Lead contamination and transfer in urban environmental compartments analyzed by lead levels and isotopic compositions

Environmental Pollution 187 (2014) 42e48

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Lead contamination and transfer in urban environmental compartments analyzed by lead levels and isotopic compositions Xin Hu a, *, Yuanyuan Sun b, Zhuhong Ding c, Yun Zhang c, Jichun Wu b, Hongzhen Lian a, Tijian Wang d a

State Key Laboratory of Analytical Chemistry for Life Science, Center of Material Analysis, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 21 0093, Jiangsu Province, PR China State Key Laboratory of Pollution Control and Resource Reuse, School of Earth Science and Engineering, Hydrosciences Department, Nanjing University, Nanjing 210093, China c School of Environment, Nanjing University of Technology, Nanjing 210009, Jiangsu Province, PR China d School of Atmospheric Science, Nanjing University, Nanjing 210093, Jiangsu Province, PR China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 16 October 2013 Received in revised form 23 December 2013 Accepted 25 December 2013

Lead levels and isotopic compositions in atmospheric particles (TSP and PM2.5), street dust and surface soil collected from Nanjing, a mega city in China, were analyzed to investigate the contamination and the transfer of lead in urban environmental compartments. The lead contents in TSP and PM2.5 are significantly higher than them in the surface soil and street dust (p < 0.05). The enrichment factor using the mass ratio of lead to the major crustal elements (Al, Sr, Ti and Fe) indicates significant lead enrichment in atmospheric particles. The plots of 206Pb/207Pb vs. 208Pb/206Pb and 206Pb/207Pb vs. 1/Pb imply that the street dust and atmospheric particles (TSP and PM2.5) have very similar lead sources. Coal emissions and smelting activities may be the important lead sources for street dust and atmospheric particles (TSP and PM2.5), while the deposition of airborne lead is an important lead source for urban surface soil. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: Lead Atmospheric particles Street dust Surface soil Source identification

1. Introduction Human exposure to lead occurs through inhalation and oral ingestion, which may pose potential adverse health effects to human, especially children. Therefore, lead contamination in urban soil, street dust and atmospheric particles have been well documented (Charlesworth et al., 2011; Cheng and Hu, 2010; Hu et al., 2010; Komarek et al., 2008; Laidlaw et al., 2012; Luo et al., 2012; Wei and Yang, 2010). However, most investigations have focused on a single urban environmental compartment to assess contamination, sources and human health risks of lead. Various natural and anthropogenic lead sources in urban environment include mining, smelting, industrial uses, waste incineration, coal burning and traffic emissions (vehicle exhaust particles), and suspension of soil particles (Charlesworth et al., 2011; Wei and Yang, 2010). It is also believed that long-range transport of airborne particles is an important external source (Hsu et al., 2006; Lee et al., 2007; Mukai et al., 1994). The dry and wet deposition of atmospheric particles is

* Corresponding author. E-mail addresses: [email protected], [email protected] (X. Hu). 0269-7491/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.envpol.2013.12.025

an important source for heavy metals in street dust and urban surface soil (Tanner et al., 2008). Street dust is easily re-suspended back into atmospheric aerosols by wind or the movement of vehicles. It can also be transfer to surface soil by urban runoff (Tanner et al., 2008). For example, re-suspended soil/road dust is a primary source accounting for 74% of the total suspended particulate (TSP) mass in Ho Chi Minh City, Vietnam (Hien et al., 1999). Therefore, interactions between urban soil, street dust and atmospheric particles may indicate the transfer of lead in the urban environment. Atmospheric particles and street dust have more lead loads due to anthropogenic activities, while the pedogenic lead from the weathering of parent rocks is an important origin of soilborne lead. A systematic investigation of lead in urban surface soil, street dust and atmospheric particles can help us to understand its environmental geochemical behavior. It also aids assessment of the health risks posed by lead in the urban environment. Lead has four stable isotopes: 204Pb, 206Pb, 207Pb and 208Pb. Generally, each Pb source has a distinct or sometimes overlapping isotopic ratio range. Physico-chemical and biological fractionation processes do not significantly alter these isotopic lead ratios (Komarek et al., 2008; Yip et al., 2008). Therefore, lead isotopic composition can be used as a “fingerprinting” to identify lead

