Suspended particles in air, which are produced by natural and anthropogenic factors, are normally referred to as “aerosols”. They might carry solid grains and also polluting and dangerous elements in urban areas. Dust enters the urban environments as particles with a diameter of less than 100 micron resulting from the resuspention of anthropogenically produced particles and particles created by storm. Such particles are the host of different contaminants such as heavy metals originated from natural and human induced sources. The objectives of this study were: (i) to investigate the seasonal variability of the concentration of selected heavy metals in the streat dust of Isfahan, Felavarjan, Khomeinishahr, Najafabad, Shahreza and Natanz cities, (ii) to determine the concentration of selected metals, (iii) to trace the potential sources of selected metals in the street dust, and (iv) to find the contamination level of street dust. It was hypothesized that the concentartion and pollution level of heavy metals in the street dust of urban areas in the rainy seasons were higher than those in other seasons and also they were also higher in more populated cities, and anthopogenic sources were the dominant sources of heavy metals. Street dust samples were taken in the 2nd month of each of the four seasons including 20 samples from Isfahan and 10 samples from each of the other cities. Besides, 30 samples were collected from non-urban areas for comparison. Physico-chemical properties of dust samples including the amounts of sand, silt, clay, pH, EC, carbonates, and gypsum were measured in all the dust samples. Also, the concentration of selected heavy metals as well as the magnetic susceptibility in low and high frequencies were determined in the samples. Collected data were eva‎luated using non-parapetric statistics including Kruskal-Wallis test, nested design, and Tucky-Kramer mean comparison approach. Results showed that carbonates, gypsum, pH, and EC values in street dust samples had considerable spatial and temporal variability, and the their variability was much higher in the street dust of the urban areas than that of non-urban sites indicating their anthropogenic origin. Magnetic susceptibility of the street dust samples from all the cities and all the seasons at both frequencies was higher than that in non-urban areas. Besdies, samples from Natanz showed the highest magnetic susceptibility which could be attributed to the igneous rocks surrounding this city likely containing a high level of feeromagnetic minerals. Both geoaccumulation and Nemerrow indices showed the following contamination trend for the street dust: Cu>Zn>Pb>Ni>Cr>Co. The concentration of Cd was not within the measurable range by atomic absorption spectrometry. The level of contamination of all the heavy metals was variable in different seasons, but the general trend was spring>winter>fall>summer. The results of this investigation indicated that the level of heavy metals in the street dust was influenced by human activities. The highest pollution level was obtained for the city of Isfahan, where as the lowest for the street dust in Natanz. Contrary to our expectation, the highest seasonal pollution of the street dust was obtained for spring for most of the heavy metals studied.

حسين خادمي

شمس اله ايوبي

حسين شريعتمداري، سعيد اسلاميان