Molecular characterization of water-soluble humic-like substances in

Jan 31, 2018 - Water-soluble humic-like substances (HULIS) in smoke particles emitted from combustion of biomass materials and coal were characterized...
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Molecular characterization of water-soluble humic-like substances in smoke particles emitted from combustion of biomass materials and coal using ultrahigh-resolution electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry Jianzhong Song, Meiju Li, Bin Jiang, Siye Wei, Xingjun Fan, and Ping'an Peng Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b06126 • Publication Date (Web): 31 Jan 2018 Downloaded from http://pubs.acs.org on January 31, 2018

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Environmental Science & Technology

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Molecular characterization of water-soluble humic-like substances in smoke

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particles emitted from combustion of biomass materials and coal using

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ultrahigh-resolution electrospray ionization Fourier transform ion cyclotron

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resonance mass spectrometry

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Jianzhong Songa,*, Meiju Lia,c, Bin Jianga, Siye Weia,c, Xingjun Fana,b, Ping’an Penga

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a

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Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, P. R. China

State Key Laboratory of Organic Geochemistry, Guangzhou Institute of

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b

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Anhui 233100, P. R. China

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c

College of Resource and Environment, Anhui Science and Technology University,

Graduate School of Chinese Academy of Sciences, Beijing 100049, P. R. China

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*Corresponding author: Jianzhong Song

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Tel.: (+86) 20 85291312

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Fax: (+86) 20 85290706

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E-mail: [email protected]

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ABSTRACT ART

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ABSTRACT

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Water-soluble humic-like substances (HULIS) in smoke particles emitted from

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combustion of biomass materials and coal were characterized by ultrahigh-resolution

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Fourier transform ion cyclotron resonance mass spectrometry. The formulae identified

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were classified into four main groups: CHO, CHON, CHOS, and CHONS. The

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average H/C and O/C ratios are 1.13-1.33, 1.01-1.13, 1.26-1.48, 1.09-1.24 and

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0.21-0.41, 0.27-0.45, 0.41-0.46, 0.44-0.61 for CHO, CHON, CHOS, and CHONS

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groups, respectively. CHO compound was the predominant component (43%–72%) of

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the smoke HULIS from biomass burning (BB) and coal combustion, followed by the

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CHON group for BB-smoke HULIS and the S-containing groups (i.e., CHOS and

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CHONS) for coal-smoke HULIS. These results indicate that the primary HULIS

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emitted from biomass burning contain a high abundance of CHON species, which

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appear to be made up mainly of oxidized nitrogen functional groups such as nitro

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compounds and/or organonitrates. The coal-smoke HULIS contained more

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compounds with relatively low molecular weight and high aromaticity index (AImod).

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They were significantly enriched in S-containing compounds with high DBE (≥4) and

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O/S ratios suggest that they are most likely made up of aromatic organosulfates and

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nitrooxy organosulfates that are usually found in polluted atmospheres. These findings

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imply that the primary emissions from combustion of biomass and coal fuels are

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potential sources of water-soluble HULIS in an atmospheric matrix and that coal

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combustion is an especially important source of sulfate compounds.

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INTRODUCTION

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Water-soluble humic-like substances (HULIS) are a group of unresolved

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polyacidic compounds identified ubiquitous in aerosol particles sampled in urban,

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rural, and marine environments and in rain, fog, and cloud water samples.1-10 They are

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termed because they have many physical and chemical properties similar to natural

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humic substances in terrestrial and aqueous environments.5,

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fraction of light-absorbing organic carbon (i.e., brown carbon), HULIS can alter the

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light absorption and radiative forcing of aerosols.12-15 HULIS are also surface-active

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and can influence the surface tension and growth of cloud condensation nuclei, thus

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playing an important role in climate and the atmospheric environment.6, 16, 17

6, 11

As an important

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Studies have suggested that atmospheric HULIS are derived from various sources,

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including primary emissions from biomass burning (BB),1, 2, 9, 18 coal combustion,9, 19

