Factors affecting spatial variation of polycyclic ... - Wiley Online Library

Environmental Toxicology and Chemistry, Vol. 31, No. 10, pp. 2246–2252, 2012 # 2012 SETAC Printed in the USA DOI: 10.1002/etc.1959

FACTORS AFFECTING SPATIAL VARIATION OF POLYCYCLIC AROMATIC HYDROCARBONS IN SURFACE SOILS IN NORTH CHINA PLAIN XILONG WANG,* QIAN ZUO, YONGHONG DUAN, WENXIN LIU, JUN CAO, and SHU TAO Laboratory for Earth Surface Processes, College of Urban and Environmental Sciences, Peking University, Beijing, China (Submitted 15 May 2012; Returned for Revision 5 June 2012; Accepted 15 June 2012) Abstract— The spatial variation in concentrations of 16 polycyclic aromatic hydrocarbons (PAHs) in surface soils P in the North China Plain and the influential factors were examined in the present study. High concentrations of the sum of 16 PAHs ( PAH16) appeared in cities and their surrounding areas. Emissions and soil organic carbon (SOC) content significantly regulated spatial differentiation of PAH contamination in soils in the study area. Compared with emissions, concentrations of individual and total PAHs in soils were more closely controlled by the SOC content. Furthermore, concentrations of PAH species with lower molecular weight (e.g., two- or threering) in surface soils were more strongly correlated with the SOC content in comparison with those of higher molecular weight (e.g., fiveor six-ring), mainly because of their higher saturated vapor pressure, thus higher mobility. The spatial variation of PAH species in soils in the North China Plain tended to be larger with increasing benzene ring numbers, and the difference in physicochemical properties of PAH species determined their distinct spatial distribution characteristics. The present study highlights the relative importance of emissions and SOC content in spatial variation of PAHs and the dependence of the spatial distribution characteristics of PAH species in surface soils on their physicochemical properties at a regional scale. Results of the present work are helpful for regional risk assessment of the contaminants tested. Environ. Toxicol. Chem. 2012;31:2246–2252. # 2012 SETAC Keywords—PAHs

Emission

Soil organic carbon content

Spatial distribution

North China Plain

Yangtze River Delta area, East China, ranged from 8,600 to 3,881,000 mg/kg, with an average level of 397,000 mg/kg [7]. The total concentrations of 16 PAHs in Ji’nan City, North China, were 1,310 to 254,080 mg/kg, with an average value of 23,250 mg/kg. Vast differences in PAH concentrations in surface soils have been observed in other countries as well [8,9]. The PAH species with strikingly different physicochemical properties (e.g., Henry’s law constant, saturated vapor pressure, hydrophobicity) might have dissimilar effects on their environmental behaviors, resulting in spatial differentiation of their concentrations in soils. However, the underlying mechanisms for this are still unclear and should be investigated further. The North China Plain is one of the major coastal areas in China experiencing rapid economic development, and the population in 2003 was 0.19 billion, with a population density 4.88 times that of the whole country [10]. Accelerated industrialization and urbanization have taken place in this area over the last decades, resulting in an increasing demand for energy. This led to the increasingly severe PAH contamination in soils, and very high PAH concentrations were also detected in other environmental compartments (e.g., atmosphere, water, and sediment) in this area [4,11,12]. Previous studies on PAH contamination in soils in this region have focused mainly on large cities and their surrounding areas, and the PAH contamination in surface soils and its spatial variation on a large regional scale in this area remain poorly understood. Based on these facts, the major objectives of the present study were to: (1) examine the PAH concentrations in surface soils in the North China Plain and their spatial variation; (2) probe the effect of emissions and soil organic carbon (SOC) content on spatial distribution of soil PAH concentrations in this area; and (3) investigate the mechanisms by which physicochemical properties of PAHs affect the spatial differentiation of their concentrations in surface soils.

