Dominant fraction of EPFRs from non-solvent-extractable organic

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Characterization of Natural and Affected Environments

Dominant fraction of EPFRs from non-solvent-extractable organic matter in fine particulates over Xi’an, China Qingcai Chen, Haoyao Sun, Mamin Wang, Zhen Mu, Yuqin Wang, Yanguang Li, Yansong Wang, Lixin Zhang, and Zimeng Zhang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b01980 • Publication Date (Web): 02 Aug 2018 Downloaded from http://pubs.acs.org on August 3, 2018

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Environmental Science & Technology

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Dominant fraction of EPFRs from

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non-solvent-extractable organic matter in fine

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particulates over Xi’an, China

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Qingcai Chen,a,b* Haoyao Sun,a Mamin Wang,a Zhen Mu,a Yuqin Wang,a,c Yanguang Li,d,e

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Yansong Wang,f Lixin Zhang,a Zimeng Zhanga

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a

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Technology, Xi’an 710021, China

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b

Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8601, Japan

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c

Department of Earth and Atmospheric Sciences, Saint Louis University, St. Louis, MO

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63108, USA

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d

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710054, China

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e

Xi’an Center of Geological Survey, China Geological Survey, Xi’an 710054, China

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f

College of Chemistry and Chemical Engineering, Shaanxi University of Science and

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Technology, Xi’an 710021, China

School of Environmental Science and Engineering, Shaanxi University of Science and

Key Laboratory for the Study of Focused Magmatism and Giant Ore Deposits, MLR, Xi’an

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*Corresponding author e-mail: [email protected]

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Phone/fax: (+86) 15029613700

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Address: School of Environmental Science and Engineering, Shaanxi University of Science

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and Technology, Weiyang District, Xi’an, Shaanxi, 710021, China

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ACS Paragon Plus Environment


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ABSTRACT: To understand the nature and possible sources of environmentally persistent

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free radicals (EPFRs) in atmospheric aerosols, the present study used a solvent extraction

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method to fractionate aerosol components with different polarities and solvent resistance in

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fine particulate matter (PM2.5) from Xi’an, China. The characteristics of EPFRs, i.e., their

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concentration, type and lifetime, were obtained based on their electron paramagnetic

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resonance (EPR) spectra. The results showed that the EPFRs in the PM2.5 samples were

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carbon-centered with a nearby heteroatom (g = 2.0031) and had a long half-life of more than

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3 years. Nearly all of the extractable EPFRs were detected in the water-insoluble organic

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fraction and showed characteristics indicating that may contain oxygen-centered radical (g =

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2.0038). Most of the total EPFRs in the PM2.5 were derived from solvent-resistant organic

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matter (88%), which likely consisted of graphene oxide (GO) analogues. The results suggest

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that previous studies may have missed the major proportion of EPFRs in atmospheric

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particulates if they only focused on solvent-extractable or metallic oxide-formed EPFRs. Our

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results showed that the EPFR concentration was significantly and positively correlated with

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the elemental carbon (EC) and NO2 concentrations, suggesting that traffic emissions may be

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an important source of EPFRs in PM2.5 over Xi’an.

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KEYWORDS: Air pollution; PM2.5; Environmentally persistent free radicals; Formation

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mechanism; Source

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1. Introduction

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Environmentally persistent free radicals (EPFRs) are present within fine atmospheric

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particles and assumed to be responsible for the detrimental effect of atmospheric particles on

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human health.1-5 Toxicity studies have demonstrated that EPFRs are directly linked to health

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effects involving cardiovascular and respiratory dysfunctions in murine models (rats and

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mice).2 Atmospheric EPFRs have lifetimes of several months or even years; thus, they may

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have long-term effects on lung tissue cells and could increase the risk of chronic lung

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disease.3-5 One possible mechanism of such health effects is the continuous EPFR-induced

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conversion of O2 molecules into reactive oxygen species (ROS),6-8 which damage cell

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components, including DNA.5,8-10 Therefore, characterizing the physicochemical properties

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of atmospheric EPFRs is important to better understand their potential human risk.

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Previous studies have reported that the EPFR concentrations in atmospheric particulates

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varied within the range of 2.02×1016 to 6.23×1020 spins/g, and these concentrations should

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depend on the sources and physiochemical processes of the atmospheric particulates in the

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atmosphere.3,11-14 EPFRs have been found in cigarette smoke,15,16 biomass and coal

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combustion,11,17 automobile exhaust,11 lead smelting ash and coking ash,11 which are

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potential primary sources of EPFRs in atmospheric particulates. Atmospheric EPFRs may

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originate from primary sources as well as secondary processing. Photochemical processes

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impact the initial radical concentration and fast decay rate of EPFRs.3 The generation of

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secondary EPFRs from polyaromatic hydrocarbon (PAH) photodegradation has been verified

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under laboratory conditions.13 Extensive research on the occurrence and source of EPFRs in 3 / 34



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various atmospheric environments is essential for understanding the risk that these EPFRs

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pose to humans and developing effective control policies for air pollution.

