PAHs content level. As shown in Table 1, according to the screening value of soil pollution risk in the standard for construction land (GB36600-2018), the value of BaP at sampling point 3 of Zhiwu Park in the soil of the park in summer is 0.944 mg/kg, which is greater than the screening value of 0.55 mg/kg. The values of Daha at sampling point 3 of Zhiwu Park and sampling point 3 of Toutunhe Park are 1.469 mg/kg and 0.615 mg/kg, respectively, which are greater than the screening value of 0.55 mg / kg, whereas BAP and Daha mainly originate from vehicle exhaust emissions, such as gasoline and diesel combustion 12. This shows that the traffic congestion near Zhiwu Park and Toutunhe Park in summer needs to be reasonably controlled. The concentrations of PAHs in other soil sampling points in the summer and winter were lower than the screening value for soil pollution risk.
PAH-contaminated soil can be divided into four levels: no pollution (< 0.200 mg/kg), slight pollution (0.200-0.600 mg/kg), medium pollution (0.600-1.000 mg/kg) and serious pollution (> 1.000 mg/kg) 13. As we can see from Fig. 1, in the summer, the sampling point No. 1 in Shuimogou Park and the sampling point No. 2 in Zhiwu Park are pollution-free, the sampling point No. 3 in Srk is moderately polluted, and the total concentration of PAHs in other park soil shuimogou Paampling points is greater than 1.000 mg/kg, reaching the degree of serious pollution. The highest pollution point was the sampling point No. 3 in Zhiwu Park. The large difference in concentration between different the sampling points in the Zhiwu Park and Shuimogou Park may be caused by factors such as the distance between the sampling points and the road and the dumping of garbage 14. Among the soil samples in the summer, 91.6% were more than moderately polluted, and only 8.4% were pollution-free. This is because the park has a large passenger flow, especially in the summer, and it is located adjacent to the main traffic road. Traffic volume is large, and traffic emissions are one of the main sources of urban PAHs 15. In the soil of the winter park, the sampling point No. 1 of South Park and sampling point No. 2 of Shihua Park were moderately polluted. The total concentration of PAHs in the three sampling points of Toutun Park was greater than 1.000 mg/kg, reaching the level of serious pollution. The most polluted point was the sampling point No. 3. Some other sampling points were slightly polluted, and some were pollution-free. Toutun Park is located near the Diwopu Airport. Due to the re-planning and reconstruction of the park in recent years, the exhaust emissions of large trucks and the wear of tires during the reconstruction process have greatly increased the accumulation of PAHs in the park 16. Shihua Park is located near Midong District. There are numerous chemical plants of petrochemical companies, plastic factories, which generate a large number of PAHs during the production and processing of products. The South Park is located in a downtown area with heavy traffic and many nearby residential areas. Compared with other parks, there are more man-made emission sources near these three parks. In winter, 58.3% of soil samples were polluted, of which 20.8% were moderately polluted or above, and 41.7% were pollution-free. Compared with the summer, the proportion of contaminated samples decreased.
As shown in Table 2,the average order of the soil PAHs in each functional area in summer is traffic area > industrial area > park area > rural area > commercial area, which is consistent with the research by Min et al. (2015). The park is located adjacent to the main traffic road. Compared with the rural and commercial areas, the park area has a larger passenger and vehicular traffic flow. Compared with the traffic area, the traffic volume in the park area is weaker, and industrial activities such as coke oven gas, organic chemical industry, petroleum industry, steelmaking, and ironmaking in the industrial area generate a large amount of PAH emissions. Conversely, plant leaves in the park absorb PAHs from atmospheric deposition and accumulate PAHs, such that the PAHs entering the soil of the park through atmospheric deposition is greatly reduced. Compared with other regions, the content of soil PAHs in the Urumqi park area in the winter is slightly different from that in Beijing and Xi'an park area, which is slightly higher than that in the Yinchuan and Fuzhou park areas. This is because the first three cities have a larger population, and heating in the winter generates more PAHs. The average value of PAHs in the summer park area was 5-6 times that in winter. This is consistent with the results of Fangfang et al. (2014), who determined that the PAH content in individual sampling sites in the park area in winter is higher than that in summer. This may be owing to climatic conditions and location of the sampling site. This location is related 26. In addition, at the end of 2019, a new type of coronavirus (COVID-19) was discovered in China for the first time, and it quickly spread to all parts of the world after its outbreak. To curb the COVID-19 epidemic, outdoor activities (transportation, industry, entertainment, etc.) in many countries were restricted 27. This also includes the city of Urumqi. Until September 2020, the epidemic in Urumqi City had been well controlled, such that outdoor activities resumed, the restrictions, greatly reduced the content of PAHs in the soil in the winter of 2020. Overall, the PAH pollution of soil in the park area of Urumqi City is relatively serious and should be considered.
