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Environmental Research

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Enhanced H-abstraction contribution for oxidation of xylenes via mineralparticles: Implications for particulate matter formation and human health

Jiangyao Chena,b, Jiajing Yia, Yuemeng Jia, Baocong Zhaoa, Yongpeng Jia, Guiying Lia,b,Taicheng Ana,∗

aGuangdong Key Laboratory of Environmental Catalysis and Health Risk Control, Guangzhou Key Laboratory Environmental Catalysis and Pollution Control, School ofEnvironmental Science and Engineering, Institute of Environmental Health and Pollution Control, Guangdong University of Technology, Guangzhou, 510006, Chinab Synergy Innovation Institute of GDUT, Shantou, 515041, China

A R T I C L E I N F O

Keywords:Xylenes oxidationMineral particlesPathway alterationAtmospheric particle formationHuman health risk

A B S T R A C T

Xylenes are important aromatic hydrocarbons having broad industrial emissions and profound implication to airquality and human health. Generally, homogeneous atmospheric oxidation of xylenes is initiated by hydroxylradical (%OH) resulting in minor H-abstraction and major OH-addition pathways. However, the effect of mineralparticles on the homogeneous atmospheric oxidation mechanism of xylenes is still not well understood. In thepresent study, the heterogeneous atmospheric oxidation of xylenes on mineral particles (TiO2) is examined indetail. Both the experimental data and theoretical calculations are combined to achieve the feast. The experi-mental results detected a major H-abstraction (≥87.18%) and minor OH-addition (≤12.82%) pathways for theOH-initiated heterogeneous oxidation of three xylenes on TiO2 under ultraviolet (UV) irradiation. Theoreticalcalculations demonstrated favorable H-abstraction on methyl group of xylenes by surface %OH with large exo-thermic energies, because of the reason that their methyl group rather than the phenyl ring is more occupied byTiO2 via hydrogen bonding. Furthermore, the particle monitor and acute risk assessment results indicated thatthe H-abstraction products significantly enhance the formation of particulate matter and health risk to humanbeings. Taken together, these results indicate that the atmospheric oxidation mechanism of xylenes is altered inthe presence of mineral particles, highlighting the necessity to re-evaluate its implication in the environment andhuman health.

1. Introduction

Aromatic hydrocarbons (AHs) constitute 20%–30% of volatile or-ganic compounds and represent a significant source for the formation ofurban particulate matter (PM), which has profound implications on airquality and human health (Ameen and Raupp, 1999; An et al., 2010,2011, 2014; Atkinson and Arey, 2003; Bianchi et al., 2019; Blount andFalconer, 2001; Chae et al., 2011). Xylenes (m-, o- and p-xylene) areconsidered as the most important ambient AHs after toluene. Everyyear, approximately ten million tons of xylenes are produced and ap-plied with significant emissions into the atmosphere during manu-facturing and utilization processes, which contribute to the formation ofPMs and cause potential threat to human health (Chen et al., 2017a,2019). According to the U.S. Agency for Toxic Substances & DiseaseRegistry, xylenes rank 62nd out of 275 on a list of substances withpotential threat to human health. Additionally, the formation of toxicby-products from xylenes poses a major human health concern (Chen

et al., 2015, 2017b, 2018; Cho et al., 2018). Therefore, the detailedunderstanding of atmospheric oxidation mechanisms of xylenes isneeded to prevent the formation of PMs and the health risk of potentialoxidation products on the environment and human beings.

The reaction of xylenes with %OH dominates atmospheric lossduring daytime via homogeneous and heterogeneous oxidation pro-cesses (Dai et al., 2019). Most studies have focused on the homogeneousoxidation of xylenes with %OH in the gas phase, which results in majorOH-addition to the phenyl ring (about 90%) and minor H-abstractionfrom the methyl group (about 10%) (Dhada et al., 2016; Fan and Zhang,2006). However, heterogeneous oxidation of xylenes with %OH on thesurfaces of atmospheric mineral particles is insufficiently explored, al-though mineral particles have the potential to significantly alter theprocess of atmospheric chemistry (Fan and Zhang, 2008; Farias et al.,2018). There have been numerous studies investigating the hetero-geneous oxidation of inorganic gases (including SO2 (Fu et al., 2007),NO2 (Gonet and Maher, 2019), carbonyl sulfide (Groppi et al., 2012))

https://doi.org/10.1016/j.envres.2020.109568Received 26 January 2020; Received in revised form 23 March 2020; Accepted 20 April 2020

∗ Corresponding author.E-mail address: antc99@gdut.edu.cn (T. An).

