Polycyclic aromatic hydrocarbons in urban soils of different land uses in Beijing, China: Distribution, sources and their correlation with the city's urbanization history

Journal of Hazardous Materials 177 (2010) 1085–1092

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Polycyclic aromatic hydrocarbons in urban soils of different land uses in Beijing, China: Distribution, sources and their correlation with the city’s urbanization history Shaoda Liu, Xinghui Xia ∗ , Lingyan Yang, Mohai Shen, Ruimin Liu State Key Laboratory of Water Environment Simulation/School of Environment, Beijing Normal University, Beijing 100875, China

a r t i c l e

i n f o

Article history: Received 3 November 2009 Accepted 6 January 2010 Available online 11 January 2010 Keywords: Polycyclic aromatic hydrocarbons (PAHs) Urban soil Land use Urbanization history Beijing

a b s t r a c t A total of 127 surface soil samples (0–20 cm) were collected from Beijing’s urban  district and determined for 16 polycyclic aromatic hydrocarbons (PAHs). The mean concentration of PAHs was 1802.6 ng g−1



with a standard deviation of 1824.2 ng g−1 . Average PAHs concentration and the percentage of highmolecular weight PAHs (4–6-rings) decreased from inner city to exterior areas. This correlated with the urbanization history of Beijing’s urban district  and inferred an increasing trend of soil PAHs with accumulation time and age of the urban area. PAHs in different land uses decreased in an order as: culture and education area (CEA) > classical garden (CG), business area (BA) > residential area (RA), roadside area (RSA) > public green space (PGS). PAHs in CEA mainly came from coal combustion, while soils of RSA exhibited clear traffic emission characteristics. PAHs in other land uses came from mixed sources. Principle component analysis followed by multivariate linear regression indicated that coal combustion and vehicle emission contributed about 46.0% and 54.0% to PAHs in Beijing’s urban soils, respectively. Risk assessment based on the Canadian soil criterion indicated a low contamination level of PAHs. However, higher contents in some sensitive land uses such as CEA and CG should draw enough attention. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Urban soils refer to all soils that generate under the urban environment and are influenced by intensive anthropogenic activities such as constructions, industrial activities, traffic and daily life [1,2]. Being a basic resource supporting the development of the city and people’s everyday life, urban soils play important roles in ensuring a healthy life for people living in cities and sustainable development of the urban ecosystem [3,4]. However, with fast economic development especially in some emerging countries, the urban environment is experiencing great challenges of potential heavy soil pollution. Polycyclic aromatic hydrocarbons (PAHs) are one group of the most notorious pollutants in the urban environment. They are mainly generated from combustion of carbon containing fuels [5,6], and many of them are mutagenic and some are carcinogenic [7]. Elevated PAH levels in urban soils are frequently reported [8–10]. For example, Wilcke [11] noted that concentration of PAHs in urban soils was frequently 10 times higher than that in natural soils. This undoubtedly means a potential risk of PAHs in urban soils and

∗ Corresponding author. Tel.: +86 10 58805314; fax: +86 10 58805314. E-mail address: [email protected] (X. Xia). 0304-3894/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2010.01.032

raises people’s great concern about PAH contamination in the urban environment. The urban area comprises a wide range of different land uses such as traffic, industry, business, residence, garden and public green space, implying different patterns of human activities and their possible impacts on soil quality [12]. Some researches [10,13,14] have demonstrated that specific land uses in the urban environment always showed higher PAH concentrations than other land uses. For example, soils collected at roadside or busy street in Shanghai [13], Dalian [10] and New Orleans [15] all showed much higher levels of PAHs than those collected from parks and residential area. Haugland et al. [16] and Jiao et al. [17] studied PAHs in urban soils from Bergen, Norway and Tianjin, China, respectively, and soils from both cities showed much higher PAHs in the industrial area than other areas. Although these studies have indicated different levels of PAHs in some land uses of urban area, research about PAH composition and sources in different land uses of urban environment is scarce; it is thus highly desired to have a better understand about how different land uses affect PAH distribution in urban soils. In addition, urban soil PAHs often show high levels in central and/or old districts of a city [14,16]. Some authors reported decreasing trends of PAH concentrations with increasing distances from the city center [8,18] or along the urban–suburban–rural transect

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Fig. 1. Schematic description of sampling sites in Beijing’s urban district.

