Seasonal difference and availability of heavy metals in street dust in Beijing.

Street dust samples were collected from nine sites with different land uses in the Beijing metropolitan area during three seasons of autumn, winter and spring in 2007 and 2008. Heavy metal concentrations for 21 bulk samples and 105 sieved sub-samples were quantified. Total heavy metal concentrations and concentrations in fractions from the optimized BCR sequential extraction procedure were measured in bulk samples and sieved sub-samples collected from nine sites in spring, 2008. The results show that the concentrations of heavy metals adsorbed on street dust in winter are 10-30% higher than those in spring and autumn. The total and labile concentrations of heavy metals in street dust increase significantly with decreasing particle sizes. The percentages of various heavy metals that are labile are in the order of Zn > Cd > Cu > Pb > Ni > Cr > Al. Zn, Cd and Cu are the most mobile elements and are found extensively in urban environments. The geoaccumulation index (I(geo)) assessment results suggest that street dust in Beijing is moderately to strongly polluted by Cd, moderately polluted by Cu and Zn, unpolluted to moderately polluted by Pb and Cr and unpolluted by Ni. Sites with intense human activities such as commercial zones and traffic zones are more polluted than parks.