Elemental Composition of HULIS in the Pearl River Delta Region

Jun 17, 2012 - The HUmic-LIke Substances (HULIS) fraction isolated from aerosol samples collected at a rural location of the Pearl River Delta Region ...
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Elemental Composition of HULIS in the Pearl River Delta Region, China: Results Inferred from Positive and Negative Electrospray High Resolution Mass Spectrometric Data Peng Lin,†,‡ Angela G. Rincon,§ Markus Kalberer,*,§ and Jian Zhen Yu*,†,‡ †

Department of Chemistry, Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China Division of Environment, Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China § Department of Chemistry, University of Cambridge, Cambridge CB2 1EW, United Kingdom ‡

S Supporting Information *

ABSTRACT: The HUmic-LIke Substances (HULIS) fraction isolated from aerosol samples collected at a rural location of the Pearl River Delta Region (PRD), China, during the harvest season was analyzed by both positive and negative mode electrospray ionization (ESI) coupled with an ultrahigh resolution mass spectrometer (UHRMS). With the remarkable resolving power and mass accuracy of ESI-UHRMS, thousands of elemental formulas were identified. Formulas detected in the positive (ESI+) and the negative (ESI-) mode complement each other due to differences in the ionization mechanism, and the use of both provides a more complete characterization of HULIS. Compounds composed of C, H, and O atoms were preferentially detected in ESI- by deprotonation, implying their acidic properties. Tandem MS and Kendrick Mass Defect analysis implies that carboxyl groups are abundant in the CHO compounds. This feature is similar to those of natural fulvic acids, but relatively smaller molecular weights are observed in the HULIS samples. A greater number of reduced nitrogen organic compounds were observed in the ESI+ compared to ESI-. Compounds with biomass burning origin including alkaloids, amino acids, and their derivatives are their probable constituents. Sulfur-containing species were dominantly detected in ESI-. The presence of sulfate fragments in the MS/MS spectra of these species and their high O/S ratios implies that they are mainly organosulfates. Organosulfates and nitrooxy-organosulfates were often the most intensive peaks in the ESI- spectra. They are believed to be products of reactive uptake of photooxidation products of reactive volatile organic compounds by acidic sulfate particles. The elemental compositions deduced from the UHRMS analysis confirm the conclusion from our previous study that biomass burning and SOA formation are both important sources of HULIS in the PRD region.

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INTRODUCTION Aerosol phase HUmic-LIke Substances (HULIS) have been demonstrated to play active roles in several areas of atmospheric chemistry.1−4 Their physicochemical properties and environmental effects depend on their chemical compositions, which have not been fully characterized yet. Determining the chemical compositions and structural information of HULIS on the molecular level is challenging due to the highly complex composition with thousands of compounds from a wide range of compound classes. Ultrahigh resolution mass spectrometry (UHRMS) coupled with soft ionization techniques such as electrospray ionization (ESI) allows characterization of complex organic mixtures at the molecular level due to the extremely high resolution and mass accuracy.5 This technique has been widely used in analyzing natural organic matter in various environments6 and led to recent progress in identifying molecular information about organic aerosol of different origin.7 In ambient aerosols the organic components are mixed with inorganic constituents (e.g., ammonium, sulfate, nitrate), which © 2012 American Chemical Society

are abundant and greatly exceed the concentrations of individual organics. As a result, solid phase extraction (SPE) is commonly used for desalting prior to direct infusion UHRMS analysis of organic aerosol components.8−11 The most hydrophilic compounds such as inorganic ions and low molecular weight organic acids are removed by SPE. The remaining subfraction of WSOC contains the relative hydrophobic species, most of which can be considered as components of HULIS. All UHRMS spectra in past studies of HULIS in ambient aerosols were acquired using negative ESI (ESI-) mode.9−11 Results from negative ESI-UHRMS analysis include observations of the typical elemental formulas consisting solely of C, H, and O atoms, which were analogous to aquatic humic substances (HS) but with relatively smaller molecular weight.9 In addition, a series of sulfated, nitrated, and Received: Revised: Accepted: Published: 7454