The PAH content of plants in the South Park of Urumqi is shown in Table 1. Comparing plant leaves and roots, we can see that whether it is summer or winter, the proportion of Σ7PAHs in Σ16PAHs is always higher than that of leaves. This is because Σ7PAHs are mainly composed of 4-6 ring aromatic hydrocarbons, and the absorption of roots from soil is the main absorption pathway of high-ring PAHs, whereas, low-ring PAHs are the result of the combined effect of leaf absorption from the atmosphere and root absorption from the soil 24. In addition, the proportion of Σ7PAHs in plant leaves in the winter to the concentration of Σ16PAHs is considerably different from the others. This is because plant leaves mainly absorb PAHs in the atmosphere 28, and PAHs in the atmosphere are more susceptible to the impact of surrounding emission source than soil. Due to the control of the epidemic, vehicle transportation restrictions led to a reduction in the proportion of high-ring PAHs in the winter.
As shown in Fig. 1, the highest total concentration of PAHs in the leaves of park plants in summer was silver edge grass, followed by Rudbeckia leaves and Clover leaves. Because these plants belong to herbaceous plants, the plants are relatively short and grow close to the ground, the content of PAHs in their leaves is greatly affected by ground dust 29. The highest total concentration of PAHs in plant leaves in winter parks is in Juniper leaves, followed by Platycladus leaves and Dragon claw willow leaves. Because these plants belong to arbor plants, the plants are relatively high and the leaves are located far away from the ground, the PAHs content in the leaves is easily affected by PAHs in the atmosphere. In winter, due to heating, the combustion of coal and biomass mainly produces tricyclic PAHs, which are volatile and easily transferred to the atmosphere 30. In addition, Juniper and Platycladus are evergreen plants in all seasons, and their leaves do not easily fall off. Compared with other plants, they are more likely to accumulate PAHs. Pine needles have a large specific surface area, high surface wax content, wide distribution, and easy collection. They have been used by many foreign environmental scientists to monitor and evaluate organic pollutants in the environment 31. We can also observe from Fig. 1 that the total concentration of PAHs in 70% of the plant leaf samples was greater in the winter than in the summer. This is owing to volatilization through stomata caused by the rising temperature in the summer. PAHs mainly migrate from leaves to the atmosphere 32. The content of PAHs in the roots of Clover and Silver edge grass was higher in different seasons. It is speculated that this may be related to the structure of the plant. These two herbaceous plants may be used to repair soil pollution caused by PAHs in the soil 21. In the study by Yuanyuan et al. (2011), it was found that legume alfalfa and gramineous perennial ryegrass exhibited a higher removal rate of soil PAHs, which was mainly because plants promoted the joint degradation of PAHs in soil by indigenous microorganisms 10.
Table 2 shows a comparison of the PAH content of plants in this study and other regions. The range of total PAHs in summer plants in Urumqi parks is 0.033-3.941 mg/kg, which is higher than the change range of PAHs in plants considered by Dong Ruibin and others in the range of 0.020-1.000 mg/kg 21. The concentration value is close to the Nanjing campus area and Guangzhou scenic area, but much lower than the Changsha traffic area. Studies have shown that when a car is driving at a non-uniform speed, the exhaust contains more PAH pollutants, and the PAH content in the exhaust particulate matter also increase relatively 33. In addition, the study area selected by Peng Gang et al. is only 300 m away from the main urban traffic road in Changsha and only 10 m away from the main campus road. The traffic volume is large, and exhaust pollution is serious. This is slightly higher than that in Qingdao agricultural area. This is because the research collected by Wei et al. (2008) is farmland soil, which is located at the edge of the city and is far away from the city center. Although there are pollution sources such as industry and traffic around some sampling points, these are relatively limited, such that the content of PAHs in plants is close to other regions with relatively low urbanization and industrialization. The average total PAH content of plants in the park area in the summer was twice that in the winter, which is consistent with the PAH content in the soil. This may be correlated with climatic conditions and control of the epidemic.
