Heavy Metals Concentrations in Deposited Dust of Typical Chinese Tree Species in Different Functional Areas in Nanjing

摘要

Foliar dust passively adsorbs anthropogenic heavy metals (HM) present in the atmosphere and thus reduces the total suspended particle (TSP) level. Urban plants have been shown to reduce the atmospheric level of ambient particulate matter (PM) via foliar dust adsorption. We studied heavy metal concentrations in the foliar dust of three typical tree species in five functional areas of Nanjing city. The highest levels of Cd (19.89 +/- 4.56 mg/kg), Pb (167.33 +/- 16.61 mg/kg) and Cr (197.42 +/- 13.96 mg/kg) were found in the Traffic Area (TA), whereas the highest levels of Cu (309.27 +/- 25.79 mg/kg) and Zn (1036.88 +/- 52.77 mg/kg) were found in the Industrial Area (IA). Significant differences were found between tree species. The amount of PM per unit leaf area generally decreased in this order: Cedrus deodaraPittosporum tobiraCinnamomum camphora. The highest mass percentages of large, coarse and fine PM were captured by C. camphora, P. tobira and C. deodara, respectively. A scanning electron microscope (SEM) was used to investigate the surfaces of the leaves, as well as the density and size of the stomata of each species. Our results suggest that an oily and coarse leaf surface is the most important factor facilitating PM accumulation, but large high-density stomata also enhance PM adsorption and thus favor HM accumulation in foliar dust. This study shows that the HM concentrations in foliar dust can act as an indicator of air pollution.