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the term HULIS is an operational definition. There are also other approaches used for HULIS isolation and definition.27 The HULIS fractions obtained in this way were also adopted by a number of research groups to study the hygroscopic growth properties,1 surface tension effects,3 light absorption properties,2 solubility, and conductivity28 as well as characteristics in NMR and UHRMS.9,10 ESI-UHRMS Analysis. The HULIS fractions were analyzed with a LTQ-Orbitrap Velos mass spectrometer (Thermo Scientific Inc., Bremen, Germany) with a heated electrospray ionization source. Samples were injected through a steel capillary by a Hamilton syringe at a flow rate of 5 μL/min and were analyzed using a spray voltage of 2.5 to 3 kV. The system was operated in both ESI+ and ESI- mode with a resolving power of 100,000 at m/z 400. Signals in the range of m/z from 90 to 1000 were recorded and used for detailed formula calculations. The instrument was calibrated using a standard mixture of n-butylamine (MW 73), caffeine (MW 194), MRFA (L-methionyl-arginyl-phenylalanyl-alanine acetate • H2O, MW 523), sodium dodecyl sulfate (MW 288), sodium taurocholate (MW 537), and Ultramark 1621 (a mixture of fluorinated phosphazines with MW in the range of 922−2021). The resulting mass accuracy was better than 2 ppm as indicated by the root-mean-square of the measured mass errors of the standard mixture. Background spectra were obtained by analyzing field blank filters, which were processed following the same SPE procedures. We have considered whether the use of ammonia and methanol in eluting HULIS from the SPE cartridge might produce artificial compounds resulting from solvent-analyte interactions. Our analysis indicates that the influence of ammonia or methanol on the major compositions of HULIS is most likely insignificant although we cannot entirely rule out the probable reactions between solvent and HULIS. The details of this analysis are given in Appendix S1 in the SI. Data Processing and Assignment of Elemental Compositions. The mass spectra were processed, and the mass lists were exported using the Xcalibur software (V2.1, Thermo Scientific). All the ions with a signal-to-noise ratio (s/ n) ≥ 10 and in the m/z range from 90 to 1000 were exported. Subsequently, all mathematically possible formulas for these ions were calculated using a mass tolerance of ±2 ppm. In other words, the mass errors of formula assignments were within 2 ppm. In the Xcalibur software, 1−80 of 12C, 1−200 of 1H, 0− 50 of 16O, 0−5 of 14N, 0−2 of 32S, 0−1 of 13C, 0−1 of 18O, and 0−1 of 34S were allowed in the molecular formula calculations. In the positive mode, 0−1 of Na was also included in the formula calculation because of the high tendency of Na to form adducts with polar organic molecules. Several conservative rules were also applied to eliminate compounds not likely to be observed in nature: The H/C and O/C ratios were limited to 0.3−3.0 and 0−3, respectively. In the negative mode data, the N/C and S/C ratios were restricted in the range of 0−0.5 and 0−0.2, respectively.11 In the positive mode data, these two ratios were limited to 0−1.3 and 0−0.8.29 The calculated formulas were also characterized by the number of rings plus double bonds (i.e., the double bond equivalents (DBE)). The DBE value was calculated as follows for the elemental composition CcHhOoNnSs

mixed sulfated and nitrated molecules were also detected abundantly in these samples. These species are rarely detected in HS (e.g., aquatic fulvic acids), indicating the different origin and formation mechanisms of atmospheric HULIS and of HS. Similar elemental compositions to those in aerosol samples were also found in rain and fogwater samples influenced by polluted urban airflows.8,12 Positive ESI-UHRMS spectra were obtained for organic aerosols from biomass burning (BB) emissions13−15 and from oxidation of VOCs in chamber studies.16−18 These studies reported detection of many reduced nitrogen containing organic compounds in BB aerosols and monomers and oligomers in secondary organic aerosols (SOA). In some UHRMS studies of chamber SOA samples, both ESI+ and ESI- spectra were obtained and compared.19−22 These two modes generated qualitatively similar mass spectra for dlimonene SOA.19,22 However, the positive ESI (ESI+) spectra of SOA from isoprene, α-pinene, and β-pinene were partly different from those obtained in negative mode.20,21 More oligomeric molecules were detected in ESI+ spectra than in ESI- spectra.21 We have previously reported the size distributions and abundances of HULIS in the Pearl River Delta (PRD), an economically developed region in Southern China.23,24 HULIS are found to be abundant in fine aerosols in this region.23,24 Analysis of spatial and temporal variation of HULIS and other aerosol constituents implies that biomass burning and secondary formation are important sources of HULIS in this region. In the current study, we use UHRMS operated in both ESI+ and ESI- modes to analyze the HULIS fraction isolated from ambient aerosols taken at a rural location in the PRD, where biomass burning is a significant aerosol source. The objective is to obtain elemental composition and structural information of HULIS at the molecular level and provide guidance to future studies on relating HULIS compositions to their sources and apparent properties (e.g., light absorption,2 surface active,25 and redox activities26).

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METHODOLOGY Sample Collection and HULIS Isolation. The UHRMS analysis was carried out on four ambient samples collected from November 17 to 22, 2007 during the harvest season at a rural location in PRD in South China. Each sample was collected for ∼24 h. A high-volume aerosol sampler (TE-6070V-BL, Tisch Environmental Inc., USA) was used to collect particles less than 2.5 μm in aerodynamic diameter (PM2.5) on prebaked quartz filters. One field blank sample was taken following the same procedure except that no air was drawn through. Fresh smoke particles emitted from rice straw burning were also collected as described previously.24 The filter samples were stored at −18 °C in a freezer until analysis. Portions (usually 16 cm2, corresponding to about 65 m3 sampled air) of the filters were extracted with ultrapure water in an ultrasonic bath for 40 min and then filtered with a 0.45 μm Teflon syringe filter to remove insoluble suspensions. The extract was acidified to pH = 2 using HCl and then loaded on a SPE cartridge (Oasis HLB, Waters, USA). The majority of inorganic ions, low molecular weight organic molecules, and sugars were not retained by the cartridge. The HULIS fraction of WSOC was retained on the cartridge and eluted by 1.5 mL of methanol containing 2% aqueous ammonia (w/w). The resulting eluate was immediately evaporated to dryness under a gentle N2 stream and redissolved in 4 mL of acetonitrile and water (1:1) for ESI-UHRMS analysis. It should be noted that