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The EPFRs in atmospheric particulates are usually found as organic free radicals with

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g-factors of 2.0033−2.0048 and line widths (∆Hp-p) of 4.7−7.9 G,11-13 which are typical of

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oxygen-centered or oxygen-containing EPFRs, e.g., phenoxyl and semiquinone radicals.11-13

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Carbon-centered free radicals generally have smaller g-factors.17 Atmospheric EPFRs are

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assumed to be generated through a series of surface-mediated reactions between transition

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metals (Cu, Fe and Zn) and organic precursors containing benzene rings.18-21 However,

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significant correlations have not been observed between EPFRs and metals in ambient

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particulates.3 An important question is whether EPFRs are necessarily associated with metal

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oxides in actual atmospheric particulates.

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Solvent extraction has been widely used to characterize atmospheric EPFRs.11,19,22 Solvent

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extraction methods can fractionate several classes of aerosol components with different

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polarities and solvent resistances, thereby enabling the extraction of specific EPFR

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species.19,23-25 However, common solvents may not be able to extract a significant portion of

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organic particles, e.g., more polar oxygenated compounds,26 and most EPFRs are unlikely to

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be extractable using the solvent extraction method for ambient particulates13. Previous studies

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may have missed the important component of EPFRs in atmospheric particulates if they only

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focused on the solvent-extractable EPFRs.11,19,22 To the best of our knowledge, the

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characteristics of nonextractable EPFRs, e.g., their type, lifetime and chemical structure in

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atmospheric particulates, remain poorly understood. 4 / 34


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To understand the nature and sources of EPFRs in atmospheric aerosols, the present study

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used a solvent extraction method to fractionate the aerosol compounds with different

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polarities and solvent resistances in fine particulate matter (PM2.5) from Xi’an, China. The

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concentrations, types and lifetimes of EPFRs in different extracts and the nonextractable

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residues were investigated based on an electron paramagnetic resonance (EPR) analysis. To

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rapidly and accurately quantify the EPFRs in atmospheric particles, a new method consisting

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of a quartz sheet-based approach and EPR spectroscopy was used.27 The possible chemical

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structures and sources of the EPFRs are discussed based on the data obtained by the EPR

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analysis and other analytical techniques, including a thermal-optical analysis, Raman analysis

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and Fourier transform infrared (FT-IR) analysis, which were applied to the fractionated

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aerosol compounds.

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2. EXPERIMENTAL SECTION

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2.1 Experimental materials

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A Mn2+ standard in ZnS and Cr3+ standard in MgO were purchased from Freiberg

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Instruments Inc., Delfter, Germany and used to correct the g-factors and absolute spin

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numbers of the samples. D-(+)-Glucose (GC purity, ≥99.5%), benzoquinone (HPLC purity,

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≥99.5%), 1,4-naphthoquinone (AR purity, ≥97%), phenanthrenequinone (HPLC purity,

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≥99.0%), ferric sulfate (metals-basis purity, ≥99.95%), and copper(II) sulfate pentahydrate

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(metals-basis purity, ≥99.95%) were purchased from Aladdin Reagent Company (Shanghai,

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China). High-purity graphite and graphene oxide (GO) with different oxidation degrees (O/C

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elemental molar ratios of 0.15, 0.25 and 0.39) were purchased from Shandong Jin Cheng

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Graphene Technology Co., Ltd (Beijing, China). Sodium borohydride (purity≥98%) and

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concentrated hydrochloric acid in water (AR purity, 37%) were purchased from Sinopharm

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Chemical Reagent Co., Ltd. (Shanghai, China). A Cleanert C18 cartridge (500 mg/6 mL,

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Agela, China) was used to separate low-polarity, water-soluble organic matter (LP-WSOM)

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from the aqueous extracts of PM2.5 samples.

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2.2 Sample collection

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The PM2.5 samples were collected using a high-volume sampler (XT-1025, Shanghai

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Xintuo, China) set approximately 40 meters above the ground atop Shaw House at the

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Shaanxi University of Science and Technology. Twelve daily samples were collected from

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December 1 to 13, 2017 (December 1, 2 and 4–13, 2017). Each sampling start time was 7:00

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local time (Beijing time), and the sampling duration was 23 h 30 min. Each sample was

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collected on a 20 × 25 cm precombusted quartz glass fiber filter (2500 QAT-UP, Pallflex

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Products Co., USA), and the filters were stored at -20 °C until analysis. Four weekly PM2.5

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samples were also collected on 90-mm-diameter Teflon filters (MS-WPTFE, Beijing Safelab

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Technology Ltd., China) using a low-volume sampler (MH1200-F, Testo, China) operating at

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a flow rate of 100 L/min. The weekly samples were collected from December 1 to 14, 2017,

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which ensured that sufficient samples were available for the structural analysis of

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non-solvent-extractable residues using FT-IR analysis.

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2.3 Sample preparation

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The samples were extracted by rinsing with different solvents. The advantages of the

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solvent rinsing method are its conductivity to the EPR analysis of nonextractable substances

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remaining in the filtration membrane and avoidance of the possible impact of ultrasonic

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extraction on the EPFR content and speciation.28 The extraction device used in this study was

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constructed of stainless steel. A schematic diagram of the extraction set-up and the sample

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preparation procedure is shown in Figure 1. During extraction, an air pump pushes the

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solvents through the filter samples slowly. The solvent-soluble constituents are leached in the

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eluant. The nonextractable residues are retained on the sample filters or PTFE membrane

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filter (0.45 µm, Millipore, U.S.).