[10]. This is generally considered as a result of stronger emission rates in the central area than in exterior areas. However, because central or old district also receive emission for longer time, high levels of PAHs in central or old district of cities may also be a result of long-term accumulation of PAHs in these areas. As cities often develop stepwise from old area to new area, the “urban age” of soils in different areas roughly correlates with a city’s urbanization history. Therefore, the distribution of PAHs in urban soils is also supposed to correlate with a city’s urbanization history. However, this is never reported for any specific city. Beijing is one of the most developed cities in China with its urban areas expanded nearly 7 times since 1970s. This raises people’s great concern about possible changes of urban soil quality in recent decades. Although Tang et al. [19] and Li et al. [20] have studied the level and distribution of PAHs in Beijing’s urban soils, neither of them reported a detailed survey of soil PAHs in different land uses and their possible correlation with the urbanization history of Beijing. In this research, 16 EPA priority PAHs were determined in soils from 6 types of land uses, including residential area (RA), culture and education area (CEA), business area (BA), classical garden (CG), public green space (PGS) and roadside area (RSA). The level of PAHs in Beijing’s urban soils was firstly compared with other cities around the world; then, we investigated spatial distribution of PAHs and correlation with the urbanization history of the city; PAHs in different land uses were studied for both levels and sources by composition and isomer ratio analysis; and sources of PAHs were apportioned by principle component analysis. The environmental and human risk of PAHs in Beijing’s urban soils was also assessed with the Canadian soil criterion [21]. 2. 2 Materials and method 2.1. Study area Beijing, the capital of China, is situated at the northern tip of the roughly triangular North China Plain, with its center located at 39.9N and 116.4E. As 1 of the 4 municipalities in China, it consists of 18 administrative districts (counties), among which 8 districts constitute the urban area. The urban area of Beijing is situated in the south–central part of the municipality and occupies an expanding

part of the municipality’s area. It spreads out of a series of concentric ring roads, of which the 5th ring road passes through several satellite towns. The city has a typical monsoon-influenced climate, characterized by hot, humid summers due to the East Asian monsoon, and generally cold, windy, dry winters due to the vast Siberian anticyclone. Its annual temperature is about 11.5 ◦ C and the annual precipitation is about 600 mm. In the past three decades, Beijing has been undergoing a fast economic development and urban construction in China, during which the urban population has reached over 15 million. 2.2. Sample collection and preparation Each type of land use contained at least 6 sampling sites, with each site having a consistent land use. We assure all the sampling sites were distributed as evenly as possible in the urban area of Beijing. A total of 127 topsoil samples (0–20 cm) were collected during April–May of 2008 with a stainless steel shovel. The coordinates of sampling site location were recorded with a GPS, and the sampling location was shown in Fig. 1. A total of 9 samples were collected in BA, 6 samples were collected in CG, 9 samples were collected in CEA, 12 samples were collected in PGS, and 12 samples were collected in RA. In RSA, 79 samples were collected at different distances from both sides of 10 roads. According to different circumstances, the closest, middle and farthest roadside samples (named as RSA C, RSA M and RSA F, respectively) were collected within 1 m, 8–30 m and 25–50 m to roads, respectively. To make the samples be representative for each type of land use, the number of sub-samples was determined based on the area of sampling site. For BA and RSA, each sample was the mixture of 5 sub-samples taken from the sample site; for RA, each sample was the mixture of 8 sub-samples taken from the sample site; for PGS, CG and CEA, each sample was the mixture of 10 sub-samples taken from the sample site. All the samples collected were kept in sealed kraft packages and transported to the laboratory immediately. Soil samples were dried in a shady place at room temperature, and impurities such as stones and tree leaves were removed from these samples. For soil pretreatment, the soils were crushed to pass through a 100 mesh sieve and then kept at room temperature until analysis.