DBE = 7455

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Table 1. Number of Compounds in Each Compound Category and the Average Values (Arithmetic Mean ± Standard Deviation) of Elemental Ratios, Molecular Weight, and DBE elemental compositions

number of formulas

molecular weight (Da)

OM/OC

H/C

O/C

DBE

DBE/C

CHO+

364

220 ± 53

1.44 ± 0.16

1.33 ± 0.33

0.25 ± 0.12

5.2 ± 2.2

0.42 ± 0.17

CHO-

691

257 ± 71

1.64 ± 0.28

1.29 ± 0.35

0.40 ± 0.21

5.6 ± 2.4

0.44 ± 0.18

CHON+

781

251 ± 60

1.59 ± 0.18

1.42 ± 0.32

0.25 ± 0.13

5.5 ± 2.2

0.44 ± 0.17

CHON-

567

233 ± 58

1.80 ± 0.28

1.15 ± 0.31

0.41 ± 0.19

6.4 ± 2.1

0.59 ± 0.16

CHN+

124

172 ± 39

1.33 ± 0.13

1.27 ± 0.38

6.0 ± 2.4

0.56 ± 0.20

CHOS-

295

309 ± 58

2.10 ± 0.28

1.67 ± 0.31

0.55 ± 0.17

3.0 ± 1.9

0.25 ± 0.16

CHONS-

126

335 ± 50

2.61 ± 0.39

1.73 ± 0.29

0.81 ± 0.22

3.1 ± 1.9

0.28 ± 0.15

all in ESI+ all in ESI-

1269 1679

236 ± 61 265 ± 71

1.51 ± 0.20 1.85 ± 0.40

1.37 ± 0.33 1.34 ± 0.39

0.22 ± 0.14 0.46 ± 0.23

5.5 ± 2.2 5.3 ± 2.6

0.45 ± 0.18 0.45 ± 0.21

DBE reflects the degree of unsaturation in a given compound.30 All the calculated formulas with DBE < 0 and that disobey the nitrogen rule for even electron ions31 were excluded from the lists. The formulas containing 13C atoms were deleted when no corresponding 12C isotope was detected under assumptions that the two molecules represent the same compound. The background spectra obtained from field blank filters were also processed using the rules mentioned above. The formula lists of the background spectra (accounting for ∼4% in ESI- and ∼3% in ESI+ in peak number and ∼12% in ESI- and ∼6% in ESI+ in total ion intensity of the sample spectra) were subtracted from the formula lists of sample spectra, regardless of peak intensity.

tentative assigned compounds C number (C#) range alcohol, carbonyl, carboxyl, ester 5−24 carboxyl, carbonyl, alcohol, ester 3−21 amino acids and their derivatives 4−21 organic nitro or nitrooxy 4−19 alkaloids 5−22 organosulfates 6−20 nitrooxy-organosulfates 6−18

peaks can be observed between the M and M+1 peak (see Figure S1 inset). However, the intensities of these “bumps” are generally very weak, with an s/n ratio less than 3, thus very close to the noise level. This is found to be the case for the entire mass range. The presence of these peaks suggests that a small fraction of ions may be doubly charged. Ions with three or more charges are not observed. Moreover, the isotope peaks are mainly detected at ∼1.0033 Da higher than their parent peaks, confirming they are singly charged isotopic peaks. Based on the very low abundance of doubly charged ions, we conclude that the peaks with s/n > 10 in the mass spectra are singly charged. The doubly charged peaks are not considered in the further analysis. Around 79% of the peaks observed in the ESI+ spectra and 74% in the ESI- spectra are assigned with unambiguous molecular formulas. The unassigned peaks account for 18% and 24% of the total peak intensity detected in the ESI+ and ESIspectra, respectively. Identified isotope peaks (containing 13C, 18 O, or 34S) account for ∼2% of the total identified peaks in ESI + and ∼4% in ESI- in terms of peak intensity. They are not included in the ensuing discussion. Overall, 1269 and 1679 monoisotopic formulas were determined in ESI+ and ESI-, respectively. It should be noted that for each elemental composition (formula) assigned, multiple structural isomers are possible. As a result, the peak numbers detected in the UHRMS could underestimate the actual number of compounds present. The list of elemental formulas determined in the ESI+ spectra is different from the list based on the ESI-. This is because there are major differences in the ionization mechanism (e.g., due to the gas phase acidity of the analytes) between the positive and negative modes.33 These differences suggest that the two ionization modes provide complementary information, and both should be considered for characterizing molecular compositions of complex mixtures such as HULIS. The identified elemental formulas are classified into seven major compound categories, including CHONS-, CHOS-, CHON+, CHON-, CHO+, CHO-, and CHN+ compounds (Table 1). CHONS- compounds refer to compounds that are detected in the ESI- mode and contain carbon, hydrogen, oxygen, nitrogen, and sulfur elements. Other compound categories are defined analogously. The percent occurrence of