Environmental Research 186 (2020) 109568

Available online 23 April 20200013-9351/ © 2020 Elsevier Inc. All rights reserved.

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and some organic compounds (such as, formaldehyde (Guan et al.,2014), butanol (Guo et al., 2008) and toluene (Han et al., 2019)) on thesurfaces of atmospheric mineral particles. The major focus of all thesestudies is to understand the effect of relative humidity, temperature,composition of mineral particles and other gases to heterogeneousoxidation itself (He et al., 2005, 2015; Hu et al., 2011; Huang et al.,2011). The research to investigate the alternation of oxidation processof gases using mineral particles is limited. He et al. reported the for-mation of HSCO2

- species from the reaction of carbonyl sulfide with %

OH on Al2O3 rather than in gas phase (Ji et al., 2019), and suggestedthat Al2O3 altered the OH-initiated oxidation process of gas. This isbecause the atmospheric mineral particles can adsorb and catalyzegases, and consequently change the oxidation pathway of them (Jiet al., 2017; Li et al., 2014, 2019; Liu et al., 2010, 2019; Ma et al.,2011). Recently, Chen et al. reported that the initial step on mineralparticles should be dominated by the oxidation of methyl group of m-xylene (Ma et al., 2008). However, no further evidences were providedby these studies. Therefore, more research should be conducted to findthe alternation possibility and detailed mechanisms of heterogeneousoxidation of gases in the presence of mineral particles to comprehen-sively understand their implications in atmospheric environment andhuman health.

In the present study, the heterogeneous oxidation of xylenes, m-xylene (MX), o-xylene (OX) and p-xylene (PX) on mineral particles(TiO2) under ultraviolet (UV) irradiation was investigated. The inter-mediates adsorbed on the mineral particles were extracted and pre-treated. They were qualitatively and quantitatively analyzed via a gaschromatography-mass spectrometer (GC-MS) and compared with thestandard samples. Then, the concentrations of OH-addition and H-ab-straction intermediates from xylene oxidation were compared to revealthe alternation of their contributions. Furthermore, by combining theintermediate data and quantum chemical calculations, the OH-additionand H-abstraction pathways of xylenes in the presence of mineral par-ticles were also demonstrated, and the enhanced H-abstraction me-chanism was found for the oxidation of xylenes by the mineral particlesunder UV irradiation. Finally, the implications of enhanced formationof H-abstraction intermediates from xylenes to the formation of PMsand human health were also investigated.

2. Materials and methods

2.1. Heterogeneous photochemical oxidation of xylenes on TiO2

The petri dish loaded with the mineral particles (TiO2) was placed atthe bottom of a self-made fixed bed Pyrex reactor (volume of 5 L)(Mendez-Roman and Cardona-Martinez, 1998), which was thoroughlycleaned using acetone and dried using high-purity dry air prior to use.Later, 5 μL of liquid MX, OX or PX was injected into the reactor full ofdry air. Then, a certain amount of water was added into the reactor toachieve 10% of relative humidity. After 30 min dark equilibrium, a UVlamp (30 W, Guangdong Cnlight Co., Ltd., China) with maximumemission wavelength at 254 nm and minor at 185 nm was turned on tovertically irradiate the petri dish through a quartz glass window fixedoutside of the top of the reactor. The concentration variations of xyleneswere monitored using a gas chromatograph (GC-9800) in real time anddetail of the analysis process is given in Supporting Information (SI). At30, 60 and 120 min, the lamps were turned off and the petri dish withTiO2 was taken out for intermediate analysis.

2.2. Qualitative and quantitative analysis of intermediates on TiO2

About 15 mL of methanol was added into the petri dish and it wassoaked for 24 h to thoroughly extract products from TiO2. The extractwas then collected by filtration through a 0.22 μm of organic filtrationmembrane, washed three times with 5 mL of methanol, and slowlyconcentrated to 2 mL using a rotary evaporator (Shanghai Yarong

Biochemical Instrument Factory, China). The concentrated solution wastransferred into a clean cell bottle, completely dried using nitrogen andfinally diluted to 1 mL with ethyl acetate. Later, the diluted sample wasdirectly injected into the GC-MS (Agilent 7890B–5977B) with a HP-5MScapillary column (30 m × 0.25 mm × 0.25 μm) for intermediateanalysis. The GC's operational parameters were as follows: initially60 °C for 1 min, then heated to 165 °C at a rate of 3 °C min−1, andfinally heated to 280 °C at a rate of 10 °C min−1 and held for 10 min.The MS was operated in full scan mode with m/z of 45–260, while thetemperature of the transfer line, ionizing energy and scan range wereset to be 250 °C, 70 eV and 45–260 me−1, respectively. The carrier gaswas ultra-high purity helium with the flow rate of 1.2 mL min−1. Thequalitative identification of the intermediates was accomplished bycomparing their mass spectra with the National Institute of Standardsand Technology (NIST) database as well as the standard samples.