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For sample extraction, two filter punches (diameter: 50 mm) per sample were

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superimposed with the sample loading surface and then washed with 30 mL (3 times × 10 mL)

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of different solvents in the following order: water, MeOH, DCM and n-hexane.

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Water-soluble matter (WSM, eluted by water from the filter punches) and water-insoluble

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matter (WISM, eluted by MeOH, DCM and n-hexane from the filter punches) were collected

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in clean glass bottles. The WSM was further processed using Cleanert C18 cartridges

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following a procedure reported elsewhere.22 The LP-WSOM fraction was eluted with 6 mL

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of MeOH from the C18 cartridge, further concentrated to approximately 0.1 mL under N2 and

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prepared for EPR analysis. The WISM was concentrated to approximately 0.1 mL using a

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rotary evaporator (Dragon RE100-Pro, Beijing, China) and prepared for EPR analysis.

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The extracts were transferred to precombusted quartz filters (size: 5 × 28 mm) and then

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dried using a rotary evaporator. The washed filter samples were also dried using a rotary 7 / 34



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evaporator to remove residual solvents. Both the prepared filter samples and original samples

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were analyzed using an MS5000 EPR spectrometer with the previously described quartz

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sheet-based approach.27 Note that the EPFR concentration reported in this paper is based on

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the measurement of solid samples, including PM2.5, and the separated components of PM2.5,

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which should avoid the possible influence of solvents on the EPR determination of EPFRs.

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The added experiment demonstrated that the solvents did not significantly influence the

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EPFR type (g value changed in ± 0.0001) and content (EPR signal intensity changed from

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0% to -11%) during solvent extraction of the PM2.5 samples (see Figure S1).

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2.4 Sample acidification and reduction

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The filter punches (size: 5 × 28 mm) of each sample were acidified and reduced by adding

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40 µL each of 6 M HCl and 10 M NaBH4 solutions to assess the contributions of metallic

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oxide and charge transfer complexes to EPFR formation. Then, the samples were placed in a

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dark environment at room temperature for 1 h to allow for adequate chemical reaction. After

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reaction with the solutions, the samples were dried using a rotary evaporator to remove

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residual water. The dried filter samples were analyzed using an MS5000 EPR spectrometer.

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2.5 Heat treatment of glucose and PM2.5 samples

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Glucose is an organic agent that forms EPFRs during pyrolysis. Forty microliters of a 50

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g/L glucose solution were added to a precombusted quartz filter (size: 5 × 28 mm) and then

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dried using a rotary evaporator in preparation for the heat treatment. Both the prepared filter

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samples and the real samples (December 1, 2 and 4, 2017) were heat treated using a

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thermal-optical carbon analyzer (DRI Model 2015) with the IMPROVE_A temperature

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protocol. The heat-treated samples were prepared for EPR analysis.

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2.6 EPR measurements and data analysis

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The quartz sheet-based EPR measurements and data analysis have been previously

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described in detail.27 The EPR measurement parameters were set as follows: magnetic field,

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330−345 mT; scan time, 180 s; modulation amplitude, 0.2 mT; and microwave power, 8 mW.

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The EPR signal for the samples was baseline corrected, and a Gaussian function was used

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to fit the EPR signal (Figure S2). Then, the characteristic parameters of the EPFRs, such as

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the g-factor, ∆Hp-p and peak height, were obtained. The atmospheric concentrations of

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EPFRs (spins/m3) were calculated as the total spin numbers divided by the total sample

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volume. The concentrations of EPFRs in PM2.5 (spins/g) were determined as the total spin

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numbers divided by the PM2.5 mass collected.

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2.7 Other analyses

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The organic carbon (OC) and elemental carbon (EC) in the original samples and prepared

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samples were quantified on a carbon analyzer (DRI 2015) using the thermal-optical

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reflectance (TOR) method and the IMPROVE_A temperature protocol.24 Metallic elements

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in the PM2.5 samples were quantified using laser ablation (LA, GeoLas Pro, Coherent,

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Germany) combined with inductively coupled plasma mass spectrometry (ICP-MS, Agilent

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7700x, USA). An FT-IR spectrometer (VECTOR-22, Bruker, Germany) was employed to

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identify the chemical functional groups in the nonextractable residues. Amorphous carbon in

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the PM2.5 filter samples was identified by a microscopic Raman imaging spectrometer

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(Thermo Fisher DXRxi, USA). Blank filters were analyzed to correct for background signals

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in the OC/EC, LA-ICP-MS, FT-IR and Raman analyses. Details of the carbon, functional

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group, metal and Raman analyses are given in the Supporting Information S2-S5.

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3. RESULTS AND DISCUSSION

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3.1 Carbonaceous components of the separated fractions

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The concentrations of the carbonaceous components (OC, EC, OC1−4 and EC1−3) in the

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original and separated samples were determined using a thermal-optical method (Figure 2

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and Table S1). The atmospheric concentrations of OC and EC in the original samples were

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8.5−37.0 (mean ± SD: 16.5 ± 6.8) and 2.3−7.9 (mean ± SD: 4.6 ± 1.6) µg m−3, respectively.

