Article pubs.acs.org/est
Cite This: Environ. Sci. Technol. 2018, 52, 2575−2585
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† †
State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, P. R. China ‡ College of Resource and Environment, Anhui Science and Technology University, Anhui 233100, P. R. China § Graduate School of Chinese Academy of Sciences, Beijing 100049, P. R. China S Supporting Information *
ABSTRACT: Water-soluble humic like substances (HULIS) in smoke particles emitted from combustion of biomass materials and coal were characterized by ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry. The formulas identified were classified into four main groups: CHO, CHON, CHOS, and CHONS. The 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 0.21−0.41, 0.27−0.45, 0.41−0.46, 0.44−0.61 for the CHO, CHON, CHOS, and CHONS groups, respectively. The CHO compound was the predominant component (43%−72%) of the smoke HULIS from biomass burning (BB) and coal combustion, followed by the CHON group for BB-smoke HULIS and the S-containing groups (i.e., CHOS and CHONS) for coal-smoke HULIS. These results indicate that the primary HULIS emitted from biomass burning contain a high abundance of CHON species, which appear to be made up mainly of oxidized nitrogen functional groups such as nitro compounds and/or organonitrates. The coal-smoke HULIS contained more compounds with relatively low molecular weight and high aromaticity index (AImod). They were significantly enriched in S-containing compounds with high double bond equivalent (≥4), and O/S ratios suggest that they are most likely made up of aromatic organosulfates and nitrooxy organosulfates that are usually found in polluted atmospheres. These findings imply that the primary emissions from combustion of biomass and coal fuels are potential sources of water-soluble HULIS in an atmospheric matrix and that coal combustion is an especially important source of sulfate compounds.
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INTRODUCTION Water-soluble humic like substances (HULIS) are a group of unresolved polyacidic compounds identified ubiquitously in aerosol particles sampled in urban, rural, and marine environments and in rain, fog, and cloudwater samples.1−10 They are termed this way because they have many physical and chemical properties similar to natural humic substances in terrestrial and aqueous environments.5,6,11 As an important fraction of lightabsorbing organic carbon (i.e., brown carbon), HULIS can alter the light absorption and radiative forcing of aerosols.12−15 HULIS are also surface-active and can influence the surface tension and growth of cloud condensation nuclei, thus playing © 2018 American Chemical Society
an important role in the climate and atmospheric environment.6,16,17 Studies have suggested that atmospheric HULIS are derived from various sources, including primary emissions from biomass burning (BB),1,2,9,18 coal combustion,9,19 vehicular emissions,20 and secondary sources such as photochemical transformation of volatile organic compounds1,21 and oxidation Received: Revised: Accepted: Published: 2575
November 28, 2017 January 22, 2018 January 31, 2018 January 31, 2018 DOI: 10.1021/acs.est.7b06126 Environ. Sci. Technol. 2018, 52, 2575−2585
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Environmental Science & Technology of soot particles.22,23 Among the various sources listed above, BB is generally considered to be one of the largest sources of atmospheric HULIS.2,6,24,25 In addition, coal combustion is also reported as a primary source of HULIS in the atmosphere.9 Recent studies have investigated the chemical and lightabsorption properties of HULIS in smoke particles emitted from biomass fuel and coal burning in a laboratory combustion chamber.9,14,25 However, these studies focused mainly on an overall description of primary HULIS and revealed less structural information at the molecular level due to the highly complex composition of these substances. Recently, electrospray ionization (ESI) coupled with ultrahigh-resolution mass spectrometry, for example, Fouriertransform ion cyclotron resonance mass spectrometry (FTICR MS), has been used for successful characterization of water-soluble organic compounds and HULIS in cloudwater,26 rainwater,27−29 BB aerosols,30,31 and atmospheric aerosols.3,4,32−34 Because of its extremely high resolution and mass accuracy, ultrahigh-resolution mass spectrometry has enabled detailed characterization of organic materials at the molecular level.3,4,29,31,34,35 In the present study, FT-ICR MS was operated in negative ESI mode to analyze the water-soluble HULIS fractions isolated from smoke particles emitted from burning of biomass materials (including rice straw, corn straw, and pine branches) and coal. Rice straw and corn straw are the dominant crop residues in China, and pine branches are an important biomass cooking fuel in rural areas. Combustion of these biomass residues is reported to have important influences on aerosols in China,36,37 and therefore these three biomass materials were used to study BB-derived HULIS. Coal was chosen because coal combustion is also an important source of atmospheric aerosols in China.19,38,39 This is the first detailed analysis of primary HULIS in coal-smoke particles by FT-ICR MS. The objective was to obtain elemental composition and structural information for water-soluble primary HULIS emitted from combustion of biomass materials and coal at the molecular level and to provide some novel insights into the characteristics of HULIS emitted from specific sources.
