北京城区行道树国槐叶面尘分布及重金属污染特征

叶面尘分布范围广、累积时间长,能够较好地指示近地面大气颗粒物污染特征和累积状况。为揭示城市道路交通的大气颗粒物污染特征,研究了北京城区不同类型道路两侧行道树国槐(Sophora japonica)的叶面滞尘量,分析了叶面尘主要重金属的浓度和污染特征,采用相关分析和主成分分析法探讨了颗粒物重金属的主要来源。结果表明:国槐叶面滞尘量在快速路、主干路、次干路、支路上的比值为100∶84∶75∶75,受车流量影响较大。行道树国槐的平均叶面滞尘量为0.68 g/m2,庭院树国槐(远离交通)叶面滞尘量是0.51 g/m2,两者差异显著。行道树叶面尘重金属元素Cu、Zn、Pb在叶面尘中的浓度达到土壤背景值的6倍,空间分布较均匀,主要来自交通排放,庭院树叶面尘重金属污染也受到交通排放的重要影响。

Distribution and heavy metal character of foliar dust on roadside tree Sophora japonica of urban area in Beijing

Foliar dust is an important indicator of particulate matter (PM) in the near ground atmosphere, it can provide valuable information about the accumulation and characteristic of PM. The study on distribution and pollution characteristics of foliar dust is important for identifying the airborne particle source. With the rapid increase of traffic flow in cities, emission from traffic has become one of the main sources of the air PM pollution, and led to the deterioration of air quality in urban area. However, characteristics of road-based particle emissions and their effects on PM pollution have not been well studied. Sophora japonica is the main species of roadside trees of the urban area of Beijing and also serves as a good indicator species for mornitoring PM pollution. In this study, we collect 105 samples of foliar dust on Sophora japonica, which contains 91 samples along different types of road and 14 samples in parks and gardens as background. Foliar dust is obtained by elution and filtration, and the dust retention is classified according to road type and analyzed by one-way Analysis of Variance(ANOVA). The total concentrations of main heavy metals Mn, Ni, Cu, Zn, Pb and Cr in foliar dust are determined by Inductively Coupled Plasma-Optical Emission Spectrometry(ICP-OES). The possible pollution sources of these metals are identified by Pearson correlation and Principal component analysis (PCA). The result shows that the highest foliar dust retention, mainly affected by the traffic volume, is found on roadside trees of freeways, followed by major arterial, minor arterial and collector road, with a ratio of 100: 84: 75: 75. Foliar dust retention by the roadside trees (0.68 g/m²) is significantly higher (with P=0.054) than that of the garden trees (0.51g/m²). The concentrations of Cu, Zn and Pb in foliar dust of roadside trees are 6 times higher than the soil background values, indicating that traffic emission is a significant source of metal pollution. The concentrations of heavy metals may affected by a number of factors and have a generally uniform spatial distribution on traffic area. The correlation among Mn, Ni, Cu, Zn, Pb and Cr is significant in traffic areas, which means they are mainly originated from particle emissions from on-road vehicles. Lots of heavy metal pollutants are generated from vent-pipe emission, tyre abrasion, brake wear abrasion, road surface abrasion and resuspension in the wake of passing traffic. More than one source of particulate heavy metals is identified for foliar dust of garden trees and traffic emissions accounts for a large proportion. The major heavy metal pollutant Pb, Cu and Zn are mainly derived from traffic emissions. By comparing the concentrations of Pb, Cu and Zn between garden samples and roadside samples, we find that Pb is most likely to transport, followed by that of Cu and Zn. This study implies that traffic emission has become the major anthropogenic source of PM and heavy metal pollution in the urban area in Beijing, corresponding measures should be adopted for the pollution control.