S. Liu et al. / Journal of Hazardous Materials 177 (2010) 1085–1092

2.3. Materials and reagents The mixed standard solution of 16 PAHs (naphthalene (Nap), acenaphthylene (Any), acenaphthene (Ane), fluorene (Fle), phenanthrene (Phe), anthracene (Ant), fluoranthene (Fla), pyrene (Pyr), benz[a]anthracene (Baa), chrysene (Chy), benzo[k]fluoranthene (Bkf), benzo[b]fluoranthene (Bbf), benzo[a]pyrene (Bap), indeno[1,2,3-cd]pyrene (I1p), dibenz[a,h]anthracene (Daa), benzo[ghi]perylene (Bgp) each at 200 ␮g ml−1 ) was purchased from AccuStandard Inc., USA and further diluted to the desired concentration as stock solution. M-terphenyl (AccuStandard Inc., USA) was chosen as the internal standard for instrumental determination and also prepared to stock solution (10 ␮g ml−1 ). Both stock solutions were then stored in a sealed brown flask with Teflon cap under −4 ◦ C until use. Acetone, n-hexane and dichloromethane were HPLC grade and purchased from J.T. Baker Inc. (USA). Silica gel for column chromatograph (100–200 mesh) was purchased from Qingdao Haiyang Chemical Co., Ltd. (Qingdao, China). It was activated at 120 ◦ C for 12 h in a muffle and then deactivated with 3–5% distilled water before use. Alumina (100–200 mesh; China National Medicines Corporation Ltd., Shanghai, China) was activated at 180 ◦ C for 4 h and then deactivated with 3–5% distilled water. Anhydrous sodium sulfate was baked at 650 ◦ C for 4 h in a muffle and used as the dewatering agent. Powdered copper (high purity, China National Medicines Corporation Ltd., China) was used as the desulfurizing agent and was treated with 1 mol l−1 HCl before use. All glasswares for experiment were soaked in a prepared nitric acid lotion for above 4 h and then washed with distilled water and oven-dried; the glasswares were washed again with acetone for 3 times before use. 2.4. PAH determination 2.4.1. Sample extraction and clean up Fourteen grams of soil sample were weighed for PAHs determination. Soil sample extraction method was modified from Popp et al. [22]. Soils were extracted with an accelerated solvent extractor (ASE300, Dionex Corp., USA) using hexane–acetone (1:1, v/v). Soil samples were extracted 2 times under 100 ◦ C, 1500 psi, each time for 6 min, and finally cleaned up with the extraction solvent of 60% cell’s volume (66 ml). Acetone and hexane pre-cleaned fossil flour were filled in the cells together with soil sample in order to reduce the consumption of organic reagents. An appropriate amount of powdered copper was also filled in order to remove sulfur from the extract. The extract was then concentrated with a rotary evaporator and solvent exchanged with 5 ml of hexane. Sample clean up procedure was modified from Guo et al. [23]. Clean up was carried out on a silica gel–alumina column of 10 mm diameter. Activated silica gel and alumina which were pre-soaked in hexane were successively filled into the column till the height of 12 cm and 6 cm, respectively. One centimeter of anhydrous sodium sulfate was finally filled onto the top of the column. The former extracted sample was then loaded onto the prepared silica gel–alumina column. Elution was performed by successively loading 15 ml hexane and 70 ml hexane–dichloromethane (3:7, v/v) and the second part of the elution was collected for PAHs determination. The elution was again concentrated and solvent exchanged with hexane. The final extract was then blown to 2 ml under soft nitrogen stream in a graduated test tube followed by 10 ␮l of internal standard (m-terphenyl) injected into each extract using a 20 ␮l injector, sealed and kept at −4 ◦ C until determination. 2.4.2. Instrumental determination The determination of PAHs was carried out with a Varian 3800 gas chromatography-4000 ion trap mass spectrometry (GC/MS) system equipped with a Varian FactorFourTM highly inert capil-

