Revista mexicana de física
Print version ISSN 0035-001X
ASPIAZU, J et al. Temporal and spatial trends studied by lichen analysis: atmospheric deposition of trace elements in México. Rev. mex. fis. [online]. 2007, vol.53, suppl.3, pp. 87-96. ISSN 0035-001X.
Ball moss on Tillandsia recurvata (Bromeliaceae), collected in an area previously identified as unpolluted, was transplanted to thirteen biomonitoring sites in the downtown and metropolitan areas of México City (which cover a surface of 9,560 km2) during the periods August 2002 - January 2003 and July 2003 - 0ctubre 2003. A total of 52 lichens (weighing 300 g) were transplanted to each place. Two were analysed as zero or reference, El Chico National Park, a location 100 Km upwind from the city and the remaining 26 were hung in nylon net bags in order to be able to collect two transplanted tree month, out of every season over a one-year period. The concentrations were measured by the quantitative PIXE method based on an external beam facility. The atmospheric deposition for trace elements was inferred by its concentration in lichen samples collected in 2002 from 13 sites in México and compared with data from a similar survey in 2003. The concentration of Cr, Cu, Co, Fe, Mn, Ni, Pb and Zn and other elements was determined for each sample. Maps for each element were drawn after a geostatistical estimate of the metal concentration in the sample was made. Maps were drawn for all elements with the estimated values. Geographical distribution patterns were obtained for the different metals, reflecting the contribution of natural and antropogenic emission sources. The deposition patterns of V, As, Se, Cd and Pb are substantially influenced by long-range transport from other parts of México City. For Cr, Fe, Co, Ni, and Cu, the deposition patterns are largely determined by contribution from point sources within México and in the metropolitan area. The lichen data for Br and, in part, Se reflect an airborne supply from the environment. Contributions to trace element concentrations in lichen sources other than atmospheric deposition are identified and discussed. The Spatial and temporal variations in the distribution of metal concentration are discussed.
Keywords : Bio-monitoring; lichens; atmospheric contamination; PIXE analysis.