Atmospheric concentration, temporal and spatial variations, and sources identication of persistent organic pollutants in urban and semi-urban areas using the passive air samplers

In this study, the ambient persistent organic pollutants (POPs) such as polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and organochlorine pesticides (OCPs) concentrations were measured for 12 months at urban and semi-urban areas using the passive air sampler. During the sampling period, a total of 14 PAH ( ∑ 14 PAH) concentrations measured in urban and semi-urban areas were found as 54.4 ± 22.6 ng/m 3 and 51.7 ± 34.3 ng/m 3 , respectively. Molecular diagnostic ratios (MDRs) were used to determine PAH sources. According to the MDR values, combustion sources are the most important PAH sources in both sampling areas. However, since the urban area is close to the industrial zone, the combustion sources occurred at high temperatures (> 800 o C), while the semi-urban area generally consisted of burning petrogenic fuels. ∑ 50 PCB concentrations measured in the urban and semi-urban areas were found as 522.5 ± 196.9 pg/m 3 and 439.5 ± 166.6 pg/m 3 , respectively. Homologous group distributions were used to determine the source of PCBs. According to homologous group distributions, Tri-, Tetra-, and Penta- chlorinated PCBs were dominant in both sampling areas. ∑ 10 OCP concentrations measured in urban and semi-urban areas were found as 242.5 ± 104.6 pg/m 3 and 275.9 ± 130.9 pg/m 3 , respectively. Also, α-HCH/γ-HCH and β-/(α + γ)-HCH ratios were used to determine the source of OCPs. Lindane was the predominated OCP in both sampling areas.


Introduction
Persistent organic pollutants (POPs) are chemicals that have biological accumulation, persistence, toxicities, carcinogenic effects, a large number of natural and anthropogenic sources and have been widely used since the Second World War (Estellano et  Polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs) and organochlorine pesticides (OCPs) are the well-known POPs (Qu et al. 2019). PAHs are contaminants formed by combining two or more benzene rings that are clustered, angularly, or linearly connected (Naing et al. 2020). PAHs have stable molecular structures since they come together with π -π bonds (Zhang et al. 2020a). There are both anthropogenic and natural sources of PAHs. Anthropogenic sources are usually caused by the incomplete combustion of coal, wood, oil, organic compounds (Naing et al. 2020). Natural sources of PAHs include volcanic eruptions and the open burning of fossil fuels (Gaurav et al. 2021).
Although PCBs have historically been widely used in dielectric uids and transformers, they were banned in the 1970s due to their high bioaccumulation and toxicity properties (Wang et al. 2011). OCPs were rst used frequently as synthetic insecticides between the 1940s and 1960s. However, it was banned worldwide in the 1980s due to its negative characteristics for the environment and human health (Chen et al. 2020a).
Molecular diagnostic ratios (MDRs) are used as a qualitative method to investigate the transport and fate of PAHs in a speci c environment (Zhang et al. 2020b). This source identi cation method plays an essential role in determining emission sources. Volatile and semi-volatile PAH compounds vary widely at greater distances from the source region due to different reaction rates (Ji et  The majority of researchers focused on pollutant concentrations and their distribution between sampling points. In this study, the concentrations of different pollutants, their distribution between the sampling points and their sources, as well as their temporal and spatial changes were investigated. The speci c objectives of the present study were to; i) measure the concentrations of PAHs, PCBs and OCPs in the urban and semi-urban areas, ii) identify the possible sources, and iii) the determination of temporal and regional changes of concentration values.

Sampling areas, program and preparation
Bursa is Turkey's 4th largest city in terms of population, with about 3 million inhabitants. Also, Bursa, according to industry statistics inventory, is Turkey's largest industrial and automotive manufacturing center. Therefore, tra c, industry, and domestic heating are among the primary sources of air pollution in Bursa. Also, air pollution is felt more with the topographic structure of Bursa and the decrease of existing winds, especially in winter. In this study, ambient air samples were taken from urban and semi-urban areas in Bursa (Turkey) using a passive air sampler (PAS). The urban sampling area (40° 17'11.16" N − 29° 5'13.20 "E) is approximately 1.5 km from the Bursa-Ankara highway, 3 km from the industrial zone and 500 m from the nearest settlement. The semi-urban sampling area (40° 10'8.30 "N − 29° 10'26.82 "E) is approximately 2 km from the nearest highway and 3 km from the nearest settlement.
Air samples were collected monthly between May 2017 and April 2018 using PASs. The PASs were placed on a branch of a tree 1-2 meters in height in the urban sampling area and on a platform 2 meters above the ground in the semi-urban sampling area. Sampling rates (R s ) values for individual POP compounds targeted in this study were calculated according to the model proposed by Herkert