In this work, methylphenol, methylbenzaldehyde, methylbenzylalcohol, benzoic acid and methylbenzoic acid were selected for quan-tifying the intermediates of phenols, aldehydes, methylbenzyl alcohols,benzoic acid and other acids, while phthalan was used for quantitativeanalysis of benzofurans. 2-Methylbenzaldehyde was used as a casestudy to illustrate the standard sample preparation methods. Initially,3.2 μL of 2-methylbenzaldehyde was added into a beaker containing400 mL of ethyl acetate. Then, the mixture was poured into a 500-mLvolumetric flask using a glass rod. About 30 mL of ethyl acetate wasused to clean the beaker and glass rod before pouring the mixture intothe flask. Finally, pure ethyl acetate was gradually added into themixture to reach a volume of 500 mL. The calculated concentration of2-methylbenzaldehyde was 6.4 ppm. Later, 0.8, 1.6 and 3.2 ppm con-centrations of 2-methylbenzaldehyde were obtained using the dilutionmethod. In a similar manner other standard samples were preparedusing the same method. The concentrations were set as follows: 2-me-thylphenol (0.8, 1.6, 3.2 and 6.4 ppm), 3-methylphenol (3.2, 6.4, 12.8and 25.6 ppm), 4-methylphenol (3.2, 6.4, 12.8 and 25.6 ppm), 3-me-thylbenzaldehyde (0.8, 1.6, 3.2 and 6.4 ppm), 4-methylbenzaldehyde(0.8, 1.6, 3.2 and 6.4 ppm), 2-methylbenzyl alcohol (4, 8, 16 and32 ppm), 3-methylbenzyl alcohol (4, 8, 16 and 32 ppm), 4-methyl-benzyl alcohol (4, 8, 16 and 32 ppm), benzoic acid (100, 150, 180 and200 ppm), 2-methylbenzoic acid (100, 150, 180 and 200 ppm), 3-me-thylbenzoic acid (100, 150, 180 and 200 ppm), 4-methylbenzoic acid(100, 150, 180 and 200 ppm) and phthalan (0.8, 1.6, 3.2 and 6.4 ppm).Then, 1 mL of the standard sample was transferred to a sterilized cellbottle for GC-MS analysis. Similarly, the GC-MS operational conditionswere used for the standard samples and their corresponding inter-mediates.

2.3. Theoretical calculations for adsorption and oxidation of xylenes onTiO2

All quantum chemical calculations were performed using theGaussian 09 software package (Miyazawa et al., 2004; Molteni et al.,2018). Geometric optimization of reactants and products involved inthe title reactions were carried out using the hybrid density functionalM06-2X method with the 6-311G(d,p) basis set (at the M06-2X/6-311G(d,p) level). Detailed information about the theoretical calcula-tions for adsorption and oxidation of xylenes on TiO2 is provided in SI.

3. Results and discussion

3.1. Identification of OH-addition and H-abstraction intermediates fromxylenes oxidation on TiO2

Fig. S1 shows the increased elimination efficiencies from 0 to 94.5%for MX, to 96.9% for OX, and to 91.8% for PX with the increase in thereaction time from 0 to 150 min, indicating that TiO2 cannot com-pletely degrade xylenes, leaving products on TiO2. As shown in Fig. 1a,eight peaks with retention times of 8.864, 9.175, 10.264, 11.330,