X. Hu et al. / Environmental Pollution 187 (2014) 42e48

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sources and to trace lead pollution in different environmental compartments such as soils, sediments and atmosphere particles (Bentahila et al., 2008; Chen et al., 2008; Cloquet et al., 2006; Margui et al., 2007; Prohaska et al., 2005; Renberg et al., 2002; Semlali et al., 2001; Soto-Jimenez and Flegal, 2009; Townsend et al., 2009; Wong and Li, 2004; Zhang et al., 2008). In the present study, atmospheric particles (TSP and PM2.5) were collected monthly for a year from two sampling sites in Nanjing located on the lower reaches of the Yangtze River, one of the fastest developing areas in China. Urban surface soil and street dust samples were also collected from various urban and suburban areas in Nanjing. The lead content and isotopic composition and the crustal elements (Al, Sr, Ti and Fe) were analyzed to investigate the contamination and the transfer of lead in urban environmental compartments.

material, National Institute of Standards and Technology, NIST, USA)). The NIST SRM 981 was analyzed repeatedly after every two real samples were measured to calculate the precision and the accuracy of the lead isotopic determination using normalization. The analytical precision of the lead isotope ratios in NIST SRM 981 at 30 ng ml
1 Pb was 0.25% for 207Pb/206Pb and 0.36% for 208Pb/206Pb.

2. Materials and methods

3.1. Lead levels in atmospheric particles, street dust and surface soil

2.3. Data analysis One-way analysis of variance (ANOVA) was used to determine the differences in the lead content and isotopic ratio in surface soil, street dust and atmospheric particles (TSP and PM2.5). Post-hoc multiple comparisons of means were conducted using the least significant difference (LSD) test. The criterion for statistical significance was taken to be p < 0.05. Statistical analysis was performed using Statistical Product and Service Solutions (SPSS) 13.0 software for Windows, and the figures were drawn using Origin 7.0.

3. Results and discussion

2.1. Sampling Surface soil samples (0e15 cm depth) were collected from a total of 61 sites using a simple random sampling method (the sampling area is shown in Fig. S1 of the supplementary materials). They were 19 samples from urban parks and greenbelts, 34 samples from suburban greenbelts and sporadic vegetable fields, and 8 samples from mining vegetable fields near the suburban Qixia leadezinc mining/ smelting plant, the largest leadezinc deposit in eastern China. Three sub-samples were collected at each sampling site using a stainless-steel hand auger and combined into one single soil sample. The soil samples were stored in polyethylene bags and brought back to the laboratory. After the removal of stones, coarse materials, and other debris, the soils were air-dried, ground and sieved through a 200 mm nylon sieve for further treatment. Street dust on road surfaces was collected using a commercial vacuum cleaner as described in the literature (Tanner et al., 2008). The detailed sampling scheme was the same as we outlined in an earlier report (Hu et al., 2011). Thirty samples were obtained (Fig. S1). In accordance with reports in the literature (Tanner et al., 2008), (McKenzie et al., 2008), particles less than 63 mm in size are considered to arise mainly from atmospheric deposition and transportation by re-suspension. Therefore, the air-dried street dust was sieved through a 63 mm nylon sieve and stored for analysis. Atmospheric particles (TSP and PM2.5) were collected from two sampling sites in Nanjing as our previous report (Fig. S1) (Hu et al., 2013). Briefly, four TSP samples and three PM2.5 samples were collected per month from June 2010 to May 2011. Overall, 48 TSP samples and 35 PM2.5 samples were obtained from each sampling site. All filter membranes were equilibrated in a desiccator for 48 h and then weighed before and after aerosol sampling to determine the aerosol mass. The filter membranes were subsequently placed into a clean glass vial with a Teflon-lined cap and stored at
20  C until required for analysis. The field blank filter membranes were set at the same time. 2.2. Analysis of metal contents and lead isotope ratios The metal content of the soil and street dust samples was analyzed using HCle HNO3eHFeHClO4 mixture acid and a microwave sample preparation system (Milestone ETHOS 1) with temperature control, as in our previous report (Hu et al., 2011). Atmospheric particles (TSP and PM2.5) were also digested using the Milestone ETHOS 1 as in our another report (Hu et al., 2013). Digestion solutions after microwave pretreatment were evaporated near to dryness, then dissolved with 65% HNO3, and afterward brought to volume with Milli Q water. Solutions were stored in 25 ml high-density polyethylene vials at 4  C prior to instrumental analysis. Precision and accuracy were verified using standard reference materials available at the National Research Center for Geoanalysis, China (GBW07405, Soil). The concentrations of Al, Sr, Ti, Fe and Pb in the digestion solutions were measured using inductively coupled plasma optical emission spectrometry (ICP-OES, PerkineElmer SCIEX, Optima 5300). Detailed information on the ICP-OES operating parameters for measurement of elemental concentrations is described in Table S1 (the supplementary materials). Although lead isotope ratios can be measured by using thermal ionization mass spectrometry (TIMS) and multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS), inductively coupled plasma quadruple mass spectrometry (ICP-QMS) is believed to be sufficient for lead isotope ratio measurement in most environmental examinations such as tracing lead sources and monitoring historical and environmental changes (Barbaste et al., 2001; Halicz et al., 1996; Komarek et al., 2008; Margui et al., 2007; Yip et al., 2008). In the present study, therefore, lead isotopic ratios of 207Pb/206Pb and 208Pb/206Pb were determined using ICP-QMS (ICP-MS, PerkineElmer SCIEX, Elan 9000) (the instrumental conditions are listed in Table S2 of the supplementary materials). During the measurement of the lead isotope ratios, the sample solutions were diluted to a concentration of approximately 30 ng ml
1 Pb. The lead isotopic ratios reported in this work were corrected using standard reference material (SRM981, common lead isotopic

