General characteristics of organic pollutants in air
Target priority PAHs were quantitated using a calibration curve with labeled internal standards. The concentrations of target priority PAHs ranged from 0.12 to 101.2 pg/m3. We used SureMass signal processing of GC/Q-TOF data to deconvolute the components and MassHunter Unknowns Analysis software to identify untargeted PAHs to briefly understand the components of pollutants in the air surrounding industries. Compounds were identified and verified via the NIST17 library using the exact mass of the molecular ion or characteristic fragments (mass error < 10 ppm) and isotopic distribution as the criteria parameters. Figure 1. shows an example compound, 9H-Fluorene, 9-methylene-. Altogether, we identified and verified 187 organic chemicals using GC/Q-TOF. Among these organic chemicals, 146 were aromatic hydrocarbons (Table 2) and 41 were aliphatic hydrocarbons (Table 4).
Table 4
Aliphatic hydrocarbons in air samples screened by gas chromatography quadrupole time-of-flight mass spectrometry
No. | Retention time | Compound Name | Match Factor | Formula | Area |
1 | 8.318 | Decane, dimethyl- and isomers | 95.2 | C12H26 | 118985 |
2 | 9.640 | Octane, ethyl-dimethyl- and isomers | 93.9 | C12H26 | 188790 |
3 | 11.337 | Hexadecane and isomers | 94.1 | C16H34 | 200837 |
4 | 11.576 | Decadiene and isomers | 78.3 | C10H18 | 38793 |
5 | 13.302 | Cyclopentane, trimethyl- and isomers | 80.0 | C8H16 | 43872 |
6 | 13.461 | Hexadecane and isomers | 93.5 | C16H34 | 457735 |
7 | 15.048 | Hexadecane and isomers | 76.9 | C16H34 | 96845 |
8 | 16.038 | Hexadecane and isomers | 96.8 | C16H34 | 619750 |
9 | 17.382 | Dodecane, trimethyl- and isomers | 81.9 | C15H32 | 16068 |
10 | 17.702 | Hexadecane and isomers | 68.3 | C16H34 | 70137 |
11 | 19.003 | Hexadecane and isomers | 94.3 | C16H34 | 846032 |
12 | 19.110 | Hexane, tetramethyl- and isomers | 75.2 | C10H22 | 246158 |
13 | 22.030 | Cyclopentane, tetramethyl-, and isomers | 71.1 | C9H18 | 120222 |
14 | 22.254 | Hexadecane and isomers | 94.1 | C16H34 | 990824 |
15 | 25.435 | Tetradecene, (E)- and isomers | 70.3 | C14H28 | 137921 |
16 | 25.663 | Hexadecane | 94.6 | C16H34 | 1304903 |
17 | 28.925 | Eicosene, (E)- and isomers | 83.5 | C20H40 | 336328 |
18 | 29.149 | Hexadecane and isomers | 91.4 | C16H34 | 2346506 |
19 | 31.328 | Decane, dimethyl- and isomers | 69.3 | C12H26 | 123564 |
20 | 32.417 | Docosene and isomers | 93.2 | C22H44 | 576200 |
21 | 32.634 | Heneicosane and isomers | 70.0 | C21H44 | 3252119 |
22 | 35.083 | Decane, dimethyl- and isomers | 67.8 | C12H26 | 160044 |
23 | 35.843 | Docosene and isomers | 93.7 | C22H44 | 1036067 |
24 | 39.181 | Docosene and isomers | 78.8 | C22H44 | 1375334 |
25 | 39.397 | Heptadecane, tetramethyl- and isomers | 80.5 | C21H44 | 2986363 |
26 | 42.429 | Docosene and isomers | 91.8 | C22H44 | 981694 |
27 | 42.620 | Hexadecane and isomers | 80.6 | C16H34 | 2536926 |
28 | 45.515 | Docosene and isomers | 89.4 | C22H44 | 1121534 |
29 | 45.672 | Nonane, heptamethyl- and isomers | 69.0 | C16H34 | 2144156 |
30 | 48.146 | Eicosene, (E)- and isomers | 88.9 | C20H40 | 870908 |
31 | 48.277 | Heptacosane and isomers | 85.5 | C27H56 | 1923006 |
32 | 50.421 | Tricosene and isomers | 87.0 | C23H46 | 571889 |
33 | 50.520 | Heptacosane and isomers | 89.7 | C27H56 | 1924755 |
34 | 52.482 | Heptacosane and isomers | 91.2 | C27H56 | 1072333 |
35 | 54.319 | Heptacosane and isomers | 87.7 | C27H56 | 177521 |
36 | 56.262 | Heptacosane and isomers | 89.8 | C27H56 | 959532 |
37 | 58.427 | Heptacosane and isomers | 91.5 | C27H56 | 1390481 |
38 | 60.886 | Heptacosane and isomers | 87.2 | C27H56 | 777026 |
39 | 63.210 | Isopentyl-tetramethyl-octahydronaphthalene and isomers | 70.1 | C19H34 | 146362 |
40 | 63.773 | Tridecane, hexamethyl-trimethylhexyl- and isomers | 80.0 | C28H58 | 397013 |
41 | 67.191 | Octacosane and isomers | 69.3 | C28H58 | 406189 |