PAHs monomer component characteristics. According to the number of rings in PAHs, the 16 types of PAHs are divided into 2-3 low-ring aromatic hydrocarbons (LMW) and 4-6 high-cyclic aromatic hydrocarbons (HMW). Among them, the 2-ring PAHs are mainly derived from the leakage of source oil, while the 3-ring and 4-ring PAHs are mainly derived from the combustion of coal and biomass, the 5-ring PAHs are mainly derived from the combustion of gasoline, and the 6-ring PAHs are mainly derived from the combustion of diesel.
As shown in Fig. 2, BbF, BkF, InP, and DahA accounted for a higher proportion in the summer soil. BbF mainly originates from the combustion of gasoline, BkF mainly originates from industrial coal, and InP and DahA mainly originate from automobile exhaust 12,34. The order of the proportion of the number of different PAHs was 5-ring PAHs > 4-ring PAHs > 6-ring PAHs > 3-ring PAHs > 2-ring PAHs. It is speculated that the main source of PAHs in the summer park soil may have originated from gasoline combustion and traffic emissions.
Fla, Phe, Pyr, and BbF accounted for a higher proportion of the soil in the winter. Fla mainly originates from the combustion of coal and petroleum and can also originate from garbage incineration. Phe mainly originates from the process of coking, which is the process of decomposing and distilling coal to produce coke, and Pyr mainly originates from industrial coal 35. The order of the proportions of the number of different PAHs was 4-ring PAHs > 5-ring PAHs > 3-ring PAHs > 6-ring PAHs > 2-ring PAHs. Compared with the summer, the proportion of the 3rd and 4th rings in the winter has increased significantly, and the 3rd and 4th rings of PAHs are mainly derived from the combustion of coal and biomass, which is caused by heating in the winter. In addition, it can be found that the contribution of HMW is higher in both summer and winter, which may be owing to the volatility of LMW 30.
We can observe from Fig. 2 that the higher proportions of summer plants were BbF, Phe, InP, and BkF. Barring Phe, the main proportion of PAHs was similar to the proportion of PAHs in summer park soil, indicating that the proportion of different PAHs in plants is greatly affected by soil. Phe occupieds a relatively small proportion in the summer soil, but a larger proportion in plants. This is because, due to its small molecular weight, Phe is easily volatilized from the soil when the temperature is high in the summer and is thereby absorbed by plant leaves 36. The order of the proportion of the number of different PAHs was 5-ring PAHs > 3-ring PAHs > 4-ring PAHs > 6-ring PAHs > 2-ring PAHs. Similar to summer soils, high-ring PAHs accounted for a higher proportion, but compared with summer soils, the proportion of LMW increased. This is because low-molecular-weight PAHs have 2-3 rings, which are mainly gaseous and are easily absorbed by plants.
Phe, NaP, Flu, and Fla accounted for a higher proportion of plants in the winter. Among them, Flu mainly originates from coking, and NaP mainly originates from coke oven products and oil spills 36. The order of the proportion of the number of different PAHs was 3 ring PAHs >4 ring PAHs >5 ring PAHs >2 ring PAHs >6 ring PAHs. Compared with the summer, the proportion of low-ring PAHs in winter plants and soil has increased significantly, which may be caused by the burning of large amounts of coal and organisms for heating in winter in the north. In winter, the roots of plants are different from those of leaves, and the proportion of HMW is still higher. This is because the roots are underground, and it is easier to absorb high-ring PAHs from the soil than leaves. In addition, the proportion of LMW in plants in both winter and summer was higher than that in the soil. This is because foliar absorption is the main mechanism by which plants accumulate PAHs 37.