The descriptive statistics of the lead contents of the atmospheric particles (TSP and PM2.5), street dust and surface soil are summarized in Table 1. Lead contents of the studied atmospheric particles (TSP and PM2.5) were significantly higher than that in the surface soil and street dust (p < 0.05). The lead contents in PM2.5 were significantly higher than them in TSP at p < 0.05. The pervious study reported that average TSP-Pb concentrations were 1135 mg kg
1 in January and 1874 mg kg
1 in August in 2007 in Nanjing (Zhang, 2009), which were in the similar range of our study. It is also similar to the report that lead contents varied between 70 and 2940 mg kg
1 (average of 1364  620 mg kg
1) in PM2.5, and between 32 and 2188 mg kg
1 (average of 920  638 mg kg
1) in TSP in Beijing, China (Widory et al., 2010). The lead contents of the suburban soil samples showed the lowest average contents among the studied urban surface soil, mining surface soil, street dust and atmospheric particles (TSP and PM2.5) (Table 1). The average coefficient of variation (CV) for lead contents in urban and suburban surface soil samples (157%) was apparently higher than that for street dust (44.0%), TSP (73.7%) and PM2.5 (67.9%). The lead content of the 8 surface soil samples from the Qixia mining district ranged from 356 to 578 mg kg
1, which was significantly higher than them in street dust and other surface soils (Table 1). Therefore, the mining/smelting may be an important lead source for soil pollution near the mining/smelting plant. The lead content of the 19 urban soil samples (47.1  27.7 mg kg
1) was significantly higher than that of the 34 suburban soil samples

Table 1 The descriptive statistics of Pb contents and isotopic ratios (206Pb/207Pb and 208 Pb/206Pb) of the studied soil, street dust and atmospheric particles (TSP and PM2.5). Pb (mg kg
1) TSP (n ¼ 96) Min 266 Max 3969 Median 936 Mean 1117 S.D. 688 PM2.5 (n ¼ 72) Min 391 Max 9840 Median 1826 Mean 2122 S.D. 1459 Street dust (n ¼ 30) Min 37.2 Max 203 Median 97.3 Mean 109 S.D. 48

206

Pb/207Pb

208

Pb/206Pb Pb (mg kg
1)