Sampling and Instrumental Analysis
The PUF disks were cleaned before being taken to the sampling area. First, the cleaning procedure was carried out with soxhlet extraction using puri ed water, then acetone (ACE) (Merck, Darmstadt, Germany) (two times), and nally hexane (HEX) (Merck, Darmstadt, Germany). Each processing step lasted 24 hours. After the cleaning process, the PUF discs were dried using a vacuum desiccator. Next, it was wrapped in aluminum foils until taken to the sampling area and kept in a deep freezer at -20 o C.
The PUF discs after sampling were extracted with the 300 mL ACE/HEX (v/v, 1/1) mixture for 24 hours using the soxhlet extraction method (Esen 2013;Sari et al. 2020a). In order to determine analytical e ciency, 1 mL of surrogate solution was added to samples before extraction. Extracted PUF discs were mL/min). MS was performed in selected ion-monitoring mode (SIM). The purpose of this is to detect PAH compounds with a very high sensitivity of MS. The GC-µECD oven temperature program for PCB analysis was: 70 o C (2 min) at the beginning, increased to 150 o C at 25 o C/min, followed by increased to 200°C at 3 o C/min, increased to 280°C at 8 o C/min, 8 minutes standby at this temperature and nally increased to 300°C at 10 o C/min and 2 minutes standby at this temperature. Similarly, GC-µECD oven temperature program for OCP analysis was: 80°C (1 min), with increases of 20°C/min to 240°C, 5 minutes standby at this temperature, followed by 5°C/min up to 270°C, then 20°C/min up to 300°C, 5 minutes standby at this temperature and nish. The injector inlet temperature was 250 o C, and the detector temperature was 320 o C (Cindoruk 2011;Sari et al. 2020a). Helium was used as the carrier gas (1.9 mL/min). High purity nitrogen gas was used as the make-up gas with helium gas. HP-5MS GC column (30m × 0.25mm × 0.25µm) was used as a capillary column in GC-MS, and HP-5 GC column (30 m × 0.32 mm × 0.25 µm) was used as a capillary column in GC-µECD. In this study, 14 PAH; Acenaphthene (Ace), Fluorene (Fln), 10 OCP; α-HCH, β-HCH, γ-HCH, δ-HCH, Heptachlor endo epoxide iso A, Endrin, Endosulfan-β, Endrin aldehyde, p,p'-DDT, and Methoxychlor were analysed. Of these compounds, Naphthalene and Acenaphthylene were not included in the calculations due to their low recovery e ciency (< 60%).

Quality Assurance /Quality Control (QA/QC)
Blank samples were used to check and prevent any possible contamination during the transportation of the PUFs, the extraction, or the quanti cation procedures. All glassware used during the preparation of samples, sampling, and laboratory analyzes were washed using tap water, distilled water, ACE, and PE, respectively. The surrogate standard was added to each sample before extraction for the determination of the analytical recovery e ciencies. Samples with recovery e ciency between 60% and 120% were included in the calculations. Surrogate standard consist of naphthalene-D8, acenaphthene-D10, phenanthrene-D10, chrysene-D12, and perylene-D12 (each 4000 ng) for PAHs; and PCB#14, PCB#65, and PCB#166 (each 4 ng) for PCBs. External recovery standard was used for OCPs, and not recoverycorrected was made since recovery e ciencies were generally higher than 70% (Cindoruk and Tasdemir 2014).
The GC-MS and GC-µECD instruments were calibrated before reading both blank and collected samples. Six level calibration standards were used for both instruments. Calibration standards were used for PAHs

