中文摘要：

街尘是引起城镇面源污染分布最为广泛、最重要的污染物载体.以城乡环境梯度为出发点,以北京地区的中心城区-城中村-郊区区县-乡镇-农村村庄的街尘为研究对象,并对街尘的粒径进行分级,分析城乡环境梯度下街尘的单位面积质量、粒径组成的分异规律和街尘的重金属含量及赋存形态,揭示城乡环境梯度对街尘及其负载的中重金属污染的影响和街尘中重金属污染的粒径效应,以期为我国城市化进程中街尘的源头控制和分层次管理提供科学依据.结果表明,街尘中Cr、Cu、Ni、Pb、Zn这5种重金属浓度以中心城区最高,农村村庄最低,单位面积街尘量则相反.街尘中Ni、Pb、Zn的弱酸可提取态比例较高（19.1%、11.1%、34.8%）,Cr和Cu则相对较低（3.7%、2.1%）.粒径〈105μm街尘对重金属总量的贡献率（32.4%～62.4%）要大于其所占总质量比例（22.9%～49.6%）.对重金属污染在不同城乡环境梯度可采取不同的控制策略：中心城区和郊区区县可加强细粒径冲刷的关注,而农村村庄和城中村则需加强环境卫生管理与改善道路清扫方式和频率.

英文摘要：

Road-deposited sediment（RDS） is an important carrier of the diffused pollution.This study analyzed the influences of the urban-suburban-rural gradient（central urban,urban village,central suburban county,rural town,and rural village areas） on the content and chemical fractions of heavy metals.The concentrations of five heavy metals（Cr,Cu,Ni,Pb,and Zn） in RDS in the central urban area were the highest,and the lowest concentrations appeared in the rural village area.The mass of RDS per unit area were opposite to the metals concentrations in distribution.The proportion of weak acid extractable Ni,Pb,and Zn in RDS was higher than that of Cr and Cu.The contribution of heavy metals in smaller grain size（32.4%-62.4%） was greater than that in coarser grain size（22.9%-49.6%）.It will be helpful to reduce the RDS pollution for runoff and apply appropriate strategies for controlling runoff pollution from RDS according to RDS and its contribution to heavy metal pollution along the urban-rural gradient.