Details of the isolation method were described in a previous paper by the authors.8,9,40 Ultrahigh-Resolution Electrospray Ionization FT-ICR MS. The isolated HULIS fractions were analyzed with a solariX XR FT-ICR MS (Bruker Daltonik GmbH, Bremen, Germany) equipped with a 9.4 T refrigerated actively shielded superconducting magnet (Bruker Biospin, Wissembourg, France) and a Paracell analyzer cell. The samples were ionized in negative ion mode using an ESI ion source (Bruker Daltonik GmbH, Bremen, Germany). The detection mass range was set to m/z 150−1000. Ion accumulation time was set to 0.65 s. A total of 100 continuous 4 M data FT-ICR transients were added to enhance the signal-to-noise ratio and the dynamic range. Field blank filters were processed and analyzed following the same procedure to detect possible contamination. The mass spectra were calibrated externally with arginine clusters in negative ion mode using a linear calibration. The final spectrum was internally recalibrated with typical O5-class species peaks using quadratic calibration in DataAnalysis 4.4 (Bruker Daltonics). A typical mass-resolving power (m/Δm50%, in which Δm50% is the magnitude of the mass spectral peak full width at half-maximum peak height) >450 000 at m/z 319 with AImod,w (CHONS) > AImod,w (CHOS). In addition, the AImod,w values of each compound group in coal-smoke HULIS were all significantly higher than for BB-smoke HULIS. These results suggested that these four compound groups have different aromaticities and that coal HULIS are characterized by relatively high aromaticity. The DBE/C ratio is also widely used to estimate the density of double bonds and the aromaticity of organic matter in natural environments.4,46 In this study, the compound groups in coal-smoke HULIS all presented relatively higher DBE/Cw values than those compound groups in the three BB-smoke HULIS, again indicating the relatively high aromaticity of coal-smoke HULIS. 2578
DOI: 10.1021/acs.est.7b06126 Environ. Sci. Technol. 2018, 52, 2575−2585
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Environmental Science & Technology
Figure 3. Double bond equivalent (DBE) vs C number for all the CHO compounds of the four smoke HULIS samples. The color bar and marker size denote the number of O atoms and the peak intensities of the compounds. The formulas for compounds (a−k) with high intensity are C9H12O4, C18H22O7, C16H32O2, C9H10O4, C15H18O8, C12H14O5, C20H26O3, C20H28O2, C20H30O2, C8H6O4, and C12H16O4, respectively. Note that the proposed structures are representative, not determined.