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11.908, 15.374, 17.319 and 23.551 min were clearly observed from thetotal ion chromatogram after MX oxidation for 30 min. Furthermore,the comparison of their mass fragments with the standard samples (Fig.S2) and NIST mass spectral library (Fig. S3) reveals that they were 3-methylbenzaldehyde (M1), 3-methylphenol (M2), 2,6-dimethylphenol(M3), 3-methylbenzyl alcohol (M4), 2,4-dimethylphenol (M5), 1,3-ben-zenedicarboxaldehyde (M7), 3-methylbenzoic acid (M8) and 4-hydroxy-3-methylbenzaldehyde (M9), respectively (Table S1). After another30 min reaction, 3-methylphenol and 2,6-dimethylphenol completelydegraded to form 3-formylbenzoic acid (M10, retention time of24.896 min). Additionally, more acids (benzoic acid (M6) and 3-hy-droxybenzoic acid (M11) with the retention times of 12.919 and26.062 min, respectively were observed by prolonging the reaction timeto 120 min. After the analysis of the structures of these eleven inter-mediates, it is found that six of these (3-methylbenzaldehyde, 3-me-thylbenzyl alcohol, benzoic acid, 1,3-benzenedicarboxaldehyde, 3-me-thylbenzoic acid and 3-formylbenzoic acid) are H-abstraction

intermediates, while the rest (3-methylphenol, 2,6-dimethylphenol,2,4-dimethylphenol, 4-hydroxy-3-methylbenzaldehyde and 3-hydro-xybenzoic acid) belong to the OH-adducts (Table S1). More H-ab-straction intermediates were found during MX oxidation on TiO2 in thissystem. Further observation indicates that their peak areas further in-creased with the extension of the oxidation time, indicating increasedconcentration of H-abstraction intermediates.

The formations of more H-abstraction intermediates than OH-ad-dition ones were also obtained from the oxidation of OX and PX onTiO2. For OX, a total of eleven intermediates were detected during the120 min reaction, whereas seven of these were the H-abstraction in-termediates (2-methylbenzaldehyde (O2), 2-methylbenzyl alcohol (O3),benzoic acid (O4), 2-methylbenzoic acid (O7), phthalic anhydride (O8),phthalide (O9) and 2-formylbenzoic acid (O11)). Furthermore, the otherfour (2-methylphenol (O1), 2,3-dimethylphenol (O5), 3,4-dimethyl-phenol (O6) and 5-hydroxy-2-methylbenzaldehyde (O10)) were the OH-adducts (see Figs. 1c, S4, S5, and Table S2). The oxidation of PX on TiO2

Fig. 1. Total ion chromatograms and mass data of OH-addition and H-abstraction intermediates on TiO2 from oxidation of MX (a, b), OX (c, d) and PX (e, f).

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results in the formation of seven H-abstraction products (4-methyl-benzaldehyde (P2), 4-methylbenzyl alcohol (P3), benzoic acid (P5), 1,4-benzenedicarboxaidehyde (P6), 4-methylbenzoic acid (P7), 4-hydro-xymethylbenzaldehyde (P8) and 4-formylbenzoic acid (O10)) and threeOH-adducts (4-methylphenol (P1), 2,5-dimethylphenol (P4) and 3-hy-droxy-4-methylbenzaldehyde (P9)) (see Figs. 1e, S6, S7, and Table S3).Similar to MX, the peak areas of H-abstraction intermediates from OXand PX oxidation increased with the increase in the oxidation time,suggesting the enhanced formation of H-abstraction products from xy-lenes’ oxidation on TiO2. However, almost all the homogeneous atmo-spheric oxidation studies of xylenes have demonstrated the dominantformation of OH-addition products, which account for about 90% oftotal intermediates (Dai et al., 2019; Mulheims and Kraushaar-Czarnetzki, 2011). Furthermore, the formation of only H-abstractionproducts is observed from the catalysis oxidation of MX (Nollet et al.,2003), OX (Pal and Ariya, 2004) or PX (Pan and Wang, 2014). Theseresults indicate that mineral particles (TiO2) may enhance the forma-tion of H-abstraction products during the oxidation of xylenes.

