Potential sources, influencing factors, and health risks of polycyclic aromatic hydrocarbons (PAHs) in the surface soil of urban parks in Beijing, China

Journal Pre-proof Potential sources, influencing factors, and health risks of polycyclic aromatic hydrocarbons (PAHs) in the surface soil of urban parks in Beijing, China Yajing Qu, Yiwei Gong, Jin Ma, Haiying Wei, Qiyuan Liu, Lingling Liu, Haiwen Wu, Shuhui Yang, Yixiang Chen PII:

S0269-7491(19)35028-6

DOI:

https://doi.org/10.1016/j.envpol.2020.114016

Reference:

ENPO 114016

To appear in:

Environmental Pollution

Received Date: 4 September 2019 Revised Date:

15 January 2020

Accepted Date: 17 January 2020

Please cite this article as: Qu, Y., Gong, Y., Ma, J., Wei, H., Liu, Q., Liu, L., Wu, H., Yang, S., Chen, Y., Potential sources, influencing factors, and health risks of polycyclic aromatic hydrocarbons (PAHs) in the surface soil of urban parks in Beijing, China, Environmental Pollution (2020), doi: https://doi.org/10.1016/ j.envpol.2020.114016. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

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Potential sources, influencing factors, and health risks of

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polycyclic aromatic hydrocarbons (PAHs) in the surface soil

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of urban parks in Beijing, China

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Yajing Qu a,b,1, Yiwei Gong a,1, Jin Ma a,*, Haiying Wei b, Qiyuan Liu a,c,

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Lingling Liu a, Haiwen Wu a, Shuhui Yang a, Yixiang Chen a

8 9 10 11 12 13 14

a

State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese

Research Academy of Environmental Sciences, Beijing 100012, China b

College of Environmental and Resource Sciences, Shanxi University, Taiyuan,

030006, China c

School of Earth Science and Engineering, Sun Yat-Sen University, Guangzhou,

510275, China

15 16 17 18 19

*Corresponding author.

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Email address: [email protected] (J. Ma)

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1

Yajing Qu and Yiwei Gong contributed equally to this work.

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1

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Abstract

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Urban parks are an important part of the urban ecological environment. The

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environmental quality of parks is related to human health. To evaluate sources of

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polycyclic aromatic hydrocarbons (PAHs) in soils of urban parks and their possible

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health risks, soil samples from 122 parks in Beijing, China, were collected and

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analyzed. The total content of 16 PAHs between 0.066 and 6.867 mg/kg. Four-ring

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PAHs were predominant, followed by 5-ring PAHs, while the fraction of 2-ring PAHs

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was the lowest. The dominant PAHs sources were found to be coal combustion and oil

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fuels such as gasoline and diesel. A conditional inference tree (CIT) was used to

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identify the key influencing factors for PAHs. Traffic emissions was the most

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important factor, followed by coal consumption, as well as the history and location of

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the park. Incremental lifetime cancer risk (ILCR) for urban park soil in Beijing were

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low under normal conditions. The soil PAHs exposure pathway risk for both children

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and adults decreased in the following order: ingestion > dermal contact > inhalation.

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The risk from soil in parks to children's health is slightly higher than that of adults,

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although the health risk due to exposure to PAHs was not extraordinary. Ecosystem

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risk was negligible.

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Main finding:

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Traffic emission was the key influencing factor influencing PAHs accumulation in

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soils of urban parks in Beijing.

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Keywords: urban parks; PAHs; sources; CIT; influencing factors; health risks

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1. Introduction

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In

cities,

especially

megacities,

anthropogenic

activities

related

to

48

industrialization and urbanization, such as construction, industrial facilities, traffic,

49

and other consequences of densely populated environments (Gu et al. 2016; Liu et al.

50

2010), can lead to the release of high concentrations of various pollutants such as

51

persistent organic pollutants (POPs) (Ciarkowska et al., 2019). Polycyclic aromatic

52

hydrocarbons (PAHs) are typical POPs, with widespread occurrence, toxicity and

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carcinogenicity, and high stability in the environment (Zhang et al., 2013). Sixteen

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PAHs have been put in the U.S. Environmental Protection Agency's (U.S. EPA)

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priority list of pollutants to control (Karaca, 2016; U.S. EPA, 1993), and seven of

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these are regarded as potential human carcinogens (U.S. EPA, 2011; IARC, 2010). As

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an important environmental medium, soil bears more than 90% of the environmental

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load of PAHs (Aichner et al., 2015; Wild and Jones, 1995). PAHs in soil can cause

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damage to human health and increase the risk of cancer (Cao et al., 2012). As an

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important part of urban soils, the prevention and control of PAHs pollution aroused

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extensive and strong attention (Wang et al., 2013).

