The major industrial organic compounds in the TSP extracts were phthalates (Ps), non-phthalates (NPs), phenyl phosphates (PhePhs), and polychlorinated biphenyls (PCBs) as illustrated in Figure 2. The dominant compounds identified are listed in Table 1. The synthetic compound groups are discussed in detail below.
The air temperatures and mass concentrations of the TSP from Dhahran city are summarized in Figure 3. The air temperatures in the city were usually hot in summer and cooler in winter. The range was wide with a summer high median of 35.0oC and lower winter median of 20.0oC (Fig. 3a). The seasonal concentrations of TSP also had a wide range with median values of 220.5 µg m-3, 152.8 µg m-3, 87.5 µg m-3, and 272.3 µg m-3 in summer, autumn, winter and spring, respectively (Fig. 3b). The data showed that there was a positive relationship between the mass concentrations of TSP and air temperature as indicated by their median values. The elevated temperatures enhanced the volatilization of the individual compound emissions.
3.1. Phthalates (Ps)
High levels of phthalates (Ps) were detected in all TSP samples with a wide range (Table 1, Fig. 4a). Twenty-three phthalate compounds (structures are shown in the Supplemental Materials section) were detected in the TSP with high mean concentrations for example 253.2 ng m-3 during summer and 388.7 ng m-3 in winter (Table 1). The major compounds were di(2-ethylhexyl) phthalate (DEHP = 80-158 ng m-3, mean), di(2-propylpentyl) phthalate (DPPP = 56-95 ng m-3), di-n-butyl phthalate (DBP = 48-87 ng m-3), di-n-octyl phthalate (DNOP = 41-88 ng m-3), diisobutyl phthalate (DIBP = 15-27 ng m-3), isoheptyl nonyl phthalate (IHNP = 1-11 ng m-3), heptyl isoundecyl phthalate (HIUP = 0.4-8 ng m-3), di-n-heptyl phthalate (DHP = 1-7 ng m-3), nonyl undecyl phthalate (NUP = 0.4-6 ng m-3), n-heptyl-n-nonyl phthalate (HNP = 1-4 ng m-3), (IHINP = 0.4-4 ng m-3), and dinonyl phthalate (DNP = 0.6-4 ng m-3). Numerous other phthalates detected at low concentrations (<3 ng m-3) are listed in Table 1.
The high concentrations of total Ps in the TSP samples confirmed that the ambient air of Dhahran city is contaminated by plastic additives. Generally, the presence of Ps in the environment is related to plastic manufacturers and markets (Sun et al. 2013; Gao and Wen 2016; Kim et al. 2020) as well as littering and plastic waste burning (Simoneit et al. 2005; Fu and Kawamura 2010; Rushdi et al. 2010; Kumar et al., 2015; Zhen et al. 2019). The major worldwide phthalate product used in PVC plastic is DEHP, representing 37.1% of the total plastic additives (ECPI 2016), and recently DNP and DUP have regularly been utilized as additives instead of DEHP (ECPI 2016). The presence of high levels of DEHP and relatively low concentrations of DNP and DUP in these TPS samples presumably indicated that DEHP was still produced as a major plasticizer additive in the region and DNP, or DUP have not completely replaced DEHP in production by the manufacturers.
The concentrations of Ps in the TSP samples were relatively similar to the concentrations reported for other major cities in the world. The DEP concentrations were much lower than the levels reported for other cities in the world such as Paris-France, Stockholm-Sweden, Berlin-Germany and Tokyo-Japan (Otake et al. 2001; Fromme et al. 2004; Teil et al. 2006; Bergh et al. 2011), probably due to lower retention on filters based on its greater volatility. The DBP concentrations were similar as measured in Nanjing-China, and Portland-Oregon-USA (Ligocki and Pankow 1989; Wang et al. 2008), lower as in Stockholm (Otake et al. 2001; Fromme et al. 2004; Bergh et al. 2011), and higher than for Paris, Yamato-Japan, Barcelona-Spain, Austin and Waco-Texas-USA, and Murry-Kentucky-USA (Toda et al. 2004; Teil et al. 2006; Subedi et al. 2017; Bi et al. 2018). The DEHP concentrations were similar as in Yamato (Toda et al., 2004), higher than Paris and Nanjing (Teil et al. 2006; Wang et al. 2015), but lower than Stockholm, Santiago-Chile, Beijing-China, Guangzhou-China, and major USA cities such as Portland, Austin, Waco, and Murry (Ligocki and Pankow 1989; Simoneit et al. 2005; Bi et al. 2008, 2018; Zhou et al. 2009; Bergh et al. 2011; Subedi et al. 2017).