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lary column VF-5ms (30 m × 0.25 mm diameter, and 0.25 ␮m film thickness). Instrumental conditions were modified from Guo et al. [23]. The injector temperature was kept at 290 ◦ C. The transfer line and trap temperature were 280 ◦ C and 200 ◦ C, respectively. Highly pure helium (99.9999%) was chosen as the carrier gas, and kept at a constant flow rate of 1 ml min−1 . The column oven temperature was programmed at 80 ◦ C initially for 3 min, then increased to 230 ◦ C at a rate of 15 ◦ C min−1 , held for 2 min, and finally increased to 290 ◦ C at 5 ◦ C min−1 , held for 8 min. Ionization was performed in internal electron impact (EI) mode. Quantification of each PAH was performed in MS/MS scan mode at normal speed. For identification of each target chemical, full scan mode from 35 amu to 550 amu was firstly performed and identification based both on retention time and characteristic ions. Concentrations of each chemical were finally calculated using the Varian WorkStation software. 2.5. Physicochemical parameter analysis Parameters such as pH, clay content, cation exchange capacity (CEC), total organic carbon (TOC) and black carbon (BC) were determined for general characters of these urban soil samples. Soil pH was measured by potential method, with 2.5:1 ratio of water and soil. Particle size distribution was analyzed by the hydrometer method (LY/T 1225-1999). For TOC and BC, the soil samples were first ground with agate mortar to pass through a 100 mesh stainless steel sieve. An elemental analyzer (Vario El, Elementar Analysensysteme GmbH, Germany) was used for the TOC analysis after the samples were treated with HCl (1:1, v/v). BC content in soil samples was determined with the chemo-thermal oxidation method [24]. Inorganic carbon in soil samples was firstly removed by HCl (1:1, v/v); amorphous organic carbon (OC) was subsequently removed in a thermal oxidation procedure at 375 ◦ C in a tube furnace for 24 h in the presence of excess oxygen (air) [25]. Then BC content in soils was determined with an elemental analyzer (Vario El, Elementar Analysensysteme GmbH, Germany). 2.6. Quality control Quality control procedure was conducted following instructions provided in EPA 8000 serial methods. Limit of detection (LOD) for 16 kinds of PAHs was calculated as the standard deviation of the signal-to-noise level in 6 duplicate uncontaminated samples. LOD for 16 PAHs ranged from 0.02 ng g−1 (nap) to 3.12 ng g−1 (bgp). Matrix spike, laboratory control (method blank) and duplicate unspiked samples were successively run along with the determination of all 127 samples. Soil used for both laboratory control and matrix spike was collected from a background site in Shunyi district of Beijing. The soil was firstly ultrasonically extracted for over 5 times by acetone, dichloromethane and hexane before use. For matrix spike, a concentration of 100 ng g−1 for each PAH was spiked into the matrix soil. No laboratory control samples had detectable contamination of any aimed PAHs, thus the results were not blank corrected. Average recoveries for 16 PAHs were 61.75 ± 5.76% (Nap), 63.41 ± 9.05% (Any), 64.48 ± 12.78% (Ane), 79.99 ± 4.54% (Fle), 102.84 ± 5.07% (Phe), 70.73 ± 6.63% (Ant), 86.73 ± 4.66% (Fla), 82.59 ± 7.99% (Pyr), 96.34 ± 5.53% (Baa), 101.11 ± 5.56% (Chy), 86.97 ± 8.24% (Bkf), 85.52 ± 11.90% (Bbf), 86.58 ± 4.08 (Bap), 95.44 ± 10.61% (I1p), 84.57 ± 5.85% (Daa) and 95.62 ± 3.08% (Bgp). 3. Results and discussion 3.1. Levels of





PAHs

PAHs in Beijing’s urban soils (Table 1) ranged from 8.5 ng g−1 to 13 126.6 ng g−1 . Frequency analysis indicated a log-normal

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Table 1 Concentrations of PAHs (ng g−1 ) and soil properties in Beijing’s urban soils.



PAHs in Beijing’s urban soil Compound Nap Any Ane Fle Phe Ant Fla Pyr Baa Chy Bkf Bbf Bap I1p Daa Bgp a

PAHs in different land uses

Mean ± S.D. 1.7 3.4 0.7 7 70.1 12.5 111.2 84.1 78.3 88.2 169.1 67.9 98.4 144.6 33.9 111.5

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

Geometric

2.6 5.9 1.5 7.7 93.6 17.6 184.8 147.7 132 137.1 331.8 143.6 209.2 267.1 67.9 196.1

Range

Land use

Total (N = 127) 1082.6 ± 1824.2 BA (N = 9) 2066.2 ± 3964 CG (N = 6) 1109.6 ± 886.2 CEA (N = 9) 1661 ± 1571.9 PGS (N = 12) 331.9 ± 331.0 RA (N = 12) 694 ± 539.9 RSA C (N = 40) 1159.5 ± 1290.5 RSA M (N = 23) 658.6 ± 590.7 RSA F (N = 16) 606.9 ± 757.1 Soil physicochemical parameters Item Mean ± S.D. pH 8.3 ± 0.2 Clay (%) 29.1 ± 5.5 11.0 ± 2.7 CEC (cmol kg−1 ) TOC (%) 1.5 ± 1.0 BC (%) 0.5 ± 0.5

a

0.2 0.7 0.1 2.2 29.6 6.4 35.3 24.1 27.3 36.5 57.1 19.0 33.2 57.5 14.9 46.0

Mean ± S.D.