PAH concentrations and possible sources
The average concentrations of total 14 PAH (∑ 14 PAH) in the atmosphere were found to be 54.4 ± 22.6 ng/m 3 (ranging from 24.4 to 91.3 ng/m 3 ) and 51.7 ± 34.3 ng/m 3 (ranging from 13.9 to 107.4 ng/m 3 ) for the urban and semi-urban areas, respectively (Fig. 1a). The highest PAH concentrations were determined in December and January, while the lowest PAH concentrations in July and August in both sampling areas. In general, high PAH levels were measured in the winter months, which are associated with increased domestic heating. The low e ciency of fossil combustion reportedly increases PAH concentrations in the winter months (Albuquerque et al. 2016). The PAH concentrations measured in December, January and February in the semi-urban area were higher than in the urban area. Similar PAH concentrations were measured in both sampling areas from March and November (Fig. 1a). The semiurban area is located on the mountainside, and for this reason, the weather is colder than the urban area.
Therefore, fossil fuel is used earlier for domestic heating in semi-urban areas than in urban areas. In addition, there are more settlements in the semi-urban area than in the urban area. For these reasons, more fossil fuels are used in the months when the temperature drops. The PAH concentrations measured from April to October in the urban area were higher than in the semi-urban area. The high PAH concentrations measured in the urban area are explained by the fact that the region represents an area where industry and tra c are dense.
In this study, the seasonal PAH concentrations obtained for both sampling areas are shown in Fig. 1b.  Fig. 1c. According to the annual distribution of PAH concentrations, there was a signi cant difference between the minimum and maximum concentrations in the semi-urban area compared to the urban area (p < 0.05). This may be due to the fact that PAH sources in the urban area did not change much throughout the year. In the semi-urban area, coal instead of natural gas, especially in winter, cause serious concentration differences between the summer and winter seasons. This situation creates differences in its annual distribution in the semi-urban area. The Ant/(Ant + Phe) ratios in the urban area were calculated as higher than 0.1. This meant that PAHs in this area were formed at high temperatures. In this case, it is consistent with the fact that this area represents a region where the industry is dense. In general, coal and biomass combustion emissions are dominant in both areas according to both Fl/(Fl + Py) and BaA/(BaA + Chr) ratios. Similarly, IcdP/(IcdP + BghiP) ratios are often higher than 0.4 for urban and semi-urban areas. Also, high correlation levels between COMPAH and total PAH concentrations meant that combustion sources were dominant ). The correlation between COMPAH and total PAH was strong, with a correlation coe cient of R = 0.97 (in the urban area) and R = 0.94 (in the semi-urban area). According to both MDR and correlation results between COMPAH and Total PAH, combustion sources dominated in both areas. Combustion sources in the urban area occurred at high temperatures (> 800 o C) originating from industry ((Ant/(Ant + Phe) > 0.1), while in the semi-urban area, it was usually caused by the combustion of petrogenic-derived fuels.

PCB concentrations and possible sources
The mean monthly concentrations of total 50 PCBs (∑ 50 PCBs) in the atmosphere were found to be 522.5 ± 196.9 pg/m 3 (ranging from 271.4 to 826.6 pg/m 3 ) and 439.5 ± 166.6 pg/m 3 (ranging from 243.2 to 727.2 pg/m 3 ) for the urban and semi-urban areas, respectively (Fig. 3a). Several studies have recently reported the PCB concentration in ambient air in Bursa (Cindoruk et al. , 2020; Günindi  The highest PCB concentrations were determined in June, while the lowest PCB concentrations were determined in March in the urban area. In the semi-urban area, the highest concentration of PCBs occurred in July, whiles the lowest concentrations were determined in November. Due to the evaporation of PCBs at high temperatures, atmospheric levels also increase in the summer months (Ozcan and Aydin 2009). Therefore, evaporation may have been effective at high PCB levels measured in the summer months in both sampling areas. Considering the seasonal distribution of PCBs, the highest concentrations were measured in summer, while the lowest concentration levels were measured in autumn and winter at both sampling areas (Fig. 3b) Considering the annual distribution of PCBs, higher concentrations were observed in the urban area. Some of the factors that have been reported in the literature as having contributed to contemporary levels of PCBs in the environment include to improper storage of PCB-containing waste, incineration of municipal or industrial wastes, evaporation from contaminated surfaces, accidental disposal of PCBcontaining waste, and old electronic equipment used (Aydin et al. 2014). As the urban area is close to industry, annual PCBs were found to be higher than in the semi-urban area. PCBs were predominant in all seasons in the semi-urban area. Also, Tetra-(ranging from 16.9-19.8%) chlorinated PCBs are predominant in the spring, summer, and autumn seasons in the semi-urban area (Fig. 4). High chlorinated PCBs (Hepta-, Octa, and Nona-CBs) were not dominant in both sampling areas. This situation could be explained by collecting the gas phase chemicals by diffusion with the PUF-disk sampler (Birgül et al. 2017). A study conducted by Sari et al. (2020a) reported that Penta-(37%) and Tetra-(22%) chlorinated PCBs were the dominant homologs group for Bursa in 2014. Another study conducted by Birgül et al. (2017) reported that Tetra-(31.5-81.6%) chlorinated PCBs were the dominant homolog groups for Bursa in 2014.  also reported that Tri-(64.6%) and Tetra-(22.9%) chlorinated PCBs were the dominant homologs group for Bursa in 2005.
Especially in the summer months, PCBs evaporating with the increase of the temperature increase the ambient air concentrations (Hogarh et al. 2013). With the increase of temperature in summer in both sampling areas, Di-chlorinated PCBs became dominant. Because of the low saturated vapour pressure of the low chlorinated PCBs, they are more likely to be moved from a source to more distant areas than highchlorinated PCBs (Hu et al. 2019). For this reason, atmospheric transport in summer is also thought to affect PCB concentrations.