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RESULTS AND DISCUSSION General Characteristics. Similar patterns in peak distribution were found among the four ambient HULIS samples. We therefore use the mass spectra obtained from only one sample for further detailed discussion. Figure S1 in the Supporting Information (SI) shows the UHRMS obtained in ESI+ and ESI- mode. Both spectra contain thousands of peaks, with the most intense peaks between m/z 90 and 500. Only 5 peaks in ESI+ and 2 peaks in ESI- were detected in the m/z 500−900 range, and all of them were weak (s/n ≈ 10). In addition, they could not be assigned with unambiguous formulas. Consequently, they are not discussed further in this study. Spectra details (see the insets in Figure S1) show the typical patterns of the observed peaks, which usually cluster around their nominal (nearest-integer) m/z with a spacing of ∼1 Da. Approximately 10 peaks are observed within 0.3 Da of each integer m/z. The ESI process can produce multiply charged ions in addition to singly charged ions. The charge status of an ion in a UHRMS spectrum can be determined by evaluating the m/z difference of a monoisotopic (e.g., all 12C) peak with its corresponding 13C12Cn‑1 isotope peak.31,32 The mass difference between these two peaks when singly charged is Δm/z = 1.00335, whereas for doubly and triply charged ions 1/2 and 1/ 3 of this value (∼0.5 and ∼0.33, respectively) is expected. Hence, the presence of doubly charged ions would generate peaks approximately halfway between the M and M+1 peaks, that is, peaks with mass defects (distance of a peak displaced from its integer mass) around 0.5 Da. In some cases, a group of 7456

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Figure 1. (a) COO based Kendrick mass defect (KMD) vs m/z for all the CHO compounds in the negative mode. (b) Enlarged segment from (a). The short dashed lines denote the compounds differ with each other by only several number of COO groups. Van Krevelen plots of the CHO compounds detected in the positive (c) and negative (d) mode. The color bar and marker size denote the double bond equivalent (DBE) and precise molecular weight of the compounds.

formulas, 26 of them are observed as [M + Na+] ions, and the majority are detected as protonated species. This is different from other studies,15,16 in which most of the oxygen-containing organic compounds were observed as [M + Na+] ions. The CHO- compounds presumably include carboxyl and/or hydroxyl functional groups because ESI preferentially generates deprotonated anions in the negative mode.33 Structural information on selected intense ions was explored by carrying out MS/MS analysis. The ions were fragmented by collision-induced dissociation (CID) in the linear ion trap (LTQ). The resulting product ions (i.e., the fragments) were then measured for their accurate mass by the high resolution Orbitrap analyzer. The MS/MS study of the even-electron [M + H]+ and [M − H]− ions were found to be helpful in deducing their structural information in previous studies.14,36 The dominant neutral loss of COO (decarboxylation) is a typical fragmentation property of aromatic carboxylic acid, while the dissociation of aliphatic carboxylic acids was usually dominated by loss of H2O.37 Figures S4a and S4b in the SI show example MS/MS spectra of two CHO ions, confirming the presence of carboxyl groups in these ions. The Kendrick mass defect (KMD) is usually used for finding relationships among a large data set of molecular formulas obtained by UHRMS.38,39 The regular spacing pattern usually used for KMD analysis is CH2. Functional group analysis

compounds in each category is shown in Figures S2a and S2b in the SI. The intensity distributions of the individual formula groups are shown in Figures S2c and S2d. In ESI-MS, peak intensity is the product of initial concentration of the neutral compound and ionization efficiency of the compound. The latter could span several orders of magnitude among different organic compounds.34,35 ESI is not sensitive to apolar compounds such as alkanes, PAHs, etc., which are present in the aerosol but not part of the HULIS fraction. It should be noted that the results shown here do not represent the entire organic aerosol composition but are biased toward HULIS compounds with high ionization efficiency in ESI. In the following sections, we will discuss in detail all seven compound categories. CHO Compounds. A total of 691 mass measurements could be assigned to elemental compositions containing only C, H, and O elements in the ESI- spectrum, and 364 formulas were assigned in the ESI+ spectrum. Among them, 262 formulas were detected in both modes. However, without any further information, it is not possible to decide whether these common formulas represent the same compounds. Molecules with a higher number of oxygen atoms are preferentially detected in the ESI- mode, while molecules with more than 12 C atoms but only one or two O atoms are only detected in the ESI+ mode (Figure S3 in the SI). Among the 364 CHO+ 7457

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0.7 usually serves as a criterion for identifying species with condensed aromatic ring structures.30 In this study, 18 of these 34 compounds in region C possess DBE/C values greater than 0.7, with the remaining greater than 0.6. These results suggest that the compounds in region C might be soot-derived materials or oxidized PAHs. Soot is emitted to the atmosphere as a byproduct of combustion processes and can be a significant portion of carbonaceous aerosols. Ozone oxidation of soot particles was demonstrated to form light-absorbing water soluble organic compounds.50 During ozone oxidation, oxygenated functional groups are added onto the condensed aromatic rings, making them soluble in water.50 CHON Compounds. Large amounts of organic nitrogen compounds have been observed in fog waters8 and continental precipitations.51 In this study, 781 and 567 measured masses are assigned to compounds containing C, H, O and N elements in the ESI+ and ESI- spectra, respectively. Almost all of the CHON+ compounds (779 of 781) are observed as protonated cations. Two [M + Na+] ions were observed but detected at extremely low intensity (s/n ≈ 12). The CHON compounds are classified into 22 subgroups according to the number of nitrogen and oxygen atoms. Figure 2a shows the number of formulas in each subgroup in ESI+ and