62

In urban areas, urban green spaces are the only ecosystem to natural system,

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which plays a crucial role in protecting the urban ecological environment and in

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maintaining residents' physical and mental health (Liang et al., 2019). Parks in a

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megacity like Beijing are really different from other urban areas, many residents live

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in high-rise buildings due to the limited living space. With the free opening of Beijing

3

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parks, the number of tourists entering the park has increased greatly, and the park has

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become an important recreational place for residents, and the park road is where

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tourists go. Thus, urban soils in green space have more direct and indirect effects on

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human health (Chen et al. 2005; Madrid et al. 2002; Miguel et al. 1997).

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Beijing is one of the most rapidly urbanizing and densely populated large cities

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in China, the per capita green space is much lower than the world average level, it is

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only 16.5 m2 at present, so it is very important to give full play to the maximum

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ecological environmental benefits of limited urban green space (Craul, 1994; Jim,

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1993; Hant et al., 1991). According to the Beijing urban master plan (2016-2035), the

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per capita green space is projected to increase from 16.2 m2/person to 16.5 m2/person

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from 2018 to 2020, and it will increase to 17 m2/person in 2035, which emphasizes

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the importance of urban parks for comfortable living. In addition, in the Soil

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Environmental Quality Risk Control standard for soil contamination of development

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land (GB36600-2018) in China, different functional zones are more carefully

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distinguished, in which park is one of the most sensitive land for the elderly and

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children. Especially for children, during the process of play, it is easy to absorb a

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large number of PAHs adsorbed in the soil and dust through hand-to-mouth channels

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into children's bodies (Jiang et al., 2014; Mielke et al., 1999). Therefore, the

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environmental quality of urban parks has a considerable impact on human health.

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As the only permanent member of the UN security council of a developing

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country, China plays an important role in the model of global environmental

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cooperative governance. In China, this research on Beijing urban parks can not only

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improve the comprehensive understanding of Beijing urban park, but also provide the

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basis for management. In developing countries around the world, and especially it can

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provide a basis for comparison of soil pollution in urban parks in other countries and

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international exchange of treatment methods.

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To date, the study of soil in Beijing city parks is not comprehensive, and the

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evaluation method is relatively old. In the current study, a new method, namely CIT

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model, was used to recognize crucial factors for the accumulation of PAHs in the soil

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(Zhong et al., 2014; Hu and Cheng, 2013). Up to now, CIT has not been applied in

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studies on urban soil PAHs pollution. Based on qualitative source identification of

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PMF model, CIT model quantitatively analyzes the importance of source. Therefore,

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the main novelty of this study is that we made an analysis of the key influencing

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factors of soil pollution in urban parks using the CIT model, and explore the problem

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of PAHs pollution in Beijing urban parks. The specific purposes of this study were to

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(1) measure PAHs content and analyze their spatial distribution in the topsoils of

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Beijing urban parks; (2) estimate probable sources of PAHs in city park soils; (3)

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analyze the key influencing factors of PAHs pollution using a conditional inference

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tree (CIT); and (4) assess health risks and potential ecological risks due to exposure to

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PAHs. We aimed to provide a scientific basis for soil management in Beijing urban

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parks.

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2. Materials and methods

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2.1 Study areas

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Study areas were selected in Beijing’s mainly urban areas covering seven

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districts, including Dongcheng, Xicheng, Chaoyang, Haidian, Shijingshan, Fengtai,

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and part of Tongzhou. It contains the sixth ring road and all the regions within.

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According to the Beijing Statistical Yearbook (2018), the total study area

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encompassed approximately 2291 km2 and the resident population was 1359.6 million.

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The urban green coverage rate was 48.4%, and the per capita park green space was

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16.2 m2/person. Beijing is also an important industrial city, with a large amount of

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industrial coal burning and developed transportation. The northwest of its jurisdiction

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is a petrochemical area, and heavy industry is found in the southwest.

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2.2 Soil sampling and preparation

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The urban area near and within the sixth ring road of Beijing was selected as the

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research area. We collected 122 surface soil samples (0–10 cm depth) from urban

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parks in the research area (Fig. 1). Each of the composite soil samples was composed

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of 3–6 sub-samples, which were thoroughly mixed. Soil samples were collected with

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a clean wooden shovel and a bamboo basket. The soil samples were air-dried at room

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temperature and sieved through a < 0.076 mm (120 mesh) sieve after removing stones

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and residual roots. Samples were then stored in glass bottles at 4

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conditions until the PAHs were analyzed (Sun et al. 2017; Wang et al. 2015).

and dark

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2.3 Measurement of polycyclic aromatic hydrocarbons Total 16 PAHs specified by U.S. EPA priority pollutants are shown in Table 1.