Fly ashes are considered to be important mediate for the catalysis of pollutants formation during thermal chemical processes. General characteristics of organic pollutants in air were summarized and compared to that in fly ashes.6 As shown in Fig. 2, pollutants in air are more diverse than that in fly ash samples, and the compound composition of air and the fly ash were different. Comparison between the screening results of air samples and fly ash samples from industrial sources showed that aliphatic hydrocarbons are more abundant in air, and few have been reported in fly ash from industrial sources [6]. Halogenated PACs are easily released from industrial activities [6], but PAHs and alkylated or heterocyclic PACs are more common in air samples. Predicted physical properties including subcooled vapour pressure and log koa of the contaminants in air were conducted in this study and compared to the pollutants in the fly ash samples from various industries by our previous studies [6]. Results showed that even though pollutants in air were more numerous, the deviation degree of the physical properties were smaller for pollutants in air compared to that in fly ashes (shown in Fig. 2). The deviation degree of pollutants in fly ashes were greater, most of which were of higher subcooled vapor pressure and lower log koa than that of pollutants in air. Therefore, some pollutants in fly ashes of higher subcooled vapor pressure and lower koa from various industries tend to volatilize to the air. The disposal of fly ash should emphasis on those pollutants of high subcooled vapor pressure and toxicity. Normal distribution test of subcooled vapor pressure of pollutants in air and fly ashes were also conducted (Fig. 2C). Results showed that the subcooled vapor pressure of pollutants in the air fit the lognormal distribution pattern, indicating multiple influence factors on the pollutants in the air. Pollutants in air came from varied sources, while differently, pollutants in fly ashes collected after pyroprocess were simpler as chemicals may decomposed under the elevated temperature, which contributed to the abnormal distribution of the organic pollutants in fly ashes. Therefore, air pollution originated from various sources contributed to numerous pollutants, whose properties such as subcooled vapor pressure and log koa were similar. Differently, thermal processes of high temperature made pollutants in the fly ashes more simpler, however, of great diversity on their properties. The pollutants of relatively higher subcooled vapor pressure and lower log koa in the fly ashes need to be concerned because their potential influence on the air pollution.
Aromatic hydrocarbons in air by non-target analysis of GC/Q-TOF
Most of the 16 priority PAHs were detected in the air samples surrounding industrial sources. Fluoranthene was a major contributor to the atmospheric PAH burden, and its peak areas accounted for 59% of the total peak areas of the 16 PAHs. A similar finding was also found by the comparison of PAHs in different areas in Indian, and the results showed that fluoranthene in the industrial sites was significantly higher than those in commercial sites [17]. Other studies concluded that atmospheric fluoranthene concentrations may have sources other than motor vehicles [18, 19]. Fluoranthene may therefore be considered an important indicator of industrial emissions. Higher molecular weight parent PAHs such as triphenylene, benzo[ghi]perylene, perylene, and benzo[e]pyrene were also abundant in the samples, indicating the remarkable influence of pyrogenic processes on the surrounding air as pyrogenic can be dominated by high molecular PAHs. Perylene was confirmed to be dominant precursor of PCNs during combustion or other industrial thermal processes [20, 21], however, these parent PAHs with high levels in air samples were neglected at present.
