Heavy metals biomonitoring in the Mexico City metropolitan area

摘要

Foliose lichens genera Parmotrema sp. and Parmelia sp. were used to monitor airborne heavy metal pollution in the Mexico City Metropolitan Area (MCMA). The samples were taken every three months in four selected places surrounding the urban areas (N, S, W, E). Also, another place far from the influence of the city was chosen as a reference site, and, lichens dwelling in Juniperus sp. twigs were transplanted from this site to our facilities in Mexico City and placed hanging from tree branches in order to measure the metal accumulation values in known time periods. Eight metals were measured (Pb, Cr, Cu, Fe, Zn, Cd, K, and Ca) due to their importance in the automobile and industrial pollution sector. The results show that lichens growing in near the MCMA get concentrations one order of magnitude over the lichens growing in the distant reference site. In particular Pb, Cu, and Zn present higher values south of the city which is in accordance with the dominant winds. Regarding the transplanted lichens, a significant increase in the concentration of Pb, Cu, and Zn was found after a nine month period.