ND –12.6 ND–40.4 ND–12.2 ND–45.8 0.505–761.1 ND–124.8 ND–1651.0 ND–1372.1 ND–843.3 ND–1079.8 ND–2929.3 ND–1376.5 ND–1917.7 ND–1958.1 ND–500.3 ND–1342.0

Geometric

Range

475.8 648.5 728.4 1170.8 212.4 475.8 586.9 279.1 257.3

8.5–13126.6 137.1–13126.6 143.3–2408.6 366.2–5942.3 39.1–1251.4 111.5–1712.2 8.5–12868.5 11.0–2050.3 8.8–2742.7

Geometric 8.3 16.8 10.9 1.3 0.4

Range 7.8–9.1 14.5–43.3 4.2–20.3 2.2–6.9 0–2.7

ND represents “not detected”.



distribution of  PAHs concentrations in 127 samples. The mean concentration of PAHs in Beijing’s urban soil was 1082.6 ng g−1 (geometric mean: 475.8 ng g−1 ) with a standard deviation of 1824.2 ng g−1 . Geometric means of individual PAH concentrations decreased in an order as: I1p (14.7%) > Bkf (14.6%) > Bgp (11.8%) > Chy (9.3%) > Fla (9.0%) > Bap (8.5%) > Phe (7.6%) > Baa (7.0%) > Pyr (6.2%) > Bbf (4.9%) > Daa (3.8%) > Ant (1.6%) > Fle (0.6%) > Any (0.2%) > Nap (0.05%) > Ane (0.02%). In respect to aromatic ring numbers, 2-, 3-, 4-, 5- and 6-rings PAHs contributed  averagely 0.16%, 8.65%, 33.43%, 34.10% and 23.65% to PAHs, respectively. The geometric mean concentrations of two most toxic PAHs, Bap and Daa, were 33.2 ng g−1 (mean: 209.2 ng g−1 ) and 14.9 ng g−1 (mean: 67.9 ng g−1 ), respectively. The sum of the seven carcinogenic PAHs (Baa,  Chy, Bkf, Bbf, Bap, I1p and Daa) accounted for about 51.6% of PAHs.  PAHs in Beijing’s urban soils were moderate compared with other cities around the world (Table 2). Industrial cities such as Glasgow (UK) and Kohtla-Järve (Estonia)  showed over 10 times  the concentration of PAHs in Beijing. PAHs in urban soils of Pärnu (Estonia) and Bergen (Norway) were also much higher.  PAHs in Beijing was much higher than that in tropiHowever, cal/subtropical cities such as Bangkok and Hong Kong. This is due to that these cities have a warm and much humid climate which allows much of the generated PAH to be vaporized  or washed away before accumulating into soils [14,26]. Similar PAHs levels were found between Beijing and the two northern China cities, Tian-

jin and Dalian; this is possibly because they share similar climates where house heating is needed during cold winter while summer is hot and humid. 3.2. Spatial distribution of PAHs and correlation with the urbanization history of Beijing As shown in Fig. 2, great spatial heterogeneity of PAH concen PAHs over 1200 ng g−1 trations can be observed. Most sites with (largest dots) were found in the central district of the city. Another area with high concentration of PAHs was Haidian district where more than 10 universities were situated. Most low concentraPAHs between 30 ng g−1 and tion sites (smallest dots, with 200 ng g−1 ) located in the exterior areas and the northeast part of the city. The urban district of Beijing generally developed from the inner city to exterior areas and showed a decreasing urbanization history with increasing distance to the city center. The inner city, which was the primary urban district of Beijing, had the longest history (over 500 hundred years) and formed its present layout in Ming Dynasty. It was confined within the present 2nd ring road and did not change until the middle of the 20th century. In 1970s, the urban district of Beijing expanded slowly and was confined within 3rd ring road. Great expansion of the city started since 1980s and the recent urban area, built on former farmlands and suburbs, has now expanded to the recently constructed 5th ring road.

Table  2 PAHs in surface soils from different cities around the world. City and country

Sample no.

No. of PAHs

Depth (cm)

Mean (ng g−1 )

Range (ng g−1 )

Ref.

Glasgow, UK Ljubljana, Slovenia Torino, Italia Tallinn, Estonia Pärnu, Estonia Kohtla-Järve, Estonia Bergen, Norway New Orleans, USA Bangkok, Thailand Dalian, China Shanghai, China Hong Kong, China Tianjin, China Beijing, China

20 20 20 41 15 16 79 19 30 24 55 138 105 127

15 15 15 12 12 12 16 16 20 14 22 16 16 16

0–10 0–10 0–10 0–10 0–10 0–10 0–2 0–2.5 0–5 0–5 0–10 0–20 0–20 0–20

11930 989 857 2240 7665 12390 6780 – 129.2 1104 3290 140 814 1082.6

1487–51822 218–4448 148–3410 35.5–26300 –a – NDb –200000 906–7285 12–380 219–18727 442–19700 ND–19500 68.7–5590 8.5–13126.6

[9] – – [8] – – [16] [15] [26] [10] [13] [14] [17] This survey

a b

“–” represents not available. ND represents not detected.