INTRODUCTION

Polycyclic aromatic hydrocarbons (PAHs) are a class of chemicals with two or more fused benzene rings, and those with three or more benzene rings are viewed as persistent organic pollutants, originating mainly from incomplete combustion, wildfires, and some anthropogenic sources (e.g., cooking and heating). Because of the carcinogenic and mutagenic characteristics of PAHs, 16 species in this group of chemicals are listed as priority contaminants by the U.S. Environmental Protection Agency (U.S. EPA). It has been reported that 20% of the total global emissions of PAHs are contributed by China [1]. Previous fugacity modeling-based studies further showed that soil compartment served as the predominant accumulating sink of PAHs in the Pearl River Delta and Tianjin areas [2,3]. Hence, much work has been done to investigate the PAH contamination in soils in China [4,5]. The PAHs in soils are readily transported to human bodies through consumption of crops and vegetables, inhalation of suspended particles, and dermal contact, exerting adverse effect on public health. Soil contamination with PAHs in China and other regions thus has been an increasing public health concern for soil and environmental scientists. Great spatial variation in PAH concentrations in surface soils in China, under the influence of multiple environmental factors, has been observed. For instance, the maximum total PAH concentrations in soils in the Beijing area reached 13,141 mg/kg, equivalent to 141 times of the minimum value (93 mg/kg) [6]. The total concentrations of 15 PAHs in surface soils in the All Supplemental Data may be found in the online version of this article. * To whom correspondence may be addressed ([email protected]). Published online 30 July 2012 in Wiley Online Library (wileyonlinelibrary.com). 2246

Benzene ring number affects PAH spatial variation in soil MATERIALS AND METHODS

Sample collection

A total of 302 soil samples were collected from an area of approximately 270,000 km2 in the North China Plain based on an equal longitude and latitude grid sampling mode design [13]. The distance between the adjacent sampling sites was about 30 km, so a sampling site covered an area of 900 km2. The geographic location of this area and the sampling sites are presented in Supplemental Data, Figure S1. To obtain representative samples, the sampling sites close to busy traffic lines, industrial plants, and populous residential areas were moved slightly away from these areas to minimize the interference of human activities with PAH concentrations in soil samples. In total, five subsamples of surface soil (0–10 cm) were collected with stainless-steel scoops from a 100- 100-m2 plot (one at the center and the other four at the corners of this plot) at each sampling site, and mixed well to obtain a composite sample of about 200 g [13]. All samples were collected and sealed in polyethylene bags immediately after collection. The samples after removal of plant roots were air dried, ground to pass through a 70-mesh sieve to remove small stones as was performed by Wang et al. [14], and stored in a refrigerator at 48C before analysis. The 302 samples were collected as follows: 15 from Tianjin in May, 2001; 19 from Beijing in July, 2003; and the remaining 268 samples in the summer of 2004. Reagents

The organic solvents n-hexane, acetone, and dichloromethane of analytical grade were purchased from the Beijing Reagent Company and purified by distillation. A standard mixture containing 16 PAHs was from J&K Chemical. Silica gel (60–80 mesh; Sinopharm Chemical Reagent Beijing Co.) was heated in a muffle furnace at 6508C for 10 h to remove impurities and then stored in a desiccator. It was reactivated at 1308C for 16 h immediately prior to use. Granular anhydrous sodium sulfate (analytical grade) obtained from Beijing Chemical Reagent was heated at 6508C in a furnace for 10 h and stored in a desiccator. All glassware was cleaned by soaking in a mixture of K2Cr2O7 and concentrated H2SO4 overnight, rinsed with tap water, and finally rinsed with deionized water. Sample extraction, cleanup, and concentration

A standard U.S. EPA 3545 method with slight modification was employed to extract 16 PAHs from soil samples using accelerated solvent extractor (Dionex ASE 300). A total of 10 g of anhydrous sodium sulfate and 10 g of soil were sequentially added to 34-ml stainless-steel vessels filled with glass fiber filter. Extraction was performed with a mixture of dichloromethane and acetone (1:1 v/v) at 1408C at a pressure of 10.3 MPa for 7 min of heating, followed by a 7-min static extraction. The vessels were then rinsed to a vial with 21 ml of the same solvent, and the extract was purged with highpurity N2 for 1 min. The extract was transferred to an eggplantshaped flask, the vial was rinsed twice with 2 ml n-hexane, and the eluate was pooled into the same flask and concentrated to 1 ml. The flask was rinsed twice with 2 ml n-hexane, and the eluate was poured into the column for cleanup. The U.S. EPA 3630C standard method using silica gel column was adopted for sample cleanup. This involved placing precleaned degreasing cotton wool at the bottom of the glass cleanup column and filling with 10 ml n-hexane in the column. Silica gel of 60 to 80 mesh size was then uniformly added to the column, with gentle tapping to remove air bubbles. After the

