| This research is based on the chemical characteristics of atmospheric particulates in the urban and rural transition belt of Xi’an, collected total suspended particulates samples from November2011to December2012and analyzed the temporal variation of the mass concentration of TSP. The concentrations of heavy metals in TSP were measured using by Atomic Absorption Spectroscopy and analyzed the characteristics of temporal variation. The water soluble ions in TSP were determined by using Ion Chromatograph. The pollution of heavy metals in TSP was assessed through Enrichment Factor and Potential Ecological Risk Index in the urban and rural transition belt of Xi’an. The main conclusions of this study are as follows:(1) The mass concentrations of TSP in the urban and rural transition belt of Xi’an were showed as follows:The annual average concentration of TSP from November2011to December2012was0.3560mg/m3, higher than the secondary standard of national atmospheric quality standard (0.20mg/m3). The average monthly concentration of TSP was highest in February (0.6070mg/m3) and lowest in June (0.2528mg/m3). The average seasonal concentration of TSP was maximum in winter, higher in spring and minimum in summer. The heating period was higher than non-heating period because of the emission of burning coal.(2) The average concentration of heavy metals in TSP in the urban and rural transition belt of Xi’an was Pb0.3154μg/m3, Cu0.0638μg/m3, Cr0.0848μg/m3, Ni0.0324μg/m3, Zn2.3173μg/m3, Co0.0339μg/m3, respectively and the order from high to low was Zn> Pb> Cr> Cu> Co> Ni. Because of more affection on Cu, Ni and Co from anthropogenic pollution in some time, the coefficients of variation of these heavy metals were relative higher. The average monthly concentrations of heavy metals were the highest in January except Zn, which mainly due to the affection from emission of burning coal, automobile and fireworks and crackers and more fog and haze; the lowest value of heavy metals were usually in May, June and August, which mainly because of clear weather, more rainfall and diffusion easily of pollutant. The average seasonal concentrations of heavy metals showed that:the concentrations of Pb, Cu and Co were highest in winter, higher in autumn and lowest in summer; the concentration of Cr was the highest in autumn, higher in winter and lowest in summer; the value of Ni and Zn were highest in winter, higher in autumn and little difference between spring and summer. The concentrations of heavy metals in heating period were higher than non-heating period and the concentration of Cu in heating period was3.3times higher than non-heating period.(3) It was showed that the heavy metals pollution degree in TSP in the urban and rural transition belt of Xi’an was different by Enrichment Factor. Among of them, Zn, Pb, Co and Cu were deeply polluted and in extreme pollution and significant pollution, respectively; Ni and Cr were in moderate pollution. The results of Potential Ecological Risk Assessment showed that the comprehensive risk index of heavy metals in TSP ranged from69.52to1192.08, with the average value443.77which was in strong risk; among of the heavy metals, the potential ecological risk of Pb was the biggest and Cr was the lowest. The order of potential ecological risk index of heavy metals was Pb> Zn> Co> Cu> Ni> Cr.(4) The concentration results of four kinds of anions (F-, Cl-, NO3-, SO42-) and five kinds of cations (Na+, NH4+, K+, Mg2+, Ca2+) in TSP in the urban and rural transition belt of Xi’an were showed that the average total concentration of water soluble ions in TSP was114.81μg/m3which occupied32.25%of TSP. The total concentration of anions was higher than cations. SO42-, NO3, NH4+and Ca2+occupied79.5%of total concentration of ions were the main ions in TSP. The distribution characteristics of monthly variation of water soluble ions was apparent, and the figure feature was nearly "W", which meant that the concentrations of ions were relative high in January, February, July, August, November and December. The source and formation mechanism were different and the trend of seasonal variation was also different. The concentrations of SO42-, NO3-, Cl-, NH4+and K+were the highest in winter mainly because of the affection of burning coal in winter, and the concentrations of Na+, Mg2+and Ca2+were the lowest in winter which were mainly from soil and construction dust. The concentrations of water soluble ions in TSP in heating period were higher than non-heating period except Ca2+, which due to little affection from burning coal in heating period.(5) The ions balance between total cations and total anions showed that TSP in the urban and rural transition belt of Xi’an was weak alkaline, which was consistent with the results of pH measurement. The correlation and ratio relationship among ions suggested that water soluble ions mainly existed in the forms of (NH4)2SO4and NH4NO3. K+, Ca2+and Mg2+may combine with NO3-due to their good correlations. Mg2+was significantly positive correlation with Ca2+and the mole ratio of Mg2+and Ca2+was lower than it of the desert of north China and surface loess because of the enrichment of Ca2+. The ratio of NO3-/SO42-indicates that the stationary source was the main source of water-soluble species and the affection of mobile source was also significant. The ratio of (NH4++Ca2+)/(SO42-+NO3-) suggested that a few acidic ions have not been neutralized because the seasonal concentration of (SO42-+NO3-) was apparent and the alkaline ions in coarse particles were not captured. |
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