Molecular characterization of water-soluble humic-like substances in

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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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ACS Paragon Plus Environment


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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

37

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

55

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.


14, 25

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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

314

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

315

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,

322

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

325

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

327

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

328

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

329

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

330

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

333

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

334

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

335

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

336

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

337

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

338

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

339

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

340

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

341

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

342

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

343

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

344

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

345

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

346

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

347

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

348

groups would also be a possibility in each molecule.

349

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

350

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

351

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

352

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

353

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,

61

355

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

356

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

357

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

358

analyzed and discussed in the current paper.

. However, the

359

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

360

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

361

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

362

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

363

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

364

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

365

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

366

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

367

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

368

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

369

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

370

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

371

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

372

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

373

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

374

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

375

(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)

377

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

378

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

379

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

380

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

381

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

382

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

383

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

384

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

385

during coal combustion.

386

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

387

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

388

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

389

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

390

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

391

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

392

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

393

photooxidation of biogenic VOCs in smog-chamber experiments conducted under

394

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

395

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

396

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

397

small number of O atoms (

Molecular characterization of water-soluble humic-like substances in

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