S. Liu et al. / Journal of Hazardous Materials 177 (2010) 1085–1092

Fig. 2. Spatial distribution of

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In order to explore possible correlation of the distribution  of PAHs soil PAHs with the urbanization history of Beijing, average in areas confined by the series of ring roads (2–5) were calcuPAHs showed a decreasing trend from lated. As shown in Fig. 3,  the interior to the exterior areas of Beijing. PAHs decreased from 1272.3 ng g−1 in area between the 2nd and 3rd ring road to 533.7 ng g−1 in area outside the 5th ring road, except that the inner city (within the 2nd ring road) showed a relatively lower concentration (1019.3 ng g−1 ) than area between the 2nd and 3rd ring road. Although the inner city had the longest history, it was the earliest area to restrict coal usage in Beijing and more environmental protection measures had been applied in this area. This contributed considerably to the lower PAHs concentration in the inner city. In addition, as shown in Table S1 (supplementary material), PAHs in each type of land use generally showed the same trend. Tak PAHs in the inner ing BA for example, average concentrations of city, the area between the 3rd and 4th ring road and the area between the 4th and 5th ring road were 947.1 ng g−1 , 697.3 ng g−1 and 382.4 ng g−1 , respectively; it decreased significantly from the inner to the exterior city (p < 0.05). As the urban district of Beijing generally shows a decreasing urbanization history with increasing distance to the city center, according to the above mentioned,  PAHs in Beijing’s urban soils generally accorded with the urbanization history of Beijing and showed an increasing trend with the age of the urban area.

PAHs in the urban district of Beijing.

Duo to different persistences, PAHs with different molecular weights can exhibit different accumulation behaviors in soils [27]. Heavy molecular PAHs (HMW PAHs), with low vapor pressure and high resistance to degradation processes, actually exhibit strong accumulation and increase significantly with time, while low molecular weight PAHs (LMW PAHs) can be volatilized and/or degraded leading to losses during their existence in soils [18,27,28]. The increasing trend of HMW PAHs/LMW PAHs ratio in soils with time was illustrated by study of archived soil samples collected at different times in history [18,28]. In order to further study the correlation of soil PAHs with the urbanization history,  average percentages of HMW PAHs (PAHs with 4–6-rings/ PAHs) in areas confined by different ring roads were calculated. As shown in Fig. 3, percentages of HMW PAHs showed a clear decreasing trend from  the inner city to outside areas. Therefore, high PAHs with high percentage of HMW PAHs in old urban areas of Beijing was probably a result of long urbanization time in these areas, which allowed a long-term accumulation of PAHs in soils. Both PAHs and percentage of HMW PAHs in area outside the 5th ring road was quite lower. Because this area was much farther from the central urban district, PAHs in soils in this area might be affected by both short urbanization time and input of PAHs from city-scale atmospheric deposition processes, during which LMW PAHs were transported from the more polluted interior areas to the exterior areas [10,30]. 3.3. PAHs in different land uses



Fig. 3. Decreasing trends of PAHs and percentage of HMW PAHs from the inner city to the outside areas in Beijing’s urban soils.

3.3.1. PAHs  levels Because PAHs in six land uses exhibited great standard deviations, geometric means were used in comparison of different  land uses (Table 1). CEA exhibited the highest concentration of PAHs among the 6 types of land uses. CG and BA showed moderate concentrations followed by RA and RSA, and the lowest was PGS. Most samples of CEA were collected from university campuses locating in Haidian district (Fig. 1). These soils might be contaminated by coal combustion facilities (such as coal-fired boilers) formerly built in campuses and used to supply energy for faculty and students. Furthermore, soils in campuses were more stable and less disturbed by construction activities, which allowed a continuous accumulation of PAHs in soils. High PAHs concentrations in CG and BA were related to heavier pollution in the central district, where soils were old and had a

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Fig. 5. Plot of PAH isomer ratios in urban soils of Beijing. Fig. 4. Individual compositions of PAHs in different types of land use in Beijing (concentrations of all PAHs are normalized to Bap).