(C16H32O2) may be due to palmitic acid, which is mainly derived from emissions from vegetation49−51 and anthropogenic sources such as coal burning.52 This compound was also identified in the other three HULIS samples. The high-intensity CHO compounds in rice HULIS were also mainly distributed at C numbers from 8 to 18. Some of the high-intensity compounds identified were C15H18O8, C9H10O4, C12H14O5, C13H14O5, C12H12O5, and C13H16O6, which had DBE values of 7, 5, 6, 7, 7, and 6, respectively. These probably are typical biogenic derivatives;53−55 some possible structures are denoted as d, e, and f in Figure 3. Compared to the two BB-smoke HULIS formed from grass straw burning, the pine-smoke HULIS contained relatively high amounts of CHO compounds in high C number ranges (approximately >30), resulting in a relatively lower O/C ratio (as shown in Table S2). Several high-intensity CHO formulas detected in pine HULIS include C20H28O2, C20H26O3, C20H28O3, C20H30O2, and C20H26O2, which have DBE values of 7, 8, 7, 6, and 8 and relatively low O numbers. These compounds are probably diterpenoid derivatives emitted from combustion of conifer plants such as pine.51,56 Some possible molecular structures, including 7oxodehydroabietic acid (C20H26O3, g), dehydroabietic acid
Cw ratios of CHO compounds were 0.21−0.41 and 1.13−1.33 for the four smoke HULIS (Table S2). It is obvious that the HULIS data in this study are similar to those for water-soluble organic compounds in atmospheric aerosols,4,34 which may indicate that the primary smoke HULIS samples are similar to those in atmospheric aerosols. As shown in Figure 3, the DBE of the four HULIS samples in the current study displayed similar changes against C number. A wide range of DBE values (0−20) was observed in the CHO compounds, with a clear trend toward increasing DBE values with increasing carbon content. As shown in Figure 3, the highintensity CHO compounds in corn HULIS were mainly detected with C numbers from 8 to 18, such as C9H14O4, C18H28O8, C11H20O5, C18H22O7, C9H12O4, C16H32O2, and C16H20O6, which had DBE values of 3, 5, 2, 8, 4, 1, and 7, respectively. The formulas (a, b) with high intensity denoted in Figure 3 and their possible structures are also shown. These probably represent typical biogenic methoxyphenols, and the formula b (C18H22O7) may be dimers of formula a (C9H12O4). According to a previous study,48 the formulas (C9H14O4, C18H28O8) may be derivatives of limonene, and the latter ones may be dimers of the former. In addition, formula c 2579
DOI: 10.1021/acs.est.7b06126 Environ. Sci. Technol. 2018, 52, 2575−2585
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Environmental Science & Technology
Figure 4. Double bond equivalent (DBE) vs C number for all the CHON compounds of the four smoke HULIS samples. The color bar and marker size denote the number of O atoms and the peak intensities of the compounds. The formulas for the compounds with high intensity (a−c) are C7H7O4N1, C15H19O8N1, and C13H10O3N1, respectively. Note: the proposed structures are representative, not determined.
OxN2 compounds. The major OxN1 compounds of the three BB-smoke HULIS covered relatively large ranges from O4N1 to O9N1, whereas the major OxN1 compounds of coal-smoke HULIS were distributed in relatively small ranges from O3N1 to O6N1. In addition, the relative abundances of OxN2 compounds in coal-smoke HULIS were much lower than those in BBsmoke HULIS, suggesting that more OxN-containing compounds with high N content are contained in BB-smoke HULIS. In this study, all the high-relative-abundance CHON subclasses were rich in oxygen with respect to nitrogen, as shown in Figure S5; almost all these classes showed oxygen-tonitrogen ratios (O/N) ≥ 3. Hence, the CHON compounds in the current study appear to contain a large number of oxidized nitrogen functional groups such as nitro compounds (−NO2) and/or organonitrates (with the functional group of −NO3). In addition, the excess of O atoms in the CHON compounds suggests that these compounds also possess other oxygenated functional groups. As shown in Table S2, the average O/Nw ratios (5.66−5.99) of BB-smoke HULIS are greater than that (4.15) of coal-smoke HULIS. Moreover, the CHON groups