PAHs source analysis. The feature ratio method is often used to identify the source of PAHs in the environment 38,39. LMW/HMW > 1 represents the crude oil source, LMW/HMW < 1 represents high-temperature pyrolysis, Ant/(Ant+Phe) < 0.1 represents the crude oil source, Ant/(Ant+Phe) > 0.1 represents the combustion source, BaP/BghiP ratio > 0.6 means traffic emission source, less than 0.6 means non-traffic emission, BaA/(BaA+Chy) < 0.2 characterizes crude oil source, 0.2 < (BaA/BaA+Chy) < 0.35 is used to characterize the combustion of coal, vegetation, etc., finally (BaA/BaA+Chy) > 0.35 is usually used to characterize the combustion of petroleum and fossil fuels.
As shown in Fig. 3, the ratio of LMW/HMW in summer soil was less than 1, 54.2% of the Ant/(Ant+Phe) ratio was greater than 0.1, and 91.7% of the BaP/BghiP ratio was greater than 0.6. All BaA/(BaA+Chy) ratios were greater than 0.35. We can infer from this that the PAHs in the soil of Urumqi City Park in the summer mainly originate from traffic emissions, that is, the combustion of crude oil fuels such as gasoline and diesel.
In winter, 79.2% of the LMW/HMW ratio in the soil was less than 1, 95.8% of the Ant/(Ant+Phe) ratio was greater than 0.1, the BaP/BghiP ratio was greater than 0.6, and 75.0% of the BaA/(BaA+Chy) ratio was between 0.2 and 0.35. We can infer from this that the PAHs in the soil of Urumqi city parks in winter mainly originate from high-temperature sources, including incomplete combustion of coal, grass, and other biomass and fossil fuels.
As shown in Fig. 4. The LMW/HMW ratios of summer plants were all less than 1, 93.3% of the Ant/(Ant+Phe) ratios were greater than 0.1, 73.3% of the BaP/BghiP ratios were greater than 0.6, and 73.3% of the BaA/(BaA+Chy) ratios were greater than 0.35. From this, we can infer that the PAHs in plants of Urumqi park in the summer are similar to those in the soil, and they mainly originate from traffic emissions.
In winter plants, 66.6% of the LMW/HMW ratios were less than 1, all the ratios of Ant/(Ant+Phe) were less than 0.1, 86.6% of the BaP/BghiP ratios were greater than 0.6, and 33.3% of the BaA/(BaA+Chy) rations were less than 0.2, 33.3% were between 0.2-0.35, 33.3% were greater than 0.35. We can infer from this that the source of PAHs in park plants in Urumqi in winter is more complicated, and the leakage of crude oil, burning of coal biomass, and traffic emissions all contribute to a certain extent.
PAHs risk assessment. Table 3 displays the toxic equivalent concentrations of PAHs in the soil of parks in Urumqi. The TEQ range of PAHs in the summer soil of the park was 0.024-2.937 mg/kg, with an average of 0.733 mg/kg, while the average TEQ of Σ7PAHs was 0.729 mg/kg, accounting for 99.5% of the Σ16PAHs, and the TEQ of the remaining PAHs only accounts for 0.5%, which shows that Σ7PAHs are the primary contributing factors to the toxic effect of the PAHs in the summer soil of the park. The contributions of different monomer PAHs to the total TEQ were as follows: DahA (42.8%) > BaP (35.6%) > BbF (7.5%) > InP (6.7%) > BkF (4.3%) > BaA (2.8%) > Chy (0.3%). Further, 25% of the total TEQ value of PAHs in the soil in Urumqi Summer Park is higher than the standard value of 1 mg/kg given by the World Health Organization (WHO). These sampling point mainly originate from South Park, Zhiwu Park, Shihua Park and Toutunhe Park. Therefore, direct or indirect contact with the soil of these four parks in the summer cause a certain risk of toxic effects to humans.
