| As the most important cell of urban eco-system and urban environment, the quality of road dust is related with aquatic, atmospheric and ecological environment including human health. Since establishment of municipality, even though some works had been conducted for understanding the fate of road dust, still no further researches were developed, especially, few reports about the persistence organic pollutants (POPs) and nutrients were found. Thus, studying road dust pollution and risk characteristics in Chongqing main districts is helpful for understanding particle compositions, distinguishing pollution sources, characterized road dust environmental and health risks, which can significantly promote urban eco-environmental quality in Chongqing, and provide theory and guide for human health protection.Through sampling road dust from different functional zones and administration areas in main districts respectively, and combining multiple-sieve, gravimeter and laser-scattering technique, the particle sizes distribution characteristics of road dust was studied. The results showed that two-peak trend was observed in dust particle size distributions, first modal number particle was0～0.01mm (36.13%), and second modal number particle was0.1-0.25mm (28.24%), suggesting that the movement of dust particles can attributed to suspended movement, possibly resulted in jeopardy of human respiratory system due to atmospheric suspended particles, which should not be neglected. In range of0～0.01mm, subgroups of particles distribution complied normal distribution form, the highest distribution frequency was observed in338.49-6169.90nm with average value2496.91nm. In addition, the average particle sizes distribution trend was showed as following order:industrial area> transportation area> residential area> commercial area>cultural and education area> agricultural area> tourist area.Sixteen types of PAHs including naphthalene (NAP), acenaphthylene (ANY), acenaphthene (ANA), fluorine(FLU), phenanthrene (PHE), anthracene (ANT), fluoranthene(FLT), pyrene (PYR), chrysene (CHR), benz(a)anthracene(BaA), benzo[b]fluoranthene (BbF), benzo [k] fluoranthene (BkF), benzo [a]pyrene (BaP), indeno (1,2,3-cd)pyrene (IPY), dibenzo (a,h) anthracene (DBA), benzo (g,h,i)perylene (BPE) in0-0.25mm particle size ranges were measured respectively. The results showed that:(1) total PHAs concentration (∑16PAHs) in road dusts sampled from investigated areas ranged from0.443～19.746mg/kg, average concentration was4.204mg/kg, indicating a lower levels. From variation coefficient, the sources of PAHs in middle-high ring groups, which showed higher risk, were similar, but sources of lower ring groups were complex.(2) In16types of PAHs, concentration ratio was highest in four rings PAHs (61.75%), three rings PAHs (14.80%) second and six rings PAHs (11.52%). Among different sampling locations, administration areas, and functional zones, the component ratio of PAHs didn't show any significantly different.(3) Though method of multiple special compound ratio based on priority principles, PAHs in road dusts mainly came from car tail gas emission and coal combustion.According special compounds ratio BaA/CHR, dusts in Baishiyi, Nanshan park, Renhe, Hualong bridge, Yanggong bridge, Beibei south new district, Chongqing power station, Jinkou industrial park, Piba Shan park possibly attributed to short distance transportation, but all of dusts sampled from Chenjiang, Southwest university of political science and law, Guanyin bridge, Southwest university, Gaojia Garden, Shibanpo Yangtze River Bridge, Caiyuanba, Tongyuanju may be from exogenous transportation. The rest of sampling location may be from mixing sources.(4) Carcinogenic risk ratio of PAHs (∑6PAHs/∑16PAHs) in road dust ranged from9.20%～44.43%, indicating slight lower level. But strong carcinogenic risk ratio (BaP/E16PAHs) showed highest7.71%, suggesting the carcinogenic risk in a certain degree. Of all sampling location, Guanyin bridge (∑6PAHs), Shengkai Garden (E6PAHs), Chaotianmen (∑6PAHs), Southwest university (BaP), Shibanpo Yangtze River Bridge (BaP), Southwest university of political science and law (BaP) and Caiyuanba(BaP) showed the most clearly carcinogenic risk. Mixing eco-toxicity in road dust was relative higher, and the main toxins were fluorine (FLU), fluoranthene (FLT) and phenanthrene (PHE), which all are low ring group of PAHs.(5) The contribution coefficient of road dust PAHs in different age stages of local people (baby, infant, juvenile, youth and adult) through hand-mouth uptake, were98.13%,99.33%,98.24%,97.78%,97.95%respectively, average value was98.25%, indicating a predominant pathway.Different heavy metals including arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), mercury (Hg), nickel (Ni), lead (Pb) and zinc (Zn) in0～2.0mm particle size of road dust, which was subgrouped into0～0.25mm,0.25～05mm,0.5～1.0mm and1.0～2.0mm ranges respectively, were analyzed, and heavy metal spatial distribution pattern in road dust was also investigated. Meanwhile particle size effect of heavy metals was discussed, and corresponding potential environmental risk of heavy metals also was characterized. The results showed that:(1) The average concentration of As, Cd, Cr, Cu, Hg, Ni, Pb and Zn in road dust were12.16,0.31,83.93,78.22,0.16,34.65,73.63和144.69mg/kg respectively, which were lower than the average concentrations of peer cities in China. Among different functional zones, only variance coefficient of Hg reached significant level, and the Hg contents order implied commercial area> residential area> transportation area> cultural and education area> tourist area> industrial area> agricultural area.