(C20H28O2, h), and pimaric acid (C20H30O2, i), are illustrated in Figure 3. Unlike the three BB-smoke HULIS, a narrow leafshaped pattern was observed for coal-smoke HULIS. The DBE ranges of each C number for coal-smoke HULIS were smaller than those for the three BB-smoke HULIS. The high-intensity CHO compounds detected in coal HULIS include C8H6O4, C13H18O4, C12H16O4, and C14H20O4, which have DBE values of 6, 5, 5, and 5 respectively. The most likely structure is one benzene ring substituted with O-containing groups such as hydroxyl, methoxyl, and carboxyl. CHON Compounds. Large amounts of organic nitrogen compounds were observed in the four HULIS samples, accounting for 18%−41% of total identified formulas in the three BB-smoke HULIS, but only 9% of the formulas identified in coal-smoke HULIS (Figure 1). These CHON compounds were classified into 25 subgroups according to their N and O numbers, including OxN1 (O2N1−O13N1) and OxN2 (O2N2− O9N2) groups. Some OxN2 compounds may be dimers of OxN1. The sums of peak intensities for each subgroup were added together and are listed in Figure S4. The four HULIS all had high contents of OxN1 compounds and low contents of 2580
DOI: 10.1021/acs.est.7b06126 Environ. Sci. Technol. 2018, 52, 2575−2585
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Environmental Science & Technology
other O-containing functional groups such as hydroxyl or carbonyl groups would also be a possibility in each molecule. As shown in Figure 1, the relative contributions of CHOS and CHONS to the total number of compounds identified were all very low (2%−6%) for the three BB-smoke HULIS. Moreover, all identified formulas in the CHOS group were less abundant than background pollution (C17H27O3S1, C18H29O3S1, and C19H31O3S1, corresponding to alkanesulfonates that are known to exist widely in the atmospheric environment) during FT-ICR MS measurement (Figure S6).27,61 However, the abundances of sulfur-containing compounds identified in coal-smoke HULIS were significantly higher, with 36% for CHOS and 12% for CHONS respectively. Therefore, only the CHOS and CHONS groups in coal-smoke HULIS were further analyzed and discussed in the current paper. CHOS Compounds. In this study, CHOS compounds accounted for 36% of the total number of compounds identified in coal-smoke HULIS, indicating that coal combustion may be an important source of OS species in the atmospheric environment. As shown in Table S2, the average AImod,w value of CHOS compounds was 0.31 for coal-smoke HULIS, which is significantly lower than the 0.45−0.56 obtained for the CHO and CHON groups. Moreover, the average H/C ratios for the CHOS compounds were also higher than the corresponding elemental ratios of the CHO and CHON compounds. These data together indicated that the CHOS compounds have a lower degree of unsaturation. Similarly, the DBEw values of CHOS species were also lower than those of CHO and CHON in the same HULIS sample. Figure 5 shows the DBE, C, and O atomic distributions in the CHOS compounds. For coal-smoke HULIS, the identified CHOS formulas were O3S1−O10S1 class species. The most abundant CHOS species class identified in the coal-smoke HULIS had 4−6 O atoms, with O5S1 being the most abundant (Figure 5). It is obvious that the high-relative-abundance CHOS species in coal-smoke HULIS are characterized by relatively low O contents. Among these CHOS compounds, the chemical formula (C9H12O4S1) denoted as a is likely consistent with an alkylbenzene ring substituted with one sulfate group (Figure 5). The chemical formula (C8H10O5S1) denoted as b could be an alkylbenzene ring substituted with one sulfate and one hydroxyl group. Note that most of the intense CHOS compounds in coal-smoke HULIS have DBE values equal to or greater than 4, with four or five O atoms. Within these chemical formulas, many aromatic organosulfate isomers with relatively high DBE can be predicted to be made up of benzene backbones with sulfates attached to side chains or to the aromatic ring, which were found and identified in laboratory studies.62 The results reported here show that these compounds may be produced from thermal reactions of benzenes with SO2 (produced by sulfur in coal reacting with oxygen) during coal combustion. CHONS Compounds. CHONS compounds were identified and accounted for 12% of overall compounds in coal-smoke HULIS. In these CHONS compounds, 31.6% of CHONS formulas had seven or more O atoms, implying that some nitrogen atoms may exist in the form of −NO3 groups and that these CHONS compounds are probably nitrooxy-organosulfates (nitrooxy-OS). These results also suggest that other O-containing functional groups existed in each formula as well. However, nitrooxy-OS derived from biogenic VOCs have been demonstrated to form by photooxidation of biogenic VOCs in
