/outdoor ratios of carbonyls exceeded 1. Formaldehyde and acetaldehyde risks were higher in smoking environments.

Measurements of formaldehyde and acetaldehyde in the atmosphere of Mexico City.

Ambient levels of carbonyls were measured at the University of Mexico campus, Mexico City. Only formaldehyde and acetaldehyde were measured, since aldehydes with higher molecular weight were not detected. The most abundant aldehyde was formaldehyde, with an overall ratio CH3CHO/H2CO of 0.43. Maximum concentrations occurred for formaldehyde at 10:00 h while for acetaldehyde at 8:00 h. Comparing the concentration measured in this work with those in urban areas it was found that the formaldehyde and acetaldehyde levels in Mexico City are among the highest reported in the literature.

Arsenic concentrations in water, soil, milk and forage in Comarca Lagunera, Mexico.

Arsenic levels were determined in seventy three samples of well water, and in fifty samples of soil, forage and cows milk collected at the most important dairy farms of the Comarca Lagunera located in Coahuila and Durango, Mexico, region naturally rich in As. The total inorganic arsenic concentration in well water ranged from 7 to 740 μg L-1 and about ninety percent of the total arsenic was found as As(V). The agricultural soil texture of the sampled area was sandy clay loam type with total arsenic levels up to 30 μg g-1, however, the extractable arsenic was not higher than 12% of the total and it was higher in the 0–30 cm depth horizon. In alfalfa, the most important crop, the total aresenic ranged from 0.24 to 3.16 μg g-1, with 40% of it accumulated at the root level. Significant correlations (p=0.05) were obtained between arsenic (III), (V) and total inorganic arsenic in groundwater with arsenic in soil (0–30 cm depth), and with arsenic in alfalfa (leaves and roots). It was also found a good correlation between extractable arsenic in soil with As concentrations in alfalfa (roots). Arsenic concentrations found in milk ranged from <0.9 to 27.4 ng g-1. The cows milk biotransfer factor for arsenic was up to 6 × 10-4, applying a pharmacokinetic approach. It was associated with the exposure not only to food but also to water arsenic.

On the evolution of aerosol properties at a mountain site above Mexico City

Size distributions, scattering and absorption coefficients, and the bulk chemical composition of aerosols have been measured at a mountain site 400 m above the southwest sector of the Mexico City basin during a two-week period in November 1997. Variations in these properties are primarily related to local meteorology, i.e., wind direction and relative humidity; however, a link was found between carbon monoxide and ozone and the partitioning of aerosols between Aitken and accumulation mode sizes. Relative humidity was also found to affect this partitioning of aerosol size and volume. In addition, the fraction of sulfate in the aerosols was much higher on a high-humidity day than on a very low humidity day; however, the fraction of the mass contained in organic and elemental carbon was the same regardless of humidity levels. The daily variations of aerosol properties are associated with the arrival of new particles at the research site transported from the city basin and their subsequent mixture with aged aerosols that remain in the residual layer from the night before.

Evidence for volcanic influence on Mexico City aerosols

In situ measurements of sulfur dioxide (SO2), carbon monoxide (CO) and sulfate mass provide evidence that aerosol composition in Mexico City is affected by emissions from the neighboring volcano, Popocatepetl. The data collected over a two-week period in November 1997 suggest that there are two distinct pathways by which SO2 is incorporated into particulates. Periods of high humidity, fog, and rain are accompanied by elevated sulfate mixing ratios, attributed to aqueous chemistry. Similarly, elevated sulfate concentrations in low humidity periods appear to be a result of adsorption onto existing particles. These two mechanisms are important for understanding the processes associated with particle formation in this highly polluted urban area. Under the influence of volcanic emissions, SO2 concentration can reach more than four times the monthly mean daily average (20ppbv) and particulate sulfate can be a factor of 2 larger.

The algal community as an indicator of the trophic status of lake Patzcuaro, Mexico

An evaluation of water quality and phytoplankton composition position was carried out in order to determine the trophic conditions of Lake Patzcuaro (2035 m above sea level), a high altitude tropical lake. Temperatures ranged from 15 to 23 degrees C. Total phosphorus and inorganic nitrogen showed a seasonal variation; highest values coincided with the rainy season (0.48 and 2.1 mg litre(-1), respectively). Dissolved oxygen ranged from 2 to 7.9 mg litre(-1) at the surface and from 0.6 to 7.3 mg litre(-1) on the bottom, the lowest values being found in shallow zones. Average transparency varied from 0.62 to 1.4 m Secchi depth. Rainfall was a primary factor in seasonal variability as it influenced both physical and biological conditions by contributing to the transport and deposition of silt, which mixed with sinking algal cells. The composition of the surface phytoplankton segregated along five major divisions comprising a total of 49 species. General seasonal patterns of dominance alternated in a sequence beginning with Bacillariophyta, through Chlorophyta to Cyanophyta. Diatoms, the dominant group from February to early June, included Melosira granulata, Stephanodiscus sp., Synedra sp. and Fragilaria sp. During the rainy season (late June to September), Microcystis aeruginosa, Oscillatoria sp., Anabaena sp., Merismopedia sp., Crucigenia cuadrata, Oocystis lacustris, Selenastrum gracile, Mallomonas sp. and Tetraediella sp. were important. Melosira granulata was present throughout the period of study. Spatial and temporal variability in both physical and biological conditions make it difficult to assign a specific trophic state to Lake Patzcuaro. Nevertheless, analysis of the algal community indicates a generally mesotrophic condition.