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silica gel had settled, anhydrous sodium sulfate (1 cm in thickness) was added to the column, followed by releasing of excess n-hexane until the solvent surface was at the top of the anhydrous sodium sulfate. The extract was eluted with 25 ml n-hexane (2 ml/min). The eluate was decanted and then eluted with a 50-ml mixture (2:3, v/v) of dichloromethane and n-hexane (50 ml) at a flow rate of 2 ml/min. The eluate was collected in an eggplant-shaped flask, and the extract was concentrated to 1 ml with a rotary evaporator. The concentrated extract was transferred with n-hexane to a graduated test tube and purged with N2, followed by fixation of the final volume to 1 ml with n-hexane. The samples were sealed in automatic sampling vials for gas chromatography analysis and stored at –48C for PAH concentration determination. Sample analysis

Quantitative analysis was conducted for 16 PAHs, including naphthalene (NAP), acenaphthylene (ACY), acenaphthene (ACE), fluorene (FLO), phenanthrene (PHE), anthracene (ANT), fluoranthene (FLA), pyrene (PYR), benz[a]anthracene (BaA), chrysene (CHR), benzo[b]fluoranthene (BbF), benzo[k]fluoranthene (BkF), benzo[a]pyrene (BaP), dibenz[a,h]anthracene (DahA), benzo[ghi]perylene (BghiP), and indeno[1,2,3cd]pyrene (IcdP). All samples were analyzed on a gas chromatography-mass selective detector (GC-MSD) (Agilent GC6890/MSD 5973), coupled with an HP-5MS capillary column (30 m 0.32 mm i.d. 0.25 mm film thickness; Agilent Technology) in splitless mode at a flow rate of 1.0 ml/min. Helium was used as the carrier gas, and its flow rate was set as 1 ml/min. The head column pressure was 0.03 MPa, and the injector port temperature was set at 3008C. The GC column temperature was programmed from 60 to 3008C at 68C/min, with a final isothermal stage held for 20 min. The mass spectrometer was operated using a selected ion monitoring (SIM) mode with an electron impact ionization of 70 eV, an electron multiplier voltage of 1,288 V, and an ion source temperature of 2308C. The total organic carbon content of soil samples was determined with a TOC-5000A analyzer. Quality control

A procedural blank was analyzed after running 10 actual samples to eliminate the environmental interference. Of the total samples, 25% were randomly selected for replicate measurements. If the relative error between replicate measurements was below 30%, the analyzed results were accepted. Otherwise, the extraction, cleanup, and analysis of this series of all samples were repeated. Recovery of 16 PAHs was tested by spiking standard mixture to three soil samples, followed by extraction, cleanup, and analysis as performed for the original samples. The concentrations of individual PAH species in blank samples corresponding to the signals with a signal-to-noise ratio of 10 were taken as the detection limits. Recoveries and detection limits of the 16 PAHs are summarized in Supplemental Data, Table S1. Emission estimation of PAHs in the study area

Ten major PAH emission sources, including straw and firewood burning, agricultural waste combustion in the field, coal burning for heating and some for cooking, combustion of fossil fuels for industrial activities, coal combustion for electricity generation and coke and aluminum production, and petroleum combustion for transportation and other nontraffic purposes, were covered in the inventory estimation. Anthropogenic emissions of 16 PAHs derived from consumption of

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various fuels and their respective emission factors defined as the amount of PAHs emitted from unit mass of fuel consumed from all 728 counties in the North China Plain in 2003 were reported in our previous articles [15–17]. Based on the county-level PAH emissions, a up-bottom method was employed for spatial allocation of emissions to a 1- 1-km grid using a sum of secondary and tertiary gross domestic products (GDPs) and urban and rural populations as proxies for consumption of various fuels. Additionally, PAH emissions from forest and grass fires were taken from Yuan et al. [18]. Matlab and ArcGIS were used for data analysis and mapping. The PAH emission data corresponding to the geographic location of the sampling sites of 302 surface soils were obtained with the aid of IDRISI32, and they were used for discussion and correlation analysis.