long urbanization history. In fact, although PAHs concentration in RA was moderate, old RA (geometric mean: 1183.7 ng g−1 ), of which most sites  located in the central district actually exhibited much higher PAHs concentration than newly built RA (geometric mean: 219.8 ng g−1 ) most sites of which distributed in the exterior areas. Most sites of PGS also located in the exterior areas and exhibited quite low PAHs concentrations. In addition, soils of PGS were supposed to be re-turfed and transported from other clean areas, as also  contributed significantly to low level of PAHs in soils of PGS.  PAHs in RSA C were much higher than those in RSA M and RSA F; PAHs in RSA showed a clear decreasing trend with increasing distance to the road in Beijing. This suggested the direct impact of traffic emission on soil PAHs. Many researches indicated that traffic emission is an important source of PAHs in the urban environment [29,30]. However, PAHs concentration in RSA was lower than CEA, CG, and BA in Beijing. This was because soils in RSA might be affected by city greening activities and some roads were built for quite short time.  It can be concluded that PAHs concentrations in urban soils were affected by both different land uses and the “urban age” of soils. Soils that were old and less disturbed often exhibited high concentrations due to accumulation of PAHs in soils. Young soils such as those transported from other cleaner places and exposed to sources for shorter time exhibit low levels of PAHs. 3.3.2. PAHs composition and isomer ratio In order to compare individual PAH composition among different land uses, the geometric means of each PAH in different land uses were normalized to Bap [5]. As shown in Fig. 4, most land uses showed quite similar PAHs compositions, except that RSA C showed obvious elevations in most PAH/Bap values compared to other land uses. Bap is a typical PAH from coal combustion [31], high PAH/Bap values of RSA suggest less contribution by coal combustion. In addition, RSA C showed especial elevation in I1p and Bgp, which are two typical PAHs from vehicular emissions [29,32]. This indicated that PAHs in soils close to road mainly come from traffic emission. Low PAH/Bap values in RA, PGS, CG, BA and CEA indicated more contribution of coal combustion to these land uses. A series of PAH isomer ratios which are able to differentiate different sources have been established to evaluate PAH sources in sediments, aquatic suspended particles, atmospheric aerosols and soils [10,33–35]. Isomer ratio of Fla/(Fla + Pyr) between 0.4 and 0.5

indicates an input of fossil fuel (vehicle and crude oil) combustion, while Fla/(Fla + Pyr) > 0.5 indicates an input of grass, wood or coal combustion [33,35]. As for Bap/Bgp, a ratio of <0.44 indicates vehicle emission while a ratio of >0.9 indicates coal combustion [31]. As shown in Fig. 5, ratios of Fla/(Fla + Pyr) were generally above 0.5 with an average value of 0.59. This indicated that PAHs in Beijing’s urban soils mainly came from coal and/or biomass combustion. The ratio of Bap/Bgp distributed with a relatively broader range (from 0.02 to 2.24). Bap/Bgp for quite a number of soil samples were between 0.44 and 0.9, suggesting a mixed contamination from coal combustion and traffic emission in these soils. Bap/Bgp values for part samples of RSA C and two of RA were less than 0.44, revealing that PAHs in these soils mainly came from traffic emission. Almost all values of CEA samples were more than 0.9, suggesting that PAHs mainly came from coal combustion in campuses. This supported the former judgment that coal and/or coal-fired boilers historically used in campuses for energy supply contributed to PAHs in soils in campuses. Most Bap/Bgp values of RA and BA, part of CG and PGS were between 0.44 and 0.9. Soils in these land uses were contaminated by mixed sources from both coal combustion and traffic emission in different extents. 3.4. Source apportionment Principle component analysis (PCA) is a commonly applied technique to extract valuable information from multivariate. By utilizing the orthogonal transformation method, principle components (PCs) are extracted with different factor loadings indicating correlations of each pollutant species with each PC [36]. Each PC is further evaluated and recognized by source markers or profiles as reasonable pollution sources. Multivariate linear regressions (MLR) followed PCA can further quantitatively assess contribution of each identified source to the sum of pollutants [36,37]. In this investigation, PCA with varimax rotation was performed for 15 PAHs (Nap excluded) of 127 samples using SPSS 16.0 software. PCs with eigenvalue greater than 1 were retained. Two PCs were finally extracted and explained 86.9% of the total variance. PC 1 explained 78.6% of the total variance and had heavier loadings on Bkf, Bbf, Bap, Daa, Chy, Baa, Bgp, I1p and Any (supplementary material, Fig. S1). Bgp and I1p are typical tracers of traffic emission [5,29,32]. Bkf and Bbf are also largely released by both gasoline and diesel engines [36,38]. Thus, PC 1 can represent contribution from traffic emission. PC 2 explained 8.3% of the total variance and had heavier loading on Phe, Ant, Pyr, Fla, Flu, and Ane. As Phe, Fla, Pyr

