Seasonal changes in the concentration and pollution level of selected heavy metals in the street dust of Isfahan and its surrounding cities

Airborne particles, commonly known as aerosols, can carry polluting and harmful particles and elements. Street dust refers to particles smaller than 70 micrometers that enter urban environments due to the resuspension of particles from human activities or dust storms. These particles host various pollutants, including heavy metals, which are produced from various anthropogenic and natural sources. The sources and amounts of these metals can be seasonally and spatially highly variable. The objectives of this research were to investigate seasonal changes in the concentration and pollution level of selected heavy metals in street dust and soil samples around a few cities in Isfahan province, and also to compare them with selected cities in the world.

Street dust samples were taken in the first half of the second month of each of the four seasons in 2018 including 80 samples from Isfahan, and 40 samples for each of the cities of Flavarjan, Najafabad, Shahreza, Khomeinishahr and Natanz Each dust sample (about 300 g) was collected from the surface of pedestrian walkways and stored in plastic containers. All the samples were then extracted by digestion with 6N nitric acid, and the concentration of selected heavy elements including copper, zinc, lead, and cobalt was measured in the extracts by an atomic absorption spectrometer. Finally, the contamination indices including geoaccumulation index and Nemerrow pollution index were calculated in the samples and the seasonal and spatial changes in the concentration and rate of heavy metal pollution were investigated.

Spatial and seasonal changes in the concentration and contamination level of copper and lead showed that the highest pollution rate of these elements happened in Isfahan in spring. They also indicated that the highest degree of street dust pollution with zinc was found for the city of Flavarjan in the spring season. As far as cobalt, the highest pollution level was found in Shahreza city in the winter season. For all the elements, the lowest concentration and level of pollution were obtained for Natanz city in the summer season, and lead showed the lowest concentration in fall. Besides, except for cobalt, soil samples taken from non-urban areas showed the least concentration and pollution level. The comparison of the obtained results with the data reported for other cities of the world showed a higher level of pollution of the street dust in the urban area of Isfahan compared to other big cities of the world. The concentration of copper in the street dust of Isfahan is almost similar to that in Ahvaz and Huainan (China), and that of zinc is lower than Avilés (Spain) and Zhuzhou (China). Furthermore, the concentration of lead is lower than that in Zhuzhou and Huludao (China). Also, cobalt concentration in the street dust of selected cities was higher than that in all the studied cities except for Rabat-Saleh highway (northwestern Morocco) and Huainan (China).

The results of this investigation and their comparison with those of other studies indicated that the mean concentration of copper, zinc, and lead elements in street dust was highest in all the cities, except for Natanz. This is mainly due of higher traffic, more population and the density of machinery, the presence of more and larger constructions in big cities, the transit of vehicles, and the presence of more intense atmospheric dust storms in the central areas of Isfahan province, drought and lack of sufficient rain. Lower concentration of selected heavy elements in the street dust of Natanz city is probably due to the lower population density, different types of parent materials as compared to other cities, and its low industrial activities.