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As shown in Figure 2a, the percentages of extractable and nonextractable OC (NE-OC) in the

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total carbon (TC) were 57 ± 9% and 23 ± 8%, respectively, on average. Approximately 29 ±

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10% of the OC could not be extracted by the solvents. We increased the volume of the

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elution solvents 2-fold for three different samples (12/10, 12/11 and 12/13, 2017), although

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the OC eluted by the increased solvent volume did not substantially increase (-1 ± 7%). The

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TC also did not decrease significantly (1 ± 8%), indicating that the remaining carbon material

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was solvent resistant.

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As shown in Figure 2b, the content of OC1−4 in the washed sample was significantly

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reduced while the EC1−3 carbon content was slightly reduced compared with that in the

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original sample. NE-OC was mainly found in OC3 and OC4, indicating that the

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nonextractable organics may have low volatility and high molecular weight. Combustion

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aerosols contain a significant amount of high-molecular-weight substances, such as large

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PAHs, asphaltenes and graphite analogues, which are solvent resistant and may not be

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extracted by the solvents.29,30

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3.2 EPFR characteristics of the separated fractions

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As shown in Figure 3a, the initial concentrations of EPFRs in the studied PM2.5 samples

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were (2.58−5.41) × 1014 spins/m3 in the atmosphere and (2.58−9.17) × 1018 spins/g in the

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studied PM2.5, which corresponded to PM2.5 mass concentrations of 76−154 µg/m3 (Table

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S2).27 The EPFR concentrations in the studied PM2.5 in this study were similar to those in

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total suspended particle (TSP) samples collected from Laibin County in Xuanwei, Yunnan,

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China.13 However, the levels of EPFRs in the PM1 and PM1-2.5 in Beijing are dozens of times

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higher than our reported EPFR concentrations from the PM2.5 samples at the same PM2.5

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pollution level.11 This result shows that the sources of EPFRs in Xi'an may not be the same as

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those in Beijing but may be similar to those in Xuanwei.

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The relative contents of EPFRs in the different fractions are different. As shown in Figures

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3b and 3c, the EPFR signal intensity of the samples before and after solvent extraction did

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not significantly decrease, and most of the EPFRs remained on the filter after extraction,

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averaging 88 ± 10% of that on the original filter (n = 12), which indicated that most of the

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EPFRs could not be extracted. This result is similar to the result of Wang et al., who showed

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that EPR signal intensities decreased only 20−30% in TSP samples after extraction with a

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mixture of n-hexane/acetone (1:1, v/v).13 We used multiple solvents (water, methanol, 11 / 34



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dichloromethane, and n-hexane, with polarity indices of 10.2, 5.1, 3.1, and 0.1, respectively31)

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and multiple extractions (3 for each sample); thus, extractable EPFRs are less likely to remain

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on the filter after extraction. To further confirm this finding, we increased the elution solvent

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volume 2-fold for three different samples (12/10, 12/11 and 12/13, 2017). The results showed

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that increasing the solvent volume did not reduce the residual EPFR content, which decreased

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by only 0.3 ± 3.3% (n = 3). Although the EPFR content in the extracts increased by 10.8 ±

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5.9% (n = 3), the increase in the EPFRs in the extracts was negligible (~0.3%) relative to the

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total EPFRs in the PM2.5 samples. This study also employed Yang's method of EPFR

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extraction in which filter punches were soaked for 3 min in 30 mL of DCM and then steeped

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for 10 h in a dark environment.11 Comparable EPFR signals were not detected from the

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extracts of 12 samples.27 The above results show that the EPFRs in PM2.5 are mainly

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nonextractable substances.

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In the extraction solution samples, the EPFRs were mainly distributed in water-insoluble

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components and the average EPFR content was 2.4 ± 0.7% of that in the original filter

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samples (Figure 3b). The EPR signal of LP-WSOM was very weak, and the average EPFR

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content in this fraction accounted for only 0.2 ± 0.1% of that in the original filter samples. A

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comparable signal was not detected in the high-polarity, water-soluble material (HP-WSM)

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for the samples. In addition, the sum of the EPFR contents in the extracts and the washed

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filter samples was smaller than the EPFR content in the original filter samples. An average of

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9.6 ± 10.0% of the EPFRs was not recovered, which may have been caused by an interaction

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between the EPFRs and soluble ions, such as Fe3+, which dissolve in the solvents during 12 / 34


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extraction. This effect mechanism was partly supported by the result of the additional

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experiment described in section 3.3.