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vehicular emissions,20 and secondary sources such as photo-chemical transformation

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of volatile organic compounds1, 21 and oxidation of soot particles.22, 23 Among the

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various sources listed above, BB is generally considered to be one of the largest

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sources of atmospheric HULIS.2, 6, 24, 25 In addition, coal combustion is also reported

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as a primary source of HULIS in the atmosphere.9 Recent studies have investigated

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the chemical and light-absorption properties of HULIS in smoke particles emitted

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from biomass fuel and coal burning in a laboratory combustion chamber.9,

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However, these studies focused mainly on an overall description of primary HULIS

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and revealed less structural information at the molecular level due to the highly

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complex composition of these substances.

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Recently, electrospray ionization (ESI) coupled with ultrahigh-resolution mass

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spectrometry, for example, Fourier-transform ion cyclotron resonance mass

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spectrometry (FT-ICR MS), has been used for successful characterization of

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water-soluble organic compounds and HULIS in cloud water,26 rain water,27-29 BB

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aerosols,30,

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resolution and mass accuracy, ultrahigh-resolution mass spectrometry has enabled

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detailed characterization of organic materials at the molecular level.3, 4, 29, 31, 34, 35

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and atmospheric aerosols.3,

4, 32-34

Because of its extremely high

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In the present study, FT-ICR MS was operated in negative ESI mode to analyze

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the water-soluble HULIS fractions isolated from smoke particles emitted from

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burning of biomass materials (including rice straw, corn straw, and pine branches) and

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coal. Rice straw and corn straw are the dominant crop residues in China, and pine

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branches are an important biomass cooking fuel in rural areas. Combustion of these

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biomass residues is reported to have important influences on aerosols in China,36, 37

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and therefore these three biomass materials were used to study BB-derived HULIS.

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Coal was chosen because coal combustion is also an important source of atmospheric

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aerosols in China.19, 38,

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coal-smoke particles by FT-ICR MS. The objective was to obtain elemental

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composition and structural information for water-soluble primary HULIS emitted

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from combustion of biomass materials and coal at the molecular level and to provide

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some novel insights into the characteristics of HULIS emitted from specific sources.

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This is the first detailed analysis of primary HULIS in

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MATERIALS AND METHODS

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Sample collection and HULIS isolation. In this study, four smoke particle

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samples were subjected to FT-ICR MS analysis. These samples were collected from

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combustion of biomass materials and fossil fuels in a laboratory resuspension

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chamber.9 They included three BB-smoke particle samples (rice straw, corn straw, and

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pine branches) and one coal-smoke particle sample. Details of these samples were

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described in a previous paper by the authors and the supporting information.9

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The smoke particles were extracted with 40 mL of Milli-Q water in an ultrasonic

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bath for 20 min, followed by filtration to remove insoluble suspensions. Then the

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extract was acidified to pH 2 with HCl and loaded onto a preconditioned solid-phase

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extraction (SPE) cartridge (Oasis HLB, Waters, Milford, USA). The most hydrophilic

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compounds such as inorganic ions, and low-molecular-weight organic molecules such

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as organic acids and sugars were removed by the cartridge, whereas the relatively

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hydrophobic fraction, generally considered as HULIS, was retained. Finally, the

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retained organics were eluted with methanol and evaporated to dryness under a gentle

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N2 stream. This isolation protocol for water-soluble HULIS has also been used by

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many other research groups.2-4, 17, 25, 33 Details of the isolation method were described

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in a previous paper by the authors.8, 9, 40

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Ultrahigh-resolution electrospray ionization FT-ICR MS. The isolated HULIS

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fractions were analyzed with a solariX XR FT-ICR MS (Bruker Daltonik GmbH,

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Bremen, Germany) equipped with a 9.4 T refrigerated actively shielded

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superconducting magnet (Bruker Biospin, Wissembourg, France) and a Paracell

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analyzer cell. The samples were ionized in negative ion mode using an ESI ion source