suggested that carboxyl groups are abundantly present in atmospheric HULIS.40,41 Here we rescale the KMD value by normalizing the carboxylate group (-COO)42 for CHO compounds in ESIKendrick mass (KM) = observed mass ×

nominal mass of COO exact mass of COO

Kendrick mass defect (KMD) = normal mass (NM) − KM

After the conversion, all molecules differing only in the number of −COO groups would exhibit the same KMD and appear on the same horizontal line in the KMD diagram (Figures 1a and 1b). Although the KMD analysis cannot provide definitive evidence for structural information, this regular −COO spacing pattern in CHO compounds may reflect the presence of abundant polycarboxylic and/or carboxylic acid ester groups in HULIS. Esters have been reported to form through esterification between carboxylic acids in smog chamber studies,43−46 and they usually contain more than one carboxyl group in addition to the ester functional group, making them soluble in water. The van Krevelen (VK) diagram is usually employed to display compositional characteristics of complex organic mixtures and to provide a broad overview on their average properties and, to some extent, used to qualitatively identify different composition domains in natural organic mixtures.47,48 For example, the most oxidized species lie at the lower right, and the most reduced/saturated species lie at the upper left of the VK diagram where the H/C ratio of each compound in the mass spectrum is plotted versus its O/C ratio (Figures 1c and 1d). In this study, we visualize the UHRMS data by slicing the VK diagrams into segments corresponding to their DBE values and molecular weight. Figures 1c and 1d show the VK diagrams for CHO compounds in both ESI+ and ESI- modes, respectively. Since the lists of the molecular formulas derived from the ESI+ and ESI- spectra do not fully overlap, different patterns are found in these two VK diagrams, and some complimentary information can be deduced. In the ESI- VK diagram (Figure 1d) more compounds are present with an O/ C > 0.5, likely due to the more favorable ionization of carboxylic acids in negative mode. The compounds present in region A with a high H/C ratio (≥2.2) and DBE = 0 correspond to saturated oxygenated species that contain at least 6 carbon and 3 oxygen atoms in each formula. The candidate compounds could be some long chain polyalcohols. The upper portion of the CHO- VK diagram (region B, H/C = 2 and DBE = 1) represents aliphatic oxygenated compounds containing at least 2 oxygen atoms in each formula. Monocarboxylic acids may be the most probable candidates. The lower left corner of the CHO- diagram (region C) contains the most unsaturated (DBE ≥ 10) compounds with both low H/C and O/C ratios. The carbon normalized double bond equivalents (DBE/C) of these formulas show high double bond densities in their structures, which is the main factor determining the light absorption properties of organic matters.49 Part of this region on the VK diagram also overlaps with the typical region of condensed hydrocarbon,47 which is analogous to soot, an important class of light absorption organic matter in the atmosphere. A threshold DBE/C value of

Figure 2. Classification of CHON species into 22 subgroups according to the number of N and O atoms in their molecules. (a) The number of formulas and (b) the sum of peak intensities, expressed as signal-tonoise ratio, in each subgroup.

ESI- mode. Sums of peak intensities for individual formulas in the same subgroup are also shown in Figure 2b. Compounds in the O3N − O9N subgroups (shown in the upper half of Figure 2) are preferentially detected in the ESI- spectra. The high O/ N ratios (≥3) of these formulas allow an assignment of one nitro (-NO2) or nitrooxy (-ONO2) group. The excess of O atoms in addition to those in the -ONO2 functionality in the O4N − O9N subgroups suggest that these compounds also possess other oxygenated functional groups. In contrast, compounds belonging to the ON4 − O2N2 subgroups (i.e., the six lower most categories in Figure 2) are mainly observed in the ESI+ spectra. The O/N ratios of these compounds are too low (≤1) to have organic nitrate functionality. This means that they instead contain reduced N functional groups (e.g., amines), which are preferentially ionized in ESI+ mode. A comparison of the ESI+ and ESI- spectrum lists reveals that 272 formulas of the CHON subgroup were found in both 7458

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spectra, suggesting that these formulas may contain both acidic and basic functional groups. One kind of such compounds are amino acids, containing both acidic (-COOH) and basic (-NH2) functionalities. Amino acids have been identified as an important class of dissolved organic nitrogen in aerosols.52 Biomass burning emissions could be a significant source of amino acids.53 Among the 272 formulas, 241 contain at least one N and two O atoms, allowing functionalities similar to amino acids. Though the number of natural amino acids only contribute a small percentage of species in the CHON class, their derivatives and oxidation products may be numerous and contribute substantially to the CHON class.51 The 3-D plot of atom distributions of the CHON compounds in Figure 3 and their VK diagrams in Figure S5

Figure 4. (a) Reconstructed mass spectrum of CHN species detected in the positive mode. The neutral form molecular formulas of some intensive ions are labeled, and the formulas differing by multiple number of CH2 are indicated by green arrows. (b) Van Krevelen plots of the CHN compounds. The color bar denotes the double bond equivalent (DBE), and the text markers denote the number of N atoms of the compounds.