Chemical substitution in PAH molecules can substantially affect their carcinogenic potential [22]. However, PAH derivatives in the environment have been studied less than the 16 priority pollutants. We have detected multiple novel aromatic hydrocarbons and substitutes of the 16 PAHs such as isopropyl-methylphenanthrene, methylphenanthrene, and ethyl-methylanthracene. Some PAH derivatives, including methyl-, dimethyl-, trimethyl-, tetramethyl-, and ethenyl- substitutes, may be more toxic than their parent compounds and contribute a large part of toxicity of the atmospheric pollutants [2]; one example is 7,12-dimethylbenz[a]anthracene, whose toxic equivalency factor was reported to be 20 times that of its parent and twice that of benzo[a]pyrene [2], but this compound is typically ignored in routine tests of PAHs in air samples. Dimethylbenzo[a]anthracene was screened out in this study, even though the methyl substitution position cannot be elucidated according to the screening result, alkyl derivatives of PAHs such as 7,12-dimethylbenzo[a]anthracene in the air need further attention.
Concentrations of phthalic acid esters such as dibutyl phthalate, dibutyl phthalate, bis(2-methylpropyl) ester-1,2-Benzenedicarboxylic acid, di(hex-3-yl) ester phthalic acid (Table 2) were higher than those of other chemicals, and their peak areas contributed 19% of all 147 chemicals detected. Phthalic acid esters are widely used as plasticizers in various industries and have been detected in water, soil, and air, because they are not chemically bound to polymers and can therefore be easily released into the environment [23]. The oral chronic reference dose of p-phthalic acid was calculated as 1 mg/kg/day, approximately four orders of magnitude higher than that of the widely recognized toxic benzo[a]pyrene (3 × 10− 4 mg/kg/day),[24] and its highest peak areas in the air samples we collected indicated higher inhalation exposure. Six phthalic acid esters have been listed as priority controlled toxic pollutants by the US and European agencies considering the corroborated endorine disrupting toxicity, and three phthalic acid esters —dimethyl phthalate, diethyl phthalate, and di-n-octylphthalate—are regulated in surface and drinking water in China [25, 26], but phthalic acid esters in the air around industrial plants has not yet become a focus of study. Our findings were in accordance with those of previous studies, which reported higher concentrations at industrial sites than at residential and trafficked areas [27]. Workers and residents in areas contaminated by phthalic acid esters will be exposed to high levels over time [28], so control of phthalic acid esters in industrial areas is essential.
We detected heteroatom-substituted polycyclic aromatic compounds, which are often neglected in studies of environmental pollution even though their toxicity is comparable to that of PAHs [29]. Oxygenated PAHs have one or more carbonyl oxygens in the aromatic ring structure and are more mobile in the environment than PAHs because of their polarity properties, easily moving from air to surface water [29]. Therefore, oxy-PAHs should be taken into consideration when assessing risks of PAHs in the air. Oxygenated PAHs have also been reported in diesel exhaust [30], stack gas from combustion processes [31], and fly ash from various industries [32]. Hydroquinone, a toxic phenolic organic compound, has been found in various industrial effluents [33]; [34, 35]; In this study, oxy PAHs such as hydroquinone with alkane substituents was detected. Apart from the known hematotoxicity and carcinogenicity of hydroquinone [36], it may also be a critical intermediate and precursor of an emerging toxic pollutants in the air-the environmentally persistent free radicals, which has already been found in the atmospheric particles [37–39]. Semiquinone free radicals and cyclopentadiene radicals attached to airborne fine particles were considered as the dominant composition of EPFRs in the air and are believed to persist in the air for a long time [39–44]. Hydroquinone molecules with alkane substituents and phenol substitutes may be precursors or products in the formation or transformation of environmentally persistent free radicals in airborne particles [45], and therefore the levels and characteristics of hydroquinone and environmentally persistent free radicals in the air should be correlated and merits further attention. Oxygenated PAHs such as benzobisbenzofuran and dibenzofuran were also detected, and may subsequently chlorinate to polychlorinated dibenzofurans. We also identified nitro- and sulfurized PAHs such as dibenzothiophene and carbazole, which have acute or long-term hazardous to the aquatic life. Furthermore, they may be further chlorinated to the toxic polychlorinated dibenzofurans, polychlorinated dibenzothiophenes, and polychlorinated carbazoles [46, 47]. This highlights the importance of studying high molecular weight PAHs, alkylated PAHs, and heteroatom-substituted PAHs in the air, beyond the standard focus on the 16 priority pollutants.