Plain. Similarly, such a test result for SOC contents from 302 sampling sites showed that the resulting p value was 2.5 106, much lower than 0.05. This suggested that SOC contents also did not follow a normal distribution. Trigonometric function transformation has been proved to be an effective method for normalization [19]. The statistical distributions of SOC content in the study area before and after trigonometric function transformation are presented in Supplemental Data, Figure S2. It is evident that SOC contents in the North China Plain were normally distributed after transforming with the trigonometric function using the equation given below, because the resulting p value (0.063) was greater than 0.05. pffiffiffiffi x0j ¼ arcsin xj

Statistical analysis

Concentrations of PAH in surface soils and their spatial distribution in the North China Plain

To examine the effect of the emissions of individual PAH species and SOC content on their concentrations in surface soils in the study area, we must know the statistical distribution of these two variables such that we are able to use the appropriate statistical analysis method. To achieve this, a Kolmogorov– Smirnov test was performed to determine whether the emission data derived from the interpolated emissions of individual PAHs at 1- 1-km resolution and SOC content followed the normal distribution.

The total concentrations of 16 PAHs in surface soils in the North China Plain ranged from 27.9 to 8430 mg/kg, with a variation of two orders of magnitude (Table 1). Nam et al. [20] measured the total concentrations of 16 PAHs in 266 soil samples collected from the whole of South Korea and reported that their concentrations ranged from 23.3 to 2830 mg/kg, with a difference also of two orders of magnitude. Trapido [21], however, reported that the total concentrations of 12 PAHs in 133 soil samples from rural, urban, and industrial areas in Estonia were from 11.2 to 153,000 mg/kg, with a difference as high as four orders of magnitude. These findings clearly show the great spatial variations of PAHs in soils in China and in the rest of the world. Based on the experimentally determined concentrations of 16 PAHs in surface soils from 302 sampling sites and the results of variation function regression, spatial distribution of these contaminants in the study area was obtained with the aid of a Kriging interpolation method. Using this approach, the whole study area was divided into 525 (from east to west) 694 (from north to south) pixels, giving a surface area of 1 km2 (1 1 km) for each pixel for interpolation. The interpolated isogram of the

RESULTS AND DISCUSSION

Statistical distribution of PAH emissions and SOC content in North China Plain

The Kolmogorov–Smirnov test results revealed that all resulting p values were lower than 0.05, even though the interpolated emission data were log-transformed, implying that emissions of individual PAHs in this area were not normally distributed (Supplemental Data, Table S2). Therefore, nonparametric Kendall’s rank correlation analysis was employed to probe the correlation between emissions of individual PAHs and their concentrations in surface soils in the North China

Table 1. Concentrations of polycyclic aromatic hydrocarbons (PAHs) in surface soils in the North China Plain (mg/kg; n ¼ 302) Percentiles PAHs

Min

P5

P25

P50

P75

P95

Max

Meana

Gmeana

NAP ACE ACY FLO PHE ANT FLA PYR BaA CHR BbF BkF BaP DahA IcdP BghiP P PAH16

ND 0.4 ND 1.1 5.4 0.8 3.1 1.2 0.6 3.1 1.2 2.8 ND ND ND ND 27.9

9.2 1.1 0.5 2.7 11.0 1.6 9.0 6.6 1.9 7.8 3.2 8.0 3.2 0.4 1.3 2.8 76.6

23.4 2.5 1.2 7.0 33.3 3.4 19.1 13.5 4.5 13.5 7.9 15.4 6.6 0.6 4.4 6.7 177.5

39.2 4.1 2.5 11.6 56.8 5.3 30.4 21.6 9.6 24.5 17.6 27.9 13.8 1.8 10.4 14.0 291.9

61.4 6.8 4.4 17.0 89.0 9.2 53.1 39.3 22.4 49.3 42.8 56.9 31.9 4.8 27.4 32.3 566.2

134.7 17.2 10.9 38.9 196.2 24.4 166.5 133.8 97.8 155.6 193.8 184.6 139.9 24.2 149.2 145.9 1,659.9

1,550.0 80.9 69.0 97.5 843.3 145.7 1,035.8 784.2 494.3 628.9 721.0 970.6 836.2 113.9 1,051.7 762.2 8,427.8