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A g-factor is a characteristic parameter that characterizes the magnetic moment and

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gyromagnetic ratio of an atom, and it can be used to roughly distinguish a few classes of

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EPFRs in complex environmental materials. As shown in Table 1, the g-factor of the EPFRs

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in the original sample ranged from 2.0028 to 2.0033 and averaged 2.0031 ± 0.0001. These

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values are typical of an organic radical that is carbon-centered with a nearby heteroatom.12

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The g-factors of the original samples were not significantly different from those of the

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samples after extraction with the solvents. In contrast, the g-factors of the EPFRs in the

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WISM ranged from 2.0036 to 2.0040 and averaged 2.0038 ± 0.0001, indicating that the

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EPFRs may contain oxygen-centered radicals.12 These g-factors are similar to those of the

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quinone reference materials benzoquinone, 1,4-naphthoquinone and phenanthrenequinone

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(Figure S3), indicating that the EPFRs in the WISM may be sourced from quinones. Reports

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have indicated that atmospheric aerosols are rich in extractive quinones.32-34 However, the

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signal intensity per milligram of three quinone standards was approximately 50 times lower

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than that of WISM (Figure S3); therefore, the EPFRs in the WISM should originate from

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other quinones and may be high-molecular-weight, medium-polarity organic compounds

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containing benzene rings and heteroatoms.24

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The EPFRs of all samples decayed within 3 months (Table 1 and Figure S4). For the

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original sample, three decay modes were observed: fast decay followed by no decay (42% of

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samples, e.g., Figure 4a), fast decay followed by slow decay (50% of samples, see Figure S4), 13 / 34



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and only slow decay (8% of samples, see Figure S4). The τ1/e values corresponding to fast

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decay and slow decay were 42 ± 21 d and 649 ± 238 d, respectively. Although the EPFRs

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decayed, the average attenuation was only 22 ± 5% of the original EPFR content within 3

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months, and all samples were in a mode of slow decay or no decay after 3 months of decay.

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According to the average decay rate, more than half of the EPFRs in the PM2.5 samples will

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be retained after 3 years. The above results show that most of the EPFRs in the samples of

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this study are very long-lived.

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The decay of samples after washing is similar to that of the original filter, and three decay

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modes were observed: only slow decay (42% of samples, Figure 4b), fast decay followed by

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no decay (42% of samples, see Figure S5), and fast decay followed by slow decay (16% of

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samples, see Figure S5). The τ1/e values corresponding to fast decay and slow decay were 81

281

± 54 d and 403 ± 155 d, respectively. The mean decline within 3 months was 19 ± 4%, which

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was similar to that of the original sample. Compared with the decay of the original and

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washed samples, the decay of the WISM was more complicated. Most of the WISM samples

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showed a rapid increase in EPFRs before entering the fast or slow decay mode (75% of

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samples, Figures 4c and S6), and the initial exponential growth rate was 0.03−0.12 d-1. The

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EPFR growth may be attributed to the oxidation of aromatic compounds, such as PAHs, to

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phenols or quinones to form semiquinone-type EPFRs or phenoxyl radical species.13 This

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speculation is supported partly by the g-factors of the EPFRs. The g-factor before and after

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the increase was approximately 2.0039, which is the same as the g-factor of the quinone

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standards (Figure S3). 14 / 34


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The decay of EPFRs should be related to their chemical composition and environmental

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factors. When the EPFRs contained in a sample are formed from volatile compounds, such as

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phenol (phenoxyl radicals can form), the EPFR content will gradually decrease as the

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compounds volatilize or decompose. This speculation is supported by the research of Gehling

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and Dellinger, who found that the decay rate of EPFRs was positively correlated with the

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phenol content.3 In addition, the adsorption of certain substances from the air may also

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reduce the content of EPFRs. Liu et al. found that the toxicity of carbon nanotubes exposed to

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air was reduced, and this result may have been related to the adsorption of organic substances

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from the air.35 Further studies are needed to study the decay mechanism of atmospheric

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EPFRs.

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3.3 Possible source and formation mechanism of EPFRs in PM2.5

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The following conclusions were drawn in section 3.2: The contribution of extractable

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components to the total EPFRs in PM2.5 is insignificant (average: 2.6%), and the EPFRs in

304

atmospheric particulate matter are mainly nonextractable. The nonextractable residues are

305

mainly OC, EC, and trace metal minerals. Two EPFR formation mechanisms can explain this

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result: the participation of metal oxides in the formation of EPFRs and the formation of

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EPFRs from carbonaceous materials. The adsorption of benzene-containing organic

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compounds by metal oxides to form EPFRs has been presumed to be an important

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mechanism of EPFR formation in atmospheric particles.18,36 However, this study suggests

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that the EPFRs in atmospheric particles in Xi’an may not be primarily formed by metal

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oxides but rather may originate from nonextractable organic substances, most likely GO

312

analogues. The evidence and reasons for this supposition will be illustrated.

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The acidification of solvent-washed samples with 6 M concentrated hydrochloric acid

314

reduced the signal intensity of the EPFRs by an average of 24 ± 10% (Figure 2). This

315

reduction may be due to the dissolution of metal oxides under strongly acidic conditions,

316

which destroy the bonding relationship between the metal oxides and the adsorbed organics

317

that form EPFRs.36 However, most of the EPFR signals (76 ± 10%) did not change under

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acidic conditions. Metallic elements, such as Fe, Cu, Al, Zn, and Ni, are commonly found in

319

atmospheric particles, and their oxides have been reported to be reactive materials that form

320

EPFRs.18,36 However, the oxides of these metal elements are all soluble in acidic solution,

321

indicating that metal oxide-based EPFRs may be not major contributors to the EPFRs in

322

PM2.5.