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The standard U.S. EPA 3550C method (U.S. EPA, 2007) was used for PAHs

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extraction from soil; then the extracts are purified on a silica-gel column using the

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standard U.S. EPA 3630C method (U.S. EPA, 1996a). Each soil sample was mixed

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with anhydrous sodium sulfate, and then add recovery indicator which were: 2-

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fluorophenol, phenol-d6, 2, 4, 6-tribromophenol, nitrobenzene-d5, 2-fluorobiphenyl,

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and p-terphenyl-d14. The mixture was extracted with acetone/n-hexane (v/v = 1:1)

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three times using an ultrasonic bath, and then the extract was concentrated by rotary

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vacuum filtration, the extract solvent was exchanged to cyclohexane. The

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concentrated extract was cleaned up using glass column fitted with anhydrous sodium

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sulfate and silica gel. Pre-elute the column with the pentane, transfer the cyclohexane

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sample extract onto the column using an additional cyclohexane to complete the

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transfer, and then add pentane and continue the elution of the column. Next, elute the

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column with methylene chloride/pentane (v/v = 2:3). The eluate was then

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concentrated under a gentle stream of nitrogen for PAHs measurement on a GC-MS

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instrument.

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The standard US EPA 8270E method (U.S. EPA, 2018) was used to measure the

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16 PAHs. The PAHs in the final eluate was separated using an HP-5 ms capillary

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column (30 m × 0.25 mm I.D., and 0.25 µm film thickness). Using the helium as the

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carrier gas. Initial temperature of the oven temperature was programmed of 40°C for

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4 min, increased to 320

153

benzo[g,h,i]perylene was eluted. To ensure data quality, duplicate samples were

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analyzed for every ten samples. Analysis of blank samples, parallel samples, and

at a rate of 10 /min, maintained for 2 min until

7

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certified reference material PAHs to quality control. The tagged recoveries for PAHs

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were in the range of 58–127%.

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[Table 1 goes here]

158 159 160

2.4 Data analysis

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Statistical analyses were performed using IBM SPSS Statistical v. 20. The spatial

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distribution of PAHs in the soil was analyzed using ArcGIS 10.2 software. The source

163

diagnostics included positive matrix factorization (PMF). USEPA PMF 5.0 was used

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to apportion sources of PAHs. CIT and random forest were implemented using R

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software. Origin 8.1 and CorelDRAW X7 were used for graphical illustrations (Syed

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et al., 2017; Zhong et al., 2014).

167 168

3. Results and discussion

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3.1 Concentration and spatial distribution of PAHs

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The concentrations and spatial distribution of 16 PAHs in the 122 soil samples

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are shown in Fig. 1. The total PAHs content ranged from 0.066 to 6.867 mg/kg, and

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the mean concentration of total PAHs was 0.460 mg/kg. The mean concentration of

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seven carcinogenic PAHs was 0.218 mg/kg, ranging from 0.033 to 4.182 mg/kg. The

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seven carcinogenic PAHs accounted for 47% of the total concentration. The total

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PAHs concentrations exhibited a wide range, with a maximum value more than 100

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times greater than the minimum value. Concentrations of the 16 PAHs in the 122 soil

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samples were all lower than the soil pollution risk screening values for construction

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areas from the Soil Environmental Quality Risk Control standard for soil

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contamination of development land (GB36600-2018) issued by the Ministry of

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Ecology and Environment of China (Table S1). To assess soil pollution more

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rigorously, Maliszewska-Kordybach (1996) proposed that soil contaminated by PAHs

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can be classified into four levels: ΣPAH concentrations < 0.2 mg/kg are

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non-contaminated; ΣPAH concentrations from 0.2 mg/kg to 0.6 mg/kg are weakly

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contaminated; ΣPAH concentrations from 0.6 mg/kg to 1.0 mg/kg are contaminated;

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and ΣPAH concentrations > 1.0 mg/kg are heavily contaminated. Based on this

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classification, 71.3% of soils samples in urban parks could be considered polluted,

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and 7.3% of samples could be considered heavily contaminated. Comparing to the

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former studies, the PAHs content in Beijing urban parks showed a slight decrease

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(Zhang et al., 2016). The sampling time of Zhang et al. (2016) was during

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March-April, the heating season had been just ended. Almost 90 percent of PAHs in

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soil was by atmosphere deposition, soil was the ultimate destination of PAHs in the

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environment (Nelson et al., 1983). According to the UN Environment in 2019

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published A Review of 20 Years’ Air Pollution Control in Beijing, the mean

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concentration of PM2.5 was dropped by 34% from 2013 to 2018 (UNEP, 2019).