57.8 106.0 6.1 8.0 3.9 5.9 14.5 12.5 75.8 83.5 9.0 14.2 54.9 91.2 41.9 70.7 23.8 46.7 46.1 67.6 46.0 83.6 53.0 85.2 34.8 70.1 6.0 13.7 34.9 85.9 37.0 74.3 545.5 853.9

37.5 2.5 4.1 2.3 2.4 2.8 10.8 2.2 52.9 2.3 5.8 2.3 33.8 2.4 24.6 2.6 10.4 3.3 27.5 2.6 19.7 3.5 30.8 2.6 15.8 3.2 1.9 4.1 11.1 4.3 15.6 3.5 325.9 2.5

a

Mean and Gmean are, respectively, arithmetic and geometric means; standard deviations are of mean and Gmean, respectively. PAHs ¼ polycyclic aromatic hydrocarbons; NAP ¼ naphthalene; ACE ¼ acenaphthene; ACY ¼ acenaphthylene; FLO ¼ fluorine; PHE ¼ phenanthrene; ANT ¼ anthracene; FLA ¼ fluoranthene; PYR ¼ pyrene; BaA ¼ benz[a]anthracene; CHR ¼ chrysene; BbF ¼ benzo[b]fluoranthene; BkF ¼ benzo[k]fluoranthene; P BaP ¼ benzo[a]pyrene; DahA ¼ dibenz[a,h]anthracene; IcdP ¼ indeno[1,2,3-cd]pyrene; BghiP ¼ benzo[ghi]perylene; PAH16 ¼ sum of 16 polycyclic aromatic hydrocarbons; ND ¼ not detected.

Benzene ring number affects PAH spatial variation in soil

total concentrations of 16 PAHs in surface soils is presented in Figure 1A. To help visualize the spatial differentiation of the total concentrations of 16 PAHs in the study area, a point hierarchical graph was also plotted (Fig. 1B). Clearly, relatively high total concentrations of the 16 PAHs appeared in the following areas: (1) Beijing–Tianjin–Tangshan–Qinghuangdao, (2) Zhangjiakou and its surroundings, (3) Jinan–Zibo, and (4) Xingtai–Handan–Shijiazhuang. These areas were exactly the places where 41 samples were collected from large cities and their surrounding areas and 38 samples from the industrial and mining areas as shown in Supplemental Data, Figure S3. The PAH emissions from these areas were also relatively higher than emissions elsewhere (Supplemental Data, Fig. S4). The Kriging interpolated isogram of the SOC content of soils also demonstrated the generally higher SOC content of these areas relative to others (Supplemental Data, Fig. S5). It is possible that the spatial variation of PAH emissions and SOC content can greatly influence the spatial differentiation of its concentrations in surface soils in the North China Plain, which is discussed below. In contrast, relatively low concentrations of a sum of 16 PAHs were observed in the mountain areas in Shijiazhuang and Chengde as well as some areas in Shandong Plain. For a comprehensive understanding of the surface soil contamination by various PAH species, the interpolated isogram of their concentrations in surface soils of the North China Plain is presented in Supplemental Data, Figure S6, as well. Because the log-transformed concentrations of 16 PAH species in 302 sampling sites were significantly positively correlated, their spatial variations were quite similar. Effect of PAH emissions on its spatial variation in surface soils in the North China Plain

The results of correlation analysis suggested that concentrations of individual and total PAHs in surface soils in the study area were significantly positively correlated with their emissions ( p < 0.05; Supplemental Data, Table S3). This implies that PAH emission was an important factor regulating the spatial differentiation in the North China Plain. It was reported that concentrations of polychorinated biphenyls (PCBs) in soils were in good agreement with their usage amounts along an altitude transition [22]. A previous study also suggested that the atmospheric bulk deposition of PAHs to soils in France


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decreased with an increase in the distance from urban areas [23]. Heywood et al. [24] observed that concentrations of PAHs in soils in the United Kingdom were generally higher at sites closer to large cities, which could be a result of their emissions from urban areas being much higher than in the rural areas. Harrison and Johnston [25] found that most of traffic-origin PAHs would generally be deposited to the soil surface at a distance 3.8 m away from the roads, thereby giving high concentrations in soils close to busy roads [26]. The PAH concentrations in soils within 50 m around an oil refinery reached 300,000 mg/kg, and those in soils 1.3 to 4.2 km away from the plants decreased to 3,000 to 14,000 mg/kg [27]. Soils with high PAH concentrations were also detected from some industrial point source (e.g., cooking furnace, gasworks, oil refinery) [28]. These studies revealed that concentrations of PAHs in soils were highly dependent on their emissions on a regional scale. Effect of SOC content on the concentrations of PAHs in soils and their spatial variation in the North China Plain