OCP concentrations and possible sources
The mean monthly concentrations of a total of 10 OCPs (∑ 10 OCPs) were found to be 242.  (Fig. 5a). Regarding the seasonal distribution of OCPs, the highest concentrations were observed in the summer season, while the lowest levels were observed in the spring season (Fig. 5b). High OCP concentrations seen in the summer season can be explained by increasing agricultural activities in this season (Qu et al. 2015). According to the annual distribution of OCP concentrations, higher levels were determined in the semi-urban area compared to the urban area (Fig. 5c). This situation is consistent with the fact that agricultural activities are more in a semi-urban area.
The ratios of α-HCH/γ-HCH and β-/(α + γ)-HCH are often used in source identi cation of HCHs (Kong et al. 2014). For example, if the α-HCH/γ-HCH ratio is less than 4, lindane is dominant, whereas if the α-HCH/γ-HCH ratio is between 4-7, technical HCHs are dominant (Da et al. 2014). Similarly, if the β-/(a + γ)-HCH ratio is higher than 0.5, it means that the pesticide has been used in the past, and if the ratio is less than 0.5, then the pesticide has been used recently (Liu et al. 2012). In this study, α-HCH/γ-HCH and β-/(α + γ)-HCH ratios calculated for urban and semi-urban areas are shown in Fig. 6.
The mean α-HCH/γ-HCH ratios were found to be 0.87 for the urban area and 1.15 for the semi-urban area.
According to the α-HCH/γ-HCH ratios, lindane was predominant in both sampling areas. A study conducted by Cindoruk (2011) in Bursa stated that α-HCH/γ-HCH ratios are generally higher than 1.0. In another study conducted in Bursa, α-HCH/γ-HCH ratios were reported to vary between 0.27-0.55 (Esen 2013). A study conducted by Kurt-Karakus et al. (2018) across Turkey reported the average α-HCH/γ-HCH ratios as 2.26 for the urban area. Although these compounds have long been banned, it is thought that high concentrations of γ-HCHs are reintroduced into the atmosphere through illicit use, pure lindane use and evaporation from formerly contaminated areas (Cindoruk 2011). According to β-/(α + γ)-HCH ratios, most the pesticides have been used recently. In a study by Wu et al. (2020), they stated that until 2019, lindane was used to control termites in China, while in Japan, it was still used for wood preservatives. It can also be explained that high lindane levels are still used in many countries even though they are banned (Yang et al. 2008).

Conclusions
In this study, concentration levels, temporal and spatial variations, and possible sources of POPs such as PAHs, PCBs, and OCPs were determined using a passive air sampler in urban and semi-urban areas for 12 months. The average annual PAH and PCB concentrations were higher in the urban area, while OCP concentrations were higher in the semi-urban area. Industrial and tra c activities were effective in the urban area, while agricultural activities were generally effective in the semi-urban area. Combustion was the primary source of PAHs in both sampling areas. Combustion sources in the urban area occur at high temperatures (> 800 o C) originating from industry ((Ant/(Ant + Phe) > 0.1), while in the semi-urban area, it is usually caused by the combustion of petrogenic-derived fuels. Low chlorinated PCBs (Tri-, Tetra-, and Penta-chlorinated PCBs) were dominant in both sampling areas. This situation was explained by the collecting of the gas phase chemicals by diffusion with the PUF-disk sampler. Isomer distributions of HCHs were used to determine the sources of OCPs. According to the α-HCH/γ-HCH ratios, lindane was determined to be effective in both sampling areas. Also, β-/(α + γ)-HCH ratios were determined higher in urban areas than in semi-urban areas. According to this result, most of the pesticides were used recently in the semi-urban area.

Declarations
Ethics approval and consent to participate Not Applicable

Consent for publication Not Applicable
Availability of data and materials The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.

Competing interests
The authors declare that they have no known competing nancial interests or personal relationships that could have appeared to in uence the work reported in this paper.   The MDR values and the correlation results between COMPAH and Total PAH Figure 3 Monthly (a), seasonal (b) and annual (c) PCB distributions for the sampling areas Seasonal homologous group distributions for urban and semi-urban areas Figure 5 Monthly (a), seasonal (b) and annual (c) OCP distributions for the sampling areas