3.2. Enhanced H-abstraction intermediate formation on TiO2

In order to accurately find whether or not the formation of higherconcentrated H-abstraction intermediates on TiO2 occurred, the massesof the intermediates were determined based on the standard samples(Figs. S8–S13). The calibration curve correlation coefficients (R2) for allstandard samples exceeded the value of 0.97 within the examinedconcentration ranges, suggesting the reliability of the calibration curves(Table S4). In addition, after calculating the peak areas of detectedsamples with the equations obtained from the calibration curves, themass of each intermediate was obtained and listed in Tables S5–S7.Fig. 1b shows the mass evolution of OH-addition and H-abstractionintermediates for MX with the passage of reaction time. As indicated,30 min reaction results in the formation of 1.30 × 10−1 mg of OH-addition intermediates and 8.84 × 10−1 mg of H-abstraction inter-mediates, accounting for 12.82% and 87.18% of total mass, respec-tively. About 6.85 times higher mass of intermediates is obtained fromH-abstraction than OH-addition, indicating that TiO2 actually enhancesthe formation of H-abstraction products during MX oxidation. In ad-dition, prolonging the reaction time to 60 min and then, to 120 minresulted in decreased masses of OH-addition intermediates with themasses of 2.66 × 10−2 and 3.14 × 10−1 mg, respectively, while themasses of H-abstraction intermediates further increased up to 5.44 and15.5 mg, respectively. Accordingly, the mass percentages of OH-addi-tion and H-abstraction intermediates change to 0.49% and 99.51% at60 min, and 1.99% and 98.01% at 120 min. These results furtherconfirmed the enhanced formation of H-abstraction intermediates byTiO2 with the increase in oxidation time. TiO2 also enhances the for-mations of H-abstraction intermediates towards other two xylenes. Asshown in Fig. 1d and f, about 5.08 × 10−1 and 2.52 mg of H-ab-straction intermediates were generated from OX and PX oxidations onTiO2 for 30 min, which finally increased to 9.11 and 39.0 mg at120 min. The mass percentages accordingly increased from 90.71% to94.03% at 30 min to 99.87% and 99.94% at 120 min. These resultsfurther support the formation of enhanced H-abstraction intermediateson TiO2 by xylenes. The highest mass of H-abstraction intermediates isobtained from PX oxidation, which however shows the lowest elim-ination efficiency (see Fig. S1), indicating facilitated formation of H-abstraction products on TiO2 from lower oxidation efficiency of xy-lenes.

Further analysis of H-abstraction products reveals that 3-methyl-benzoic acid, 2-methylbenzoic acid and 4-methylbenzoic acid ac-counted for 95.05% (Fig. S14), 79.93% (Fig. S15) and 92.86% (Fig.S16), respectively for the H-abstraction intermediates from MX, OX andPX, indicating major contribution of methylbenzoic acids to H-ab-straction by-products from the oxidation of xylenes. Followed are for-mylbenzoic acids with the average percentages of 3.45% for 3-

formylbenzoic acid, 11.62% for 2-formylbenzoic acid and 3.71% for 4-formylbenzoic acid. Notably, for OX oxidation, an average of1.76 × 10−1 mg of benzofuran intermediates (phthalic anhydride andphthalide) are formed on TiO2, and accounted for 4.99% of the H-ab-straction intermediates (Fig. S15), indicating them as an importantgroup of H-abstraction products obtained from the oxidation of OX.Actually, the oxidation of OX to phthalide and phthalic anhydride isone of the important industrial synthesis processes (Ponczek andGeorge, 2018; Ran et al., 2018), proving relatively high productionpercentages of benzofuran intermediates from OX oxidation in thepresent study. In the case of OH-addition intermediates, dimethylphe-nols are the most dominant contributors, and accounted for 46.17%,68.88% and 42.53% in average of the OH-adducts from MX, OX and PXoxidation, respectively (Figs. S14–S16). In short, the heterogeneousphotochemical oxidation of three xylenes on TiO2 certainly results inthe enhanced formation of H-abstraction products, indicating the pro-moted H-abstraction pathways than OH-addition pathways of xylenes.

3.3. Enhanced H-abstraction pathway for oxidation of xylenes on TiO2

In order to accurately elucidate the enhanced H-abstractionpathway of xylenes than OH-addition pathway on TiO2, quantum che-mical calculations were performed on the above-mentioned inter-mediate data. Since our previous work has proved the major con-tribution of •OH to heterogeneous photochemical oxidation routes ofgaseous AHs on TiO2 (Mendez-Roman and Cardona-Martinez, 1998),the oxidation pathways of xylenes on TiO2 in the present study are alsoinitiated by •OH. Fig. 2a shows the proposed H-abstraction and OH-addition pathways of MX. In Pathway I that was initiated by H-ab-straction, H atom of methyl group of MX is firstly abstracted by •OH toform 3-methylbenzyl alcohol after losing one H2O, and releases com-paratively high energy (△Er = −110.45 kcal mol−1). Furthermore,•OH further abstracts H atom of alcohol group of 3-methylbenzyl al-cohol to generate 3-methylbenzaldehyde and then 3-methylbenzoicacid, which were accompanying by the released energies of 117.33 and67.74 kcal mol−1, respectively. The transformation of 3-methylben-zaldehyde to 3-methylbenzoic acid during MX oxidation is also reportedby Sun et al. (Nollet et al., 2003), which is similar to the results ob-served in the current study. The oxidation of benzaldehyde to benzoicacid has also been reported from the oxidation of other AHs on TiO2