Because both trigonometric function-transformed SOC contents and log-transformed PAH concentrations in these samples followed a normal distribution, as stated above, the Pearson correlation analysis was adopted to explore the correlation between these two parameters. Our observations showed that concentrations of individual and total PAHs in soils were significantly positively correlated with SOC content in the North China Plain (p < 0.001; Supplemental Data, Table S4). A positive correlation between SOC content and the concentrations of organic pollutants has consistently been observed by other investigators [26,29,30]. These findings indicated that SOC content was an important factor governing the concentrations of organic contaminants in soils. One reason is that organic matter is a dominant sorption medium for organic compounds in soils, unless the organic carbon content is below 0.1% by mass [31]. Sorption of organic chemicals to the organic matter would have a great influence on their concentrations and spatial distributions in the environment [26]. A portion of organic compounds would be sequestered in organic matter, thereby preventing the compounds from being degraded by microorganisms, which in turn has effects on their concentrations and spatial variations in soils [22,32].

Fig. 1. Interpolated isogram of the total concentrations of 16 polycyclic aromatic hydrocarbons (PAHs) in surface soils (mg/kg) in the North China Plain at log scale (A) and their point hierarchical graph (B). Note that the Universal Transverse Mercator (UTM) geographical coordinate system was used.

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Table 2. Multivariate correlation analysis of the concentrations of polycyclic aromatic hydrocarbons in surface soils with their emissions and soil organic carbon (SOC) content in the North China Plain (n ¼ 302) P NAP ACE ACY FLO PHE ANT FLA PYR BaA CHR BbF BkF BaP DahA IcdP BghiP PAH16 r (C-SOC, E)a 0.404 0.362 0.425 0.384 0.342 0.390 0.375 0.385 0.355 0.393 0.378 0.393 0.371 0.164 0.143 0.181 0.105 0.116 0.132 0.206 0.188 0.180 0.210 0.188 0.170 0.214 r (C-E, SOC)b c r (C, SOC & E) 0.567 0.478 0.617 0.519 0.440 0.534 0.509 0.528 0.468 0.545 0.512 0.543 0.503

0.355 0.174 0.468

0.384 0.173 0.524

0.392 0.184 0.541

0.395 0.171 0.547

a

r (C-SOC, E) is the partial correlation coefficient between PAH concentrations in soils and SOC content without taking their emissions into account. r (C-E, SOC) is the partial correlation coefficient between PAH concentrations in soils and their emissions without taking SOC content into account. r (C, SOC&E) is the multiple correlation coefficient between PAH concentrations in soils with their emissions and SOC content. NAP ¼ naphthalene; ACE ¼ acenaphthene; ACY ¼ acenaphthylene; FLO ¼ fluorine; PHE ¼ phenanthrene; ANT ¼ anthracene; FLA ¼ fluoranthene; PYR ¼ pyrene; BaP ¼ benz[a]anthracene; CHR ¼ chrysene; BbF ¼ benzo[b]fluoranthene; BkF ¼ benzo[k]fluoranthene; BaP ¼ benzo[a]pyrene; DahA ¼ P dibenz[a,h]anthracene; IcdP ¼ indeno[1,2,3-cd]pyrene; BghiP ¼ benzo[ghi]perylene; PAH16 ¼ sum of 16 polycyclic aromatic hydrocarbons. b c

Multiple correlation analysis of the concentrations of PAHs in soils with their emissions and SOC content in the North China Plain