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Previous studies showed that the spatial variations of PAHs showed the same trends

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with PM2.5 mass concentrations (Shen et al., 2019). Also, the PAHs content had been

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on a downward trend (Zhang et al., 2017). Therefore, the decrease of soil PAHs

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concentration could be partially explained by the decrease of haze weather. In more

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recent years, Beijing’s haze weather decrease, which might be the reason to

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explaining this decrease.

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The highest concentration of ΣPAHs was found in Dongba Country Park (yellow

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point in Fig. 1), located in the north of Chaoyang district. This park was unregulated

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for a long time and garbage accumulated there. Many organic pollutants have been

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found in municipal solid waste leachates (Han et al., 2013); both the total amount of

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PAHs and the content of 4-6 ring PAHs were higher than those of uncontaminated soil

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at different distances from the domestic landfill (Han et al., 2009), indicating that the

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landfill leachate increased the content of PAHs in the soil. Dongba Country Park is

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surrounded by the airport’s second expressway, and has high traffic flow and heavy

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traffic loads. Vehicle exhaust was found to be the primary source of PAHs in the

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topsoils alongside roads (Yang et al., 1991), indicating that high vehicle exhaust

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emissions will increase the PAHs content of soil. The second concentration gradient

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points were more dispersed, including the Beijiao Park, the Shangdi Park, and the

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Beijing World Flower Garden (black points in Fig. 1). Beijiao Park, formerly a coking

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plant, was founded in 1958. It was the largest independent coking plant in China, the

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largest coke supply and export base in China, and the main energy supply base in the

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capital. It mainly produced coke, as well as more than 20 kinds of chemical raw

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materials. PAHs were the most common pollutants in the soils of coking industrial

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sites. Shangdi Park and the Beijing World Flower Garden are located at the

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intersections of main roads with high grades and heavy traffic. Suman et al. (2016)

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found that the heavy traffic load and congestion at intersections can slow traffic

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speeds, leading to an increase in PAHs emissions. Meanwhile as the largest botanical

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garden within the fourth ring road of Beijing, the Beijing World Flower Garden

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receives a large number of tourists every day. Population size and activities are

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positively correlated with PAHs concentrations produced (Saltiene et al., 2002).

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The third concentration gradient points were mostly located in the central urban

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area (red points in Fig. 1), in Dongcheng district and Xicheng district. The central

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urban area is the oldest area in Beijing. Research shows that the oldest districts have

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the highest accumulation of PAHs in the soil, due to long-term accumulation (Liu et

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al., 2010). The central urban area is also a major cultural and tourist area. Moreover,

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there is a small resident population and a high degree of population mobility. As

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mentioned above, urban population activities result in PAHs accumulation. The

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concentrations of the remaining points were low. PAHs concentration distribution was

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spatially specific, and traffic emissions were a common source of high concentrations.

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In conclusion, the soil quality of Beijing urban parks was generally good, although

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pollution prevention and control should be carried out at some locations. High PAHs

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content was concentrated in the central urban areas, which require greater attention.

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[Fig. 1 goes here]

239 240

3.2 PAH molecular composition analysis

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The composition characteristics of PAHs are shown in Fig. 1. Fla accounted for

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the largest proportion, with an average value of 13.81% of the ΣPAHs concentration,

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followed by Pyr, Bbf, and Chy, with mean proportions of 13.28%, 11.49%, and

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11.42%, respectively. In a general way, PAHs in urban soils are classified in terms of

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the number of aromatic rings. The contribution of different number of aromatic rings

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was 4-ring PAHs (45.95%) > 5-ring PAHs (27.38%) > 3-ring PAHs (16.43%) > 6-ring

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PAHs (6.38%) > 2-ring PAHs (3.86%). The low molecular weight PAHs (LMW 2-3

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rings) have high volatility. The total PAHs concentration of higher molecular weight

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(HMW 4-6 rings) was 45.03 mg/kg, comprising 79.71%. The contribution of HMW

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PAHs was significant. The proportions of PAHs in each administrative region of

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Beijing are shown in Fig. 1. The content of 4-ring PAHs showed the pattern Chaoyang

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district > Xicheng district > Dongcheng district > Haidian district > Fengtai district >

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Shijingshan district > Tongzhou district. According to the Beijing Statistical Yearbook

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(2018), Chaoyang district had the highest total energy consumption (886.1 tons

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standard coal) in the city, which includes the burning of coal, oil, and natural gas. The

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density of car ownership (12956.2, 8965.9) and tourist populations (2.6312, 0.4253)

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in Dongcheng and Xicheng districts, respectively, were higher than in other

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administrative regions.