(Roldin et al., 2019; Saha et al., 2018; Shang et al., 2017), furtherproving the observed results. During this reaction, H atoms of methylgroup of 3-methylbenzaldehyde and 3-methylbenzoic acid are ab-stracted by •OH to individually form 1,3-benzenedicarboxaldehyde and3-formylbenzoic acid after losing H2O, which result in the release of225.24 and 225.57 kcal mol−1 of energies, respectively. Furthermore,•OH replaces the methyl group of 3-methylbenzoic acid to form OH-adduct: 3-hydroxybenzoic acid, releasing 9.81 kcal mol−1 of energy.During the transformation of 3-methylbenzoic acid to 3-formylbenzoicacid, energy released was 22.99 times higher than the 3-hydro-xybenzoic acid, demonstrating more thermodynamically favorable H-abstraction reaction on 3-methylbenzoic acid.

In Pathway II that was initiated by OH-addition, •OH adds onto thephenyl ring of MX to form two OH-adducts: 2,6-dimethylphenol and2,4-dimethylphenol after releasing one H2O. Theoretical calculationsrevealed that the energies released were 119.14 and 118.72 kcal mol−1,respectively, suggesting spontaneous occurrence of above processes.Considerably lower mass percentage of 2,6-dimethylphenol (2.94% inaverage) is obtained as compared to 2,4-dimethylphenol (43.23% inaverage) among OH-addition intermediates (Fig. S14), which might bedue to the spontaneous transformation of the former to 4-hydroxy-3-methylbenzaldehyde (accounting for 7.17% of OH-addition products)through H-abstraction process (△Er = −228.88 kcal mol−1). Mean-while, the replacement reaction of methyl group for MX by •OH takesplace to form 3-methylphenol, releasing 10.88 kcal mol−1 of energy.The obtained 3-methylphenol accounts for 14.62% of OH-addition

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products, which is greater than 2,6-dimethylphenol, but lower than 2,4-dimethylphenol, indicating that its formation process is a second im-portant OH-addition reaction.

OX and PX undergo similar H-abstraction and OH-addition path-ways. As shown in Fig. 2b and c, the methyl groups of OX and PX gothrough a series of H-abstraction processes to successively form 2-

Fig. 2. Proposed H-abstraction and OH-addition oxidation pathways of MX (a), OX (b) and PX (c) on TiO2 (The energy data are given in kcal mol−1).

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methylbenzyl alcohol, 2-methylbenzaldehyde, 2-methylbenzoic acid, 2-formylbenzoic acid, phthalide and phthalic anhydride for OX, and 4-methylbenzyl alcohol, 4-methylbenzaldehyde, 4-methylbenzoic acid, 4-hydroxymethylbenzaldehyde, 1,4-benzenedicarboxaidehyde and 4-for-mylbenzoic acid for PX (Pathway I). In the meantime, •OH adds tophenyl ring of OX to form 2,3-dimethylphenol, 3,4-dimethylphenol(then to 5-hydroxy-2-methylbenzaldehyde) and 2-methylphenol,whereas PX forms 2,5-dimethylphenol (then to 3-hydroxy-4-methyl-benzaldehyde) and 4-methylphenol (Pathway II). All reactions releasedsignificant energies (△Er ranges from −7.39 to −228.70 kcal mol−1),indicating thermodynamically favorable H-abstraction and OH-additionprocesses. Further comparison demonstrates that H-abstraction andOH-addition intermediates of MX, OX and PX from Pathways I and IIaccount for 94.58% and 5.10%, 94.77% and 3.24%, and 95.98% and1.95% of the total mass, respectively (Fig. S17), thus proving the en-hanced H-abstraction pathway during the oxidation of xylenes on TiO2.

In addition to these two leading pathways, demethylation reactionof MX, OX and PX also takes place to generate gaseous toluene. Due tothis reaction the photon of UV lamp with the energy of 647 kJ mol−1

easily breaks the C–C bond (356 kJ mol−1) of xylenes. The theoreticalcalculations demonstrated that 8.98, 8.92 and 8.67 kcal mol−1 of en-ergies were released during the demethylation processes for MX, OXand PX, respectively (Fig. S18), proving easy formation of toluene fromxylenes. Dhada et al. reported the formation of gaseous toluene duringOX oxidation (Smith et al., 1999), supporting the current hypothesisfrom experiments. Toluene undergoes a series of H-abstraction reac-tions to finally generate benzoic acid. The release of very high energy(△Er = −294.85 kcal mol−1) solidly proves easy transformation oftoluene to benzoic acid through H-abstraction reactions by •OH. How-ever, benzoic acid only contributes about 0.32%, 1.99% and 2.07% tothe total intermediate mass for MX, OX and PX, respectively, indicatingthat its formation pathway was a minor H-abstraction process.