323

In this study, the characteristics of the EPFRs in the PM2.5 samples are obviously different

324

from the characteristics of EPFRs formed by metal oxides, primarily in terms of the g-factor

325

and decay rates. The average g-factor of the PM2.5 samples in this study was approximately

326

2.0031, which is typical of a C-centered free radical.13 According to the literature, the

327

g-factors of EPFRs formed by the adsorption of benzene-containing compounds on metal

328

oxides range from 2.0029 to 2.0067,18,20,36,37 and most are greater than 2.0031; moreover,

329

EPFRs based on metal oxides are considered O-centered radicals, including phenoxyl

330

radicals20,36,37 and semiquinone-type radicals20,37. In addition, the decay of the EPFRs in the

331

PM2.5 samples in this study was very slow, with the half-lives of the EPFRs for most samples 16 / 34


Page 16 of 34

Page 17 of 34


332

greater than 3 years. In contrast, the decay rate of the EPFRs formed by metal oxides is very

333

high, with reported half-lives of a few hours to a few months.18,20,36 Therefore, the EPFRs in

334

PM2.5 are less likely to be primarily metal oxide-derived based on the types of EPFRs

335

identified.

336

A correlation analysis of the initial concentrations of EPFRs with the metals, OC, EC,

337

thermally derived carbon fractions (OC1−4 and EC1−3) and other common pollutants (PM2.5,

338

SO2, NO2, CO and O3) was performed (Table S3). The EPFR concentrations were

339

significantly correlated with the OC, OC3, OC4, and EC1 concentrations (at the 0.01 level

340

(2-tailed)) but poorly correlated with the contents of the 24 metal elements (of which only Sn

341

had a correlation at the 0.05 level (2-tailed)). This result is consistent with the extraction

342

results, which showed that the major carbon components that were nonextractable were OC3,

343

OC4, and EC1 (Figure 2) and the vast majority of EPFRs were nonextractable (Figure 3).

344

Therefore, the EPFRs in the PM2.5 samples may have been primarily derived from the carbon

345

materials in OC3, OC4, and EC1. In addition, our results showed that the EPFR

346

concentration was significantly and positively correlated with the concentrations of EC and

347

NO2 (at the 0.05 level (2-tailed), Table S3); thus, traffic emissions may be the important

348

source of the EPFRs in the PM2.5 over Xi’an. Note that other sources may also have

349

contributed to the EPFRs in the studied aerosols, such as biomass11 and plastic combustion

350

emissions38, and the EPFRs may have formed from atmospheric chemical processes, e.g.,

351

secondary organic aerosol formation39 and PAH photodecomposition13. In another study, we

352

assessed the contributions to the PM2.5 in Xi'an based on a full year of observational data 17 / 34



353

collected in 2017 and discussed in detail the primary sources of EPFRs, including coal

354

combustion, biomass combustion and vehicle emissions, as well as the secondary sources,

355

including ozone-involved aerosol ageing processes and photochemical processes (Chen et al.,

356

Submitted manuscript).

357

To demonstrate that pure organic matter rather than metal oxides or EC can produce

358

EPFRs, glucose was subjected to heat treatment under inert gas and different temperature

359

conditions to produce EPFRs.40 As shown in Figure 5c, glucose did not undergo coking at

360

room temperature and 140 °C (OC1 stage, Figure S7), and no EPFRs were generated. At

361

280 °C, glucose started coking (OC2 stage, Figure S7), and the EPFR signal appeared with a

362

g-factor of 2.0034, which is close to that of the PM2.5 samples (2.0031). Pronounced coking

363

occurred at 480 °C (OC3 stage, Figure S7), and the EPFR signal intensity reached a

364

maximum. This result illustrates that the formation of EPFRs may not require the

365

participation of metal elements. When temperatures continued to increase to 580 °C, the

366

EPFR content was greatly reduced (OC1234, Figures 5c and S7), thus prolonging the coking

367

time of OC4, and the EPFR signal then disappeared completely (result not shown). Because

368

the final coking product is EC, pure EC is not a source of EPFRs. Note that the production of

369

EPFRs via glucose pyrolysis was also reproduced in an atmospheric PM2.5 sample as shown

370

in Figure 5a. Therefore, the EPFRs in PM2.5 are likely primarily formed by

371

solvent-nonextractable OC3 and OC4 organic substances and not metal oxides or EC.

372

We assume that the OC3 and OC4 organic compounds that likely produce EPFRs are GO

373

analogues with a benzene ring structure and heteroatom functional groups. The content of 18 / 34


Page 18 of 34

Page 19 of 34


374

EPFRs was analyzed in graphene and GO with different oxidation degrees to elucidate the

375

chemical structures required for the formation of EPFRs in PM2.5. As shown in Figure 5b,

376

graphite (G, typical pure EC material) did not produce an EPR signal, which supports the

377

previous conclusion that pure EC is unlikely to produce EPFRs. GO produced a significant

378

EPFR signal, which should be assigned to the single electrons induced by the surface oxygen

379

functional groups.41 The g-factor of GO is 2.0033, which is close to the g-factor of the

380

atmospheric samples (2.0031), and GO exhibited no decay characteristics. This result

381

supports the hypothesis that the EPFRs in atmospheric PM2.5 may be derived from GO

382

analogues. Soot particles are ubiquitous in the atmosphere and rich in graphite structures.42-44