The MW of the most intense peaks typically ranges from 120 to 220 Da. Both the arithmetic mean MW and the intensity weighted average MW of these compounds are smaller than all other categories (Table 1). Most of the intense CHN compounds contain two N atoms in the formula, and they usually belong to several homologous series, in which the formulas only differ by the number of CH2 groups (two example series are indicated by the dark green arrows in Figure 4a). A VK diagram of all CHN+ compounds is shown in Figure 4b where the N/C ratio was plotted versus the H/C ratio. This plot clearly separates different compound classes according to the number of nitrogen atoms and the degree of unsaturation (i.e., DBE). Data points corresponding to the compounds within the same CH2 homologous series appear along the lines that intersect at H/C = 2 on the y-axis. Four formulas appear in the upper region of VK diagram (H/C > 2). They have either one N atom with DBE = 0 or two N atoms with DBE = 1, indicating they are aliphatic amines. A total of 34 formulas have a high degree of unsaturation (DBE= 8, 9, and 10), implying the presence of fused aromatic structures and N-heterocyclic rings. Since CHN compounds can only be detected in the ESI+ mode and most of them (120 of 124) having a DBE value ≥2, nitrile and amine species are their probable candidates. The structural information of these compounds was probed by selecting some of the intense ions containing two N atoms for the MS/MS study. [M + H]+ ions of aromatic/aliphatic nitriles and primary aromatic amines frequently decompose by neutral loss of HCN, while loss of NH3 is more pronounced for aliphatic amines in their MS/MS spectra.37 None of the 12 ions examined in the MS/MS study showed HCN or NH3 losses,

Figure 3. The 3-D plot of atom distributions of the CHON compounds detected in the positive and negative modes.

in the SI show clearly that molecules detected in the ESI+ and ESI- spectra do not exactly overlap. A total of 509 CHON+ formulas are detected only in the ESI+ mode, mainly observed in the right part of Figure 3 and in the upper left region of the VK diagram (Figure S5a). These compounds have high H/C but low O/C ratios, consistent with reduced nitrogen species (e.g., amines). Fewer formulas, at a total of 295, are observed only in the ESI- mode. They concentrate more in the left part of Figure 3, indicating relatively higher O/C ratios and lower H/C ratios (Figure S5b). Most of these compounds (94%) have at least three O atoms (Figure 3). Their placements on the VK diagram also indicate a high degree of oxidation. Organic compounds with oxidized nitrogen functional groups such as nitro compounds and organonitrates are in this category. The sources and formation mechanisms of these compounds in the atmosphere have been well documented.54 CHN+ Species. In the ESI+ mode, 124 measured masses can be assigned with formulas containing only carbon, hydrogen, and nitrogen elements. These compounds are usually the most intense ions in the ESI+ spectrum (Figures S6a and S6c), with 26 of the top 50 intense ions being CHN compounds. The MW distributions of the CHN compounds are shown in the reconstructed mass spectrum in Figure 4a. 7459

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which suggests nitriles and primary amines are unlikely part of their structures. Because of the limited amount of HULIS samples, we could not conduct MS/MS study for more CHN species. Figures S4c and S4d show example MS/MS fragmentation patterns of two selected ions, characterized by neutral losses of alkyl radicals and alkenes such as •CH3, •C2H5, C3H6, •C3H7, C4H8, and C5H10. The lack of neutral losses of N-containing moieties implies that N atoms are embedded into heterocyclic rings, which are relatively stable under CID conditions. The dominant alkyl and alkene losses imply the presence of alkyl substitutions on the rings. The DBE values of the ions remain unchanged during their fragmentation processes, and the daughter ions with the smallest m/z usually have DBE of 3 or 4. These compounds may possess five-member (e.g., imidazole) or six-member (e.g., pyrazine) rings with alkyl substitutions. Another joint name of these N-containing heterocyclic compounds is alkaloids. Alkaloids are produced by a large variety of organisms, especially by vascular plants that contain lignified tissues.55 They are generally basic compounds derived from amino acids that contain one or more heterocyclic nitrogen atoms.55 Smoldering fires are typical burning conditions of rice straw in the PRD region. The N-heterocyclic cores of these compounds may be stable during smoldering burning to allow them to be emitted with minor pyrolytic and oxidative processing. In the HULIS fractions isolated from fresh rice straw burning aerosols, there are 173 CHN compounds detected in the ESI+ mode. Almost all CHN formulas (115 of the 124) detected in ambient HULIS are also found in rice straw burning HULIS. Similar peak distributions of CHN species are observed between the HULIS fractions of the rice straw burning emission aerosols and the PRD ambient aerosols (Figures S6a and S6b in the SI). They are also the most intense peaks in the rice straw burning HULIS spectrum (Figures S6b and S6d in the SI). Their fragmentation patterns are similar to those shown in Figures S4c and S4d. These species may potentially serve as tracers of fresh biomass burning aerosols. Alkaloid compounds were also previously detected as abundant constituents of aerosols emitted from various biofuels.14 Sulfur Compounds in the Negative Mode (CHOS- and CHONS- Species). Another interesting class of compounds is sulfur-containing species detected in the ESI- mode. These species usually produce intensive ions in the UHRMS of HULIS. A total of 421 S-containing peaks (25% of all compounds identified in the ESI- mode) are detected in the 90−500 Da range, including 295 CHOS formulas and 126 CHONS formulas. In the 500−900 Da range, no CHOS or CHONS formulas were detected above s/n of 10. The average MW and OM/OC ratio of these compounds are much higher than those in the other categories (Table 1), in accordance with the addition of oxidized forms of nitrogen and sulfur atoms to the molecules. The higher average O/C ratio reflects their higher oxidation state. These species usually contain one S atom and various numbers of O atoms. Out of the 421 Scontaining species, 419 (>99%) have O/S ratios of 4−11, which allows the assignment of a sulfate group. Among them, 18 intense S-containing peaks were selected to perform MS/ MS analysis, and all of them show intense •OSO3− or HSO4− product ions (Figures S4e and S4f in the SI), indicating the presence of a sulfate group in the parent ions. Some of these organosulfates have been identified as important components of SOA in chamber studies56 and in ambient samples.10,12