Although SOC content in the North China Plain can predict the concentrations of PAHs in soils in this area, there are still some places (i.e., the northwestern part of the study P area) where the SOC content is relatively high whereas the PAH16 is low. This is mostly because the PAH emissions in this area are relatively low. It is most likely that emissions, organic carbon content of soils, and probably other environmental factors may jointly determine concentrations of PAHs in soils. A multivariate correlation analysis was performed to gain insight into the relative importance of these two parameters in total concentrations of 16 PAHs in the North China Plain. The multiple correlation coefficients between PAH concentrations in soils with their emissions and SOC content were all greater than the boundary value of 0.101 derived from a sample size of 302 associated with a significance level of 0.01, implying that PAH emissions and SOC content as a whole significantly affected the PAH concentrations in the North China Plain as shown in Table 2. Under conditions without considering the influence of emission, SOC content was still significantly positively correlated with the concentrations of individual PAH species in surface soils because their Kendall’s rank correlation coefficients r (C-SOC, E) were all above 0.101 (Table 2). Likewise, concentrations of individual PAH species in surface soils were also significantly positively correlated with their respective emissions if SOC content was taken as a controlling factor (Table 2). However, r (C-SOC, E) was generally greater than r (C-E, SOC) for individual and total PAHs (Table 2), suggesting that, relative to emissions, SOC content had a greater influence on the concentrations of individual and total PAHs in surface soils in the North China Plain.

differentiation of PAH concentrations in surface soils on their physicochemical properties. Here, R95, R05, and R50 are the 95th, 5th, and 50th percentiles, respectively, of the original and SOC content-normalized concentrations of PAHs in soils or their emissions derived from the 302 sampling sites. The original and SOC content-normalized (R95–R05)/R50 values of PAHs in soils increased with an increase in their benzene ring number, but the increasing slope for the original values was much more pronounced (Fig. 2). In contrast, no clear difference in the (R95–R05)/R50 values of the emissions of various PAHs was observed. Interestingly, the original and SOC content-normalized (R95–R05)/R50 values of two- or three-ring PAHs were lower than the corresponding values of emissions, whereas the PAHs with four to six rings exhibited the opposite trend. Such a difference in PAHs with different benzene ring numbers could result from their distinct physicochemical properties. Compared with PAHs with four to six benzene rings, those with two or three benzene rings had relatively higher Henry’s law constant and saturated vapor pressure. They would more readily be transported to remote areas away from their emission sources [33], and the PAHs with two or three benzene rings deposited to soil matrix would be reevaporated and transported through air diffusion, thereby forming the secondary pollution [34]. These processes resulted in the more homogeneous spatial distribution of PAHs with two or three benzene rings in soils as compared with PAHs with four to six benzene rings and the (R95–R05)/R50 values of the original and SOC content-normalized concentrations of PAHs with two or three benzene rings lower than the corresponding values of their emissions. In contrast, the PAHs with four to six benzene rings would more preferentially be deposited to the soil surface but would not readily be transported to remote areas [33].

Effect of physicochemical properties of different PAHs on spatial differences of their concentrations in surface soils in the study area

The 16 PAHs have different physicochemical properties such as Henry’s law constant, saturated vapor pressure, and octanol–water distribution coefficient (KOW). It is hypothesized that the physicochemical properties of different PAHs would have distinct effects on their regional environmental behaviors, leading to spatial variations of these compounds in surface soils in the study area. Concentrations of various PAHs and their variations in surface soils collected from 302 sampling sites were different. An index of (R95–R05)/R50 was employed to describe the relative variation of PAH emissions and their concentrations in soils as well as dependence of the spatial

Fig. 2. Comparison of the (R95–R05)/R50 values of polycyclic aromatic hydrocarbon (PAH) emissions and the original and soli organic carbon (SOC) content-normalized concentrations of PAHs with different number of benzene rings in surface soils in the study area.