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Wagrowski and Hites (1997) showed that PAHs composition could effectively

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indicate the source of pollutants. Oil spills mainly produce 2-ring PAHs, coal and

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biomass combustion mainly produce 3- and 4-ring PAHs, gasoline combustion mainly

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produces 5-ring PAHs, and diesel combustion mainly produces 6-ring PAHs (Wang et

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al., 2008; Yu et al., 2007). Individually, coke ovens and oil spills have high Nap

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content (Khalili et al., 1995). Nap is also produced during incomplete combustion

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(Simcik et al., 1999). Acy is a characteristic component of fuel wood burning (Biache

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et al., 2014). Fla and Phe are mainly derived from coking (Ciaparra et al., 2009). BkF,

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Chy, and Pyr come mainly from industrial coal burning (Brown et al., 2012). The

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source of InP, BghiP, and DahA is automobile exhaust (Hong et al., 2007). Through a

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preliminary analysis of the contribution of each PAHs in each region, it can be seen

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that the contribution of 4-6 ring PAHs in soils was far greater than that of 2-3 ring

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PAHs, indicating that the PAHs in Beijing parks mainly come from high temperature

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combustion of fossil fuels such as coal, gasoline, and diesel.

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3.3 Positive matrix factorization (PMF)

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Positive matrix factorization (PMF) is a widely used technique to proportion the

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source of PAHs presented in different environmental media. By comparison between

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Q true and Q robust values to determine the number of PMF factors (Kwon and Choi,

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2014). When the PMF model was running, the uncertainty calculation was carried out

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with 10% of the content, and the operation was run 20 times. It was possible to extract

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three main factors and the mean contributions of different individual PAHs for these

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three PMF factors are presented in Fig. 2.

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Factor 1 was dominated by InP, DahA, and BghiP, and moderately weighted by

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BbF. Ind and BghiP are major markers for gasoline emissions (Sadiktsis et al., 2012;

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Harrison et al., 1996) and burning of heavy oil (Marr et al., 1999). DahA and BbF are

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representative compounds for diesel emissions (Larsen and Baker, 2003). Therefore,

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factor 1 represents combustion of liquefied petroleum and fossil fuels from traffic

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emissions.

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Factor 2 was predominantly loaded on Fla and moderately weighted by Ant. Fla

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and Ant are signs of coke production (Mu et al., 2013; Wang et al., 2013; Yang et al.,

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2013). Therefore, factor 2 represented coking sources.

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Factor 3 had the highest fractions of Nap and Ace, followed by BkF. Nap is

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produced during incomplete combustion (Simcik et al., 1999). Ace is a sign of

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biomass (including wood) burning (Biache et al., 2014) and BkF is a typical marker

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for industrial coal combustion (Brown et al., 2012). Biomass combustion was

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dominated by a predominance of LMW-PAHs (Wang et al., 2015; Zhao et al., 2014).

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Therefore, factor 3 was designated as coal and biomass combustion.

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The mean contributions of each source to the Σ16PAHs in the soils were

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determined by PMF analysis. The contributions differed among sites and 30.8%,

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30.3%, and 38.9% of PAHs in urban parks were from traffic emissions, coking, and

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coal and biomass combustion, respectively. There was no significant difference

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between the three sources. Coal and biomass combustion showed the highest

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contribution, especially coal burning. These results suggest that soils in Beijing parks

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are generally polluted by PAHs originating from combustion sources and the

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consumption of energy resources.

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[Fig. 2 goes here]

307 308

3.4 Conditional inference tree (CIT)

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Evaluating the factors influencing PAHs levels is a significant step in preventing

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the detrimental effects of pollution. For a more detailed understanding of the key

311

influencing factors of pollution, the impact degree of different factors was evaluated.

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With a reasonable selection of predictors corresponding to the influencing factors and

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energy resources of PAHs, such as consumption of energy resources, local

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socio-economic conditions, traffic conditions, and soil properties. In the current study,

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a recently developed tree method—the conditional inference tree (CIT) (Hu and

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Cheng, 2013)—was used to recognize crucial factors for the accumulation of PAHs in

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the topsoils of the Beijing parks. Fig. 3 shows the details of the CIT.

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For PAHs, the most important splitting factor of the root nodes was the length of

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road. Samples containing road lengths ≤ 0.693 km were separated to terminal node

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2, and the average concentration of PAHs in the left branch (0.747 mg/kg) was more

321

than twice that in the right one (0.378 mg/kg). Short roads are generally low grade

322

and have a high traffic index, resulting in congestion. Morillo et al. (2007) found that

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the PAHs content in heavy traffic areas was very high. Hence, traffic emissions have

324

an important impact on the accumulation of PAHs in the soil on both sides of the road.