1.151 1.185 1.165 1.165 0.006

2.075 2.124 2.103 2.103 0.009

1.150 1.175 1.166 1.165 0.005

2.079 2.118 2.102 2.102 0.007

1.160 1.172 1.164 1.164 0.003

2.093 2.115 2.105 2.105 0.006

206

Pb/207Pb

208

Pb/206Pb

Urban surface soil (n ¼ 19) 22.4 1.162 2.090 126 1.186 2.115 37.0 1.176 2.102 47.1 1.175 2.103 27.7 0.007 0.006 Suburban surface soil (n ¼ 34) 19.3 1.172 2.084 43.4 1.194 2.114 29.2 1.185 2.098 30.1 1.184 2.098 6.5 0.006 0.009 Mining surface soil (n ¼ 8) 356 1.125 2.158 578 1.132 2.171 417 1.128 2.167 431 1.128 2.166 73 0.002 0.004

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X. Hu et al. / Environmental Pollution 187 (2014) 42e48

(30.1  6.5 mg kg
1) (p < 0.05). The geochemical background values of the lead in Nanjing are 24.8  16.3 mg kg
1 (GSBVCAS, 1979), 25.9  21.0 mg kg
1 (Xia et al., 1987), 17.5 mg kg
1 (non-urban soils Nanjing) (Lu et al., 2003), and 26.2 mg kg
1 (Luhe County, Nanjing) (Wang et al., 2011), respectively. Therefore, the lead content in 34 suburban soil samples was similar or slightly higher than its geochemical background value in Nanjing, suggesting that lead pollution in those suburban soils is not significant. The monthly variation of lead in TSP and PM2.5 was shown in Fig. 1. Average lead concentrations were 137 ng m
3 in TSP and 162 ng m
3 in PM2.5 in this study (Fig. 1). Average TSP-Pb concentrations were 79.1  38.3 ng m
3 (n ¼ 35) in Xiamen, China (Zhu et al., 2010). The PM2.5-Pb concentrations ranged from 0.10 to 0.18 mg m
3 in Beijing (Widory et al., 2010) and from 0.176 to 0.213 mg m
3 in Shanghai, China (Li et al., 2009). However, the PM2.5-Pb concentrations from 2007 to 2009 in Xi’an, China ranged from 0.021 to 2.63 mg m
3 (Jan. 21, 2007) with an average value of 0.31  0.27 mg m
3. Those indicate the spatial and temporal difference of airborne lead in Chinese cities. The ambient air quality standard of airborne lead is 0.15 mg m
3 (rolling 3 month average) for US EPA and 0.5 mg m
3 (annual mean) for European Commission, respectively. Compared with the monthly variation of lead in TSP and PM2.5 in the present study (Fig. 1), airborne lead was all below 0.5 mg m
3 and above 0.15 mg m
3 in some months, suggesting that the pollution of airborne lead was not serious in Nanjing. Lead enrichment in atmospheric particles (TSP and PM2.5) was investigated using enrichment factor e the ratio of Pb to the major crustal elements (Al, Sr, Ti and Fe) (Fig. 2). Compared with soil and street dust, the obvious lead enrichment was shown in TSP and PM2.5 (Fig. 2). The average Pb/Fe, Pb/Al, Pb/Sr and Pb/Ti ratios for the soil background in Jiangsu Province were 0.0009, 0.0004, 0.20 and 0.006, respectively (CNEMC, 1990), which are lower than their average values in our studied urban and suburban surface soils. Lead enrichment in the street dust was similar to that in the suburban surface soils, except for the 8 mining suburban surface soil samples from Qixia District, which was above the Grade 3 standard of Chinese soil environmental quality (300 mg kg
1 for lead) (Table 1). Therefore the potential risks of lead in atmospheric particles to human health and eco-environment were higher than that from urban and suburban surface soil and street dust. 3.2. Lead isotope composition in atmospheric particles, street dust and surface soil The descriptive statistics of 206Pb/207Pb and 208Pb/206Pb in the soil, street dust and atmospheric particles (TSP and PM2.5) samples

Fig. 2. Plots of Pb/Sr vs. Pb/Ti and Pb/Fe vs. Pb/Al of the surface soil, street dust and atmospheric particles (TSP and PM2.5).

are also summarized in Table 1 and a plot of 206Pb/207Pb vs. 208 Pb/206Pb is shown in Fig. 3 (a and b). No significant differences were found in the lead isotope composition (206Pb/207Pb and 208 Pb/206Pb) between TSP and PM2.5 at p < 0.05 (Table 1). The 206 Pb/207Pb ratio for soil is different to those from street dust and atmospheric particles (TSP and PM2.5). The plot of 206Pb/207Pb vs.