383

The graphite structures present in the studied PM2.5 were confirmed using Raman

384

spectroscopy (Peaks 2 and 4 in Figure S8).45 The signal intensity ratio of the G-band at ~1570

385

cm-1 (graphite carbon, SP2 hybridization) to the D-band at ~1354 cm-1 (diamond carbon, SP3

386

hybridization) calculated based on the average FT-Raman spectra was ID/IG = 0.71, which is

387

similar to that of GO with a low degree of oxidation (Figure S9), suggesting that graphite

388

structures may be present in the studied PM2.5 and a certain degree of disorder occurs in the

389

carbon-based structures. The black carbon present in atmospheric particles contains not only

390

carbon atoms but also oxygen atoms, nitrogen atoms, and other heteroatoms.46 The FT-IR

391

analysis showed that the nonextractable residues were rich in benzene ring structures

392

(benzene ring C=C) and heteroatom functional groups (strong signal for −C−OH or

393

−C−O−C−, may be part of −C=O and −CN, etc.) (Figure S10). These functional groups exist

394

on the surface of graphene-like carbon structures and lead to disorder and defects in 19 / 34



395

carbon-based structures, thus contributing to the formation of EPFRs in PM2.5.41,47-49 As the

396

O/C ratio increases, the EPFR content of GO decreases gradually (Figure 5b), which

397

indicates that too many oxygen-containing functional groups are not conducive to the

398

production of EPFRs. When the O/C ratio increases, the SP2 hybrid planar structure is

399

destroyed, thereby resulting in decreased aromaticity, and the number of single electrons also

400

decreases, which increases the difficulty of EPFR formation. In the thermal-optical analysis,

401

the OC3 and OC4 organics were coked during the two heating stages, indicating that OC3

402

and OC4 organics have low volatility, large molecular weights, and features rich in

403

oxygen-containing functional groups. In summary, the above results indicate that the majority

404

of EPFRs in PM2.5 were derived from OC3 and OC4 organic compounds that are likely GO

405

analogues.

406

Different oxygen-containing functional groups may contribute by different degrees to the

407

production of atmospheric EPFRs. When NaBH4 was used to reduce the atmospheric samples,

408

the EPFR signal increased by 14% (n = 12, Figure S11). This result suggests that phenolic

409

(rather than quinonic or ketonic) functional groups may play an important role in the

410

formation of EPFRs in PM2.5. This experiment also ruled out the possibility that charge

411

transfer is involved in the formation of EPFRs because electron acceptors, such as C=O

412

bonds, are reduced by NaBH4, thus preventing electron transfer between the electron acceptor

413

(C=O) and electron donor (C−OH) functional groups.50

414

This paper also explored the effect of metal ions on EPFR formation. During glucose

415

coking to produce EPFRs, metallic Cu2+ promoted (~30% increase) EPFR formation (Figure 20 / 34


Page 20 of 34

Page 21 of 34


416

S12b). The spectra of the coking sample with added Fe3+ contained a signal with a very wide

417

half-width, which is different from the signal in the spectrum of coked glucose alone;

418

however, this effect is not clear. Previous studies have shown that the formation of EPFRs

419

from the oxidative pyrolysis of 1-methylnaphthalene can be promoted by the addition of

420

Fe2O3 nanoparticles in a reaction system that contains Fe2O3 nanoparticles and gas organic

421

molecules in a flow reaction tube and an oxygen atmosphere;51,52 however, in this study, the

422

pyrolysis of the mixture of glucose and Fe2(SO4)3 solutions occurred in an oxygen-free

423

atmosphere. Soluble metal ions had an inhibitory effect on EPFR formation. After GO was

424

mixed with Cu2+ or Fe3+, the intensity of the EPFR signal decreased by approximately

425

10-fold (Figure S13). The metal ions may bind to a single-electron site on the surface of GO

426

via coordination such that a single electron chelated in the empty d orbital of the metal ion

427

stabilizes the free electrons and causes some of the EPFRs to disappear. This mechanism can

428

partly explain the disappearance of certain EPFRs during the extraction of the atmospheric

429

samples as explained in section 3.2 (Figure 3b).

430

4. Environmental Implications

431

In this study, we demonstrated that the EPFRs in PM2.5 are mainly derived from

432

solvent-resistant organic matter. This finding furthers our understanding of the possible

433

sources and formation mechanisms of EPFRs in atmospheric particulates and the potential

434

environmental impacts of these components. This study showed that atmospheric EPFRs are

435

not primarily formed from metal oxides but rather from nonextractable organics and

21 / 34



436

suggested that previous studies may have missed the major proportion of EPFRs in

437

atmospheric particulates if they only focused on solvent-extractable or metallic oxide-formed

438

EPFRs. A better understanding of the sources, formation mechanisms, and potential

439

environmental impacts of atmospheric EPFRs may require a characterization of the

440

nonextractable organic matter in atmospheric particulates. Our results suggested that

441

incomplete combustion of vehicle emissions may represent an important source of the EPFRs

442

in the studied samples. This finding provides basic evidence for the development of effective

443

air pollution control measures in Xi’an.