3.2. Non-phthalates (NPs)
The relative seasonal concentrations of non-phthalates (NPs) in the TSP were comparatively low comprising 1.7±0.6 to 2.0±0.4% of the total concentrations (Table 1), with low levels and medians of 5.6 ng m-3 in summer and 9.5 ng m-3 in winter (Fig. 4b). Only di(2-ethylhexyl) adipate (DEHA = 4.7-10 ng m-3) and tri(2-ethylhexyl) mellitate (TEHM = 0.6-1.6 ng m-3) were detected in these samples. The presence of NPs in these TSP samples suggested that alternative plasticizers were a minor component in the Dhahran atmosphere but also used as additives for plastic production in the region. The concentrations of DEHA in the TSP of Dhahran were lower than that reported in the cities of Waco-Texas and Murry-Kentucky (Subedi et al. 2017).
3.3. Phenyl phosphates (PhePhs)
The phenyl phosphates (PhePhs) in the TSP were also low, ranging from 1.1±1.2 to 3.4±1.7% of the total concentrations (Table 1), with similar levels for all seasons (Fig. 4c). The major compound was triphenyl phosphate (TPhePh) with concentrations ranging from 3.9 ng m-3 in spring to 6.5 ng m-3 in winter (Table 1). Diphenyl p- and m-tolyl phosphates (DPhepTPh and DPhemTPh) were relatively significant at 0.7 ng m-3 in spring to 4.5 ng m-3 in autumn. Phenyl di(p- and m-tolyl) phosphates (PheDpTPh and PheDmTPh) were low ranging from 0.3 ng m-3 in spring to 2.5 ng m-3 in summer, with traces of tri(p- and m-cresyl) phosphates (TpCPh and TmCPh) (Table 1). The concentration of TPhePh was similar as in Islamabad, Pakistan (Faiz et al. 2018) and higher than in Nanjing-China (Wang et al. 2008; Faiz et al. 2018). TPhePH was the dominant compound of the phenyl phosphates emitted in TSP from an e-waste recycling facility in South China (Bi et al. 2010).
3.4. Polychlorinated biphenyls (PCBs)
Polychlorinated biphenyls (PCBs) were major compounds in the TSP samples and comprised 9.1±2.7% to 21.6±21.0% of the total concentrations (Table 1), with amounts ranging from a high median of 88.7 ng m-3 in summer to a low of 17.8 ng m-3 in spring (Fig. 4d). The total concentrations of the trichlorobiphenyls (TrCB = 8 congeners) were highest in summer (20.3 ng m-3) and the major compound was 2,3',4-trichlorobiphenyl. Tetrachlorobiphenyls (TeCBs = 12 congeners) were also significant in summer (40.2 ng m-3), with 3,3',4,5'-tetrachlorobiphenyl and 2,3,3',5'-tetrachlorobiphenyl as major compounds. The total concentrations of pentachlorobiphenyls (PeCBs = 8 congeners) varied from 8.9 ng m-3 in spring to 18.8 ng m-3 in winter, with a dominance of 2,3,3',4,4-pentachlorobiphenyl. The total PCB concentrations in Dhahran TSP were higher than those reported in TSP from Bermuda, Bloomington-Indiana-USA, Northern China, and Prague-Czech Republic (Panshin and Hites 1994a, b; Shahpoury et al. 2015).
3.5. Correlation and source comparison
Pearson's correlation statistical analysis was used to assess the association and relationship between these POP groups detected in the TSP samples. The outcome of Pearson's correlation (Table 2) showed that the correlations between the TSP and these groups were insignificant (r < 0.439). Also, insignificant correlations were found between air temperatures and different groups (r < 0.377). The correlations between total concentrations and different groups were found to be significant (r = 0.537 to 0.985). The insignificant correlation among TSP, air temperature, and these POP groups demonstrated that these were not important factors in controlling the concentrations and distribution of POPs in the atmosphere of Dhahran. On the other hand, the significant correlations between total compounds and their different groups indicated that they apparently were from similar local sources.