Besides the sulfate ions, neutral losses of HNO3, HNO2, and •NO2 fragments (Figure S4f) in MS/MS spectra of some precursor ions (e.g., m/z = 294, C10H16O7NS−) imply the presence of organic nitrate (ONO2) and/or nitro-functionality. In this study, 125 of the 126 CHONS compounds have O/S ratios ≥7, which allows the assignment of both −OSO3H and −ONO2 groups. The MS/MS spectrum of C10H16O7NS− is identical to those of the 3 isomers produced by oxidation of monoterpenes.56 A total of 39 S-containing compounds are observed to have elemental formulas consistent with organosulfates identified in smog chamber studies of SOA formation from biogenic VOCs.56 Overall Elemental Composition Characteristics. Elemental formulas detected in the ESI+ and ESI- spectra probe different compound classes in HULIS from the PRD region. The average OM/OC and O/C ratios of different formula categories listed in Table 1 show that the ESI+ mode favors the ionization of primary organic aerosol compounds, as evidenced by lower OM/OC and O/C ratios in the detected formulas. In ESI- mode, ionization of secondary organic aerosol compounds seems to be favored. The molecular formulas detected in ESIusually have higher OM/OC and O/C ratios, e.g., carboxylic acids, nitrates, and sulfates (Table 1). This finding suggests that ESI+ and ESI- modes complement each other, and the combined information from the two is needed for a more complete HULIS characterization. In this study, elemental compositions of HULIS revealed in ESI- spectra are similar to those found in aerosol samples collected in North America and central Europe.8−11 Large numbers of N-containing, S-containing, and N- and Scontaining compounds are detected in all of these samples, suggesting that they are the ubiquitous feature of atmospheric HULIS. The elemental compositions of HULIS obtained in ESI + spectra revealed distinctive features of organic aerosols emitted directly from biomass burning, which has been proven to be a major source of atmospheric HULIS.27 Organic compounds containing reduced-N functional groups (e.g., alkaloids) were detected at strong intensities in both our ambient and BB HULIS samples as well as in other type of BB aerosols.14 ESI+ is not used as widely as ESI- in UHRMS characterization of ambient HULIS; therefore, a comparison of our ESI+ results with others is not feasible. Compared to HULIS from ambient aerosols (Table 1), the elemental composition of HULIS in rain waters show significantly larger O/C and OM/OC ratios,12,51 suggesting that the latter are more oxidized. These differences seem to suggest that aqueous reactions may play an important role in the evolution of HULIS compositions in the atmosphere. Implications. This work shows that atmospheric HULIS contain numerous compounds that have multiple carboxyl groups and reduced nitrogen groups. These functional groups possess electron lone pairs and can serve as ligands to form complexes with transition metals, thereby influencing the atmospheric chemistry57 and redox chemistry of metals, which in turn has implications to health effects imposed by aerosols.58,59 The coordination effects of ligands coexisting in aerosols are important factors influencing solubility, oxidative potential, and reactivity of transition metals in the atmosphere. 57 Ligands in aquatic HS has been plentifully demonstrated to affect oxidation state, speciation, and mobility of metal ions in aquatic environments.60 A few studies in the literature have examined the metal−ligand complexing in atmospheric samples based on thermodynamic data,57 but 7460