Fig. 1. Monthly variation of lead in TSP and PM2.5 (ng m
3; n ¼ 72 for PM2.5 and n ¼ 96 for TSP).

X. Hu et al. / Environmental Pollution 187 (2014) 42e48

Fig. 3. Plot of 208Pb/206Pb vs. 206Pb/207Pb of the surface soil, street dust, atmospheric particles (TSP and PM2.5) (a), and the potential lead sources (b). Major Source data were taken from Chen et al. (2005), Mukai et al. (1993) and Zheng et al. (2004), others see Table S3. 208

Pb/206Pb displays a visible difference among surface soil, street dust and atmospheric particles, suggesting the different sources of lead (Fig. 3 (a)). The lead isotope ratios in TSP and PM2.5 were similar to those in TSP in Nanjing reported by Mukai et al. (Mukai et al., 2001). The 206Pb/207Pb ratios in the neighborhood of Shanghai ranged from 1.1612 to 1.1692 (average ¼ 1.1643) for TSP (Chen et al., 2005), from 1.157 to 1.170 (average ¼ 1.162) for PM10 (Zheng et al., 2004) and had an average of 1.159 for PM2.5 (Chen et al., 2008). These 206Pb/207Pb ratios in atmospheric particles in other Chinese cities are similar to our results, suggesting the similar dominant lead sources in these Chinese cities. The 206Pb/207Pb ratio ranges from 1.1617 to 1.1706 (average ¼ 1.1660) in Xiamen, a southeastern Chinese city (Zhu et al., 2010). The 206Pb/207Pb ratio in ambient samples from Xi’an, a northwestern Chinese city, ranged from 1.1274 to 1.1891 (average ¼ 1.1628) in summer and from 1.1455 to 1.2804 (average ¼ 1.1862) in winter (Xu et al., 2012). They are similar to a narrow range of those in atmospheric particles from other Chinese cities (Chen et al., 2008; Tan et al., 2006; Zheng et al., 2004; Zhu et al., 2010). This implies the existence of common dominant lead sources in those cities.

45

supplementary materials. Lead isotope ratios (208Pb/206Pb vs. 206 Pb/207Pb) in some potential lead sources are plotted in Fig. 3(b). Fig. 3 (a and b) shows the difference of the lead isotope ratios in the urban and suburban surface soil, mining soil, street dust and atmospheric particles. This is confirmed by plotting 206Pb/207Pb ratios vs. 1/Pb (reciprocal of lead contents in samples) (Fig. 4), which has been used to differentiate lead sources for different endmembers in environmental researches (Roussiez et al., 2005). For example, the average mixing 206Pb/207Pb ratios of multiple interrelated endmembers can be deduced from the line regression of 206Pb/207Pb ratios vs. 1/Pb (Roussiez et al., 2005). Therefore, the average mixing 206 Pb/207Pb ratios suggested by this plot was 1.166 for PM2.5, 1.168 for TSP, 1.161 for street dust, 1.159 for urban surface soil, 1.184 for suburban surface soil and 1.133 for mining surface soil. Those suggest the differences on interrelated endmembers among our studied samples. As shown in Fig. 3 (a and b) and Fig. 4, the lead isotope ratios in mining surface soil differ significantly from them in atmospheric particles, street dust, urban and suburban surface soil. It is in the same range of local galena ores of leadezinc mining/smelting in Qixia, Nanjing which has lower 206Pb/207Pb ratios and higher 208 Pb/206Pb ratios (Pan and Dong, 1999) (Table S3 and Fig. 3(b)), suggesting that the local mining/smelting ores were the main lead sources of the mining soil contamination. Compared with local galena ore, the other studied samples showed higher 206Pb/207Pb ratios and lower 208Pb/206Pb ratios, similar to sediments and suspended particle materials in Nanjing in the previous reports (Table S3). Table S3 shows that local parent rocks such as diorite, granite, gabbro and cenozoic basalts have similar lead isotope ratios, so the weathering of those parent rocks may be the important sources of urban and suburban surface soil. Fig. 3(b) also shows that atmospheric particles and street dust may have similar lead source and differ from urban and suburban surface soil. As can be seen from Fig. 3(b) the lead isotope ratios for street dust and atmospheric particles (TSP and PM2.5) are in a narrow range comparing with potential anthropogenic sources (e.g. vehicle exhaust (unleaded), gasoline and oil combustion, unburned coal and coal combustion, and metallurgical dust) and natural sources (e.g. galena in Nanjing) in the related literatures. Fig. 3(b) shows that the lead isotope ratios in the present study overlapped with those from Chinese coal (unburned coal and coal combustion) and metallurgic dust and partly overlapped with those from oil