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(Bruker Daltonik GmbH, Bremen, Germany). The detection mass range was set to

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m/z 150–1000. Ion accumulation time was set to 0.65 s. A total of 100 continuous 4M

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data FT-ICR transients were added to enhance the signal-to-noise ratio and the

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dynamic range. Field blank filters were processed and analyzed following the same

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procedure to detect possible contamination. The mass spectra were calibrated

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externally with arginine clusters in negative ion mode using a linear calibration. The

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final spectrum was internally recalibrated with typical O5-class species peaks using

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quadratic calibration in DataAnalysis 4.4 (Bruker Daltonics). A typical mass-resolving

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power (m/∆m50%, in which ∆m50% is the magnitude of the mass spectral peak full

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width at half-maximum peak height) >450,000 at m/z 319 with AImod,w (CHO) > AImod,w (CHONS) > AImod,w

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(CHOS). In addition, the AImod,w values of each compound group in coal-smoke

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HULIS were all significantly higher than for BB-smoke HULIS. These results

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suggested that these four compound groups have different aromaticities and that coal

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HULIS are characterized by relatively high aromaticity. The DBE/C ratio is also

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widely used to estimate the density of double bonds and the aromaticity of organic

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matter in natural environments.4, 46 In this study, the compound groups in coal-smoke

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HULIS all presented relatively higher DBE/Cw values than those compound groups in

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the three BB-smoke HULIS, again indicating the relatively high aromaticity of

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coal-smoke HULIS.

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The van Krevelen (VK) diagram, which plots the H/C ratio as a function of the

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O/C ratio for individual molecular formulae, has been widely used to illustrate the

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chemical properties of organic matter.4, 47 In this study, the four primary HULIS had

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similar VK patterns. Six composition domains, including lipids, proteins, lignins,

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carbohydrates, condensed aromatic structures, and condensed hydrocarbons, were

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identified in every HULIS sample (Figure S3).4, 28, 32, 46 Most of the compounds had

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O/C ratios < 1.0 and H/C ratios in the 0.4 to 2.0 range, which are similar to those

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obtained by FT-ICR MS for water-soluble organic compounds in cloud water and

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aerosols.4, 26, 27, 34 However, for different compound groups, the distribution pattern

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presented some differences (Figure S3). The CHO and CHON groups were present in

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more formulae in the VK diagrams of BB-smoke HULIS, whereas CHOS and

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CHONS compounds were the significant components in coal-smoke HULIS. In

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addition, most of the CHONS compounds in coal-smoke HULIS were observed at

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relatively lower O/C ratios than in BB-smoke HULIS. These observations suggest that

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HULIS derived from different sources have their own distinct elemental

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characteristics.

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CHO compounds. CHO compounds, which probably include carboxyl and

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hydroxyl functional groups, have been widely identified in water-soluble organic

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compounds in cloud water, rain water, and organic aerosols.4, 26 As shown in Table S2,

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a total of 1514, 1733, 2296, and 918 ions could be assigned to CHO groups in the

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smoke HULIS samples emitted from combustion of corn straw, rice straw, pine

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branches, and coal, which accounted for 66%, 53%, 72%, and 43% of the overall

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compounds in each HULIS. The average O/Cw and H/Cw ratios of CHO compounds

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were 0.21–0.41 and 1.13–1.33 for the four smoke HULIS (Table S2). It is obvious

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that the HULIS data in this study are similar to those for water-soluble organic

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compounds in atmospheric aerosols,4, 34 which may indicate that the primary smoke

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HULIS samples are similar to those in atmospheric aerosols.