325

This study shows that the coal consumption was followed by the length of road,

326

the mean concentration of PAHs in the right terminal node of this branch of coal

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consumption > 7163.8 kg (1.018 mg/kg) was approximately twice as many as that in

328

the left terminal node (0.571 mg/kg). Dongcheng district, Xicheng district, and

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Shijingshan district had coal consumption ≤ 7163.8 kg. These administrative districts

330

have fewer residents and, therefore, used less coal than other districts. The higher the

15

331

amount of coal used, the higher the PAHs content. The Beijing-Tianjin-Hebei (BTH)

332

region is an important city agglomeration in China, where coal is still the primary fuel

333

for residences in both urban and rural areas (Tian et al., 2018). The policy of replacing

334

coal with gas in China could reduce PAHs pollution.

335

The first influencing factor of the right branch was age of park. The mean

336

concentration of PAHs in the right terminal node of age of park was > 17 (0.549

337

mg/kg), which was higher than that in the other length of road ≤ 17 (0.235 mg/kg).

338

Liu et al. (2010) inferred a trend of increasing soil PAHs with time and age of urban

339

areas. These results also illustrate that there was a significant correlation between the

340

building age and the content of PAHs in urban soils. As mentioned above, the older

341

the park, the greater the accumulation of PAHs in the soil.

342

The next splitting factor was city center distance. Peng et al. (2013) pointed out

343

that road density in urban areas and distance from the city center are significantly

344

correlated with PAHs. Soil PAHs content in areas farther away from the urban center

345

were much lower than those in urban areas. This finding explains why terminal node

346

7, which is constituted by soils from the areas more than 12.8 km from the city center,

347

showed a relatively higher average PAHs content (0.272 mg/kg).

348

Park area was the last splitting variable, separating soils from parks with areas >

349

80 ha (0.218 mg/kg) from those from parks with areas < 80 ha. Larger parks were

350

mostly ornamental parks, such as Xiangshan Park and Badazhu Park, which have

351

unique viewing programs that attract large numbers of tourists, and therefore, have a

352

high degree of population mobility. As mentioned above, the impact on PAHs in the

16

353

soil was greater in larger parks than in smaller parks.

354

For PAHs, the key influencing factors were the above five, of which the most

355

important was traffic emissions, including the combustion of gasoline and diesel,

356

followed by the burning of coal. Therefore, energy consumption is the main source of

357

soil PAHs in urban parks. The next three factors related to the park’s characteristics,

358

include history and location. CIT served as an efficient tool for assessing the

359

correlation between PAHs content and sources, screened out the key influencing

360

factors affecting PAHs content in the natural environment and anthropogenic activities,

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and apportioned their contributions.

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[Fig. 3 goes here]

363 364 365

3.5 Risk assessment of PAHs

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3.5.1 Toxic equivalent concentration

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The carcinogenicity and endocrine disruptive activity of PAHs are documented

368

(Davis et al., 1993). Due to the different toxic effects of different individual PAHs,

369

such as acute toxicity of LMW-PAHs and carcinogenicity of some HMW-PAHs, it is

370

not possible to simply sum the potential effects of these compounds (Kuang et al.,

371

2011). Using the toxic equivalence factors (TEFs) to compute toxic equivalent

372

concentrations (TEQBaP) of soil samples to characterize the toxic potency, for the sake

373

of comparison and quantification.

374

Table 1 shows the TEF values for PAHs in this study and their TEQBaP

17

375

concentrations. The total TEQBaP of 16 PAHs in soil samples was between 0.005 and

376

0.726 mg/kg, with a mean of 0.049 mg/kg. The total TEQBaP of 7 carcinogenic PAHs

377

in soil samples was between 0.005 and 0.0721 mg/kg, with a mean of 0.048 mg/kg.

378

The total TEQBaP of the 7 carcinogenic PAHs was approximately the same as that of

379

all 16 PAHs; the TEQBaP of the 7 carcinogenic PAHs accounted for 98.77% of the

380

total TEQBaP. This result indicates that the 7 carcinogenic PAHs were the main

381

contributors to the total carcinogenic potency of PAHs. The contributions of different

382

PAHs to the total TEQBaP decreased as follows: BaP (60.2%) > BbF (11.0%) > DahA

383

(10.3%) > BaA (7.1%) > BkF (5.6%) > InP (3.6%) > Chy (1.1%). The 16 PAHs in the

384

urban soil samples had TEQBaP under the WHO standard value of 1 mg/kg. Therefore,

385

direct or indirect exposure to these soils poses little risk to human health.