3.4. Enhanced H-abstraction mechanism for oxidation of xylenes on TiO2

Evidently, the results of the oxidation pathways confirmed that H-abstraction is entirely dominant oxidation pathway for all three xyleneson TiO2. Previous theoretical calculations consistently proved that OH-addition more easily takes place on xylenes than H-abstraction duringatmospheric homogeneous reaction (Song et al., 2007, 2019; Sun et al.,2010, 2015). From these results, it is concluded that the presence ofTiO2 alters the dominant oxidation pathway of xylenes from OH-addi-tion to H-abstraction. In order to understand the enhanced mechanism,the adsorption configurations and energies of xylenes on (101) facet ofanatase TiO2 were investigated using the quantum chemical calcula-tions. In the case of MX, the H atoms of its methyl group would interactwith the O atom of (101) facet of anatase TiO2 to form TiO2-MX com-plex, whereas the adjacent H–O bond distances were 2.67 and 2.79 Å(Fig. 3a). The calculated adsorption energy of TiO2-MX was negative(△E = −2.56 kcal mo1−1), suggesting spontaneous occurrence of theadsorption of MX on (101) facet of anatase TiO2. Due to the physi-sorption with low adsorption energies of −1.20 to 9.56 kcal mol−1 andchemisorption with −9.56 to −191.30 kcal mol−1 (Tremblay, 2013),the adsorption performance of MX on (101) facet of anatase TiO2 be-longs to weak chemisorption. Both adsorption configuration and energyresults disclose the weak chemisorption of MX adsorbed on (101) facetof anatase TiO2 through its methyl group. Similar results were observedin OX and PX with the formation of TiO2-OX and TiO2-PX complexeshaving different adjacent H–O bond distances and negative adsorptionenergies (Fig. 3b and c). Therefore, TiO2 prefers to adsorb the methylgroup of xylenes through hydrogen bonding, favoring subsequent H-abstraction by surface •OH on TiO2 to contribute in the formation ofhigher concentrated H-abstraction intermediates and enhanced in-volvement of H-abstraction pathway. These results also suggest that theadsorption configuration of AHs onto mineral particles determines theiroxidation contributions. AHs with different side-chain sites and

composition may display various adsorption configurations onto dif-ferent mineral particles, leading to distinct oxidation mechanisms.Further research is needed to comprehensively find the adsorption andoxidation mechanisms as well as their relationship with other AHs.

3.5. Implications for PM formation and human health

The results obtained in this study confirmed that the H-abstraction(rather than OH-addition) is the dominant pathway for the oxidation ofxylenes on mineral particles. This result is contrary to that of homo-geneous atmospheric oxidation, indicating that mineral particles effi-ciently alter the oxidation process of xylenes. Although the experi-mental conditions in the present study, such as the initial concentrationof xylenes (ppm level) and wavelength of lamp (254 nm and 185 nm),are more or less different to that of the real atmospheric environment(xylene in ppb level and solar light irradiation). Furthermore, the oxi-dation pathway and the mechanism investigations in the present studyinvolved •OH, which is the dominant daytime oxidant in the atmo-sphere (Wang et al., 2015). Additionally, compared with the alreadypublished data, the results indicate that mineral particles promote theformation of highly oxygenated organic molecules (HOM) from H-ab-straction of AHs with •OH rather than that of unsaturated carbonylcompounds with low oxygenated organic molecules from OH-addition.The obtained HOM further condenses on the mineral particles andcould have been involved in the formation of new particles, thus con-tributing to the formation of PMs. This makes it a ubiquitous compo-nent of atmospheric particles known to affect the earth's radiationbalance and climate (Wang et al., 2007; Wei et al., 2019; Wen et al.,2019; Wu et al., 2004). As shown in Fig. 4a, the significantly increasedPM concentrations (particle size of 2.5 μm detected using a particlecounter (CEM DT-9880)) from 5, 10 and 11 μg m−3 at 0 min to 18519,26167 and 23562 μg m−3 at 30 min are observed during the oxidationof MX, OX and PX on TiO2, and further confirm the significant con-tribution of HOM to PM formation. Therefore, our results suggest sig-nificant underestimation of the contribution of HOM from H-abstrac-tion of AHs on mineral particles to form PMs, and more attention isneeded to understand the altered oxidation processes of gases by mi-neral particles and re-evaluate their effect on the formation of PMs.