3.3. Potential sources of lead in atmospheric particles, street dust and surface soil The lead isotope compositions in some environmental matrices in Nanjing taken from the literature are listed in Table S3 of the

Fig. 4. Plot of 206Pb/207Pb vs. 1/Pb of the surface soil, street dust, and atmospheric particles (TSP and PM2.5).

46

X. Hu et al. / Environmental Pollution 187 (2014) 42e48

combustion and vehicle exhaust (unleaded gasoline), which have lower 206Pb/207Pb ratios. It has been reported that Chinese coal has a wide variations in its lead isotope ratios (Mukai et al., 1993, 2001). For example, the 206 Pb/207Pb ratio in coal ranged from 1.081 to 1.176 with higher values in southern China and lower values in northern China (Mukai et al., 2001). It has also been reported that this ranged from 1.140 to 1.208 (Chen et al., 2005) and 1.1534e1.1825 (Zheng et al., 2004). The consumption of raw coal was 1.46  107 ton in Nanjing in 2009. The main consumers of raw coal then were thermal power plants (8.25  106 ton), non-metallic mineral manufacturers (2.03  106 ton), the petroleum processing, coking and nuclear fuel processing industry (1.93  106 ton), and chemical raw material and chemical manufacturers (1.56  106 ton) (NMBS, 2009). It has been reported that the arithmetic mean of the lead content in Chinese coal is 13 mg kg
1, that there is no obvious difference in the distribution of lead in different coalfields in China (Huang and Ao, 2010), and that the rate of lead emission from coal combustion is about 66% (Huang and Ao, 2010). So the annual lead emission from coal combustion in thermal power plants into the atmosphere is about 71.8 ton in Nanjing. There are several coal burning power plants in Nanjing (such as Huaneng Nanjing Power Plant, Huaneng Jinling Gas Turbine Power Plant, China Resources (Nanjing China) Thermal Power Company Limited and Datang Nanjing Power Plant) that release lead-bearing particulate matters into the environment as a result of coal combustion. According to our primary survey, there is no exclusive provenance for the raw coal supplied to those thermal power plants and some other plants. Nanjing also imports a certain amount of coal from Australia and other foreign sources. Therefore, the great variation in the lead isotope composition in coal makes it hard to calculate their contributions to the lead content of street dusts and atmospheric particles (TSP and PM2.5) in Nanjing. However, coal is certainly an important lead source in Nanjing. Mining/smelting is considered to be one of the most important anthropogenic pollution sources. Leadezinc and the iron/steel mining/smelting are the main mining/smelting sources e there are no other non-ferrous mining/smelting activities in Nanjing. The local ores mainly include galena ore in Qixia, CueFe-multimetal deposits and magnetite ores. The isotope ratios for galena ore in Qixia in Nanjing are 1.124 (1.099e1.138) for 206Pb/207Pb and 2.171 (2.148e2.230) for 208Pb/206Pb (Pan and Dong, 1999). The mean values for the 206Pb/207Pb and 208Pb/206Pb ratios calculated from the data for 13 CueFe-multimetal deposits in middle-lower area of Yangtze River where Nanjing is located are 1.1218 and 2.1588, respectively (Wang et al., 1995). Those from magnetite ores in the Ning-Wu area (Nanjing and Wuhu) are 1.1765 and 2.0958, respectively (Ma et al., 2006). Therefore, the lead isotope ratios in the studied street dust and atmospheric particles were out of the range of those in local lead ores and more consistent with those in local magnetite ores. Chinese lead ores have a wide range of 206 Pb/207Pb ratios from 1.081 to 1.176, with higher values in southern China and lower values in northern China (Mukai et al., 2001). For example, most of the northern Chinese lead ores have lower 206Pb/207Pb ratios (1.02e1.08) and higher 208Pb/206Pb ratios (2.22e2.27). On the other hand, typical samples from southern China are 1.17 (206Pb/207Pb) and 2.11 (208Pb/206Pb) for Fankou in Guangdong Province in southern China, and 1.18 (206Pb/207Pb) and 2.10 (208Pb/206Pb) for Jinding in Yunnan Province in southwestern China (Cheng and Hu, 2010). Nanjing is situated at the northe south transition region of China. The lead isotope ratios in our studied samples are closer to them in lead ores in the southern China. After the leaded gasoline was banned, vehicle exhaust should not represent a significant anthropogenic lead source (Chen et al.,