386 387

3.5.2 Health risk assessment

388

Environmental health risk assessments are based on quantifying the degree of

389

risk to describe the threat and risk level of exposure of different populations to

390

pollutants (Wu et al., 2018). In general, the body can be exposed to pollutants in the

391

soil in three ways: ingestion, dermal contact, and inhalation. As there are physical

392

differences between age groups, the integrated lifetime cancer risks (ILCRs) was

393

computed for both children and adults, respectively (Wang et al., 2018). The detailed

394

parameters values were obtained from the Environmental site assessment guideline of

395

Beijing (DB11/T 656-2009) (Table S1). Generally, a value of ILCRs less than or equal

396

to 10-6 was taken as non-significant or essentially negligible (Asante-Duah, 2002). An

18

397

ILCRs value of 10-5 is the critical value for health risk, and ILCRs between 10-6 and

398

10-4 indicate a low-risk or critical health level, respectively. ILCRs exceeding 10-4

399

signify potentially high risk and are deemed to be of grave concern, with potential

400

health problems (U.S. EPA, 1996b). As children are most at risk for PAHs exposure,

401

the division of health risk levels is mainly targeted at determining the risk toward

402

children (Wang et al., 2018). The results of the equivalence calculations are shown in

403

Fig. 4(A).

404

In this study, the mean ILCRs for children and adults in all soil samples were

405

0.225 × 10-6 and 0.184 × 10-6 for all groups, which were lower than the baseline

406

values. The range of ILCRs was estimated to be 2.365 × 10-8 to 3.367 × 10-6, and

407

1.935 × 10-8 to 2.754 × 10-6, for children and adults respectively. Of the cumulative

408

probability ILCRs for children and adults, 97.5% were less than or equal to 10-6.

409

There were only three samples with values greater than 10-6, but less than 10-5. These

410

results indicated that almost all soil samples contaminated with PAHs had ILCRs

411

lower than the acceptable risk levels. The soil PAHs exposure pathway risk for both

412

children and adults decreased in the following order: ingestion > dermal contact >

413

inhalation. Inhalation of PAHs via the nose was almost negligible when compared

414

with the other pathways. In terms of the overall ILCR value, the risk of soil in parks

415

to children's health is slightly higher than that of adults. The ILCRs for ingestion were

416

greater for children than adults due to their hand-to-mouth activity (Jiang et al., 2014).

417

It appears that PAHs may be pervasive in the soils of Beijing parks, however, the

418

cancer risk due to PAHs exposure is not extraordinary.

19

419

[Fig. 4 goes here]

420 421 422

3.5.3 Potential ecosystem risk

423

To estimate the risk posed by certain PAHs, the Nemerow Integrated Pollution

424

Index (NIPI) was used to evaluate the ecological risk of PAHs in the surface soil of

425

Beijing parks. Soil contaminated by PAHs can be classified into five levels: NIPI ≤

426

0.7, safe; NIPI 0.7 to 1.0, warning-line; NIPI 1.0 to 2.0, weakly contaminated; NIPI

427

2.0 to 3.0, moderately contaminated; and NIPI > 3.0, heavily contaminated. The

428

calculation results are shown in Fig. 4(B). With the exception of the 10th sample

429

(Dongba Country Park), the NIPI range for each point in the study area was 0.012–

430

0.337, and the mean NIPI was 0.062. The NIPI of the 10th sample (1.898) was quite

431

different from that of the other samples, indicating weak contamination, and all other

432

points were at safe levels. This result was consistent with the PAHs concentrations

433

and spatial distribution. The results of the ecosystem risk assessments indicate that the

434

surface soils of Beijing parks were almost unpolluted by PAHs. Dongba Country Park

435

should consider various control measures and strict management to reduce pollution.

436 437

4. Conclusions

438

The main sources of PAHs in the topsoils in Beijing parks were pyrogenic

439

sources, consisting of a mix of coal, biomass, petroleum, and traffic-related sources.

440

There were five key influencing factors, among which the most important was traffic

20

441

emissions, including the combustion of gasoline and diesel, followed by the burning

442

of coal. The next three factors related to the park’s characteristics, including history

443

and location. In Beijing, the estimated ILCRs associated with PAHs exposure in

444

adults and children are acceptable. However, the risk to children's health is slightly

445

greater than that to adults, as ingestion is the most important route of soil PAHs

446

exposure. The overall potential ecological risk of soil PAHs pollution in Beijing is

447

low.

448 449

Acknowledgments

450

This work was supported by Central Level, Scientific Research Institutes for

451

Basic R&D Special Fund Business (2019YSKY006) and National Key Research and

452

Development Program of China (2019YFC180022).

453 454 455

Appendix A. Supplementary data Supplementary data of this article can be found in the supplementary materials.