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and Ant are all predominant emissions of coal combustion [6,39]; PC 2 was deduced to represent coal combustion. MLR was then performed  on these two identified components (factor scores) and PAHs. PAHs was normally standardized to Z scores i.e., the mean standardized PAHs was 0 and the standard deviation was 1. The regression equation was expressed as: PAHs = 0.781PC 1 + 0.665PC 2 (R2 = 0.985, Standardized p < 0.001). Contributions from the two PCs could be calculated from the coefficients; PC 1 (traffic emission) contributed 54.0% and PC 2 (coal combustion) contributed 46.0% to PAHs in Beijing’s urban soils. This apportionment of PAH sources was further compared with energy consumption structure of Beijing in 2003 (supplementary material, Fig. S2). The source apportionment of coal combustion corresponded well with coal usage in the energy consumption structure. However, over estimation of traffic emission indicating unidentified sources existed. As Beijing is transforming its energy structure to cleaner patterns, energy such as natural gas and fuel oils are used more and more in the city [40]. Emission of PAHs from these sources may come from oil-fired boilers, industrial fuel oils burning, and gas consuming home appliances. These all may be included in the unidentified sources of PAHs in Beijing’s urban soils.

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level in different types of land use was influenced by both the source and “urban age” of soils. (4) PCA/MLR analysis indicated that traffic emission and coal combustion contributed about 54% and 46%, respectively to PAHs in Beijing’s urban soils as a whole. (5) Risk assessment based on the Canadian soil criterion indicated a low contamination level of PAHs, but higher contents in some sensitive land uses such as CEA and CG should draw enough attention. Acknowledgements This research was supported by the Major State Basic Research Development Program (no. 2009CB421605) and National Science Foundation of China (no. 40871228). The authors thank Dr. Guo Wei for his help in laboratory analysis. We also thank the editor and anonymous reviewers for their comments that improved this paper. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.jhazmat.2010.01.032. References

4. Risk assessment The Canadian soil criterion [21] was used to evaluate PAHs contamination in urban soils of Beijing. This is a risk-based soil criterion for protection of both environmental and human health. The safe level of Bap in soil is 700 ng g−1 . Toxic equivalency concentrations of Bap (Bapeq ) were calculated using toxic equivalency factors (TEFs) provided by Nisbet and LaGoy [41]. Bapeq in 127 soil samples ranged from 0.7 ng g−1 to 3240 ng g−1 with an average of 180.7 ng g−1 . About 96.1% of the samples met the protection level of 700 ng g−1 , indicating a light risk level of PAHs in Beijing’s urban soils as a whole. However, five soil samples (one in BA, two in CEA and two in RSA) in this investigation exceeded the Canadian criterion, and PAHs in one sample of BA even showed over 4 times the level of safe. Exposure to these soils in various ways undoubtedly poses a significant risk to human health. Furthermore, high level of soil PAHs were found in some more sensitive land uses such as CEA, CG and BA. This should draw enough attention because these areas are places where dense population lives and/or children often play. Especially CG, as an area where citizens and tourists often visit, ingestion of soils from these areas may do much harm to people’s health. 5. Conclusions The level, distribution, and source of PAHs in Beijing’s urban soils have been studied, and PAHs’ correlation with the urbanization history has been investigated in this research. The main conclusions have been  drawn as follows: (1) PAHs in Beijing’s urban soils ranged from 8.5 to 13 126.6 ng g−1 , with a mean concentration of 1082.6 ng g−1 . It is moderate compared with other cities around the world. (2) The spatial distribution of PAHs in Beijing’s urban soils accorded with the city’s urbanization history, indicating an increasing trend of soil PAHs with the urbanization history of the city and the “urban age” of soils. (3) PAHs in different land uses decreased in an order as: CEA > CG, BA > RA, RSA > PGS. PAHs in different land uses came from different sources. PAHs in soils of CEA mainly came from historical coal combustion in campuses. Soils of RSA showed clear characteristic of traffic emission, while the other land uses were contaminated by both coal combustion and traffic emission in different extents. PAH

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