2005; Wang et al., 2006). But the release of from unleaded gasoline was also reported (Table S3 and Fig. 3(b)). The local vehicular fleet in 2009 was composed of approximately 0.60 million passenger cars (private cars: 0.47 million), 0.40 million motorcycles and 58,194 trucks (NMBS, 2009). Light-duty vehicles use unleaded gasoline, diesel oil or compressed natural gas while diesel oil is the fuel mainly used by trucks and buses. Therefore, the vehicle exhausts from unleaded gasoline might also be a potential lead source in Nanjing. For example, the consumption of gasoline, diesel oil and fuel oil was 3.38  104, 1.23  105, and 2.37  105 ton in Nanjing in 2009, respectively (NMBS, 2009). Therefore, vehicle exhaust from unleaded gasoline may be also an anthropogenic lead source in Nanjing. 3.4. Environmental implications for lead transfer among surface soil, street dust and atmospheric particles (TSP and PM2.5) Lead is bound to environmental medium such as soil, street dust, fly ash and atmospheric particles and transfers along with them in unban environmental compartments. The atmospheric particles may enter into soil through the dry and wet deposition, while the suspended soil particles become atmospheric particles. The deposition of atmospheric particles and the weathering of surface soil may be the main sources of street dust. Therefore, as the supporter of lead, the interaction of soil, street dust and atmospheric particles have great effects on lead transfer in unban environment. As shown in Table 1, average lead content in TSP (and PM2.5) is 23.7 (70.5), 37.1 (45.1) and 10.3 (19.5) times higher than that in urban surface soil, suburban surface and soil street dust, respectively. Although the soil particles less than 100 mm can be suspended into atmosphere, it was reported no great difference on lead content between bulk urban soil and <100 mm -fraction particles or <63 mm -fraction particles or fine particles (Acosta et al., 2009; Luo et al., 2011; Zhang et al., 2013). Therefore, the significant difference of lead contents between atmospheric particles and urban and suburban surface soil indicates that the suspended soil particles can not offer enough content of lead to atmospheric particles and can not be the main lead source for atmospheric particles. It is also confirmed by the difference of lead isotope ratios (Fig. 3). However, lead contents in urban surface soil are higher than them in suburban surface soil and lead isotope ratios in urban surface soil was also within the range of them in atmospheric particles and suburban surface soil (Table 1, Figs. 3 and 4). As mentioned above, lead contents in suburban surface soil are similar to the local soil lead background values. Those may imply that lead in urban surface soil is the mixture of local soil background lead and lead from the deposition of atmospheric particles. Supposing isotope ratios of soil background lead is equal to that in suburban surface soil and deposition of atmospheric particles is the only endmember of lead load, the contribution of airborne lead to total lead in urban surface can be calculated according the simple binary isotope model (Komarek et al., 2008). Therefore, the contribution was 47.4% based on the average lead isotope ratios in Table 3. Suburban surface soil also receives some particle bound lead due to the deposition, which may influence the calculation of the contribution of airborne lead to urban surface soil. Fig.3 showed that lead isotope compositions in street dust were similar to them in atmospheric particles (TSP and PM2.5), indicating the similar source. Considering lead contents, the deposition of particle bound lead is the main source for street dust although street dust is easily resuspended due to the winds and the traffics. Based on lead content and isotopic fingerprint, the deposition of particle bound lead is the important sources for street dust and urban surface soil while lead in street dust and surface soil has less effect on lead in atmospheric particles.