Moreover, our results highlight the necessity to re-examine thehuman health impact of enhanced HOM from H-abstraction pathways.This is due to the reason that higher mass of organics may lead to higherhealth risk to human beings due to their diffusion into the atmosphere.Furthermore, acute risks of H-abstraction and OH-addition inter-mediates are evaluated to investigate the potential acute effects onhuman beings after short-time interaction. The corresponding targetcompound risk quotient (RQ) is the ratio of target compound mass (mg)to its 50% lethal dose (LD50 × 70 kg of human body) (Tables S5–S7). Asshown in Fig. 4b, the RQs of H-abstraction intermediates from MX, OXand PX oxidations are 4.85 × 10−5, 2.71 × 10−5 and 1.44 × 10−4,respectively, which account for 96.57%, 98.15% and 98.96% of totalRQ. However, such tremendous contribution to health risk from H-ab-straction intermediates is scarcely reported in previous studies, as mostof the attention has been focused on the OH-adducts during AH oxi-dation in the homogeneous atmosphere (Chen et al., 2017b; Dai et al.,2019; Fan and Zhang, 2006; Mulheims and Kraushaar-Czarnetzki,2011). Some H-abstraction intermediates obtained in the present study,such as benzofurans, are known as promising insecticides having hightoxicity to aquatic animals (Wu et al., 2013), and even pose adulticidalthreat to terrestrial animals (Yang et al., 2019; Zhang et al., 2019a).This study discovered that the presence of TiO2 effectively enhances theformation of H-abstraction products from the oxidation of xylenes,which poses acute risk to human beings. Although, OH-adducts onlycontribute 2.11% (on average) to total RQ for xylenes intermediates,the acute threat of phenolic OH-addition intermediates to humanbeings should be given serious consideration at the initial oxidationprocess of xylenes on mineral particles, because phenols show

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noticeable mutagenicity (Zhang et al., 2019b; Zhao et al., 2019). Al-togather, these findings suggest that H-abstraction rather than OH-ad-dition represents the dominant pathway for xylenes’ oxidation on

mineral particles, and more systematic and theoretical analysis is re-quired to obtain a clear understanding of the influence of mineralparticles on the oxidation mechanisms and the potential impact on

Fig. 3. The optimum adsorption configurations and energies of MX (a), OX (b) and PX (c) on TiO2 (The energy data are given in kcal mol−1).

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atmospheric environment and human health.

4. Conclusions

In the present study, the heterogeneous atmospheric oxidation ofxylenes on TiO2 was examined. The heterogeneous oxidation of threexylenes on TiO2 under UV irradiation resulted in the major H-abstrac-tion and minor OH-addition pathways. This was due to the reason thattheir methyl group rather than the phenyl ring was more occupied byTiO2 through hydrogen bonding, and favored the H-abstraction onmethyl group of xylenes by surface •OH with large exothermic energies.The H-abstraction products significantly enhance the formation of PMsand health risk to human beings. The results revealed that the atmo-spheric oxidation mechanisms of xylenes were altered in the presence ofmineral particles, suggesting the need for additional studies to morecomprehensively understand the effect of this change on the environ-ment and human health.

CRediT authorship contribution statement

Jiangyao Chen: Formal analysis, Methodology, Writing - originaldraft. Jiajing Yi: Formal analysis, Methodology. Yuemeng Ji: Datacuration. Baocong Zhao: Investigation. Yongpeng Ji: Investigation.Guiying Li: Validation. Taicheng An: Conceptualization, Supervision.

Declaration of competing interest

The authors declare that they have no known competing financialinterests or personal relationships that could have appeared to influ-ence the work reported in this paper.

Acknowledgement

This work was financially supported by National Natural ScienceFoundation of China (21777032 and 41425015), Local Innovative andResearch Teams Project of Guangdong Pearl River Talents Program(2017BT01Z032), National Key R&D Program of China(2019YFC0214402), Key-Area Research and Development Program ofGuangDong Province (2019B110206002) and The Innovation TeamProject of Guangdong Provincial Department of Education, China(2017KCXTD012).

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.envres.2020.109568.

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