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As shown in Figure 3, the DBE of the four HULIS samples in the current study

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displayed similar changes against C number. A wide range of DBE values (0–20) was

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observed in the CHO compounds, with a clear trend towards increasing DBE values

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with increasing carbon content. As shown in Figure 3, the high-intensity CHO

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compounds in corn HULIS were mainly detected with C numbers from 8 to 18, such

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as C9H14O4, C18H28O8, C11H20O5, C18H22O7, C9H12O4, C16H32O2, and C16H20O6, which

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had DBE values of 3, 5, 2, 8, 4, 1, and 7 respectively. The formulae (a, b) with high

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intensity denoted in Figure 3 and their possible structures are also shown. These

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probably represent typical biogenic methoxyphenols, and the formula b (C18H22O7)

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may be dimers of formula a (C9H12O4). According to a previous study,48 the formulae

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(C9H14O4, C18H28O8) may be derivatives of limonene, and the latter ones may be

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dimers of the former. In addition, formula (c) (C16H32O2) may be due to palmitic acid,

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which is mainly derived from emissions from vegetation49-51 and anthropogenic

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sources such as coal burning52. This compound was also identified in the other three

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HULIS samples. The high-intensity CHO compounds in rice HULIS were also mainly

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distributed at C numbers from 8 to 18. Some of the high-intensity compounds

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identified were C15H18O8, C9H10O4, C12H14O5, C13H14O5, C12H12O5, and C13H16O6,

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which had DBE values of 7, 5, 6, 7, 7, and 6 respectively. These probably are typical

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biogenic derivatives53-55; some possible structures are denoted as d, e, and f in Figure

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3. Compared to the two BB-smoke HULIS formed from grass straw burning, the

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pine-smoke HULIS contained relatively high amounts of CHO compounds in high C

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number ranges (approximately > 30), resulting in a relatively lower O/C ratio (as

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shown in Table S2). Several high-intensity CHO formulae detected in pine HULIS

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include C20H28O2, C20H26O3, C20H28O3, C20H30O2, and C20H26O2, which have DBE

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values of 7, 8, 7, 6, and 8 and relatively low O numbers. These compounds are

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probably diterpenoid derivatives emitted from combustion of conifer plants such as

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pine51, 56. Some possible molecular structures, including 7-oxodehydroabietic acid

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(C20H26O3, g), dehydroabietic acid (C20H28O2, h), and pimaric acid (C20H30O2, i), are

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illustrated in Figure 3. Unlike the three BB-smoke HULIS, a narrow leaf-shaped

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pattern was observed for coal-smoke HULIS. The DBE ranges of each C number for

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coal-smoke HULIS were smaller than those for the three BB-smoke HULIS. The

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high-intensity CHO compounds detected in coal HULIS include C8H6O4, C13H18O4,

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C12H16O4, and C14H20O4, which have DBE values of 6, 5, 5, and 5 respectively. The

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most likely structure is one benzene ring substituted with O-containing groups such as

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hydroxyl, methoxyl, and carboxyl.

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CHON compounds. Large amounts of organic nitrogen compounds were

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observed in the four HULIS samples, accounting for 18%–41% of total identified

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formulae in the three BB-smoke HULIS, but only 9% of the formulae identified in

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coal-smoke HULIS (Figure 1). These CHON compounds were classified into 25

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subgroups according to their N and O numbers, including OxN1 (O2N1–O13N1) and

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OxN2 (O2N2–O9N2) groups. Some OxN2 compounds may be dimers of OxN1. The

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sums of peak intensities for each subgroup were added together and are listed in

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Figure S4. The four HULIS all had high contents of OxN1 compounds and low

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contents of OxN2 compounds. The major OxN1 compounds of the three BB-smoke

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HULIS covered relatively large ranges from O4N1 to O9N1, whereas the major OxN1

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compounds of coal-smoke HULIS were distributed in relatively small ranges from

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O3N1 to O6N1. In addition, the relative abundances of OxN2 compounds in coal-smoke

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HULIS were much lower than those in BB-smoke HULIS, suggesting that more

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OxN-containing compounds with high N content are contained in BB-smoke HULIS.