The TEQ range of PAHs in the winter soil was 0.003-0.656 mg/kg, with an average value of 0.096 mg/kg, the average TEQ of Σ7PAHs was 0.095 mg/kg, accounting for 99.0% of the Σ16PAHs, and the TEQ of other PAHs only accounts for 1.0%, and similar to summer soil, Σ7PAHs are almost all contributing factors to the toxic effect of PAHs in the winter soil. The contribution of different monomer PAHs to the total TEQ was in the following order: BaP (41.8%) > DahA (37.8%) > BbF (7.2%) > BaA (4.6%) > InP (4.2%) > BkF (2.7%) > Chy (0.7%). The total TEQ value of PAHs in the soil in Urumqi winter is lower than the standard value of 1 mg/kg given by the World Health Organization (WHO). Therefore, direct or indirect contact with Urumqi park soil in winter will not cause toxic effects to humans.
ILCRs less than or equal to 10-6 are considered to be basically negligible lifetime cancer risk, ILCRs between 10-6 and 10-4 indicate low risk, ILCRs greater than 10-4 indicate high potential health risks 40.
As shown in Fig. 5, the mean values of ILCRs for adults and children regarding PAHs in the summer soil were 2.783 × 10-6 and 3.400 × 10-6, respectively. ILCRs in adult, 83.3% were greater than 10-6, and in children, 87.5% were greater than 10-6, reaching a low risk level. Among them, the maximum ILCRs in adults and children were greater than 10-5 at sampling point 3 of the Botanical Garden. The mean values of ILCRs for adults and children regarding PAHs in the winter soil were 3.632 × 10-7 and 4.439 × 10-7, respectively, which were lower than the minimum ILCRs. Only 12.5% of adult and child ILCR values greater than 10-6 reached a low risk level, and the sampling points were all in Toutunhe Park. In addition, we can observe that the carcinogenic risk of soil in the park is much higher in children than in adults. This is because children’s organs, nerves and immune systems may be more sensitive to pollutants, as children are developing, at the same time, children move easily on the ground and have greater access to contaminated soil. The carcinogenic risk of the summer soil to the human body was much higher than that of winter soil. This is inconsistent with the results of Fangfang et al. (2014). The reason may be that Urumqi is affected by the new coronavirus (COVID-19), and the amount of artificially produced PAHs decreased. As shown in the figure, the ILCRs in adults and children in both summer and winter are as follows: ingestion > dermal > inhalation. Compared with other exposure routes, the lifetime carcinogenic risk of inhalation is nearly negligible. This is because the lifetime carcinogenic risk caused by children's frequent hand-to-mouth contact while playing in the park is much higher than that of inhalation. The results show that the ILCRs of the PAH exposure pathway of soil samples from Urumqi parks in winter were all lower than the minimum acceptable risk level. The lifetime carcinogenic risk of adults and children is negligible, but high ILCRs in summer require particular attention.
The Nemerow multifactor index of PAHs in soil can be divided into five levels: NIPI is less than or equal to 0.7, where soil pollution is at a safe level, NIPI is equal to 1.0, where soil pollution is at the warning line, NIPI between 1.0-2.0, where soil pollution is at a weak level, NIPI between 2.0-3.0, where soil pollution is at a moderate level, and NIPI greater than 3.0, where soil pollution is severe 41.
As shown in Fig. 6, in summer, the NIPI range of park soil in Urumqi is 0.025-3.532, with an average value of 0.715. The NIPI value of 75% of the sampling points was less than 1.0, and the ecological risk of the soil was safe. The NIPI value of 20.8% of the sampling points is between 1.0-2.0, mainly located in South Park, Zhiwu Park, Shihua Park and Toutunhe Park. The soils of these parks have more PAH accumulation factors than the soils of other parks. At the same time, the NIPI The maximum value is 3.532, which is a serious pollution level. This may be related to the pile of garbage in the park. The NIPI range of soil in Urumqi city parks in winter was 0.004-0.564, with an average value of 0.095. The NIPI value of all sampling points was less than 0.7, indicating that the ecological risk of soil in all parks in winter is at a safe level. The ecological risk of park soil in Urumqi in winter is much lower than that in summer.
In general, the ecological risks of soil in Urumqi parks can be ignored. Among them, South Park, Zhiwu Park, Shihua Park and Toutunhe Park should adopt a variety of control measures to reduce the ecological risk of soil PAHs through strict environmental management.