Oyster (Crassostrea virginica) as indicator of heavy metal pollution in some lagoons of the Gulf of Mexico

The littoral lagoons of the Gulf of Mexico have been investigated to determine the amount of heavy metal pollution. Some lagoons have been contaminated by liquid wastes mainly originating from sugar refining and petrochemical industries, as well as petroleum exploitation and refining. Since mollusks concentrate trace elements, determination of levels of Cd, Cr, Hg, and Pb in oysters (Crassostrea virginica) were measured in some of these lagoons. The results have shown that the concentrations of these metals in lagoon water were below the recommended levels to protect aquatic life, and it was found that oysters are able to concentrate metals, mainly Cd and Pb above the maximum permissible limits for edible consumption.

Rainwater chemistry at the eastern flanks of the Sierra Madre Oriental, Veracruz, Mexico

A rainwater chemistry study was carried out at a single site in Xalapa located at the eastern flanks of the Sierra Madre Oriental, Veracruz, Mexico, during the rainy and dry seasons under the influence of different meteorological conditions. Rain samples were analyzed for the most important ions. The statistical results indicated that there were significant differences at the 95% confidence level between the ionic concentration measured in samples collected in both seasons. The lower concentration values found during the rainy season could be attributed to dilution processes since in this season 79% of the annual precipitation occurs, compared to only 21% in the dry season. High and positive ion-pair correlations were found among the most important ions in both seasons. Although Xalapa is situated at less than 100 km from the coast, marine contribution of SO42− and Ca2+ is negligible compared with the nonmarine contribution. In the rainy season, enrichment of SO42−, NH4+, and NO3− is due to upwind medium- and long-distance emission sources of Veracruz, as back trajectories of the predominant winds from the southeast indicate. On the other hand, in the dry season this enrichment is due to emission sources situated in Tampico, Madero, and Poza Rica and south-southeastern Texas, as indicated by the predominant winds from the northeast. Finally, wet deposition rates were, as expected, higher during the rainy season in spite of the dilution processes.

Some aspects of the environmental exposure to chromium residues in Mexico

Research was conducted to determine the environmental effects of Cr residues from a chromite-ore processing plant (Cromatos de México) on the human population living in the surrounding area. The results of this study provided the first quantitative evidence of the transfer of Cr to the atmosphere, soil and subsequently to ground water. The atmospheric concentration of Cr ranged from 0.25 to 0.39 μg m−3, seven times higher than the level found in the control area. Most of the well water samples (90%) had a higher Cr concentration than the threshold limit value for drinking water of 0.05 mg L−1. The high Cr concentration in the environment may be responsible for the high concentration of this metal in urine (18.6 ng mL−1) and hair (4.3 μg g−1) found in the residents of this polluted area. These results were compared with those of people living in a Cr free zone through statistical analysis (Wilcoxons signed-ranks test). A significant difference of p<0.01 was found between these two populations. In relation to the average concentrations in hair (1.4 μg g−1) and urine (2.0 to 6.0 ng mL−1) considered as normal for a non exposed population in the USA, 70 to 80% of the residents in the Mexican urban-industrial area registered higher values. The high Cr concentration in urine found in workers from the chromite-ore processing plant show the lack of an efficient pollution control system. Therefore the Cr levels found in urine and hair can be used as indicators of environmental exposure.

Chemical composition of precipitation at two sampling sites in Mexico: A 7-year study

Abstract This paper describes the results of a study of the chemical composition of bulk and wet precipitation, collected for 7 years in a large urban area and a forested rural site in Central Mexico. In rain samples from both sites, sulfate was the most abundant ion followed by ammonium. Seasonal patterns of monthly volume-weighted concentration show that the highest values of SO2−4 NO3−, Ca2+ and NH4+were measured in May, the beginning of the rainy season in Central Mexico. The lowest values were observed from July to September, the months when most thunderstorms occurred. Multiple regression correlation analysis was applied to all data and a hierarchical model was used to select the variables included in the final model. In Rancho Viejo the model showed that SO2−4 contributed with 41% in the H+ concentration prediction. In Mexico City, despite the high SO42− concentration in rain water, this ion only contributed 0.46% whereas Ca2+ contributed in wet and bulk precipitation, 61 and 41%, respectively. Finally no long-term trends were noticed.