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4. Conclusions Lead is one of the most important pollutants in airborne particles and is also the only metallic pollutant in the ambient air quality standards of the most countries. Lead exposure via inhalation and ingestion of atmospheric or soil particles can pose health risks to urban residents. Average lead contents were 1117, 2122, 109, 47.1, 30.1 and 431 mg kg
1 for TSP, PM2.5, street dust, urban surface soil, suburban surface soil and mining surface soil, respectively. The lead content in TSP and PM2.5 is significantly higher than in the surface soil and street dust (p < 0.05). The enrichment factor using the mass ratio of lead to the major crustal elements (Al, Sr, Ti and Fe) indicates significant lead enrichment in atmospheric particles. The median values of 206Pb/207Pb and 208Pb/206Pb were 1.165 and 2.103 for TSP, 1.166 and 2.102 for PM2.5, 1.164 and 2.105 for street dust, 1.176 and 2.102 for urban surface soil, 1.185 and 2.098 for suburban surface soil and 1.128 and 2.167 for mining surface soil, respectively. The plots of 206Pb/207Pb vs. 208Pb/206Pb and 206Pb/207Pb vs. 1/Pb imply that the street dust and atmospheric particles (TSP and PM2.5) have very similar lead sources. Coal emissions and smelting activities may be the important lead sources for street dust and atmospheric particles (TSP and PM2.5), while the deposition of airborne lead is an important lead source for urban surface soil. Therefore, more attention should be paid to airborne particles due to their potential impact in the unban environment. Acknowledgments The work was supported by the National Natural Science Fund of China (41030746 and 21275069), the Natural Science Fund of Jiangsu Province (BK20131268) and Special Open Fund of State Key Laboratory of Analytical Chemistry for Life Science, Nanjing University. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.envpol.2013.12.025. References Acosta, J.A., Cano, A.F., Arocena, J.M., Debela, F., Martinez-Martinez, S., 2009. Distribution of metals in soil particle size fractions and its implication to risk assessment of playgrounds in Murcia City (Spain). Geoderma 149, 101e109. Barbaste, M., Halicz, L., Galy, A., Medina, B., Emteborg, H., Adams, F.C., Lobinski, R., 2001. Evaluation of the accuracy of the determination of lead isotope ratios in wine by ICP-MS using quadrupole, multicollector magnetic sector and time-offlight analyzers. Talanta 54, 307e317. Bentahila, Y., Ben Othman, D., Luck, J.M., 2008. Strontium, lead and zinc isotopes in marine cores as tracers of sedimentary provenance: a case study around Taiwan orogen. Chem. Geol. 248, 62e82. Charlesworth, S., De Miguel, E., Ordonez, A., 2011. A review of the distribution of particulate trace elements in urban terrestrial environments and its application to considerations of risk. Environ. Geochem. Health 33, 103e123. Chen, J., Tan, M., Li, Y., Zhang, Y., Lu, W., Tong, Y., Zhang, G., 2005. A lead isotope record of Shanghai atmospheric lead emissions in total suspended particles during the period of phasing out of leaded gasoline. Atmos. Environ. 39, 1245e 1253. Chen, J.M., Tan, M.G., Li, Y.L., Zheng, J., Zhang, Y.M., Shan, Z., Zhang, G.L., Li, Y., 2008. Characteristics of trace elements and lead isotope ratios in PM2.5 from four sites in Shanghai. J. Hazard. Mater. 156, 36e43. Cheng, H.F., Hu, Y.A., 2010. Lead (Pb) isotopic fingerprinting and its applications in lead pollution studies in China: a review. Environ. Pollut. 158, 1134e1146. Cloquet, C., Carignan, J., Libourel, G., 2006. Isotopic composition of Zn and Pb atmospheric depositions in an urban/periurban area of northeastern France. Environ. Sci. Technol. 40, 6594e6600. CNEMC (China National Environmental Monitoring Centre), 1990. The Elemental Background Values of Chinese Soil. Environmental Science Press of China, Beijing (in Chinese). GSBVCAS (The Group of Soil Background Values of Chinese Academy of Sciences), 1979. The natural background values of some trace elements in the important

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