| Fine particulate matter in urban, suburban and rural areas in Xi'an was regarded as the study object in this study and collected synchronously in three sampling sites of Beishaomen ?BSM?, Gewu building ?GWL? and Nandoujiao village ?NDJC? from November 2014 to October,2015. The concentration levels, the characteristics of the seasonal and the spatial variations of PM2.5 and its components during the whole sampling time were analyzed; in addition, the combination ways and sources of water soluble inorganic ions in PM2.5 were discussed preliminary; at the same time, the health risk of heavy metals in PM2.5 was assessed. The concentration levels, the characteristics of the spatial variations of PM2.5 and its components the periods of New Years' Day and May Day were studied. The main conclusions are as follows:?1? During the period of sampling time, the mean concentration of PM2.5 in Xi'an was 106.11 ± 70.92 ?g/m3 and the seasonal variation showed:winter> autumn> spring> summer, as the same as each sampling site's. The average concentrations of PM2.5 in BSM, GWL and NDJC was 114.17 ± 73.17,108.49 ± 72.71 and 95.67±65.33 ?g/m3, respectively, showing the trend of BSM> GWL> NDJC. In the view of each season, the spatial variation of PM2.5 in winter was the most obvious; the space variation of concentrations of PM2.5 in winter and spring revealed BSM> GWL> NDJC; showed GWL> BSM> NDJC in autumn and BSM> NDJC> GWL in summer.?2? In the whole collecting period, the mean concentration of the total water soluble inorganic ions ?TWSS? of PM2.5 in Xi'an was 51.43 ?g/m3, accounting for 48.47% of the annual mean concentration of PM2.5; the ratio of the sum of mean concentrations of SNA and TWSS was 48.47%, stating SNA as the main component. The seasonal change of TWSS in PM2.5 in Xi'an showed winter> summer> autumn> spring and the variation of each water soluble ion was not all the same. From the perspective of the single sampling site, the seasonal variation of every inorganic ion in each site was basically consistent with the parallel ion in Xi'an overall, only individual differences. The spatial variation of the mean concentration of the total water soluble inorganic ions of PM2.5 presented BSM> GWL> NDJC; whereby, the change of every ion was not identical and not remarkable. For single season, the variable coefficients of Na+, F-, NCVand Cl- in summer were larger and the mean concentrations in BSM were the highest; however, the space changes of every ions in other seasons were not obvious.?3? Cl-, water soluble inorganic ion in PM2.5 in Xi'an, was highly related to K+ and equivalence ratio of them was 0.28, indicating that Cl- can combine with K+ producing KC1 and remain part Cl-; NH4+ showed remarkable correlations with NO3-, Cl- and SO42- and in the view of correlation indexes, NH4+ firstly combined with NO3-to make NH4NO3 and then reacted successively with remainder Cl- and SO42- to produce NH4Cl and ?NH4?2SO4; but still remaining NH4+may combine with F+. Mg2+ correlated significantly with Ca2+ and the molar ratio between them was 0.3, stating Ca2+ and Mg2+ in PM2.5 in Xi'an may come from other common sources, except soil dust. The ratio of mass concentration of NO3- and SO42- was 0.96, illustrating the contribution of stationary sources like fuel coal were more than the contributions of mobile sources like motor vehicles.?4? The mean mass concentrations of As, Cr, Cu, Mn, Ni, Pb and Zn in PM2.5 in Xi'an during the period of sampling were 541.05,332.61,667.93,466.73,269.56, 181.09 and 805.41 ng/m3, respectively. The seasonal variations of As, Cu, Mn, Ni and Zn among these heavy metals were the same, which was winter> spring> autumn> summer; as for Cr, it was winter> summer> autumn> spring; the change of Pb was winter> autumn> summer> spring. From the perspective of the single sampling site, the seasonal variation of every metal in each site was basically consistent with the parallel metal in Xi'an overall, only individual differences. In a word, the most average concentration in winter were the maximum. In the whole sampling time, heavy metals of As, Mn, Ni, Pb and Zn in PM2.5 in Xi'an showed the same space change which was BSM> NDJC> GWL; the variation of Cu was BSM> GWL> NDJC; as for Cr, it was NDJC> BSM> GWL. In terms of different season, the space changes of different metals in different seasons were not all the same.?5? During the whole period of sampling, the total annual average excess risk of the seven heavy metals and the cancer risk of single pollutants of Cr and As in PM2.5 in Xi'an to males, females and children by the way of breathing exceed the acceptable level of 10-6; the risks of other five metals were in the safe range. Comparing with the three exposure populations, the excess risk of seven heavy metals and the risk of As, Ni and Cu to males were the highest, but the non-carcinogenic risks of Cr, Mn, Pb and Zn to children were the highest.As for three types of people, the space variation of the total annual average excess risk of the seven heavy metals and the cancer risk of single pollutants of Cr were GWL> NDJC> BSM, exceeding the acceptable level of human ?10-6?; the cancer risk of As in GWL was the highest; the excess risks of other five metals to the three groups were in the save range. In the aspect of single sampling sites, the total annual average excess risk of the seven heavy metals and the cancer risk of single pollutants of heavy metal in PM2.5 in Xi'an to three groups presented different orders, probably due to the different parameters of respiratory rates, exposure durations and weight, and so on.?6? During the period of sampling in New Year's Day, the trend of PM2.5 in Xi'an and each sampling site revealed that PM2.5 in holiday was higher than in pre-and post-holiday; the concentrations of the total water soluble inorganic ions and NH4+, K+ Cl-, NO3- and SO42- accorded with the rule; but the concentrations of Na+, Ca2+ and F-decreased gradually and the minimum mean concentration of Mg2+ was in the time of holiday, all the changes of which were not significant. Furthermore, the mean contents of Cu, Pb and Zn during holiday was more than pre-and post-holiday; the concentrations of As, Cr, Mn and Ni grew persistently, but only Cr was remarkable. In the view of space, the mean concentrations of PM2.5 in each period in BSM were close to GWL, but both of them were higher than NDJC. As for water inorganic ion components, the space changes of the nine ions were not the same, but the level of mean concentrations in every sites were similar. For heavy metals component, the average concentrations of As, Cr and Ni in PM2.5 in NDJC, the mean concentrations of Cu and Mn in PM2.5 in GWL and the contents of Pb and Zn in PM2.5 in BSM were the highest among the three sites. |
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