The data were analyzed by principal component analysis (PCA), using Varimax rotation to examine the similarities and distinctions between the levels and sources of the plasticizer groups in the TSP of Dhahran. The PCA output identified three significant components (C1, C2, and C3) elucidating 90.69% of the variance at an Eigen value of > 1 (Table 3, Fig. 5). We have interpreted the data using factor loadings of > 0.65 for each component. The C1 revealed a variance of 54.39% with phthalates and non-phthalates as dominant, indicating that they were from the same sources. The C2 showed a variance of 25.06% with TSP and PCBs specifying that the concentration of TSP had an effect on the levels of PCBs in the atmosphere of the city. The C3 revealed a variance of 11.32% with only phenyl phosphates confirming that these plasticizers were from different sources. Therefore, phthalates and non-phthalates were likely released mainly from the plastic manufacturers around the city (Rushdi et al. 2017; Saini et al. 2019) or as a result of the combustion of plastic waste in the region (Simoneit et al. 2005). The similarity and correlation between TSP and PCBs were probably due to spills, leaks, or disposal of PCBs from paint and metallurgical industries (Zhao et al. 2019) into local topsoil and resuspension of soil particles into the atmosphere. The sources of phenyl phosphates were likely emissions from residential sources (Saini et al. 2019).
3.6. Plasticizer and flame retardant levels relative to other urban organic compounds
The total extractable organic matter of the atmospheric TSP from Dhahran city also contained compounds from different natural and anthropogenic sources. These compounds included n-alkanes, hopane biomarkers, PAHs, unresolved complex mixture (UCM), methyl n-alkanoates, and n-alkanones (Table 1).
The dominant n-alkanes ranged from C16 to C40 with maximum (Cmax) concentrations of tetracosane or nonacosane (Cmax = 24 or 29) and total concentrations varying from 216.5 ng m-3 in winter to 382.9 ng m-3 in summer (Table 1). The Cmax of the n-alkanes indicated that the major sources were from terrestrial plants (Cmax = 29) and petroleum residues (Cmax = 24) (Scalan and Smith 1970; Simoneit 1989). The carbon preference index (CPI(o/e) = ΣCi(o)/ΣCi(e), Mazurek and Simoneit 1984) values for the entire n-alkane range were 1.5±0.3 in autumn to 2.1±0.4 in spring, also confirming a mixture of natural and petroleum sources. The concentrations of terrestrial plant wax n-alkanes were calculated according to Simoneit et al. (1991) and estimated to range from 42.0 ng m-3 in winter to 111.6 ng m-3 in summer. n-Alkanes from fossil fuel emission sources ranged from 105.2 ng m-3 in spring and 239.3 ng m-3 in summer (Table 1). This indicates that the major source of n-alkanes in the Dhahran TSP is crude oil and petroleum products.
The methyl n-alkanoate (e.g., fatty acid methyl ester) concentrations in the atmospheric TSP samples were 80.6 ng m-3 in spring to 134.8 ng m-3 in summer (Table 1). They ranged from C10 to C32 with Cmax at 16 (as acids), and their even-to-odd carbon preference indices (CPI(e/o) = TCeven/TCodd) varied from 7.9±3.9 in summer to 10.1±5.8 in spring (Table 1). Methyl n-alkanoates may be formed by transesterification in the extraction solvent from fatty acids or wax esters present. Their sources are similar to n-alkanoic acids from biogenic sources, including terrestrial vegetation, marine plants, microbial mats, and bacteria (Simoneit 1978; Perry et al. 1979; Volkman et al. 1980; Kharlamenko et al. 1995; Budge and Parish 1998). The total concentrations of n-alkanones in the TSP varied from 8.1 ng m-3 in spring to 16.8 ng m-3 in summer with a Cmax at 18 (Table 1), indicating that they also were mainly from biogenic sources.
Hopane biomarkers were detectable in the TSP samples ranging from C27 to C35 (no C28) with Cmax at 29 or 30 and concentrations varying from 2.36 ng m-3 in spring to 47.90 ng m-3 in winter. The C31 and C32 S/(S+R) ratios of the extended hopanes were ~0.6 for all seasons (Table 1). This confirmed that petroleum was the source of the hopanes in the TSP samples because they are typical for mature crude oils and petroleum-derived hydrocarbons (Peters and Moldowan 1993; Rushdi and Simoneit 2002a, b).