456 457

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Table 1 Levels of PAHs in soils of the studied region (mg·kg-1) and toxic equivalent concentrations

Compounds

English

Aromatic

abbreviations

ring

Concentration

Range/(mg/kg)

Average/ (mg/kg)

TEQBaP/(mg/kg)

SD

GB36600 -2018

TEFc

Range/(mg/kg)

Average/ (mg/kg)

Naphthalene

Nap

2

0.004 ~ 0.110

0.018

0.014

25

0.001

0.0001 ~ 4.024E-06

1.786E-05

Acenaphthene

Ace

3

0.001 ~ 0.032

0.003

0.005

－

0.001

0 ~ 3.179E-05

2.160E-06

Acenaphthylene

Acy

3

0.001 ~ 0.040

0.005

0.007

－

0.001

0~ 3.956E-05

5.255E-06

Fluorene

Flu

3

0.001 ~ 0.119

0.008

0.015

－

0.001

0 ~ 0.0001

7.596E-06

Phenanthrene

Phe

3

0.005 ~ 0.478

0.043

0.074

－

0.001

5.031E-06 ~ 0.001

4.352E-05

Anthracene

Ant

3

0.001 ~ 0.090

0.017

0.018

－

0.01

1.006E-05 ~ 0.001

0.0002

Fluoranthene

Fla

4

0.001 ~ 1.341

0.063

0.141

－

0.001

1.006E-06 ~ 0.001

6.396E-05

Pyrene

Pyr

4

0.006 ~ 0.963

0.061

0.101

－

0.001

5.534E-06 ~ 0.001

6.147E-05

Benzo[a]anthracene

BaA

4

0.003 ~ 1.002

0.035

0.094

5.5

0.1

0 ~ 0.100

0.003

Chrysene

Chr

4

0.009 ~ 1.325

0.053

0.122

490

0.01

0 ~ 0.013

0.001

Benzo[b]fluoranthene

BbF

5

0.002 ~ 0.870

0.053

0.086

5.5

0.1

0.0002 ~ 0.087

0.005

Benzo[k]fluoranthene

BkF

5

0.005 ~ 0.358

0.027

0.035

55

0.1

0 ~ 0.036

0.003

Benzo[a]pyrene

BaP

5

-0.001 ~ 0.406

0.029

0.047

0.55

1

-0.001 ~ 0.406

0.029

Indeno[1,2,3-cd]pyrene

InP

5

0.0001 ~ 0.158

0.020

0.025

5.5

0.1

0 ~ 0.016

0.002

Dibenzo[a,h]anthracene

DahA

6

0.001 ~ 0.063

0.007

0.010

0.55

1

0 ~ 0.063

0.005

Benzo[g,h,i]perylene

BghiP

6

0.001 ~ 0.205

0.028

0.034

－

0.01

0 ~ 0.002

0.0002

－

∑7PAHa

－

0.033 ~ 4.182

0.219

－

－

－

0.005 ~ 0.721

0.048

－

∑PAHb

－

0.066 ~ 6.867

0.460

－

－

－

0.005 ~ 0.726

0.049

a. Σ7PAHs: concentrations of 7 carcinogenic PAHs ( BaA，Chr，BbF，BkF，BaP，IcdP，DahA ) ; b. ΣPAHs: total concentrations of 16 PAH; c. TEF: toxic equivalency factors．

Figure 1 The concentration distribution of sampling sites and composition profile of PAHs groups in each administrative regions

Figure 2 Source profiles of each PMF factor

Figure 3 Regression tree for PAHs (n: the number of samples; unit: mg/kg)

Figure 4 Incremental lifetime cancer risks (ILCRs) for PAHs: ILCRs for children and adults (A) and Nemerow Integrated Pollution Index (NIPI) at the sampling sites (B). (A)The blue horizontal line represents the ILCRs value was 10-6. Below the blue horizontal line represents essentially negligible. (B) The blue horizontal line represents the NIPI value was 0.7, safe. The black horizontal line represents the NIPI value was 1, warning-line. The NIPI value was between 1.0 and 2.0 represents weakly contaminated.

Highlights


PAHs concentrations were lower than national soil standards (GB36600-2018).




Vehicular emissions and pyrogenic source were found to be the main sources.




Traffic emission was suggested to be the primary key factor according to CIT.

Conflict of Interest

This manuscript is an original work, it has not been previously published, and it is not under consideration for publication elsewhere. All authors have read the manuscript, agree that the work is ready for submission to a journal, and accept responsibility for the manuscript’s contents. All authors have disclosed any competing financial interests in this work, and in fact, there were none.

Author Statement Jin Ma designed research; Yajing Qu, Yiwei Gong, Lingling Liu, Haiwen Wu, Shuhui Yang and Yixiang Chen collected soil samples; Yajing Qu, Yiwei Gong, Qiyuan Liu and Haiying Wei conducted the experiments; Yajing Qu and Yiwei Gong analyzed data and wrote the paper.