(2) Higher heavy metal distribution ratio was observed in fine particle sizes. Except for Cr, the all heavy metals distribution ratio were predominant in0-0.25mm. In0-0.25mm range, distribution ratio implied as Cd>As>Ni>Pb>Cu>Hg>Zn>Cr.(3) Using the deep soil background values in main districts as assessment standard, the road dust pollution level order was commercial area> industrial area> residential area> transportation area> tourist area> cultural and education area> agricultural area, and corresponding heavy metal order was Hg>Cu>Pb>Cd>Zn>As>Cr>Ni. Meanwhile, using exhibition conference soil standard as assessment reference, the corresponding pollution order was industrial area> transportation area> residential area> commercial area> cultural and education area> agricultural area> tourist area, and heavy metal order was Cu>Zn>Ni>Pb>As>Cr>Cd>Hg with evidently difference.(4) non-carcinogenic risk through respiration and skin exposure was not high, but non-carcinogenic risk of Cr and Pb were higher as compared with the others through hand-mouth exposure. Among different exposure pathways, the risk order was hand-mouth>skin>respiration. In addition, in different age stages of people, risk order was child>adult. In Chongqing, the main risk pathway of road dusts was hand-mouth exposure, and the main risk target is children. The total carcinogenic risk level of heavy metal was6.52×10-5, but the carcinogenic risk of Cd, Cr and Ni ranged from10-6～10-4, suggesting a lower carcinogenic risk, and do not resulted in risk for human health.The total nitrogen (TN), total phosphorus (TP), and organic matter concentrations in0～2.0mm particle size of road dust, which was subgrouped into0～0.25mm,0.25～05mm,0.5～1.0mm and1.0～2.0mm ranges respectively, were analyzed, and nutrients spatial distribution pattern in road dust was also investigated. Meanwhile particle size effect of nutrients was discussed, and corresponding potential environmental risk of nutrients also was characterized. The results showed that:(1) N, P and organic matter contents of road dust in Chongqing main districts were1.28g/kg,1.69g/kg and8g/kg respectively with a evidently spatial variance. The content of nutrients in different functional zones complied the following order:tourist area> commercial area> residential area> cultural and educational area>agricultural area> transportation area> industrial area.(2) Similar to particle size effect of heavy metal contents, most of nutrients including N, P and organic matter were distributed in0-0.25mm ranges, of which total N was highest. In particle size less than0.25mm, the nutrient distribution ranked as industrial area>residential area>commercial area> transportation area> cultural and educational area>tourist area>agricultural area, which was a slight different with heavy metals distribution.(3) Acid rain-extract nutrients were all observably higher than water-extract contents, suggesting that potential runoff pollution risk resulted from road dust should need more attention in Chongqing due to its most serious acid rain pollution in Chongqing. Using extraction rate as investigation index, the rank of potential runoff pollution risks in different functional zones were agricultural area>transportation area>industrial area>cultural and education area>tourist area>commercial area>residential area.This paper highlighted the spatial distribution characteristics of PAHs, heavy metals; nutrients in Chongqing Main districts through systemically analysis, and particle size effect, meanwhile healthy risk characteristics of targeted pollutants were all characterized. Additionally, though multiple techniques including laser-scattering, particle sized distribution in Chongqing main districts were also investigated.Due to the limitation of financial support, in this study only dust samples in typical seasons in2011were collected, and PAHs was analyzed in0-0.25mm ranges of road dust, but no PAH was investigated based on various ranges of particle sizes, which would be helpful for understanding the effect of particle sizes on PAHs changes. Thus, in future studies, more experiments, which are based on sampling in different seasons, hygienics investigation, simulating experiments, are needed for understanding time-scale changing patterns of road dust accumulation, and also the source distinguishing is proposed to speculate on road dust in Chongqing main districts, which validates healthy risk of road dust, and corresponding environmental risk pathway of atmosphere and runoff resulted from road dust.
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