PAHs were detected in these TSP samples at total concentrations from 5.6 ng m-3 in spring to 42.7 ng m-3 in winter. The major PAH compounds included fluoranthene, pyrene, benzo[g,h,i]fluoranthene, cyclopenta[c,d]pyrene, chrysene, benzo[k]fluoranthene, benzo[b]fluoranthene, benzo[e]pyrene, benzo[a]pyrene, perylene, anthanthrene, indeno[c,d]pyrene, and benzo[g,h,i]perylene. The parent and alkyl phenanthrenes were at low concentrations (0.2 ng m-3) only in winter. The absence of such volatile low molecular weight PAHs was likely due to their state in the vapor phase not as TSP under the ambient high temperature of the region.
The unresolved complex mixture (UCM) of branched and cyclic hydrocarbons had concentrations from 209 ng m-3 in spring to 1231 ng m-3 in summer. The major sources of the UCM (Fig. 2) were generally due to emissions from utilization of fossil fuels and lubricant oils (Simoneit 1984, 1985; Tolosa et al. 2004; Harji et al. 2008).
The total concentrations of POPs in the TSP of Dhahran were very high compared to the compound levels from natural and urban anthropogenic sources. The relative concentrations of POPs ranged from 17% in winter to 47% in autumn, whereas the natural inputs were 9% in summer to 21% in spring, and petroleum-related products varied from 39% in autumn to 67% in winter (Fig. 6). Therefore, the dominant sources of organic matter in atmospheric TSP of Dhahran city were inputs from fossil fuel emissions and synthetic industrial organic chemicals.
3.7. Health effects
Continuous exposure and uptake of these POP compounds through inhalation and dermal absorption will ultimately accumulate considerable quantities of these chemicals in the human body leading to potential health issues (WHO 1998, 2000; Ghisari and Bonefeld-Jorgensen 2009; Aneck-Hahn et al. 2018; Doherty et al. 2019). The health effects of POPs may include skin lesions, respiratory dysfunctions, heart diseases, neuropsychological disorders, reproductive system diseases, and infant development (Foster et al. 2001; Latini 2005; Hauser and Calafat 2005; Kim and Park 2014). Phthalates are toxicants to humans and animals and can affect reproductive systems as endocrine disruptors in humans (Foster et al. 2001; Latini 2005; Heudorf et al. 2007; Kay et al. 2013). They may produce skin irritation with extended exposure to the chemicals (David and Gans 2003; Hauser and Calafat 2005), respiratory disease, childhood obesity and neuropsychological disorders (Kim and Park 2014; Katsikantami et al. 2016). There is a relationship between phthalate exposure, mainly inhalation, and asthma (Jaakkola and Knight 2008; Bornehag and Nanberg 2010; Ventrice et al. 2013). Inhalation of phthalate aerosols increases the levels of inflammatory cells in the lung and bronchoalveolar lavage fluid (Kimber and Dearman 2010).
Phenyl phosphates are generally related to their contents of o-phenolic residues (Craig and Barth 1999; Mackerer et al. 1999). For instance, tricresyl phosphates are reported as neurotoxicants to humans and animals (Craig and Barth 1999; Patisaul et al. 2013). Recent studies have shown that there were significant correlations between neurodevelopment impairments in the exposure of children to phenyl phosphates (Castorina et al. 2017; Alzualde et al. 2018). These plasticizer and flame retardant compounds have been detected in tissue, milk, and urine samples (Sundkvist et al. 2010; Ding et al. 2016; Zhao et al. 2019). On the other hand, polychlorinated biphenyls were reported to have different toxic effects such as acne and pigmentation in various parts of the body (Kuratsune et al. 1972), skin rashes and irritation (Fischbein et al. 1979, 1982; Smith et al. 1982), cardiovascular disease and blood pressure increase (Kreiss et al. 1981; Smith et al. 1982), pulmonary dysfunction (Lawton et al. 1986), pregnancy loss and infant development (Allen et al. 1979; Fein et al. 1984; Gladen et al. 1988), and memory and cognitive function impairments (Kilburn et al. 1989; Nicholson and Landrigan 1994).
Further studies are crucial to access the toxicity of these plasticizers on humans due to inhalation by investigating the levels of these substances in smaller sized TSP such as PM2.5 and PM10.