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In this study, all the high-relative-abundance CHON subclasses were rich in

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oxygen with respect to nitrogen, as shown in Figure S5; almost all these classes

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showed oxygen-to-nitrogen ratios (O/N) ≥ 3. Hence, the CHON compounds in the

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current study appear to contain a large number of oxidized nitrogen functional groups

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such as nitro compounds (-NO2) and/or organonitrates (with the functional group of

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-NO3). In addition, the excess of O atoms in the CHON compounds suggests that

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these compounds also possess other oxygenated functional groups. As shown in Table

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S2, the average O/Nw ratios (5.66–5.99) of BB-smoke HULIS are greater than that

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(4.15) of coal-smoke HULIS. Moreover, the CHON groups with high O/Nw ratio (≥ 3)

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account for 85.8%–96.3% of overall compounds for BB-smoke HULIS, which is

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significantly higher than 76.7% for coal-smoke HULIS. These results together suggest

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that the CHON compounds in the three BB-smoke HULIS likely exhibit a higher

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degree of oxidation, whereas the coal-smoke HULIS may feature a relatively low

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degree of oxidation and some reduced N functional groups. According to previous

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studies, these reduced nitrogen compounds in smoke particles emitted from

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combustion processes may be associated with alkyl amides and alkyl nitrile31, 57 as

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well as heterocyclic aromatic compounds with a single N atom.4, 31

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The CHON compounds span a wide DBE range (3–20), indicating a high

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prevalence of double bonds and/or ring structures (Figure 4). It is obvious that the

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most abundant CHON species exhibit DBE 5–9, which may indicate that CHON

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species in primary HULIS contain mainly mono- and di-nitro substituted phenols and

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benzoic acids. However, each HULIS CHON group has its own distinct properties. As

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shown in Figure 4, several high-intensity formulae were detected, such as C6H5O5N1,

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C7H7O4N1,

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C13H12O8N2, C14H13O3N1, C15H15O3N1, C9H5O4N1, and C13H17O3N1, which have DBE

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values of 5, 5, 5, 7, 7, 7, 9, 9, 9, 9, 8, and 6 respectively. Figure 4 shows some possible

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chemical structures. Compound (a) in Figure 4 was identified mainly in the three

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BB-smoke HULIS, but not in coal-smoke HULIS. In addition, some other biogenic

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compounds such as C15H19N1O8 (b) are present with relatively high intensity in Figure

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4.

C8H9O4N1,

C15H19O8N1,

C16H21O8N1,

C17H23O8N1,

C12H10O8N2,

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The CHON compounds observed in the three BB-smoke HULIS had relatively

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low AImod,w values (0.44–0.46) compared to coal-smoke HULIS. The CHON fraction

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with high AImod,w (≥0.5) accounted for 37.2%–43.4% of total CHON in the three

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BB-smoke HULIS, which was also lower than the percentage (67.4%) in coal-smoke

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HULIS. In addition, the BB-smoke HULIS had relatively higher O/Cw and O/Nw

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ratios than coal-smoke HULIS, as indicated in Table S2. These differences suggest

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that the CHON compounds in BB-smoke HULIS are mainly made up of molecules

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with low aromaticity and high degree of oxidation, whereas larger quantities of high

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aromatic compounds with a lower degree of oxidation are contained in coal-smoke

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HULIS.

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Sulfur-containing compounds. In the four HULIS samples, hundreds of ions

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were assigned formulae containing sulfur atoms, accounting for 6%–48% of the total

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number of assigned formulae (Figure 1). All the sulfur-containing compounds

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contained only one sulfur atom in each formula. These sulfur-containing compounds

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had been detected in water-soluble organic compounds derived from biomass burning

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and formed secondarily by reactions between the oxidation products of VOCs and

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acidified sulfate seed particles or sulfuric acid.3, 48, 58-60 According to whether nitrogen

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atoms were present in the formula, these sulfur-containing compounds were classified

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into two subgroups, denoted as CHOS and CHONS. The average OM/OC ratios of

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these compounds were much higher than those in the other categories, which is

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consistent with the addition of sulfur atoms onto the molecule. Among these

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sulfur-containing compounds, more than 91% of the CHOS formulae had O/S ratios

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greater than 4, and more than 31% of the CHONS formulae had O/S ratios greater

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than 7. Because a sulfate group (OSO3H) carries four oxygen atoms and readily

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deprotonates in ESI, the sulfur-containing compounds are more likely organosulfates.