Benzene ring number affects PAH spatial variation in soil


In addition, the KOC (SOC content-normalized distribution coefficient) values of PAHs with four to six benzene rings by soils could be much higher than those with two or three benzene rings because of their higher KOW values. They would be more strongly protected and sequestrated in the soil organic matter because of their stronger interactions. It was thus evident that, aside from the influence of the spatial variation in emissions, the spatial differentiation of PAHs with four to six benzene rings would also be affected by the spatial differentiation of organic carbon content in soils, making their spatial differentiation less uniform in comparison with that of PAHs with two or three benzene rings. This also resulted in much higher (R95–R05)/R50 values of PAHs with four to six benzene rings in soils relative to the corresponding values of their emissions, and such values for PAHs with four to six benzene rings in soils are markedly reduced after normalization with SOC content, close to those of their emissions (Fig. 2). The magnitude of the Kendall’s rank correlation coefficient between the concentrations of PAHs with different benzene ring numbers and their emissions can be also used to compare the closeness of their correlations, because the sample size for each PAH species was identical (n ¼ 302). It is clear that the correlation coefficient between emissions of PAHs with two or three benzene rings and their concentrations in soils was generally lower than that between emissions of PAHs with four to six benzene rings and their concentrations in soils (Supplemental Data, Table S3), further supporting the idea that mobility and degradation had greater influence on concentrations of lowermolecular-weight PAHs in soils relative to those with higher molecular weight because they were more readily transported to remote areas and degraded. The Pearson correlation coefficient between the concentrations of PAHs with SOC content decreased from 0.580 for the species with two benzene rings to 0.482 for those with five or six benzene rings, and the associated p values increased from 1.49 1028 to 5.94 1019 (Fig. 3). This suggests that concentrations of PAHs of lower molecular weight were more closely correlated with SOC content compared with those of higher molecular weight. Because PAHs of lower molecular weight (i.e., two or three ring) have relatively higher saturated vapor pressure relative to those of higher molecular weight (i.e., four to six ring), they would have higher mobility and be more readily transported to remote areas and interact with soil organic

lg(PAHs)

4

matter. Hence, their concentrations in soils are more closely dependent on SOC content relative to those of higher molecular weight (e.g., four to six ring) [33]. On the other hand, the PAHs with higher molecular weight (i.e., four to six ring) are more preferentially deposited to soils close to their emission sources because of the diffusion limitation, thereby exhibiting lower mobility and poor correlation with SOC content. The difference in correlation between the concentrations of PAHs with distinct benzene ring numbers in soils and SOC content further indicates the dependence of the spatial variation of PAH concentrations in soils in the study area on their physicochemical properties. Likewise, Meijer et al. [35] reported that concentrations of PCBs in 191 global background surface soils were positively correlated with the SOC content. The correlation appeared to be poorer with increasing number of substituted chlorine atoms. Wilcke and Amelung [26] found that concentrations of PAHs of relatively lower molecular weight (i.e., NAP, ACY, FLO, PHE, and ANT) in 18 soils in North America were significantly positively correlated with SOC content, but no correlation was observed for the higher molecular weight PAHs. CONCLUSIONS

The spatial variation of 16 PAHs in surface soils in the North China Plain and its influencing factors were investigated in the present study. P It was observed that concentrations of the sum of 16 PAHs ( PAH16) in cities were higher than in the surrounding areas. Emissions and SOC content significantly governed spatial variation of PAHs, as supported by the significantly positive correlation of these two variables with the concentrations of individual and total PAHs. The spatial variation in emissions of PAHs with various benzene ring numbers was relatively small. However, spatial differentiation of PAHs in surface soils tended to be greater with increasing benzene ring number, indicating the major role of physicochemical properties of distinct PAH species in their spatial distribution characteristics. Compared with PAHs of higher molecular weight (e.g., five or six ring), concentrations of those with lower molecular weight (e.g., two or three ring) in surface soils were more strongly correlated with the SOC content, mainly because of their higher saturated vapor pressure and thus greater mobility. Overall, the results of the present work demonstrate the relative importance of emissions and SOC content in regulating

2-ring

3-ring

3 2

r=0.510 p=2.03h10-21 n=302

r=0.580 p=1.49h10-28 n=302

1 0 4

lg(PAHs)

2251

5-6 ring

4-ring

3 2

r=0.499 p=1.98h10-20 n=302

1 0 0.00

0.05

0.10

0.15

arcsin TOC(%)

0.20

0.25 0.00

r=0.482 p=5.94h10-19 n=302 0.05

0.10

0.15

0.20

0.25

arcsin TOC(%)

Fig. 3. Comparison of the correlation between concentrations of polycyclic aromatic hydrocarbons (PAHs) with different benzene ring numbers in soils in the North China Plain with trigonometric function-transformed total organic carbon (TOC) content of soil.

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concentrations of various PAH species in surface soils in the North China Plain, and this is possibly applicable for other areas. The spatial variation of different PAH species in surface soils in a large area differed with the benzene ring numbers, illustrating dependence of the spatial distribution characteristics of PAH species in surface soils on their physicochemical properties. Results of the present study are critical for prediction of the environmental processes of organic pollutants on a regional scale. SUPPLEMENTAL DATA

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