with high O/Nw ratio (≥3) account for 85.8%−96.3% of overall compounds for BB-smoke HULIS, which is significantly higher than 76.7% for coal-smoke HULIS. These results together suggest that the CHON compounds in the three BB-smoke HULIS likely exhibit a higher degree of oxidation, whereas the coal-smoke HULIS may feature a relatively low degree of oxidation and some reduced N functional groups. According to previous studies, these reduced nitrogen compounds in smoke particles emitted from combustion processes may be associated with alkyl amides and alkyl nitrile31,57 as well as heterocyclic aromatic compounds with a single N atom.4,31 The CHON compounds span a wide DBE range (3−20), indicating a high prevalence of double bonds and/or ring structures (Figure 4). It is obvious that the most abundant CHON species exhibit DBE 5−9, which may indicate that CHON species in primary HULIS contain mainly mono- and dinitro substituted phenols and benzoic acids. However, each HULIS CHON group has its own distinct properties. As shown in Figure 4, several high-intensity formulas were detected, such as C 6 H 5 O 5 N 1 , C 7 H 7 O 4 N 1 , C 8 H 9 O 4 N 1 , C 15 H 19 O 8 N 1 , C 16 H 21 O 8 N 1 , C 17 H 23 O 8 N 1 , C 12 H 10 O 8 N 2 , C 13 H 12 O 8 N 2 , C14H13O3N1, C15H15O3N1, C9H5O4N1, and C13H17O3N1, which have DBE values of 5, 5, 5, 7, 7, 7, 9, 9, 9, 9, 8, and 6, respectively. Figure 4 shows some possible chemical structures. Compound a in Figure 4 was identified mainly in the three BBsmoke HULIS, but not in coal-smoke HULIS. In addition, some other biogenic compounds such as C15H19N1O8 (b) are present with relatively high intensity in Figure 4. The CHON compounds observed in the three BB-smoke HULIS had relatively low AImod,w values (0.44−0.46) compared to coal-smoke HULIS. The CHON fraction with high AImod,w (≥0.5) accounted for 37.2%−43.4% of total CHON in the three BB-smoke HULIS, which was also lower than the percentage (67.4%) in coal-smoke HULIS. In addition, the BBsmoke HULIS had relatively higher O/Cw and O/Nw ratios than coal-smoke HULIS, as indicated in Table S2. These differences suggest that the CHON compounds in BB-smoke HULIS are mainly made up of molecules with low aromaticity and high degree of oxidation, whereas larger quantities of high aromatic compounds with a lower degree of oxidation are contained in coal-smoke HULIS. Sulfur-Containing Compounds. In the four HULIS samples, hundreds of ions were assigned formulas containing sulfur atoms, accounting for 6%−48% of the total number of assigned formulas (Figure 1). All the sulfur-containing compounds contained only one sulfur atom in each formula. These sulfur-containing compounds had been detected in water-soluble organic compounds derived from biomass burning and formed secondarily by reactions between the oxidation products of VOCs and acidified sulfate seed particles or sulfuric acid.3,48,58−60 According to whether nitrogen atoms were present in the formula, these sulfur-containing compounds were classified into two subgroups, denoted as CHOS and CHONS. The average OM/OC ratios of these compounds were much higher than those in the other categories, which is consistent with the addition of sulfur atoms onto the molecule. Among these sulfur-containing compounds, more than 91% of the CHOS formulas had O/S ratios greater than 4, and more than 31% of the CHONS formulas had O/S ratios greater than 7. Because a sulfate group (OSO3H) carries four oxygen atoms and readily deprotonates in ESI, the sulfur-containing compounds are more likely organosulfates. Sulfonates with 2581
DOI: 10.1021/acs.est.7b06126 Environ. Sci. Technol. 2018, 52, 2575−2585
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Environmental Science & Technology
Figure 5. Double-bond equivalent (DBE) vs C number for all the CHOS compounds of coal-smoke HULIS (a). Classification of CHOS species into different subgroups according to the numbers of S and O atoms in their molecules (b). The color bar and marker size denote the number of O atoms and the peak intensities of the compounds. The height of each column represents the sum of peak intensities in each subgroup. The formulas of compounds a and b are C10H12O4S1 and C9H12O5S1, respectively. Note: the proposed structures are representative, not determined.
Figure 6. Double-bond equivalent (DBE) vs C number for all the CHONS compounds of coal-smoke HULIS (a). Classification of CHONS species into different subgroups according to the numbers of S and O atoms in their molecules (b). The color bar and marker size denote the number of O atoms and the peak intensities of the compounds. The height of each column is the sum of peak intensities in each subgroup. The formulas of compounds (a−d) are C9H7O5N1S1, C10H7O7N1S1, C13H13O4N1S1, and C15H15O5N1S1, respectively. Note: the proposed structures are representative, not determined.
smog-chamber experiments conducted under high nitrogen oxide (NOx) concentrations in previous studies.3,48 The data in this study indicated that coal-combustion emissions are also important sources of these classes of compound. Note that 68.4% of the CHONS formulas have a relatively small number of O atoms (
