spatial and temporal distribution of heavy metals concentration in atmospheric dust in kerman city

introduction atmosphere health is an important environmental issue and air pollution is not a local problem in a given area of the world. particles produced by either humans or the nature enter the atmosphere and return the earth’s surface at local, regional, or global scales after physico-chemical interactions. according to the reports by who, each year 2.4 million people lose their lives worldwide due to air pollution. in recent decades, numerous investigations have been carried out on the status of heavy metals in atmospheric dust. this is mainly due to their importance and also impacts on the environment and humans. heavy metals present in water, air, dust, soil, and sediments play an important role in human life. not much information is available on the concentration of heavy metals in atmospheric dust and the total inputs of heavy metals from atmospheric deposition to the urban area of the kerman city. therefore, the objective of this research was to evaluate the rate and spatial and temporal variability of selected heavy metals in atmospheric dust of kerman city. materials and methods this study was carried out in the urban area of kerman city having an area about 13100 km2. kerman is an important industrial city where a lot of industries and mines including jalalabad iron ore, zarand coal mine, barez tire producing plant, kerman cement factory, sarcheshmeh copper mine, and momtazin cement production company. they are actively functioning in the area. to collect dust samples and investigate heavy metals, dust collectors were set up at 35 different locations distributed in the city. dust samples were taken monthly from april 20 to november 20, 2012. to compare the concentration of heavy metals in dust with that of soils in the region, 60 surface soil samples (0-10 cm) were also taken along 3 different transects including kerman rafsanjan, kerman-zarand and kerman-mahan. besides, 35 surface soil samples were collected from kerman urban area. the concentration of major heavy metals was determined in all the dust and soil samples using an atomic absorption spectrometer following digestion with 6n hno3. results and discussion  statistical description of the concentration of heavy metals indicated thatthe concentration of metals in dust followed the order of mn>zn>cu>pb>ni. the results also showed that, with exception of ni and mn, the concentration of the heavy metals decreased as the temperature increased. there was a significant correlation among cu, pb, and zn and also cu, mn, and ni concentrations suggesting two different sources for heavy metals. the calculation of total heavy metals entered the soil through dust deposition indicated that the highest rate of heavy metal deposition occurred in april as well as in september when the activity of polluting sources restarted due to changes in climate. the results also indicated that the total concentration of the heavy metals in urban soils was much higher than that of the rural soils. furthermore, the concentration of heavy metals in dust was very much higher than that of the soils studied. in addition to natural parameters, anthropogenic sources seem to have greatly influenced the status of heavy metals in atmospheric deposition in the city. the spatial distribution patterns of dust heavy metals (fig. 1) indicate that the concentration of cu, zn, and pb follow the same spatial distribution which is different from that of ni and mn. therefore, it can be concluded that the source of these two groups of heavy metals in atmospheric dust is different. besides, the concentration of the heavy metals in dust is higher in the western part of kerman city as compared with the east of the city.                                       figure 1. maps showing the spatial variability of mean heavy metals concentration (mg/kg) in atmospheric dust in kerman during the 7 months studied.   the comparison of dust heavy metals concentration and rates of deposition in kerman have been compared with those of other areas of the world in table 1. the results clearly show that the amount of heavy metals deposited through dust in the study area is very much higher than that of the european and industrial countries. this is mainly due to higher dust deposition rate as well as more polluted dust in kerman. table 1 also indicates that atmospheric dust in kerman is less polluted as compared with other industrial cities in iran such as isfahan and zanjan. table 1. the deposition rate and the concentration of heavy metals in dust from kerman city (current study) as compared with findings from other areas in iran and the world. mn cu pb zn ni unit area nd nd 1043.9 299.3 nd g ha-1 yr-1 zanjan 520.6 67.1 220.3 453.8 79.8 g ha-1 yr-1 isfahan nd 57 54 221 16 g ha-1 yr-1 england nd 34 38 227 10 g ha-1 yr-1 europe(average) 20 7 4.4 41 2.4 g ha-1 yr-1 virginia)usa) 223.5 62.4 47 356 36.8 g ha-1 yr-1 kerman 341.8 60.4 45 214.4 35.5 mg kg-1 318.9 36.4 66.8 141.8 26 mg kg-1 saudi arabia 540 71 223.5 470.3 82.2 mg kg-1 isfahan 1384 215.5 54.7 706.6 78.4 mg kg-1 india (industrial area) 29.5 521.6 576.9 4443.3 40 mg kg-1 mazowieckie                      nd: not detected   conclusions this investigation shows that human activities around the study area have caused a great degradation of soil, which in turn, resulted in a high rate of dust deposition in urban areas of kerman. among the heavy metals studied, pb, zn, and cu in dust appear to have been derived from the same source, mostly anthropogenic activities. in contrast, other heavy metals, namely ni and mn, seems to have been mostly originated from natural sources mostly the erosion of barren agricultural and desert soils in the area. to reduce the risk of air pollution in populated urban areas in kerman, it is necessary to establish less contaminating industries. besides, more attention has to be paid to control wind erosion of barren soils in the region.