Characteristics of urban pollution under arid and semi-arid condition on the Loess Plateau in northwestern China:examples form a city in northwestern China

Abstract：Lanzhou is located in the western part of the Loess Plateau in northwestern China. The factors of urban pollution are mainly amog (smoke and dust in air), fine sand flying up in air and secondary dust (ground dust), as well as dispersion of solid waste and sewage irrigation. The main polluting elements are Hg, Sb, Pb, Be, As, Cd and F. The characteristics of pollution of elements dominated by heavy metals are as follows：(1) the pollution degree is closely related to the population density and flux of people; (2) the pollution degree in the city district where the population is concentrated and the commerce is well developed is higher than that where the population is less dense and industries are more concentrated; (3) for industrial pollution the pollution degree of the heavy industry is higher than that of the light industry; (4) for agricultural pollution the pollution degree in sewage irrigation areas is higher than that in non-sewage irrigation areas； and (5) heavy metals in pollutants increase with fining granulation of grains and then their harming degree also increases. In the evaluation of the pollution degree, in addition to the pollution index, the areal metal quantity and background index of exploration geochemistry are also introduced, which makes the evaluation have more regionalized and comprehensive effects.