Aliphatic hydrocarbons in air by non-target analysis of GC/Q-TOF
We detected 41 aliphatic hydrocarbons by GC/Q-TOF (Table 4). The specific positions of substitutional groups in the chemicals identified that had multiple isomers could not be determined by CG/Q-TOF without confirmation by chemical standards. The aliphatic hydrocarbons were mainly alkanes and olefins with 8–28 carbon atoms. The carbon preference index is generally used to estimate alkane sources and derived by dividing the total concentration of n-alkanes with odd numbers of carbon atoms by the total concentration of n-alkanes with even numbers of carbon atoms [48, 49]. Airborne alkanes are assumed to originate from man-made sources such as incomplete combustion and fossil fuel utilization if the carbon preference index is near 1 and to originate from vascular plant wax if the carbon preference index is much higher than 1 [48]. We roughly estimated the carbon preference index as 0.7, indicating the influence of man-made sources of aliphatic hydrocarbons in the air around the industrial plants. Previous studies that screened organic pollutants in fly ash from various industries reported the dominance of short-chain aliphatic hydrocarbons, with mostly 7–9 carbon atoms [6]. This showed obvious differences of aliphatic hydrocarbons characteristics from the nature sources and the man-made sources. The aliphatic hydrocarbon profiles in the air can be the reference for evaluation of the influence of industrial pollution.
Occurrences of chlorinated and brominated PAHs in air by target analysis of GC/Q-TOF
Cl/Br-PAHs are halogenated derivatives of PAHs, which can be emitted as by-products of thermal industries and formed through photochemical reactions in the air [50]. Because of the large numbers of Cl/Br-PAHs congeners and extremely trace levels in environmental media, it is difficult to accurately quantify and characterize these compounds. In addition, there are no standardized methods for extraction and instrument analysis of Cl/Br-PAHs. Existing accurate analysis for multiple Cl/Br-PAH congeners is mainly conducted by HRMS [51]. We used GC/Q-TOF to analyze 21 chlorinated PAHs (including eight PCNs) and 17 brominated PAHs and quantitated them with 12 labeled internal standards (Fig. 3A). Mass spectrum parameters were shown in Table 3. The calibration curve range was set to 5–500 pg/µL. The most dilute calibration solution (5 pg/µL) was sequentially injected eight times to evaluate the stability of the analysis, and the relative standard deviations of almost all congeners were lower than 10% (Table 3). An accurate mass extraction window ± 15 ppm was used to eliminate the matrix noise. Figure 3B shows the chromatograms of extracted ions in air samples. Figure 3C shows the chromatograms of specific PCNs homologs, indicating sufficient resolution and sensitivity of the GC/Q-TOF method for the synchronization analysis of those trace pollutants containing multiple congeners. Further studies can be conducted on the development of simultaneous analysis of the widely concerned persistent organic pollutants of trace levels in the environment.
The total concentration of the 13 chlorinated PAHs and 17 brominated PAHs (shown in Table 3) in air samples was 818.9 and 294.9 pg/m3, respectively. These levels are similar to those estimated previously in our laboratory by isotope dilution high-resolution gas chromatography and HRMS (987.4 pg/m3 for 13 chlorinated PAHs and 429.6 pg/m3 for 17 brominated PAHs) in air [51]. Concentrations of chlorinated PAHs were approximately three times higher than those of brominated PAHs, because chlorine levels are typically higher than bromine levels in the natural environment and in thermal-related activities. Monochlorinated anthracene was the most abundant congener, contributing 20–50% of the total chlorinated PAHs in the samples, and its fractions were higher than those of dichlorinated or tetrachlorinated anthracene, indicating that chlorination may not be favored during the formation of the chlorinated compound. Less-chlorinated polychlorinated naphthalene congeners were dominant in the gas phase, while more highly chlorinated congeners dominated the particle phase. For example, 70% of 2-chloronaphthalene was in the gas phase. 54% of more highly chlorinated congeners (hexa- to octa-) existed in the particle phase. The phenomenon may be contributed by the physiochemical properties of Cl/Br-PAHs that highly chlorinated congeners with lower vapor pressure tend to be absorbed into the particle phase.