Air pollution in cities is one of the major problems throughout the world both in developing and in developed countries. Trees as ecological indicators have been mostly used for biological monitoring of urban air quality. Thus, tree organs as leaf and bark have been applied as accumulative biomonitors of toxic metal contamination and have achieved noticeable ecological importance. Leaves and barks of common broad leaved (Ash: Fraxinus excelsior) and conifer (Tehran Pine: Pinus eldarica) trees and corresponding soil samples from three districts within the urban areas of Bojnourd (Iran) were collected. Comparison of the three districts obviously exhibited the effects of anthropogenic activities on the metal concentrations in the studied surface soils. Our results indicated that the level of toxic metals in corresponding soil of Pinus eldarica was higher than the Fraxinus excelsior. The tree barks accumulated a greater load of Pb than the tree leaves and higher concentration of Cu and Zn was observed in leaves compared to bark. For Ash and Pine the bioconcentration factor (BCF) of Pb in barks was higher than the BCF in leaves. Conversely, BCF for Cu and Zn in leaves was higher than in barks. The mean BCF for Pb and Cu in leaves of Ash was much higher than that of Pine, while it was vice versa for Zn. In the present study washing the leaves removed of toxic metals about 20% - 46% that indicated considerable content of toxic metals was on the leaves surface as dry deposition. Higher BCF values of Pb than those of Cu and Zn affirmed these two tree species could be considered as Pb accumulators. Both broad leaved and conifer tree are good biomonitors of urban air pollution. The outcomes of this research demonstrated Fraxinus excelsior as more suitable than Pinus eldarica for biomonitoring Pb atmospheric pollution in the urban areas, while Pinus eldarica more appropriate for Zn biomonitoring. Also the tree leaves is relatively more suitable than tree bark for biomonitoring.