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(9) Reemtsma, T.; These, A.; Venkatachari, P.; Xia, X. Y.; Hopke, P. K.; Springer, A.; Linscheid, M. Identification of fulvic acids and sulfated and nitrated analogues in atmospheric aerosol by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. Anal. Chem. 2006, 78, 8299−8304. (10) Schmitt-Kopplin, P.; Gelencser, A.; Dabek-Zlotorzynska, E.; Kiss, G.; Hertkorn, N.; Harir, M.; Hong, Y.; Gebefugi, I. Analysis of the Unresolved Organic Fraction in Atmospheric Aerosols with UltrahighResolution Mass Spectrometry and Nuclear Magnetic Resonance Spectroscopy: Organosulfates As Photochemical Smog Constituents. Anal. Chem. 2010, 82, 8017−8026. (11) Wozniak, A. S.; Bauer, J. E.; Sleighter, R. L.; Dickhut, R. M.; Hatcher, P. G. Technical Note: Molecular characterization of aerosolderived water soluble organic carbon using ultrahigh resolution electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. Atmos. Chem. Phys. 2008, 8, 5099−5111. (12) Altieri, K. E.; Turpin, B. J.; Seitzinger, S. P. Oligomers, organosulfates, and nitrooxy organosulfates in rainwater identified by ultra-high resolution electrospray ionization FT-ICR mass spectrometry. Atmos. Chem. Phys. 2009, 9, 2533−2542. (13) Chang-Graham, A. L.; Profeta, L. T. M.; Johnson, T. J.; Yokelson, R. J.; Laskin, A.; Laskin, J. Case Study of Water-Soluble Metal Containing Organic Constituents of Biomass Burning Aerosol. Environ. Sci. Technol. 2011, 45, 1257−1263. (14) Laskin, A.; Smith, J. S.; Laskin, J. Molecular Characterization of Nitrogen-Containing Organic Compounds in Biomass Burning Aerosols Using High-Resolution Mass Spectrometry. Environ. Sci. Technol. 2009, 43, 3764−3771. (15) Smith, J. S.; Laskin, A.; Laskin, J. Molecular Characterization of Biomass Burning Aerosols Using High-Resolution Mass Spectrometry. Anal. Chem. 2009, 81, 1512−1521. (16) Reinhardt, A.; Emmenegger, C.; Gerrits, B.; Panse, C.; Dommen, J.; Baltensperger, U.; Zenobi, R.; Kalberer, M. Ultrahigh mass resolution and accurate mass measurements as a tool to characterize oligomers in secondary organic aerosols. Anal. Chem. 2007, 79, 4074−4082. (17) Sadezky, A.; Winterhalter, R.; Kanawati, B.; Rompp, A.; Spengler, B.; Mellouki, A.; Le Bras, G.; Chaimbault, P.; Moortgat, G. K. Oligomer formation during gas-phase ozonolysis of small alkenes and enol ethers: new evidence for the central role of the Criegee Intermediate as oligomer chain unit. Atmos. Chem. Phys. 2008, 8, 2667−2699. (18) Tolocka, M. P.; Jang, M.; Ginter, J. M.; Cox, F. J.; Kamens, R. M.; Johnston, M. V. Formation of oligomers in secondary organic aerosol. Environ. Sci. Technol. 2004, 38, 1428−1434. (19) Bateman, A. P.; Walser, M. L.; Desyaterik, Y.; Laskin, J.; Laskin, A.; Nizkorodov, S. A. The effect of solvent on the analysis of secondary organic aerosol using electrospray ionization mass spectrometry. Environ. Sci. Technol. 2008, 42, 7341−7346. (20) Heaton, K. J.; Sleighter, R. L.; Hatcher, P. G.; Hall, W. A.; Johnston, M. V. Composition Domains in Monoterpene Secondary Organic Aerosol. Environ. Sci. Technol. 2009, 43, 7797−7802. (21) Nguyen, T. B.; Bateman, A. P.; Bones, D. L.; Nizkorodov, S. A.; Laskin, J.; Laskin, A. High-resolution mass spectrometry analysis of secondary organic aerosol generated by ozonolysis of isoprene. Atmos. Environ. 2010, 44, 1032−1042. (22) Walser, M. L.; Desyaterik, Y.; Laskin, J.; Laskin, A.; Nizkorodov, S. A. High-resolution mass spectrometric analysis of secondary organic aerosol produced by ozonation of limonene. Phys. Chem. Chem. Phys. 2008, 10, 1009−1022. (23) Lin, P.; Engling, G.; Yu, J. Z. Humic-like substances in fresh emissions of rice straw burning and in ambient aerosols in the Pearl River Delta Region, China. Atmos. Chem. Phys. 2010, 10, 6487−6500. (24) Lin, P.; Huang, X. F.; He, L. Y.; Yu, J. Z. Abundance and size distribution of HULIS in ambient aerosols at a rural site in South China. J. Aerosol Sci. 2010, 41, 74−87. (25) Facchini, M. C.; Decesari, S.; Mircea, M.; Fuzzi, S.; Loglio, G. Surface tension of atmospheric wet aerosol and cloud/fog droplets in

consideration was limited to only inorganic ligands and a few simple organic ligands (e.g., oxalate) due to a lack of knowledge on molecular structures of potential ligands in the atmosphere. Many metal-containing organic species were detected in the WSOC fraction of biomass burning aerosols in a more recent study,13 implying the existence of metal−ligand complexes. Our study has identified HULIS as a group of organic compounds rich in ligand functional groups, and the molecular structural information inferred from the UHRMS data could provide guidance to future studies of metal−ligand interactions in atmospheric samples.

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ASSOCIATED CONTENT

S Supporting Information *

Analysis of probable influence of ammonia and methanol on the HULIS matrix (Appendix S1), experimental details of MS/ MS analysis (Appendix S2), six supporting figures (Figures S1− S6), and m/z data of all the assigned formulas. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: M.K. [email protected], J.Z.Y. jian. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was partly supported by the Research Grants Council of Hong Kong (621510) and the HKUST Oversea Research Awards for PhD students. We thank Prof. Zhenyang Lin for helpful discussions on metal-HULIS interactions.

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REFERENCES

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