444

EPFRs were detected mainly in the non-solvent-extractable organic matter of atmospheric

445

particulates; thus, EPFRs may play a catalytic role in the formation of ROS on the surface of

446

particulate nuclei and thus participate in multiphase chemical reactions occurring inside

447

particulates.6-8 We also found that the EPFR contents in the PM2.5 samples significantly

448

increased after excitation by visible light (data reported in detail elsewhere27), indicating that

449

visible light can induce certain substances in aerosol samples to produce EPFRs and thus

450

accelerate the photochemical aging of atmospheric particulates; for example, photochemical

451

processes can affect the initial radical concentration and fast decay rate.3 Further studies may

452

be needed to explore the role of EPFRs in heterogeneous reactions of atmospheric aerosols.

453

Acknowledgments

454 455

This work was supported by the National Natural Science Foundation of China (grant number: 41703102).

22 / 34


Page 22 of 34

Page 23 of 34

456


Supporting information

457

Additional details, including the acronym definitions, the methods of performing the carbon,

458

chemical functional group, and metal concentration analyses and Raman analyses of the

459

PM2.5 samples and the standards comparison for EPFR quantification are explained. Figures

460

include the influence of solvents on the EPR determination of EPFRs, the EPR spectra of

461

reference quinones, temporal variations in the EPFR content in the original and washed PM2.5

462

samples and WISM extract, reflected light intensity changes during the heating of glucose,

463

Raman spectra of PM2.5 samples, graphite and GO, FT-IR spectra of nonextractable residues,

464

EPFR contents before and after sample reduction by NaBH4, EPR spectra of the EPFRs

465

produced after heat treatment of glucose and in graphite and GO, and contents of metal

466

elements in the PM2.5 samples. Tables showing the concentrations of conventional

467

atmospheric pollutants; the characteristics and concentrations of EPFRs in the PM2.5 samples;

468

the correlations of the EPFR concentrations with the metal, OC, EC, thermally derived

469

carbon fractions and other common pollutants; the concentrations of metallic elements in the

470

PM2.5 samples; and the saturation curve of Cr3+ standard are also included.

471

References

472 473 474 475 476 477 478 479 480

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632 633

Figure 1. Schematic diagram of the sample preparation for the EPFR study.

29 / 34



634 635

Figure 2. Relative contents of carbonaceous components in the original and separated

636

samples were determined using a thermal-optical method (n = 12). Washed samples represent

637

the sample remaining after washing the original particles. The bars indicate standard

638

deviations.

30 / 34


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639 640

Figure 3. Atmospheric concentrations of EPFRs (a), average relative contents of EPFRs (b),

641

and an example (2017/12/13) EPR spectra (c) for the fractionated and processed samples.

642

Washed samples represents the remaining sample after washing the original particles. The

643

bars indicate standard deviations.

644

31 / 34



645 646

Figure 4. Typical EPFR decay type of (a) fast decay followed by no decay for an original

647

filter sample (2017/12/05), (b) only slow decay for a washed sample (2017/12/06), and (c)

648

fast growth followed by fast and then slow decay for an extractable WISM sample

649

(2017/12/01). The r (d-1) value present in panel c represents the exponential growth rate of

650

EPFRs content. The bars indicate standard deviations.

651

32 / 34


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Page 33 of 34


652 653

Figure 5. Average EPR spectra of the (a) EPFRs produced in each stage of pyrolysis of PM2.5

654

samples (2017/12/01, 2017/12/02 and 2017/12/04), (b) graphite (G) and GO with different

655

oxidation degrees (LGO: O/C = 0.15, MGO: O/C = 0.25, HGO: O/C = 0.39), and (c) EPFRs

656

generated in each pyrolysis stage of glucose (n = 3).

657 658

33 / 34



659

Table 1. Characteristics of the EPFRs and decay types in the PM2.5 samples (n = 12). g

Type of sample

Range

τ1/e

∆Hp-p Mean± SD

Range

τ1/e

Fast decay (d)

Mean± SD

Range

a

Mean± SD

No

Slow decay (d) Range

b

decay

Mean±

(% of

SD

samples)

Original

2.0028−

2.0031±

5.0−

5.4±

19−

42±

292−

649±

samples

2.0033

0.0001

5.8

0.19

90

21

1000

238

Washed

2.0027−

2.0030±

5.2−

5.7±

35−

81±

96−

403±

samples

2.0032

0.0001

6.2

0.31

168

54

694

155

Acidified

2.0029−

2.0031±

4.3−

4.8±

samples

2.0033

0.0001

5.3

0.22

---

---

---

---

2.0036−

2.0038±

5.8−

6.6±

7−

42±

102−

377±

42% c

2.0040

0.0001

7.4

0.49

91

30

685

201

(75%) d

WISM

660 661 662 663 664 665 666

Page 34 of 34

42% c 33% c

a

Fast decay—the τ1/e of the samples was less than 100 d in the first few days after collection;

b

Slow decay—the τ1/e of the samples was more than 100 d and less than 1000 d for decay patterns of only

slow decay and of slow decay after fast decay; c

No decay—the τ1/e of the samples was more than 1000 d for a decay pattern of fast decay followed by no

decay; d

No decay—the EPR signal intensity of the samples increased instead of decreasing in the first few days

after sample preparation.

34 / 34


Dominant fraction of EPFRs from non-solvent-extractable organic

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