| In nowadays, City served as a hub where not only being featured by highly populated, intense industrial productions, social and economic activities, but also a sensitive areas under the tremendous pressures from natural resources and urban ecological environment. Shanghai, as the Chinese largest industrial city, have persistently been gone through the vast and various persistent organic pollutants which being poured into urban ecological system due to the joint effort of urban characteristics of densely populated, large number of vehicles, frequently infrastructure construction, massive energy consumption and diverse human activities. In cities, road dusts accumulated in the impervious surface could be under subject to various sources (e.g. weathered materials of road surfaces, vehicular emissions, lubricating oils, used engine gasoline, diesel fuels, tire debris, domestic cooking, heating system, construction materials and deposited matters). Because of the large number of pollution origins are present within the urban atmosphere, road dust has a high potential for containing these compounds, making the road dusts become a significant environmental medium of carrying those pollutants and a typical non-point pollution source. Moreover, road dust frequently interacted with other urban environmental media, exhibiting a closely mutual relationship between them. For example, Road dusts could release back into the urban atmosphere through re-suspension by vortex disturbances. In addition, during the rainy period some surface dusts will be washed off and finally enter the aqueous environment, becoming a potentially significant pollutant source of PAHs to estuaries, surface water, sediments and consequently the food chain. Road dusts could impose some healthy issues on urban residents by means of direct or indirect daily contact. Consequently, it is of uttermost significance to carry out a research that is dedicated to study PAHs accumulated on road dusts in Shanghai.This study innovatively utilized a newly sample pre-treatment technique called gas purge micro-syringe extraction (GP-MSE) to systematically reveal the seasonal variation in the distribution and characteristics of PAHs in road dusts in Shanghai based on a newly developed purification method for road dusts. Arcgis software was used to visualize the seasonal spatial distribution characteristics of PAHs and tentatively discussed the reasons behind each season’s certain spatial pattern and seasonal change. Finally, the source analysis work for road dusts PAHs in Shanghai carried out by applying a variety of approaches and the results from different approaches were in accordant with one another.This study employed a new sample pre-treatment technique called GP-MSE which integrated extraction, cleanup, concentration and apparatus analysis procedures. The major advantage of this new purification approach was easily manipulated, less time and solvent consuming compared to the conventional silica-alumina column method. The extraction efficiency rate of target compounds was significantly increased after a series of optimizing experiments on parameters and the final set as follows:gas flow: 1.80 ml/min; extraction time:2 min; heating temperature:280℃; cooling temperature:-2℃. The optimal results showed that detection limit was 0.3～2.0 pg/g, the recovery rate in spiked blank samples were 85.7～105.2% with relative standard deviation (RSD) of 1.6-8.2%; the recovery rate in spiked real samples were 81.5～110.3% with RSD all below 8%. Each individual PAHs compound showed a satisfactorily linear relationship (R2= 0.9807～0.9978). At last, we conducted several groups of comparison between the new GP-MSE method and traditional silica-alumina chromatographic cleanup step. It can be observed from the results that the concentration of each PAH species from GP-MSE was basically the same as that of traditional cleanup method, revealing GP-MSE is a suitable and useful tool to conduct the cleanup procedure in road dust samples.The analysis for 18 PAHs in road dusts in Shanghai was meticulously conducted based on GP-MSE purification technique. The average PAHs concentration exhibited the following seasonal trend:winter> summer> autumn> spring. The total PAHs concentration in spring was in the range of 1.13～63.18 μg/g with a mean of 11.58 μg/g; those of that in summer ranged from 1.04 to 134.02 μg/g with a mean of 13.84μg/g; those of that in autumn was in the range of 1.87～163.57 μg/with a mean of 12.23μg/g; Those of that in winter was between 3.69 and 110.33 μg/g with a mean of 19.29 μg/g. Seasonal variation of the content of each PAHs species was basically the same. The highest average concentrations for PAHs species in Shanghai’s road dust were for Flu and Pyr; Meanwhile, the lowest species in dust were Acy and Ace. Low molecular weight PAHs homologues (2-3 rings) took up lowest proportion in total mass PAHs in all four seasons. The fraction of 4-rings PAHs most predominantly occupied in total mass PAHs both in spring and autumn; The other two seasons were found to be most predominantly comprised of 5- and 6-rings PAHs.The seasonal spatial distribution characteristics of PAHs in Shanghai’s road dusts was graphically presented by using inverse distance weighted (IDW) interpolation method. The results showed that high value areas in four seasons primarily distributed in Jinshan chemical industry zone, Baoshan industry zone, Lujiazui economic center, Pudong international airport and Waigaoqiao areas. Those certain areas were all shared in common characteristics featured by either massive industrial activities or densely populated and highly traffic flow. While low value areas mainly distributed in the vast suburban and rural areas where were far from the urban downtown center. In autumn, there were some high concentrations points sporadically appearing at rural areas due to the wide range of opening straws incineration. In summer, high values of the downtown area disappeared due to the effect of "urban heat/rain island". Because of seasonal changes in the prevailing wind direction, downwind regions correspondingly formed a high value area.The analysis of source identification in Shanghai’s road dusts was comprehensively conducted using three different methods which have been widely used in related research. The results from ratio of LMW/HMW turned out to be all below 1, suggesting that the predominantly high fraction of high molecular weight PAHs indicated that they mostly resulted from the combustion of petroleum fuels. The correlation efficient matrix among the individual PAHs as well as ΣPAHs reflected that nearly all of the variables were significantly correlated with one another at the 0.01 level. However, the higher correlations represented by r-value (r>0.8) were observed only among HMW compounds group, which is simultaneously and significant (p<0.01) correlated with ΣPAHs. In most cases, low molecular weight (2～3 rings) group did not correlate well among the compounds and with the HMW group. The poor correlation reflected the different origins of these two groups. Multiple diagnostic ratios implied that the PAHs be from mixture sources of traffic emissions (from gasoline and diesel engine) and coal & biomass combustion in this study area besides that of autumn was mainly from wood, coal and biomass combustion. In addition to use the qualitative diagnostic ratios, PCA was undertaken to further identify the specific sources of pollutant emissions in this region. The PCA/MLR results which was consistent with the outcome from diagnostic ratios method showed that PAHs in road dusts were mainly from mixture sources in spring and summer, accounting for 64.97% and 66.90% of PAHs environmental burden, respectively. While single source primarily contributed to dust PAHs concentration in autumn and winter, accounting for 60.58% and 59.12%, respectively. |
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