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Sulfonates with other O-containing functional groups such as hydroxyl or carbonyl

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groups would also be a possibility in each molecule.

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As shown in Figure 1, the relative contributions of CHOS and CHONS to the

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total number of compounds identified were all very low (2%–6%) for the three

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BB-smoke HULIS. Moreover, all identified formulae in the CHOS group were less

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abundant than background pollution (C17H27O3S1, C18H29O3S1, and C19H31O3S1,

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corresponding to alkane sulfonates that are known to exist widely in the atmospheric

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environment) during FT-ICR MS measurement (Figure S6)27,

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abundances of sulfur-containing compounds identified in coal-smoke HULIS were

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significantly higher, with 36% for CHOS and 12% for CHONS respectively.

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Therefore, only the CHOS and CHONS groups in coal-smoke HULIS were further

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analyzed and discussed in the current paper.

. However, the

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CHOS compounds. In this study, CHOS compounds accounted for 36% of the

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total number of compounds identified in coal-smoke HULIS, indicating that coal

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combustion may be an important source of OS species in the atmospheric

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environment. As shown in Table S2, the average AImod,w value of CHOS compounds

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was 0.31 for coal-smoke HULIS, which is significantly lower than the 0.45−0.56

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obtained for the CHO and CHON groups. Moreover, the average H/C ratios for the

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CHOS compounds were also higher than the corresponding elemental ratios of the

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CHO and CHON compounds. These data together indicated that the CHOS

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compounds have a lower degree of unsaturation. Similarly, the DBEw values of CHOS

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species were also lower than those of CHO and CHON in the same HULIS sample.

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Figure 5 shows the DBE, C, and O atomic distributions in the CHOS compounds.

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For coal-smoke HULIS, the identified CHOS formulae were O3S1–O10S1 class species.

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The most abundant CHOS species class identified in the coal-smoke HULIS had 4–6

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O atoms, with O5S1 being the most abundant (Figure 5). It is obvious that the

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high-relative-abundance CHOS species in coal-smoke HULIS are characterized by

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relatively low O contents. Among these CHOS compounds, the chemical formula

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(C9H12O4S1) denoted (a) is likely consistent with an alkylbenzene ring substituted

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with one sulfate group (Figure 5). The chemical formula (C8H10O5S1) denoted (b)

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could be an alkylbenzene ring substituted with one sulfate and one hydroxyl group.

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Note that most of the intense CHOS compounds in coal-smoke HULIS have DBE

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values equal to or greater than 4, with four or five O atoms. Within these chemical

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formulae, many aromatic organosulfate isomers with relatively high DBE can be

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predicted to be made up of benzene backbones with sulfates attached to side chains or

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to the aromatic ring, which were found and identified in laboratory studies.62 The

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results reported here show that these compounds may be produced from thermal

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reactions of benzenes with SO2 (produced by sulfur in coal reacting with oxygen)

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during coal combustion.

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CHONS compounds. CHONS compounds were identified and accounted for 12%

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of overall compounds in coal-smoke HULIS. In these CHONS compounds, 31.6% of

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CHONS formulae had seven or more O atoms, implying that some nitrogen atoms

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may exist in the form of -NO3 groups and that these CHONS compounds are probably

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nitrooxy-organosulfates (nitrooxy-OS). These results also suggest that other

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O-containing functional groups existed in each formula as well. However,

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nitrooxy-OS derived from biogenic VOCs have been demonstrated to form by

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photooxidation of biogenic VOCs in smog-chamber experiments conducted under

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high nitrogen oxide (NOx) concentrations in previous studies.3, 48 The data in this

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study indicated that coal-combustion emissions are also important sources of these

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classes of compound. Note that 68.4% of the CHONS formulae have a relatively

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small number of O atoms (